Shallow Foundation and Deep Foundation

TYPES OF FOUNDATION MADE BY WEISIONG

Contents Chapter Subject

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Shallow foundation

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2

Deep Foundation

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Reference

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TYPES OF FOUNDATION a) Shallow Foundation System i) Spread Foundation ii) Mat / Raft Foundation b) Deep Foundation System i) Pile

iii) Diaphragham wall

ii) Pile walls

iv) Caissons

Chapter 1 i

Shallow foundation

Introduction

Those which transfer the loads to subsoil at a point near to the ground floor of the building such as strips and raft. A shallow foundation is a type of foundation which transfers building loads to the earth very near the surface, rather than to a subsurface layer or a range of depths as does a deep foundation. Shallow foundations include spread footing foundations, raft foundation known as mat-slab foundations, slab-on-grade foundations, strip foundations, buoyancy foundations, pad foundations, rubble trench foundations, and earth bag foundations. These foundation is according to BS 8004 : 1986.

Shallow foundations are taken to be those where the depth below finished ground level is less than 3 m and include strip, pad and raft foundations. Shallow foundations where the depth breadth ratio is high may need to be designed as deep foundations . Shallow foundations are those foundations that have a depth-of-embedment-to-width ratio of approximately less than four. Shallow foundations are those founded near to the finished ground surface; generally where the founding depth (Df) is less than the width of the footing and less than 3m. These are not strict rules, but merely guidelines: basically, if surface loading or other surface conditions will affect the bearing capacity of a foundation it is 'shallow'. Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts. Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc.

Advantage of using shallow foundation i. Cost is affordable. 2

ii. Construction procedure is simple.

iii. Materials is mostly concrete. iv. Labour does not need expertise. Disadvantages of using shallow foundation i. ii. iii. iv.

Settlement. Limit capacity soil structure. Irregular ground surface for example slope and retaining wall. Foundation subjected to pullout, torsion and moment.

Footing Foundations

A footing is that part of a structure which serves to transmit the weight of the structure to the natural subsoil. A footing that supports a single column is an isolated footing or a spread footing, one that support a group of columns is a combined footing and one that support a wall is a continuous or strip footing. The depth of footing, Df, is the vertical distance between the base of the footing and the ground surface. If Df is less width of the footing it is called a shallow footing. The behaviour of shallow continuous footings will be presented first.

COMBINATION FOOTINGS

Combination, or combined, footings are similar to isolated spread footings except that they support two or more columns and are rectangular or trapezoidal in shape (Figure 2-5). They are primarily used when the column spacing is non-uniform (Bowles, 1996) or when isolated spread footings become so closely spaced that a combination footing is simpler to form and construct. In the case of bridge abutments, an example of a combination footing is the socalled “spill-through” type abutment (Figure 2-6). This configuration was used during some of the initial construction of the Interstate freeways on new alignments where spread footings could be founded on competent native soils. Spill-through abutments are also used at stream crossings to make sure foundations are below the scour level of the stream. Due to the frame action that develops with combination footings, they can be used to resist large overturning or rotational moments in the longitudinal direction of the column row. There are a number of approaches for designing and constructing combined footings. The choice depends on the available space, load distribution among the columns supported by the footing, variations of soil properties supporting the footing and economics.

Types of Combined Footing: i.

Rectangular Combined Footing 3

ii.

Trapezoidal Combined Footing

iii.

Cantilever Footing

iv.

Mat Foundation ( raft foundation )

Ultimate Bearing Capacity

Terzaghi (1943 ) first published an approximate method of computing ultimate bearing capacity of soils. He made the following assumptions in his analysis.

i. The base of the footing is rough. This assumption is fully justified in practice. ii. The soil around the footing, above its base can be replaced by equivalent surcharge. iii. The footing is shallow. Terzaghi assumed that a footing is shallow if Df < B. If Df >

B the error because of neglecting shear resistance along AA' and BB' becomes appreciable. iv. The footing is continuous. This simplifies the analysis because the problem becomes two dimensional. For isolated and spread footings a correction factor based on experience has been recommended by Terzaghi. Criteria of Satisfactory Action of a Footing A footing must satisfy two general requirements which are as follow: i. The soil supporting the footing must be safe against shear failure. An adequate factor of safety is provided while assigning allowable loads to a footing. ii. The footing must not settle more than a specified amount. The usual procedure of design of a footing consists of the following : i. Select a suitable width and depth for the footing. ii. Determine the allowable soil pressure for maximum settlement of the footing. iii. Determine the ultimate bearing capacity of the soil. iv. Determine allowable soil pressure by applying a suitable factor of safety to ultimate bearing capacity. Test Cube 4

Design of Footings Load Maximum (Newton) Compression strength = ----------------------------------Brick width surface (PxL)

(P) --(A)

Depth of Footing The depth to which foundations should be carried depends upon the following factors : i. The securing of adequate bearing capacity. ii. In the case of clayey soils, depth of zone in which the shrinkage and swelling due to seasonal weather changes are likely to cause appreciable movement. iii. In fine sands and silts, depth of zone up to which frost trouble may be encountered. iv. The depth up to which excavations are likely in future in close proximity. Indian Standard Code of Practice, provides for a minimum depth 50 cms below the natural ground. Subject to the requirement of the code, the footing is ordinarily located at the highest point, where adequate bearing capacity is obtained. In some instances if an especially firm layer is available at a greater depth, it may be more economical to establish the footing at a lower elevation because the area required for the footing would be smaller. SPREAD FOOTING FOUNDATION

a. Also known as a footer or footing b.

It’s an enlargement at the bottom of a column/

c. bearing wall that spreads the applied structural loads over a sufficiently large soil

area.

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d. Each column & each bearing wall has its own spread footing, so each structure may

include dozens of individual footings.

SPREAD FOUNDATION

I.

The foundation consists of concrete slabs located under each structural column and a continuous slab under load-bearing walls.

II. For the spread foundation system the structural load is literally spread out over a

broad area under the building III. Most common type of foundation used due to their low cost & ease of construction.

IV. Most often used in small to medium size structure with moderate to good soil condition V. Spread footings may be built in different shapes & sizes to accommodate individual

needs such as the following: a) Square Spread Footings / Square Footings b) Rectangular Spread Footings c) Circular Spread Footings d) Continuous Spread Footings e) Combined Footings f) Ring Spread Footings

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a) Square Spread Footings / Pad Foundation - support a single centrally located column - use concrete mix 1:2:4 and reinforcement - the reinforcement in both axes are to resist/carry tension loads.

