Under Reamed Pile-5108

Construction Technology-IV

Under-Reamed Pile

Under-Reamed Pile 1. GENERAL: Under-reamed piles are (1) bored cast-in-situ pile and (2) bored compaction concrete piles having one or more bulbs formed by suitably enlarging the borehole of the pile stem. With the provision of bulbs, sufficient bearing or anchorage is available. Typical details of bored cast-in-situ under-reamed pile foundation are shown in figure 1.0.

Ø1 2 1a. Section of single Under-Reamed Pile

u=

Normally 2.5 D 1b.Section of Multi Under-Reamed Pile

Fig 1 .TYPICAL DETAILS OF BORED CAST in situ UNDER-REAMED PILE Foundation All dimensions are in millimetres

Ankit Patel (5108)

Construction Technology-IV

Under-Reamed Pile

These piles find application in widely varying situations in different types of soils where foundations are required to be taken down to a certain depth in view of considerations like the need (a) To avoid the undesirable effect of seasonal moisture changes as in expansive soils; (b) To reach firm strata; (c) To obtain adequate capacity for downward, upward and lateral loads and moments; or (d) To take the foundations below scour level. When the ground consists of expansive soil, for example black cotton soil, the bulb of the under-reamed pile provides anchorage against uplift due to swelling pressure apart from the increased bearing. In case, of filled-up or otherwise weak strata overlying the firm strata, enlarged base in the form of under-reamed bulb in firm strata provides larger bearing area and piles of greater bearing capacity can be made. In loose to medium pervious sandy and silty strata, bored compaction piles can be used as the process of compaction increases the load bearing capacity of the piles. Under-reamed piles may also be used under situations where the vibration and noise caused during construction of piles are to be avoided. The provision of bulb(s) is of special advantage in under-reamed piles to resist uplift and they can be used as anchors.

2. TERMINOLOGY: Batter Pile (Raker Pile): the pile which is installed at an angle to the vertical. Bearing Pile: a pile formed in the ground for transmitting the load of structure to the soil by the resistance developed at its tip or along its surface or both. It may be formed either vertically or at an inclination (Batter Pile) and may be required to take uplift. When it is primarily meant for esisting u lift ull it is called an ‘ nch Pile’. If the ile supports the load primarily by resistance developed at the pile point or base it is efe ed t as an ‘End Bea ing Pile’ and if the l ad is supported ima ily by f icti n al ng its su face the ile is te med as ‘F icti n Pile’. Bored Cast in situ Pile: a pile formed within the ground by excavating or boring a hole within it, with or without the use of a temporary casing and subsequently filling it with plain or reinforced concrete. When the casing is left permanently it is termed as cased pile and when the casing is taken out it is termed as uncased pile. In installing a bored pile, the sides of the borehole (when it does not stand by itself) is required to be stabilized with the aid of a temporary casing, or with the aid of drilling mud of suitable consistency. Bored Compaction Pile: a bored cast in situ pile with or without bulb(s) in which the compaction of surrounding ground and freshly filled concrete in pile bore is simultaneously achieved by suitable method. If the ile with bulb(s) it is kn wn ‘unde - eamed b ed c m acti n ile’. Under-Reamed Pile: a bored cast in situ or bored compaction concrete pile with an enlarged bulb(s) made by either cutting or scooping out the soil or by any other suitable process. Multi-Under-Reamed Pile: an under-reamed pile having more than one bulb. The pile having two bulbs is known as double under-reamed pile. Cut-Off Level: It is the level where the installed pile is cut-off to support the pile caps or beams or any other structural components at that level.

Ankit Patel (5108)

Construction Technology-IV

Under-Reamed Pile

3. MERITS & DEMERITS: MERITS:         

length can be varied removed soil can be compared with design data penetration tests can be carried out in boreholes very large bases can be formed in favourable ground drilling tools can break up boulders and other obstructions pile is designed to working stresses very long lengths possible little noise and vibration during construction no ground heave

DEMERITS:      

piles liable to squeezing and necking in soft soils special techniques required for concreting in water bearing ground concrete cannot be inspected after installation enlarged bases cannot be formed in collapsible soil cannot be easily extended above ground boring may cause instability and settlement of adjacent structures

4. POSSIBLE APPLICATION OF U/R PILE: 

Where a soil layer of adequate bearing capacity lies too deep for the economic use of conventional footings.

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Where the soil layer(s) immediately underlying a structure are soft or poorly compacted.

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Where the soil layer(s) immediately underlying a structure are moderately or highly variable in nature.

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On sites where the soil strata, and in some cases the ground surface are steeply inclined.

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On river or shoreline sites where tidal or wave action or scouring may vary the amount of material near the surface.

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For structures transmitting very high concentrated loads.

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For structures transmitting significant horizontal or inclined loads.

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For structures which structurally or functionally may be sensitive to differential settlement.

Ankit Patel (5108)

Construction Technology-IV

Under-Reamed Pile

5. REQUIRED EQUIPMENT FOR CONSTRUCTION OF U/R PILE: 

Normally the following equipment will be required in manual operation: a) An auger; b) An under-reamer; c) A boring guide; and d) Accessories like spare extensions, cutting tool, concreting Funnel, etc; e) Drilling bucket.

 For the piles of size larger than 30 cm and for larger depths additional equipment required will be a portable tripod hoist with a manually operated winch. For piles in high ground water table and unstable soil conditions, boring and under-reaming shall be carried out with bentonite slurry using suitable equipment. Tremie pipe shall be used for concreting.

Short Flight Clay Auger

Drilling Bucket

 The additional equipment normally required for under-reamed compaction pile are the following: a) Drop weight for driving the core assembly, and b) Pipe or solid core.

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The selection of equipment and accessories will depend upon the type of under-reamed piles, site conditions and nature of strata. Also it will depend on economic considerations and availability of manually or power operated equipment.

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Bore holes may be made by earth augers. In case of manual boring, an auger boring guide shall be used to keep the bores vertical or to the desired inclination and in position.

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After the bore is made to the required depth, enlarging of the base shall be carried out by means of an under-reaming tool.

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In ground with high water table having unstable pile bores, boring and under-reaming may be carried out using a suitable drilling mud. In normally met soil strata, drilling mud can be poured from top while boring and under-reaming can be done by normal spiral earth auger and under-reamer. The level of drilling mud should always be about one metre above water table or the level at which caving in occurs. In case of very unstable strata with excessive caving in, continuous circulation of drilling mud using suitable pumping equipment and tripod, etc, along with modified auger and under-reamer may be used.

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Sometimes permeable strata overlying a rim clayey stratum may be cased and normal boring and under-reaming operation may be carried out in clayey stratum.

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To avoid irregular shape and widening of bore hole in very loose strata at top, a casing pipe of suitable length may be used temporarily during boring and concreting.

Ankit Patel (5108)

Construction Technology-IV

Under-Reamed Pile

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For improved control over the inclination of batter piles, a tripod hoist with fixed pulley should be used for lowering in of under-reaming tools.

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For placing the concrete in bore holes full of drilling mud or subsoil water, tremie pipe of not less than 150 mm diameter with flap valve at the bottom should be used.

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For batter under-reamed piles, the reinforcement cage should be placed guiding it by a chute or any other suitable method. If concreting is not done by tremie, it should be done by chute. In under-reamed compaction piles, suitable devices should be used for guiding the movement of drop weight and specified core assembly for its vertical driving. For operating the drop weights of adequate capacity, suitable winch with hoisting attachment should be used.

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6. DESIGN CONSIDERATION: The diameter of Under-reamed bulb (Du) is normally 2.5 times (may vary from 2 to 3 times) the stem diameter (D) depending on the feasibility of construction and design requirements. For piles up to 30 cm diameter, the spacing of the bulbs should not exceed 1.5 times the diameter of the bulb. For piles greater than 30 cm, spacing can be reduced to 1.25 times the stem diameter. The minimum diameter of stem for borehole needing stabilization by drilling mud should be 25 cm. The top most bulbs should not be at a min. depth of two times the bulb diameter. In expensive soils, it should not be less than 1.75 m below G.L. the minimum clearance below the underside of the pile cap embedded in the ground and the bulb should be a minimum of 1.5 times the bulb diameter.

7. DESIGN OF PILE CAP: Pile caps are generally designed considering pile reaction as either concentrated loads or distributed loads. The depth of pile cap should be adequate for the shear for diagonal tension and it should also provide for necessary anchorage of reinforcement both for the column and the piles. The pile caps may be designed by assuming that the load from column or pedestal is dis e sed at from the top of the cap up to the mid-depth of the pile cap from the base of the column or pedestal. The reaction from piles may also be taken to be distributed at 45º from the edge of the pile, up to the mid-depth of the pile cap. On this basis, the maximum bending moment and shear forces should be worked out at critical sections.