PAD FOUNDATION

b) Rectangular Spread Footings - Useful when obstructions prevent construction of a square footing with a sufficiently large base area and when large moment loads are present

c) Circular Spread Footings - are round in plan view - most frequently used as foundation for light standards, flagpoles and power transmission lines.

d)Continuous Spread Footings / Strip Foundation 7

- Used to support bearing walls

e) Combined Footings - support more than one column - useful when columns are located too close together for each to have its own footing f) Ring Spread Footings - continuous footings that have been wrapped into a circle - commonly used to support the walls above-ground circular storage tanks. - The contents of these tanks are spread evenly across the total base area and this weight is probably greater that the tank itself - Therefore the geotechnical analyses of tanks usually treat them as circular foundations with diameters equal to the diameter of the tank.

Factors of Safety For safety against bearing capacity failure a factor of safety of 3 is recommended under the probable maximum loads. For structures of minor importance and in subsoils of uniform character whose properties are very well known, a smaller factor of safety may be 8

used. A factor of safety of three is comparable with the value commonly used for the design of superstructure. Settlement According to Indian Standard Code of Practice referred to previously, for most of the ordinary concrete structures, such as office buildings, flats and factories, differential settlement may be permissible such that the angular distortion of the frame of the buildings does not exceed 1/250 normally and 1/1000 where it is particularly desired to avoid any kind of damage. Table below show the limiting total settlement of isolated footings, as recommended in the I.S code. Soil

Total Settlement

Non Cohesive

4.0 cm

Cohesive

6.5 cm

Table 1 Premissible Settlements of Isolated Footings Gross and net load The total load on the footing including its own weight is gross load, and the unit load is the grass intensity of loading. If however, the weight of the soil excavated for constructing the footing is deducted from the gross loads, it is called the net load and the unit load is then the net intensity of loading. Net ultimate bearing capacity It has been defined as pressure at the base of footing in excess of that at the same level due to surrounding surcharge. The use of net bearing capacity is sometimes more convenient in design.

Thus qa ( net ) = qa – yDf =cNc + ½ yBNy + yDf (Nq-1) According to Terzaghi’s equation. 9

Settlement

Types of Foundation Settlement The settlement of a foundation consists of two parts. They are i. Elastic settlement ( Se ) ii. Consolidation settlement ( Sc ) Elastic settlement is caused by the elastic deformation of dry soil and of moist and saturated soils without any change in moisture content. Consolidation divide into two group namely primary consolidation and secondary consolidation. Primary consolidation is the result of a volume change in saturated cohesive soils because of the expulsion of water that occupies the void spaces. Secondary consolidation settlement is observed in saturated cohesive soils and is the result of the plastic adjustment of soil fabrics. It follows the primary consolidation settlement under constant effective stress.

Figure 1.1 Elastic settlement profile.

Raft Foundation 10

RAFT FOUNDATION ➢ A foundation system in which essentially the entire building is placed on a large

continuous footing. ✔

It is a flat concrete slab, heavily reinforced with steel, which carries the downward loads of the individual columns or walls.

✔ Raft foundations are used to spread the load from a structure over a large area,

normally the entire area of the structure.

MAT/RAFT FOUNDATION

 It is normally consists of a concrete slab  which extends over the entire loaded area.  It may be stiffened by ribs or beams  incorporated into the foundation.  Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions.  They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.

Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. They are used when column loads or other structural loads are close together and individual pad foundations would interact. A raft foundation normally consists of a concrete slab which extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation. Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.

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Raft foundation are shallow foundation, is a combined footing that may cover the entire area under a structure supporting several columns and walls. Raft foundation are sometimes preferred for soils that have low load-bearing capacities but that will have to support high column and wall loads. Under some conditions, spread footings would have to cover more than half the building area, and raft foundations might be more economical. Raft foundations are used to

distribute heavy column and wall loads across the entire building area, to lower the contact pressure compared to conventional spread footings.

Raft foundations can be constructed near the ground surface, or at the bottom of basements. In high-rise buildings, raft-slab foundations can be several meters thick, with extensive reinforcing to ensure relatively uniform load transfer.This type of foundation is often used on poor soils of lightly loaded buildings and is capable of accommodating small settlement of soil. In poor soil the upper crust of soil (450600mm)is often stiffer than the lower subsoil and to build a light raft on this crust is usually better then penetrating it with a strip foundation. If the building loads or the allowable soil pressure low and the area of isolated footings exceeds about one half the area of the building it may be economical to provide a raft foundation. If the centre of gravity of the loads coincides with the centre of the raft, the distribution below the raft is usually assumed to be uniform. It has been shown that the pressure distribution below the base of a raft is generally not uniform. This may result in increased moments than those computed on the assumption of uniform pressure distribution.

If a raft foundation is located at such a depth that the unit load of the superstructure and the raft equals the weight of the soil removed, ( yDf ), it is called floating foundation. In this case there is no increase in the pressure intensity on the soil under the raft, and there is no problem of stability. Settlements are restricted to the extent of any swelling which might have taken place after excavation. The floating foundation is useful in every weak soil. Several types of raft foundations are currently used. Some of the common types are the following : i. Flat plate. The raft foundations is uniform thickness. ii. Flat plate thickened under column. iii. Beams and slab. The beams run both ways, and the columns are located at the intersection of the beams. 12

iv. Flat plates with pedestals. v. Slab with basement walls as a part of the raft foundations. The walls act as stiffeners for the raft foundations. Raft foundations may be supported by piles. The piles help in reducing the settlement of a structure built over highly compressible soil. Where the water table is high, raft foundations are often placed over piles to control buoyancy.

Suitably designed raft foundations may be used in the following circumstances For lightly loaded structures on soft natural ground where it is necessary to spread the load, or where there is variable support due to natural variations, made ground or weaker zones. In this case the function of the raft is to act as a bridge across the weaker zones. Rafts may form part of compensated foundations. ii. Where differential settlements are likely to be significant. The raft will require special design, involving an assessment of the disposition and distribution of loads, contact pressures and stiffness of the soil and raft c) Where mining subsidence is liable to occur. Design of the raft and structure to accommodate subsidence requires consideration by suitably qualified persons; the effects of mining may often involve provision of a flexible structure(NCB 1965). iii. When buildings such as low rise dwellings and lightly framed structures have to be erected on soils susceptible to excessive shrinking and swelling, consideration should then be given to raft foundations placed on fully compacted selected fill material used as replacement for the surface layers. iv. For heavier structures where the ground conditions are such that there are unlikely to be significant differential settlements or heave, individual loads may be accommodated by isolated foundations. If these foundations occupy a large part of the available area they may, subject to design considerations, be joined to form a raft. i.