8. SPACING: Generally the C/C spacing for bored under-reamed piles in a group should be two times the bulb diameter (2Du). It should not be less than 1.5Du. For under-grade beams, the spacing of pile should generally not exceed 3 m. In under-reamed compaction piles, the spacing should not be less than 1.5D u.

Ankit Patel (5108)

Construction Technology-IV

Under-Reamed Pile

9. GROUP EFFICINCY: The group efficiency of under-reamed piles at a normal spacing of 2Du is equal to the safe load of an individual pile multiplied by the number of piles in the group. For piles at spacing of 1.5Du, the safe load assigned per pile in a group should be reduced by 10 %. In under-reamed compaction piles, at the usual spacing of 1.5Du, the group capacity will be equal to the safe load of an individual pile multiplied by the number of piles in the group.

10. CONSTRUCTION: Normally the equipment required for manual operations are mentioned in TOPIC 5. For piles of deeper and larger size than 30 cm, a portable tripod hoist with manually operated which is required. For compaction piles, the additional equipment required is drop weight for driving core assemblies and pipe or solid core. When employing the rotary excavation method, an enlarged base (under-ream), also known as bell, can be created, which increases the base bearing capacity of piles in competent soil strata. In its closed position, the under-reamer fits inside the straight section of a pile shaft, and can be expanded at the base of the pile to produce the enlarged base. The under-ream can be of different shapes. The most common is the standard reamer, usually cut at 45 or 60 degree angle, with the maximum diameter of the under-ream being not more than three times the diameter of the shaft. Another type is the bucket reamer, which has a bell shape. The belling tool has hinged arms, which can be pushed outward by a downward force on the Kelly (drill rod). The rotation of the tool in the borehole cuts away the soil, which is carried to the center. When an upward force is applied to the Kelly, the cutter arms are retracted and the underreaming tool is lifted. The spoil is then removed by unhinging the bottom of the tool. Cutting of a bell can be a time-consuming process, compared to excavating a straight shaft, because only a limited amount of soil can be removed at one pass. Belling buckets normally cut base diameters of up to 3700 mm, although diameters of as much as 7300 mm are possible with special equipment. It is

Ankit Patel (5108)

Construction Technology-IV Under-Reamed Pile usually not practical to form bells on piles having shafts of less than 760 mm diameter. Although the base of a mechanically under-reamed pile can be cleaned by specially designed mechanical tools, this is a somewhat tedious operation. It is also possible to clean the base manually, but this excavation technique can be hazardous and costly. Under-reaming is primarily used in stiff cohesive soils. During the construction process in unstable soils (loose and water-saturated), there is a danger of collapse of the bell; the strength of the soil, the presence of soil layers, and the possible inflow of groundwater in pervious strata are important factors, which need to be considered. Close attention should be given to the planning and execution of the belling operation. Besides the danger of collapse of the excavation and decompression of the soil adjacent to the bell, it is also possible that loose soil is deposited beneath the underreaming tool, causing the tool to "ride up" and affecting the bearing capacity of the pile base. The cost advantages of under-reamed piles are due to the reduced pile shaft diameter, resulting in less concrete needed to replace the excavated material. However, in difficult soil conditions such as boulder clays containing lenses of silt or sand, or in loose, non-cohesive soils, it is practically impossible to form an under-reamed pile base. The following operations are involved in construction of under-reamed pile foundation: a) Boreholes may be made with earth augers. In manual boring, an auger boring guide is used to keep the bores vertical or at the desired inclination and in position. b) Drilling mud (bentonite) may be used for boring and under-reaming at a site with high water table. c) To avoid irregular shape and widening of bore hole in very loose strata at top, a casing pipe of suitable length may be used. d) The reinforcement cage should preferably be placed guiding it by a chute or any other means. e) In order to achieve proper under-reamed bulb, the depth of borehole should be checked before starting under-reaming. It should also be checked during underreaming. Any extra soil at the bottom of borehole should be removed by auger before re-inserting the under-reaming tool. f) Completion of the desired under-reamed bulb is ascertained by vertical movement of the handle and when no further soil is cut. g) In multi under-reamed piles, the boring is first completed to the depth required for the first bulb (top). Only after completing under-reaming bulb, the boring is extended further down to second bulb and so on.