Service connections The layout of service pipes, drains, etc., should be considered at the design stage so that the structural strength of a raft does not become unduly reduced by holes, ducts, etc. In soft soils particularly, provision should be made for relative movement where services and drains are supported partly on a raft and partly on soil, so that they may not be damaged by unequal settlement.The layout and design of service pipes and ducts should be such as to allow their future maintenance without the necessity for breaking through the raft. To design a raft foundations 1- Determine the bearing capacity of the foundation 2- Determine the settlement of the foundation 3- Determine the differential settlement 4- Determine the stress distribution beneath the foundation 5- Design the structural components of the mat foundation using the stress distribution obtained from (4). 13

From Step (4) a- The raft foundation is assumed to be a rigid foundation b- The raft foundation is assumed to be a Flexible Foundation; here use Beam on Elastic Support Method. Bearing Capacity of raft foundations: The gross ultimate bearing capacity of a mat foundation is same as for shallow foundations: qu = CNc F F F + q Nq F F F + 0.5 B N F F F The net ultimate capacity is qult,net= qult - q Factor of Safety: For sand and clay F.S. = 3 In most of the cases FS> 1.75 to 2 Bearing Capacity of Raft Foundations

The gross ultimate bearing capacity of a raft foundations can be determined by the same equation used for shallow foundation, or Qu = c'NcFcsFcdFci + qNqFqsFqdFqi + ½ y BNyFysFydFyi Rafts on sands Since the ultimate bearing capacity of sand increases with width ( B ) and rafts are usually of large dimensions, a bearing capacity failure of raft on sand is practically ruled out. Since a raft covers a large area, the stresses in the underlying soil are high to a considerable depth. The differential settlements of a raft are smaller than those of a footing under the same unit pressure as a raft eliminates the influences of local loose pockets. Hence higher allowable soil pressure can be adopted for raft design. It has been recommended ( Peck,Hansen and Thornburn 1953 ) that a pressure approximately twice as great as that allowed for individual footings may be used, because it does not lead to detrimental settlements. Therefore, a raft in sand can be designed by using the chart in figure below. The allowable pressure for raft foundations is twice the allowable pressure obtained from the charts.

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If the water table is located at the base of the raft, the allowable pressure calculated above is reduced by one half. For intermediate location of water table, a linear interpolation may be made. The value of N for fine sand below the water table should be corrected in accordance with equation ( N = 15 + ½ ( N' – 15 ) ). If N-value is less than 10, either the sand should be compacted before constructing a raft or a deep foundation should be used.

Chart 1.1 for estimating allowable load on footing in sands.

Raft on clay The ultimate bearing capacity of raft on clay is given by equation ( qd ( net ) = c Ne ). This is the ultimate bearing capacity in excess of the existing overburden. By increasing the depth of foundation, the pressure exerted by the building is correspondingly reduced. Thus in every soft soils, the factor of safety can be increased by increasing the depth of the foundation. The recommended factor of safety against failure is three. The settlement of the raft is checked by methods outlined.

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Figure 1.2: Raft Foundation and Raft Foundation for Wall

Figure 1.3: Common type of raft foundations Pad Foundation

Pad foundations are used to support an individual point load such as that due to a structural column. They may be circular, square or reactangular. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. Pad foundations are usually shallow, but deep pad foundations can also be used.

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This type of foundation is used to support and transmit the loads from piers and column.The most economiced plan shape is a square but if the columns are closer to the site boundry, it may be necessary to use a rectangular plan shape of equivalent area. The reaction of the foundation on the load and ground pressures is to cup, similar to a saucer and therefore main steel is required in both directions. The depth of the base will be governed by the anticipated moment and shear force, the calculation involved is beyond the scope of this volume. For buildings such as low rise dwellings and lightly framed structures, pad foundations may be of unreinforced concrete provided that the angle of spread of load from the pier or baseplate to the outer edge of the ground bearing does not exceed one (vertical) in one (horizontal) and that the stresses in the concrete due to bending and shear do not exceed those in Table 11 of Civil Engineering Code of Practice No. 2 1951. Where brick or masonry foundations have been used, the same rules apply with permissible stresses as given in Civil Engineering Code of Practice No. 2. For buildings other than low rise and lightly framed structures, it is customary to use reinforced concrete foundations. The thickness of the foundation should under no circumstances be less than 150 mm and will generally be greater than this to maintain cover to reinforcement where provided.

Figure 1.4: Type of Pad Foundation

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Strip Foundation Strip foundations are used to support a line of loads, either due to a load-bearing wall, or if a line of columns need supporting where column positions are so close that individual pad foundations would be inappropriate. The oldest and the most common form of foundation is a strip foundation where a trench is excavated, concrete placed in the bottom and the wall built upon it. The depth is determined by the need to place the strip below the level where expansion due to frost will affect its stability (usually 1m) and the nature of the sub-soil. The width is governed by the relationship between the imposed load and the bearing capacity of the ground and also by the practical necessity of making it wide enough for a man to work it. Similar considerations to those for pad foundations apply to strip foundations. On sloping sites strip foundations should be on a horizontal bearing, stepped where necessary to maintain adequate depth. In continuous wall foundations it is recommended that reinforcement be provided wherever an abrupt change in magnitude of load or variation in ground support occurs. Continuous wall foundations will normally be constructed in mass concrete provided that the angle of spread of load from the edge of the wall base to the outer edge of the ground bearing does not exceed one (vertical) in one (horizontal). Foundations on sloping ground, and where regrading is likely to take place, may require to be designed as retaining walls to accommodate steps between adjacent ground floor slabs or finished ground levels. At all changes of level unreinforced foundations should be lapped at the steps for a distance at least equal to the thickness of the foundation or a minimum of 300 mm. Where the height of the step exceeds the thickness of the foundation, special precautions should be taken. The thickness of reinforced strip foundations should be not less than 150 mm, and care should be taken with the excavation levels to ensure that this minimum thickness is maintained. For the longitudinal spread of loads, sufficient reinforcement should be provided to withstand the tensions induced. It will sometimes be desirable to make strip foundations of inverted tee beam sections, in order to provide adequate stiffness in the longitudinal direction. At corners and junctions the longitudinal reinforcement of each wall foundation should be lapped.In wide strip foundation is where the use of ordinary strip foundations would overstress the bearing strata, wide strip foundations designed to transmit the foundation loads across the full width of the strip may be used. The depth below the finished ground level should be the same as for ordinary strip foundations. For trench fill or deep strip footings is where the nature of the ground is such that narrow trenches can be neatly cut down to the bearing stratum, an economical foundation may be achieved by filling the trenches with concrete. When deciding the trench width, account should be taken of normal building tolerances in relation to setting out dimensions. Where the thickness of such a foundation is 500 mm or more, any step should be not greater than the concrete thickness and the lap at such a step should be at least 1 m or twice the step height, whichever is the greater.Where fill or other loose materials occur above the bearing stratum adequate support is required to any excavation. Consideration may be given to the use of lean mix mass concrete replacement under ordinary strip footings placed at shallow depth. This mass concrete can be poured against either permanent or recoverable shuttering. This form of foundation provides a method of dealing with local areas where deeper foundations are required.

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Figure 1.5: Strip Foundation Buoyancy foundations If a building is required to have a basement storey, the formation of that basement will involve the removal of a large quantity of sub-soil. The mass of this sub-soil, in many cases, can equal or exceed the total mass of the finished building and its designed imposed loads. If this occurs then the completed structure will impose no greater stress on the sub-soil through its basement floor than did the soil it replaced. Therefore the capacity of the soil to carry the load is not in doubt and the building 'floats' in the ground precisely the same way that an ocean liner floats in the sea. As the sub-soil at the levels to which this form of substructure is carried is almost always water-bearing the basement would need to be lined with a waterproof membrane which would then cause the upward hydrostatic pressure to counter-balance a proportion of the downward thrust of the building. 19

Figure 1.6 : Buoyancy foundations

Spread footing foundations

Spread footing foundations, or simply footings, consists of strips or pads of concrete (or other materials) which transfer the loads from walls and columns to the soil or bedrock. Embedment of spread footings is controlled by several factors, including development of lateral capacity, penetration of soft near-surface layers, and penetration through near-surface layers likely to change volume due to frost heave or shrink-swell.These foundations are common in residential construction that includes a basement, and in many commercial structures. But for high rise building it is not sufficient. Slab-on-grade foundations Slab-on-grade foundations are a structural engineering practice whereby the concrete slab that is to serve as the foundation for the structure is formed from a mold set into the ground. The concrete is then placed into the mold, leaving no space between the ground and the structure. This type of construction is most often seen in warmer climates, where ground 20

freezing and thawing is less of a concern and where there is no need for heat ducting underneath the floor. The advantages of the slab technique are that it is cheap and sturdy, and is considered less vulnerable to termite infestation because there are no hollow spaces or wood channels leading from the ground to the structure (assuming wood siding, etc., is not carried all the way to the ground on the outer walls). The disadvantages are the lack of access from below for utility lines, the potential for large heat losses where ground temperatures fall significantly below the interior temperature, and a very low elevation that exposes the building to flood damage in even moderate rains. Remodeling or extending such a structure may also be more difficult. Over the long term, ground settling (or subsidence) may be a problem, as a slab foundation cannot be readily jacked up to compensate; proper soil compaction prior to pour can minimize this. The slab can be decoupled from ground temperatures by insulation, with the concrete poured directly over insulation (for example, Styrofoam panels), or heating provisions (such as hydronic heating) can be built into the slab (an expensive installation, with associated running expenses). Slab-on-grade foundations are commonly used in areas with expansive clay soil, particularly in California and Texas. While elevated structural slabs actually perform better on expansive clays, it is generally accepted by the engineering community that slab-on-grade foundations offer the greatest cost-to-performance ratio for tract homes. Elevated structural slabs are generally only found on custom homes or homes with basements.Care must be taken with the provision of services through the slab. Copper piping, commonly used to carry natural gas and water, reacts with concrete over a long period, slowly degrading until the pipe fails. Copper pipes must be lagged (that is, insulated) or run through a conduit or plumbed into the building above the slab. Electrical conduits through the slab need to be water-tight, as they extend below ground level and can potentially expose the wiring to groundwater. The rubble trench foundation The rubble trench foundation, a construction approach popularized by architect Frank Lloyd Wright, is a type of foundation that uses loose stone or rubble to minimize the use of concrete and improve drainage. It is considered more environmentally friendly than other types of foundation because cement manufacturing requires the use of enormous amounts of

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energy. However, some soil environments (such as particularly expansive or poor loadbearing (< 1 ton/sf) soils) are not suitable for this kind of foundation. A foundation must bear the structural loads imposed upon it and allow proper drainage of ground water to prevent expansion or weakening of soils and frost heaving. While the far more common concrete foundation requires separate measures to ensure good soil drainage, the rubble trench foundation serves both foundation functions at once. To construct a rubble trench foundation a narrow trench is dug down below the frost line. The bottom of the trench would ideally be gently sloped to an outlet. Drainage tile, graded 1":8' to daylight, is then placed at the bottom of the trench in a bed of washed stone protected by filter fabric. The trench is then filled with either screened stone (typically 1-1/2") or recycled rubble. A steel-reinforced concrete grade beam is poured at the surface to provide ground clearance for the structure. If an insulated slab is to be poured inside the grade beam, then the outer surface of the grade beam and the rubble trench should be insulated with rigid XPS foam board, which must be protected above grade from mechanical and UV degradation. The rubble-trench foundation is a relatively simple, low-cost, and environmentally-friendly alternative to a conventional foundation, but may require an engineer's approval if building officials are not familiar with it. Frank Lloyd Wright used them successfully for more than 50 years in the first half of the 20th century, and there is a revival of this style of foundation with the increased interest in green building. Earthbag foundation The basic construction method begins by digging a trench down to undisturbed mineral subsoil. Rows of woven bags (or tubes) are filled with available material, placed into this trench, compacted with a pounder to around 1/3 thickness of pre-pounded thickness, and form a foundation. Each successive layer will have one or more strands of barbed wire placed on top. This digs into the bag's weave and prevents slippage of subsequent layers, and also resists any tendency for the outward expansion of walls. The next row of bags is offset by half a bag's width to form a staggered pattern. These are either pre-filled with material and delivered, or filled in place (often the case with Superadobe). The weight of this earth-filled bag pushes down on the barbed wire strands, locking the bag in place on the row below. The

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same process continues layer upon layer, forming walls. A roof can be formed by gradually sloping the walls inward to construct a dome. Traditional types of roof can also be made.

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Chapter 2 Deep Foundation

Introduction  Extend several dozen feet below the building a) Piles b) Piers c) Caissons d) Compensated Foundation

Deep foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. They include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations. Deep foundations can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface. Those which transfer the loads to a subsoil some distance below the ground floor of the building such as piles. A deep foundation is a type of foundation distinguished from shallow foundations by the depth they are embedded into the ground. They are either endbearing if they extend all of the way to rock or hard soil, or they are friction piles if they are mainly supported by friction along the sides, although friction piles also usually develop some end support. Timber piles are driven top down to take advantage of their taper for increasing friction, and a taper often is incorporated of their taper for increasing friction, and a taper often is incorporated into concrete piles. Compaction piles are driven into loose san to densify it and increase its bearing capacity. Modern piles are wood,steel,concrete, or composite if they are composed of more than one material. An example of a composite pile is when a wood pile section is used under a groundwater table where it is preserved by reducing 24

condition, and is connected to a concrete section that extends through aerobic surroundings above the water table.There are many reasons a geotechnical engineer would recommend a deep foundation over a shallow foundation, but some of the common reasons are very large design loads, a poor soil at shallow depth, or site constraints (like property lines). There are different terms used to describe different types of deep foundations including the pile (which is analogous to a pole), the pier (which is analogous to a column), drilled shafts, and caissons. Piles are generally driven into the ground in situ; other deep foundations are typically put in place using excavation and drilling. Whereas piles are driven or can be jacked into the ground, piers are large-diameter supports that are placed in pre-bored holes. Caissons are large tubes used for construction below water, as in rivers, and may be pressurized to keep water out. Caisson foundation sometimes is applied to bored piers, and piers also are sometimes called shafts or piles, leading to some deep confusion. The naming conventions may vary between engineering disciplines and firms. Deep foundations can be made out of timber, steel, reinforced concrete and prestressed concrete. These foundation is according to BS 8004 : 1986 which mean more than 3 meter. However, the use of deep foundations may permit much higher loadings, reduce settlements, give greater certainty of support and allow the use of additional facilities. For example, underground car parks, thus increasing the value of the development.