Ankit Patel (5108)

Construction Technology-IV

Under-Reamed Pile

h) The pile is to be installed as correctly as possible, both at the correct location and truly vertical. Piles should not deviate by more than 75 mm or one-quarter the stem diameter whichever is less. For piles of diameter more than 600 mm, the deviation may be 75 mm or 10% of the stem diameter. Any deviation beyond permissible limits from the designed location, alignment or load capacity shall be noted and adequate measures taken well before the concreting of the pile cap and plinth beams. When defective piles are formed, they shall be removed or left in place whichever is convenient without affecting the performance of adjacent piles or the cap as whole. i) The top of the conc. Pile should be brought above the cut-off level to permit removal of all laitance and weak concrete before capping an to ensure good concrete at the cut-off level for proper embedment into pile cap. j) Where cut-off level is less than 1.5 m below working level, concrete shall be cast to minimum of 300 mm above cut-off level. For every excess of 0.3 m over 1.5 meter, additional 50 mm shall be cast over and above 300 mm. When tremie pipe is employed, it shall be cast to piling platform level to permit overflow of concrete for visual inspection or to a minimum of 1 m above the cut-off level. When the cut-off level is below the ground water level, there is a need to maintain a pressure on the unset concrete equal to or greater then the water pressure and a length of extra concrete above the cut-off level may be permitted to provide this. k) The pile should be subjected 50 mm into the cap concrete.

11. CONCRETING: Concreting should be done as soon as possible after completion of the borehole. The borehole full of drilling mud should be concreted between 12 to 24 Hour depending on the stability of the hole. The method of concreting should be such that the entire volume of pile bore is filled up without formation of voids or mixing of soil and drilling mud in concrete. For placing concrete in pile bore, funnel should be used. In the empty boreholes for under-reamed piles a small quality of concrete is poured to give about a 100 mm layer of concrete at the bottom. Reinforcement is lowered next and positioned correctly. The concrete is poured to fill the borehole. Care shall be taken to ensure that soil is not scrapped from sides if rodding is done for compaction. Vibrators shall not be used. If the subsoil water level is confined to the bucket length portion at the toe, the seepage is low and the water should be bailed out before commencing concreting. In case the pile bore is stabilized with drilling mud or by maintaining water head with in the

Ankit Patel (5108)

Construction Technology-IV

Under-Reamed Pile

borehole, the bottom of borehole should be cleaned by flushing it with fresh drilling mud and the pile bore be checked before concreting. Concreting shall be done by tremie method.

12. CONCRETING BY TREMIE PIPE: Slump of concrete shall range between 150 mm to 200 mm. In case of tremie concreting of piles of smaller diameter and depth up to 10 m, the minimum cement content should be 350 kg/m3 of concrete. For piles of larger diameter and/or deeper piles, the minimum cement content should be 400 kg/m3 of concrete. The tremie has a valve at the bottom. It is lowered with valve closed at the start and filled up with concrete. The valve is then opened to permit concrete to flow out, which causes upward displacement of drilling mud. The pouring should be continuous and the tremie is gradually lifted up such that the pipe opening remains always in concrete. In the final stage the quantity should be sufficient so that on final withdrawal some concrete spills on the ground. The following precautions should be taken while concreting with a tremie pipe: a) Tremie should be always penetrate well into the concrete with an adequate margin of safety against withdrawal of pipe. The tremie method should not be changed for a given pile, to prevent the laitance from being entrapped in the pile. b) In the case of withdrawal of tremie pipe accidently or to remove a choke, the tremie may be reintroduced in a manner to prevent fragmentation of laitance or scrum lying on the top of the concrete deposited already in the bore. c) In the exceptional case of interruption of concreting which can be resumed in one or two hours, the tremie should be taken out of concrete. Instead, it should be raised and lowered slowly, from time to time, to prevent concrete around tremie from setting. Concreting should be resume by introducing a slightly of the party set concrete. If concreting cannot be resumed before the final set of concrete, the pile may be rejected or used with modification. The whole of tremie tube should be cleaned before and after use.