Figure 2.1 Deep,intermediate, and shallow foundations. Types of deep foundation Deep foundations may be classified as follows. i. Deep pad or strip footings. ii. Basements or hollow boxes (compensated foundations). 25

iii. Caissons. These may be open well caissons or pneumatic caissons. iv. Cylinders and piers. These may be excavated in the dry by hand, by mechanical

means or, in the wet by mechanical means including boring and slurry wall techniques. v. Piles. vi. Peripheral walls. Concrete walls constructed in a slurry-filled trench or as contiguous bored piles, as well as being used as basement or retaining walls, may also carry vertical loads in conjunction with their retaining functions. vii. Mixed foundations. These may be a combination of any of a) to f). viii.Ground improvement or replacement. Where the ground does not have adequate bearing characteristics and stability, consideration may be given to general or local improvement of the bearing characteristics or to replacement of the ground in depth. It may then be possible to use a shallow foundation or a cheaper type of deep foundation. Basements or hollow boxes Basement or hollow boxes is the groundwater table is at or below formation level or can be lowered economically for construction, then a basement or hollow box foundation may be constructed in open excavation if the site is large enough. The techniques are then very similar to those for building above ground level, except that special attention will have to be paid to water-proofing when a watertight structure is required and allowance will need to be made for the lateral earth pressure and any negative skin friction which may develop after backfilling. The effect of possible fluctuations in the groundwater table and of flooding during and after construction should be considered.Where there is insufficient room for open slopes or where the groundwater table is inconveniently high, some form of water retaining structure such as a cofferdam or diaphragm wall will be necessary. It should be borne in mind that constructing a basement below groundwater level is a difficult undertaking so one should consider whether the required accommodation can be provided elsewhere. Basement construction can also present problems of watertightness, even when not below the water table (as defined by site investigation) since the water table can vary at a later date, and burst water mains are always possible. A basement or part basement designed to be merely damp-proof is likely to leak if there is a positive head of water, even if this is only temporary. Compensated foundations is addition to the extra space gained by basement construction a further advantage is the reduction in net loading applied to the supporting ground by the removal of the excavated soil and water. Tall structures may then be built on ground which would otherwise be incapable of carrying the same load on shallow foundations without excessive settlement or failure (D’Appolonia and Lambe 1971). A foundation in which the weight of the structure and substructure, including any imposed loading, balances the total weight of the excavated soil and water such that the net applied load is zero is said to be fully compensated. When the gross load exceeds the weight of the excavated soil and water, as is generally the case, the foundation is partially compensated, the degree of compensation being expressed by the ratio of the total excavated weight to gross load. A foundation or structure in which the total excavated weight exceeds the gross load is over-compensated. Where the variation in the gross load is significant, consideration may be given to varying the degree of compensation so as to achieve a more uniform distribution of net loading. The problems of heave and swell of clay soils have to be 26

carefully considered as well as buoyancy at times of possible high groundwater levels, particularly in the case of over-compensated foundations such as empty submerged tanks, swimming baths and buried garages. The depth of the basement is economically limited by the stability of the excavation and the question of groundwater control (D’Appolonia and Lambe 1971). Considerably deeper excavations are easier in stronger soils than in weaker soils, but even in the former case support measures are necessary for the walls of deep excavations. Deep foundations can be constructed in soft soils, for example by sinking cellular units in which the soil is removed from within the cells until the foundation sinks to the required depth; but even here the depth is limited by the question of base stability and the inflow of ground, particularly into the perimeter cells. Many foundations of this type below a high groundwater level will become buoyant when partially completed if the groundwater is allowed to rise too early in the construction programme. Relief wells and/or bleeder holes may be introduced into the bottom slab in order to prevent uplift when the groundwater level is not reduced externally. Alternatively, the bottom slab may be loaded temporarily with soil or building materials. Where site conditions permit and the ground at formation level provides adequate bearing for constructionaloperations, and where use can be made of the extra space, the partially compensated foundation may show some economy over other forms of deep foundation. However, where methods of construction such as the sinking of cellular units are necessary, these foundations are rarely economic unless the depth to an adequate bearing stratum is great. A further advantage of compensated foundations lies in the possibility of reduction in the long-term settlement in compressible soils; the greater the degree of compensation, the smaller the long-term settlement. It should be recognized, however, that with increasing compensation the problems of heave, swell, external ground movement and groundwater control become rapidly more severe. If there is appreciable heave on excavation, the settlement may be large even if the net ground movement is small (Burland et al. 1977). Caissons  It’s a prefabricated hollow box or cylinder.  It is sunk into the ground to some desired depth and then filled with concrete thus

forming a foundation.  Most often used in the construction of bridge piers & other structures that require foundation beneath rivers & other bodies of water.  This is because caissons can be floated to the job site and sunk into place.  Basically it is similar in form to pile foundation but installed using different way used

when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat.  It’s a form of deep foundation which are constructed above ground level, then sunk to

the required level by excavating or dredging material from within the caisson.  A caisson foundation consists of concrete columns constructed in cylindrical shafts

excavated under the proposed structural column locations. 27



Caissons are drilled to bedrock or deep into the underlying strata if a geotech eng. find the soil suitable to carry the building load.

 It’s created by auguring a deep hole in the ground.  Then, 2 or more ‘stick’ reinforcing bar are I inserted into and run the full length of

the hole and the concrete is poured into the caisson hole.  The caisson foundations carry the building loads at their lower ends, which are often

bell-shaped. Caissons are a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson. Caissons are structural elements of a foundation which are wholly or partially constructed at a higher level (sometimes in a different position as well) and are then sunk to their final position by various expedients. They are used when the final foundation level is at some depth below the water table. They may consist of a single cell or be subdivided into a number of cells. Open well caissons (monoliths) are sunk to their final position by excavating or dredging material from within the cells. Pneumatic caissons are provided with an airtight deck and airlock; they are sunk by excavation from within, under compressed air which is kept at sufficient pressure to exclude water. Physiological restrictions prevent the use of pneumatic caissons to much more than 25 m below water level. Open well caissons have no such restrictions and have been used to 80 m below water level. Caissons are frequently used for bridge piers, particularly where the foundation needs to be some depth below sea or river bed level to avoid the effects of scour at flood times. Caissons are sometimes sunk through artificial sand islands in order to make possible the construction of the caisson from a working level above water level; they have also been used to form the foundations for buildings. Owing to the high cost of labour in working under compressed air, other possible forms of foundation should be investigated carefully before a pneumatic caisson is adopted. In addition, certain hazards to health exist which have to be considered.

Cylinders and piers PIERS  It’s a vertical bridge support. 28

 It’s a foundation for carrying a heavy structural load which is constructed in site in a

deep excavation. Piers are foundations for carrying a heavy structural load which is constructed insitu in a deep excavation. Cylinders are essentially small open well caissons comprising single cells which are of circular cross section. (The term pier is sometimes used for similar foundations of circular or other shaped cross section.) The distinction between cylinders and caissons is one of size and is necessarily arbitrary. Because of their smaller size, cylinders lend themselves more readily to the use of precast elements in their construction. For example, complete precast rings may be added at the top as the cylinder is being sunk. Cylinders are sometimes constructed by the method in which the ground beneath the lower edge of the cylinder so far constructed is removed and a further ring lowered in sections is installed below it, and so on. This method is likely to be employed only in appropriate ground conditions in the dry, or where pumping, grouting or freezing is practicable. Another method is to excavate to a small depth with an unsupported face, then to place formwork, behind which concrete is placed to support the soil. Below the water table, methods of sinking cylinders are usually similar to those used for open well caissons. Sinking cylinders (or large bored piles) by mechanical drilling or grabbing methods has largely superseded hand methods of excavation. Cylinders are often filled with concrete and are sometimes reinforced.

Compensated foundations Compensated foundations are deep foundations in which the relief of stress due to excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement. Peripheral walls With such walls, formed by excavating under a slurry, care is necessary to ensure that slurry is not trapped between the concrete and the base of the excavation. Diaphragm walls are relatively slender structural members and they should be capable of withstanding earth pressure and hydrostatic pressure at each stage in the sequence of excavation and support by anchorages and by temporary and permanent shoring. 29

At the final stages of excavation of a substructure with peripheral diaphragm walls, the walls are restrained from inward movement by the passive resistance of the soil below the base of the excavation until such time as the floor slab of the substructure is placed. The depth of penetration of the peripheral walls should be adequate to mobilize the required passive resistance of the soil and also to prevent erosion by seepage of water beneath the toe of the wall into the excavation. Where inclined ground anchors are used to provide support against external pressures, the thickness of the wall at the toe should be sufficient to prevent excessive settlement due to forces induced by the vertical component of the anchor stress. The thickness of the wall at the toe should also be sufficient to prevent excessive settlement resulting from vertical loading applied to the crest or intermediate levels in a wall from floors or columns. Diaphragm wall techniques are used to form load bearing foundation structures (sometimes referred to as barrettes) which may be of rectangular, cruciform, hollow box or other plan shapes (Corbett et al. 1974). Mixed foundations on non-uniform sites When site investigation indicates that ground conditions are not uniform, it is important to provide the type and size of foundations appropriate to the ground conditions existing on the site, and it should be recognized that the extent of ground variation is not always fully revealed in a site investigation. On any site further information about ground conditions becomes available during construction, and on a non-uniform site information can often make changes in foundations necessary. A typical example is a change in level of the ground-bearing stratum, revealed during foundation construction, leading to revised reinforcement requirements and problems in coping with groundwater, with consequent delays on site. A form of construction which can easily be varied to suit the local ground conditions has advantages in such a case. Conversely, if the choice of foundations is of a kind that cannot be easily modified to suit variation in ground conditions, then a more sensitive site investigation would be appropriate. A knowledge of previous site history and site geology are helpful in providing a sense of the extent of variation of ground conditions that may be expected. When ground conditions are not uniform, the use of more than one type of foundation could result in greater economy. For example, if an old buried river channel crosses a site where a satisfactory bearing stratum exists elsewhere near ground level, it may be economical to use shallow foundations combined with piles in the deeper part. When using mixed foundations, special attention will need to be paid to the effect on the structure, particularly where differential settlements can occur. If the foundation and the supporting ground are looked at as one entity, as they should be, it will be seen that any foundation may behave in a complex way. In particular, differential settlements may occur for a variety of reasons. However, there is generally some uncertainty as to what extent a calculated settlement relates to the actual settlement which will occur, and this uncertainty may lead to the decision to articulate or separate the superstructure into sections. Articulation of a superstructure, however, may easily prove ineffective if the forces required to initiate joint movement are excessive so that the building may be damaged without movement or damage in the joint areas. Since the cross-sectional area of a joint filler may be considerable, the force required to compress a joint filler by, for example, 50 % could amount to several tonnes and there may thus be less resistance to damaging the structure elsewhere rather than compressing the filler. 30

Pile foundation

 A slender, structural member consisting steel or concrete or timber.  It is installed in the ground to transfer the structural loads to soils at some significant

depth below the base of the structure.

They include large bored piles which normally differ in scale but not in principle from bored piles of conventional size. The piles may be divided into the following categories depending upon their use : i. Bearing piles is a pile when a pile passes through a poor material and its either rests on hard stratum like rock or penetrates a small distance into a stratum of good bearing capacity, it is called a bearing pile. ii. Friction piles is a pile when the piles are driven through a soft soil and develop their carrying capacity by friction on the sides of the piles, they are called friction piles.

Piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.

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Piles are structural members that are made of steel,concrete, or timber. They are used to build pile foundations, which are deep and which cost more than shallow foundations. Despite the cost, the use of piles often is necessary to ensure structural safety. A pile can be loosely defined as a column inserted in the ground to transmit the structural loads to a lower subsoil. Piles have been used in contact two hundred years ago and until the twentieth century were invariably of driven timber. The following list identifies some of the conditions that require pile foundations ( Vesic, 1977 ) i. When one or more upper soil layers are highly compressible and to weak to support the load transmitted by the superstructure, piles are used to transmit the load to underlying bedrock or a stronger soil layer. When bedrock is not encountered at a reasonable depth below the ground surface, piles are used to transmit the structural load to the soil gradually. The resistance to the applied structural load is derieved mainly from the frictional resistance developed at the soil-pile interface. ii. When subjected to horizontal forces, pile foundations resist by bending, while still supporting the vertical load transmitted by the superstructure. This type of situation is generally encountered in the design and construction of earth-retaining structures and foundations of tall structures that are subjected to high wind or to earthquake forces. iii. In many cases, expansive and collapsible soils may be present at the site of a proposed structure. These soils may extend to a great depth below the ground surface. Expansive soils swell and shrink as their moisture content increases and decreases, and the pressure of the swelling can be considerable. If shallow foundations are used in such circumstances, the structure may suffer considerable damage. However, pile foundations may be considered as an alternative when piles are extended beyond the active zone, which is where swelling and shrinking occur. Soils such as losess are collapsible in nature. When the moisture content of these soils increases, their structures may break down. A sudden decrease in the void ratio of soil induces large settlement of structures supported by shallow foundation. In such cases, pile foundations may be used in which the piles are extended into stable soil layers beyond the zone where moisture will change. iv. The foundation of some structures, such as transmission towers, offshore platforms, and basement mats below the water table, are subjected to uplifting forces. Piles are sometimes used for these foundations to resist the uplifting force.