13. BORED COMPACTION PILE: In this pile compaction of the surrounding ground as well as fresh concrete in the bore simultaneously accomplished. The pile is filled up with concrete, without placing reinforcement. Immediately the core assembly shall be driven and extra concrete shall be poured in simultaneously to keep the level of concrete up to ground level. If a hollow driving pump is used in core assembly, the pipe is withdrawn after filling it with fresh concrete. In this compaction piles it shall be ensured that concreting is done uninterruptedly. Accidental withdrawal should be completely avoided.

Ankit Patel (5108)

Construction Technology-IV 14. GRADE BEAMS: The grade beams supporting the walls shall be designed taking due account of arching effect due to masonry above beam. The beam with masonry due to composite action behaves as a deep beam. For the design of beams, a maximum bending moment of wl2/50, where w is uniformly distributed load per metre run (worked out by considering a maximum height of two storeys in structures with load bearing walls and one storey in framed structures) and l is the effective span in metres, will be taken if the beams are supported during construction till the masonry above it gains strength. The value of bending moment shall be increased to wl2/30 if the beams are not supported. For considering composite action the minimum height of wall shall be 0.6 times the beam span. The brick strength should not be less than 30 kgf/cm2. For concentrated loads and other loads which come directly over the beam, full bending moment should be considered.

2a. Beam in Expansive Soil

2b. Beam in Non-Expansive Soil All dimensions are in millimetres

FIG. 2 TYPICAL SECTIONS OF GRADE BEAMS

Ankit Patel (5108)

Construction Technology-IV The minimum overall depth of grade beams shall be 150 mm. The reinforcement at bottom should be kept continuous in all the beams and an equal amount may be provided at top to a distance of quarter span both ways from the pile centres. The longitudinal reinforcement both at bottom and top should not be less than three bars of 10 mm diameter mild steel (or equivalent deformed steel). Stirrups of 6-mm diameter bars should be at 300 mm spacing which should be reduced to 100 mm at the door openings near the wall edge up to a distance of three times the depth of beam. No shear connectors are necessary in wall. In expansive soil, the grade beams shall be kept a minimum of 80 mm clear off the ground. In other soils, the beams may rest on ground over a levelling concrete course of about 80 mm (see Fig. 2). In this case, part load may be considered to be borne by the ground and it may be accounted for in the design of piles. However, the beams should be designed as per SECOND PARAGRAPH OF 14. GRADE BEAM. In case of exterior beams over piles in expansive soils, a ledge projection of 75 mm thickness and extending 80 mm into ground (See Fig. 2) shall be provided on outer side of beam.

15. REINFORCEMENT OF PILE: The provision of reinforcement will depend on nature and magnitude of loads, nature of strata and method of installation. It should be adequate for vertical load, lateral load and moments, acting individually or in combination. It may be curtailed at appropriate depth subject to provision given below first three paragraphs. The minimum area of longitudinal reinforcement (any type or grade) within the pile shaft shall be 0.4 percent of the sectional area calculated on the basis of outside area of the shaft or casing if used. Reinforcement is to be provided in the full length irrespective of any other considerations and is further subject to the condition that a minimum number of three 10mm diameter mild steel or three 8-mm diameter high strength steel bars shall be provided. The transverse reinforcement as circular stirrups shall not be less than 6-mm diameter mild steel bars at a spacing of not more than the stem diameter or 30 cm whichever is less. In under-reamed compaction piles, a minimum number of four, 12-mm diameter mild steel or four 10-mm diameter high strength steel bars shall be provided. For piles of lengths exceeding 5 m and of 37.5 cm diameter, a minimum number of six 12-mm diameter bars of mild or high strength steel shall be provided. For piles exceeding 40 cm diameter, a minimum number of six 12-mm diameter mild or high strength steel bars shall be provided. The circular stirrups for piles of lengths exceeding 5 m and diameter exceeding 37.5 cm shall be of 8 mm diameter bars. For piles subjected to uplift loads, adequate reinforcement shall be provided to take full uplift which shall not be curtailed at any stage.

Ankit Patel (5108)

Construction Technology-IV For piles up to 30 cm diameter, if concreting is done by tremie, equivalent amount of steel placed centrally may be provided. The minimum clear cover over the longitudinal reinforcement shall be 40 mm. In aggressive environment of sulphates, etc, it may be increased to 75 mm.

Ankit Patel (5108)