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v. Bridge abutments and piers are usually constructed over pile foundations to avoid the loss of bearing capacity that a shallow foundation might suffer because of soil erosion at the ground surface. Although numerous investigations, both theoretical and experimental, have been conducted in the past to predict the behavior and the load-bearing capacity of piles in granular and cohesive soils, the mechanisms are not yet entirely understood and may never be. The design and analysis of pile foundations may thus be considered somewhat of an art as a result of the uncertainties involved in working with some subsoil conditions. The main function of bearing piles is to transfer the load to lower levels of the ground which are capable of sustaining the load with an adequate factor of safety and without settling at the working load by an amount detrimental to the structure that they support. Piles derive their carrying capacity from a combination of friction along their sides and end bearing at the pile point or base. The former is likely to predominate for piles in clays and silts and where long sockets are formed in soft rocks. The latter applies to piles terminating in a stratum such as compact gravel, hard clay or rock. When friction piles are installed in a deep deposit of fairly uniform consistency in order to transfer the foundation pressure to the lower levels, they should be long enough to ensure a substantial advantage over a shallow foundation. In these circumstances it should be borne in mind that for the same superficial area of pile surface, a few long piles forming a piled foundation are more effective and will support the load with smaller settlements than many short piles. Piles should be installed to the prescribed depth, resistance or set per blow without damage to the pile shafts or the bearing stratum and records of the installation process should be maintained. The piles should be able to carry their design loads without exceeding the permissible working stresses in the material of the pile, but the stresses during driving may exceed these. The stresses during pitching and handling should be within the safe bending stresses prescribed in the design. The load should be applied concentrically with the axis of the pile or the centre of gravity of the pile group. Allowance should be made in the design for inaccuracies in positioning the piles, particularly for isolated single piles or pairs of piles. Where permanent eccentric loads have to be carried, due allowance should be made in the design. In certain situations vertical piles may be subjected to horizontal or eccentric loads. If the ground is unable to resist the resultant forces without excessive lateral movement, additional resistance should be provided. This can be achieved by increasing the number of vertical piles or increasing their stiffness, by the use of raking piles or ground beams or by replacing the upper soil layer with soil more capable of resisting horizontal forces. Special consideration should be given to effects such as ground heave due to frost or the behaviour of expansive clays, temperature variations in the ground and alterations in groundwater level, in case they are detrimental to the bearing capacity or effectiveness of the piles. Temperature changes may also affect the horizontal dimensions of the superstructure, thereby distorting the piles and causing additional stresses.

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Figure 2.2: Piling

Classification of Piles Piles may be classified by the way in which they transmit their loads to the subsoil or by the way they are formed. Piles may transmit their loads to a lower level by: a) End Bearing The shafts of the piles act as column carrying the loads through the overlaying weak subsoil to firm strata into which pile toe has penetrated. This can be a rock strata or a layer of the firm sand or gravel which has been compacted by the displacement and vibration encountered during the drive.

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Figure 2.2: Friction Pile b) Friction Any type of foundation imposes on the ground a pressure which spreads out to form a bulb of pressure. If a suitable load bearing strata cannot be found at an acceptable level, particularly in stiff clay soils, it is possible to use a pile to carry this bulb of pressure to a lower level where a higher bearing capacity is found. The friction of floating pile is mainly supported by the adhesion or the friction action of the soil around the perimeter of the pile shaft.

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Figure 2.3: End Bearing Pile Types of bearing piles The classification of bearing piles is related to the effect on the soil. There are two main types: displacement piles and replacement piles. a) Diplacement Piles A displacement pile is either driven, jacket , vibrated or screwed into the ground. This section displaces the soil outwards and downwards but the material is not actually removed. There are two types of displacement pile: large displacement piles which includes all solid driven piles and small displacement pile, in which very little soil is displaced. This would include the screwed piles and H piles.

b) Replacement Piles Replacement piles may be classified as Supported or Unsupported.In both cases a hole is formed in the ground by some form of cutting or boring tool and is then filled with reinforced concrete. The unsupported hole will normally require a short tube at the top to prevent debris from falling into the concrete during placing. Support to holes may be provided by means of medium or heavy sectional casing, sscrewed together as boring proceeds,or by means by a head of drilling mud (usually bentonite suspension). Types Of Pile Driven Piles a) Timber piles Timber piles are tree trunks that have had their branches and bark carefully trimmed off. The maximum length of most timber piles is 30 ft to 65ft. to qualify for use as a pile, the timber should be straight, sound, and without any defects. Timber piles are usually square sawn hardwood or softwood in lengths up to 12.000m in sections,with sizes ranging from 225 x 225 mm to 600mm x 600 mm .Most timber piles are fitted with an iron or steel driving shoe 36

and have an iron ring around the head to prevent splitting due to driving. Although not particularly common they are used in sea defences such as groynes and sometimes as guide piles for large in conjunction with steel sheet piling. The American Society of Civil Engineers “Manual of Practice, No. 17 ( 1959 ) divided timber piles into three classes : i. Class A piles carry heavy loads. The minimum diameter of the butt should be 14 in. ii. Class B piles are used to carry medium loads. The minimum butt diameter should be 12 to 13 in. iii. Class C piles are used in temporary construction work. They can be used permanently for structures when the entire pile is below the water table. The minimum butt diameter should be 12 in. In any case, a pile tip should not have a diameter less tan 6 in. Timber piles cannot withstand hard driving stress; therefore, the pile capacity is generally limited. Steel shoes may be used to avoid damage at the pile tip ( bottom ). The tops of timber pile may also be damaged during the driving operation. The crushing of the wooden fibers caused by the impact of the hammer is referred to as brooming. To avoid damage to the top of the pile, a metal band or a cap may be used. The usual length of wooden piles is 15 ft to 50 ft. The maximum length is about 100 ft to 130 ft. The usual load carried by wooden piles is 65 kip to 115 kip.

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Load bearing capacities can be up to 350 kn per pile depending upon section size and or species.There are two types of timber piles: Natural logs named as Bakau Piles, and treated timber piles which are chemically treated against the decay. b) Bakau Piles The bakau pile is generally tapered and has a diameter of 75 to 125mm.. The piles are generally used as friction piles at poor ground condition which have a high ground water table.The bakau piles are generally used for light buildings (column load of approximately 30 tonnes).Suitable in soft clay areas. c) Treated Timber Pile The piles are made from kempas, a kind of broadleaf tree. The cross sectional area of the pile is 5 inches by 5 inches and six inches by 6 inches,and the pile is 20 to 24 feet long. The permissible degree of bow or wrap of the pile within 20 feet long is 1½ inches from a straight axis through the pile. The permissible degree of wrap of a pile more than 20 feet long is 2 inches. Design working ads of 5 inches by 5 inches piles an d6 inches by 6 inches piles are the 15 ton/pile and the 20 tonnes respectively. d) Composite Piles The upper and lower portions of composite piles are made of different materials. For example, composite piles may be made of steel and concrete or timber and concrete steeland-concrete piles consist of a lower portion of steel and upper portion of cast-in-place concrete. This type of pile is used when the length of the pile required for adequate bearing exceeds the capacity of simple cast-in-place concrete piles. Timber-and-concrete piles usually consist of a lower portion of timber pile below the permanent water table and an upper portion of concrete. In any case, forming proper joints between two dissimilar materials is difficult, and for that reason, composite piles are not widely used.It is composite wood and concrete pile. The timber is kept below groundwater and a greater over-all length is achieved. A closed-end pipe pile may be used in place of the timber section. Combination of two or more of a preceding type or combination of different materials in the same type of pile. 38

Composite piles are used in ground conditions where conventional piles are unsuitable or uneconomical concrete and timber are the type used because it is cheap and easy to handle of the timber piles with the durability concrete. The timber is terminated below ground water level and the an upper portion formed in concrete. e) Steel Piles Steel piles, like timber, are driven by percussion means and have a variety of suitable cross-sections. In addition to the common sheet piles, the three main types are H sections, Box piles and tube piles. The main use of steel piles is for temporary works, retaining walls and marine structures. The problem of corrosion of the steel can be overcome by suitable protection. Sheet piles have the advantages of being robust, light to handle capable of carrying high compressive loads when driven on to a hard stratum, and capable of being driven hard to a deep penetration to reach a bearing stratum or to develope a high skin frictional resistance, although their cost per metre run is high compared with precast concrete piles. f) Concrete Piles Concrete piles may be divided into two basis categories to precast piles and cast-in-situ piles. Precast piles can be prepared by using ordinary reinforcement, and they can be square or octagonal in cross section. Reinforcement is provided to enable the pile to resist the bending moment developed during pickup and transportation, the vertical load, and the bending moment caused by a lateral load. The piles are cast to desired lengths and cured before being transported to the work sites. Some general facts about concrete piles are as follow : i. Usual length : 30 ft to 50 ft ii. Usual load : 65 kip to 675 kip iii. Advantages : a) Can be subjected to hard driving b) Corrosion resistant c) Can be easily combined with a concrete superstructure i. Disadvantages : a) Difficult to achieve proper cutoff 39

b) Difficult to transport Precast piles can also be prestressed by the use of high-strength steel prestressing cables. The ultimate strength of these cables is about 260ksi. During casting of the piles, the cable are pretensioned to about 130 to 190 ksi, and concrete is poured around them. After curing, the cables are cut, producing a compressive force on the pile section. Precast concrete piles used on medium to large contracts where soft soils overlaying a firm strata are uncountered and at least 100 piles will be required. The precast concrete driven pile has a little frictional bearing strength since the driving operation moulds the cohesive soils around the shaft which reduces the positive frictional resistance. Some general facts about precast prestressed piles are as follow : i. Usual length : 30 ft to 150 ft ii. Maximum length : 200 ft iii. Maximum load : 1700 kip to 1900 kip The advantages and disadvantages are the same as those of precast piles. Cast-in-situ,or cast-in-place piles are built by making a hole in the ground and then filling it with concrete. Various types of cast-in-place concrete piles are currently used in construction, and most of them have been patented by their manufacturers. These piles may be divided into two broad categories cased and uncased. Both types may have a pedestal at the bottom. Cased piles are made by driving a steel casing into the ground with the help of a mandrel placed inside the casting. When the pile reaches the proper depth the mandrel is withdrawn and the casting is filled with concrete. The pedestal is an expanded concrete bulb that is formed by dropping a hammer on fresh concrete. Some general facts about cased cast-in-place piles are as follow : i. Usual length : 15 ft to 50 ft ii. Maximum length : 100 ft to 130 ft iii. Usual load : 45 kip to 115 kip iv. Approximate maximum load : 180 kip v. Advantages : a) Relatively cheap b) Allow for inspection before pouring concrete c) Easy to extend i. Disadvantages : a) Difficult to splice after concreting 40

b) Thin casings may be damaged during driving

Driven Cast-In-Place Pile i. Driven cast-in-place pile installed by driving,to the desired penetration,a close ended steel tube or concrete shell and the void created is filled with concrete. Steel tube or concrete shell can be withdrawn or left in place. ii. Readily adjustable in length to suit the desired depth of penetration. iii. Economic if no casing required. Bored Cast-In-Place Pile i. A borehole is formed in the ground by sugar etc and the void concrete to form bored pile. ii. Usual sizes varies from 400mm diameter to 1000mm diameter. iii. Allowable load varies from 800kN to 1500kN. iv. Length of bored piles easily adjustable to suit the penetration depth. v. Suitable in redidual soil vi. Uses high slump self compacting concrete. vii. Trem concreting if water in borehole. The uncased piles are made by first driving the casing to the desired depth and then filling it with fresh concrete. The casing is then gradually withdrawn. Following are some general facts about uncased cast-in-place concrete piles : i. Usual length : 15 ft to 50 ft ii. Maximum length : 100 ft to 130 ft iii. Usual load : 67 kip to 115 kip iv. Approximate maximum load : 160 kip v. Advantages : a) Initially economical b) Can be finished at any elevation i. Disadvantages : a) Voids may be created if concrete is placed rapidly 41

b) Difficult to splice after concreting c) In soft soils, the sides of the hole may cave in, squeezing the concrete g) Steel H- Section Piles H section piles are in the form of wide- flanged steel section and rolled in accordance with standard. The displacement piles, and the H section piles may be driven by any type of hammer, but the head of the pile should be protected by a helmet.

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Reference Code of practice for Foundations —(Formerly CP 2004) BS 8004 : 1986 Foundations.pdf Module C2001 Engineering Construction and Material 2 http://en.wikipedia.org/wiki/Shallow_foundation http://en.wikipedia.org/wiki/Deep_foundation http://isddc.dot.gov/OLPFiles/FHWA/010943.pdf http://www.eng.fsu.edu/~tawfiq/ceg4111/ShallowFoundation.html Soil mechanics and foundation engineering by Bharat Singh and Shamsher Prakash Introduction to Geotechnical Engineering by Braja M. Das Geotechnical Engineering Soil And foundation Principles And Practice by Richard L. Handy and M. G. Spangler Kejuruteraan Asas By Bujang B.K Huat, Shukri Maail and Azlan A.Aziz http://www.slideshare.net/stootypal/types-of-foundation http://environment.uwe.ac.uk/geocal/foundations/Fountype.htm http://www.ornl.gov/sci/roofs+walls/foundation/ORNL_CON-295.pdf

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