Which Foundation and Why
The Fundamentals of Foundation Design or
Which Foundations and Why
Scott Stewart Atkins Middle East PhD BE
Tuesday 27th March • Part 1 – Basic Principles and Design Relationships • Part 2 – Design Examples
Tuesday 3rd April • Part 3 – Fundamentals of Foundations (Which Foundations & Why) • Part 4 – IStructE Exam questions
Part 3 – Fundamentals of Foundations (Which Foundations & Why) 1. 2. 3. 4.
Key Issues for Consideration Foundation Types Excavations Retaining Walls
Key Issues for Consideration Uncertainties Significant and varying degrees of uncertainty are inherent in the geotechnical design process and allowances must be made for these uncertainties. Examples include : • • • •
estimating loads variability of the groundwater and ground conditions evaluation of geotechnical properties and behaviour of the founding strata compared to the analytical model chosen
Engineering judgement and experience are an essential part of geotechnical engineering and vital for controlling the safety of geotechnical structures. Get your geotechnical engineer involved!
Key Issues for Consideration Think about • • • • • • • • • •
Loading Movement / Serviceability Stratigraphy Groundwater Previous use of the site Topography and Geomorphology Site Access Environmental Considerations Economy / Reliability / Durability Construction Design Mgmt (CDM) / Health and Safety
Key Issues for Consideration V
Loading • Vertical • Compression • Tension • Horizontal • Seismic / Dynamic (V, H) • Combinations of above
Key Issues for Consideration Movements • Total settlement • Differential movement • Rotation and angular distortion • Horizontal Strain
Excavations Movements • Caused by; • Relaxation of soil towards excavated material • Vertical and horizontal movements • Groundwater drawdown Water drawdown = settlement
• Heave and settlement
Key Issues for Consideration Movements • Excavation – Flexible vs Stiff walls – Propping vs Excavation access • Dewatering – Ground water table drawdown – Erosion during dewatering (piping) • Horizontal Strain
Key Issues for Consideration Movements
Key Issues for Consideration Movements Type of structure
Framed buildings and reinforced load bearing walls
Type of damage
Values of relative rotation (angular distortion) Skempton and Meyerhof MacDonald 1/150 1/250
Cracking in walls and partitions
1/300 (but 1/500 recommended)
Values of deflexion ratio
Burland and Wroth
Unreinforced loadbearing walls
Cracking by sagging
Cracking by hogging
At L/H = 1 At L/H = 5
D / L = 0.4 X 10 -3 D / L = 0.8 X 10 -3
At L/H = 1 At L/H = 5
D / L = 0.2 X 10 -3 D / L = 0.4 X 10 -3
Key Issues for Consideration Stratigraphy • How much information is available ? Soil or rock ? Variable or uniform ? Depth to founding strata • Sources of information geological maps, borehole records etc.
Key Issues for Consideration Groundwater • Have a sound understanding of the hydrogeological conditions • Consider seasonal and long term water pressure changes • Check on the location and size of any man-made water sources • Consider the potential rise in water level caused by global damming effect of the structure •
Is groundwater flow rate static?
Key Issues for Consideration
Previous Site Use • Sources – historical maps, archive records etc. • Often sites will have had a long and varied past. - previous residential use - industrial / old infrastructure - historic mining • Implications - obstructions - aggressive ground /contamination
Key Issues for Consideration Topography & Geomorphology • Is the site level or sloping ? • What are the ground surface conditions? (boggy, hardstanding etc. )
Key Issues for Consideration Site Access and Constraints • Is the site green field or urban • Head room restrictions – e.g. overhead power lines
Key Issues for Consideration Environmental Considerations Pollution • BS5228 Part 4 1992, Code of practice for noise and vibration control applicable to piling operations • CIRIA TN142, 1992, Ground-borne vibrations arising from piling •
Sensitivity of people ~ 0.15 mm / sec
Sensitivity of equipment ~ 5mm / sec for telephone exchange Sensitivity of buildings > 10 mm / sec
• Dust / Contamination
Key Issues for Consideration Economy / Reliability / Durability • Economy selection of the most appropriate foundation solution
• Reliability / Durability linked to construction methods / workmanship aggressive ground and groundwater conditions
Key Issues for Consideration Construction Design Management (CDM) Regulations (Health and Safety) CIRIA Report 166, Feb 1997, CDM Regulations work sector guidance for designers • Plant • Lifting/pitching piling equipment • Open bores Health and Safety issues associated with ground contamination
Foundation Types Foundation
Piles, Anchors, etc
Foundation Types Shallow Foundations Typically 1m - 3m deep and generally more economical if competent strata are located near the ground surface. Require excavations so generally used when the water table is at depth. Types • Mass Footings • Pads • Strips • Rafts
Foundation Types Shallow Foundations – Things to Note • Frost susceptibility - expansion of soils when frozen due to formation of ice. Keep foundation depth at least 450mm below ground level (BS8004 : 1996) • Change in ground moisture content leading to shrinking and swelling seasonal wetting and drying (min depth of foundation 900mm (BS8004 : 1996) • Trees (removal will change conditions, e,g, heave, softening)
Foundation Types Deep Foundations • Inadequate bearing capacity • Excessive settlements • Swelling or shrinking clays • Founded on non engineered fill • High ground water
Foundation Types - Deep Foundations Mini-pile
Barrette / diaphragm wall
Large diameter bored pile
Foundation Types Deep Foundations – Mini piles • Small diameter bored piles < 0.3m dia • Small rigs < 2m high • Capacities up to 1MN • Confined spaces / poor access • Drilling water / settling ponds required • Note: Durability
Foundation Types Deep Foundations – Driven Piles • Precast concrete piles (typically 0.3m square) - capacities up to 3MN • Steel piles bearing piles (H) or tubular piles - capacities up to 8MN • Cheap and quick to install • Need to consider installation forces (pile head damage, bending in shaft) • Noise and vibration issues • Unsuitable for soils with boulders • Fixed lengths • Progressively harder to drive groups
Foundation Types Deep Foundations – Bored Piles • Capacities up to 25MN • straight shafted 0.45m - 2.5m diameter • under-reamed (up to 3 x shaft diameter) • base or shaft grouted (for extra capacity) • Need support fluid where drilling below the water table • Casings (temporary or permanent) • Can reinforce full depth
Foundation Types Deep Foundations – Continuous Flight Auger (CFA) Piles • • • •
Capacities up to 7MN Sizes up to 0.9m diameter Relatively fast and cheap Reinforcement needs to be “plunged” typically 12m max. • No need for casings or support fluid • Requires experienced drilling crew • Not able to check base cleanness
Foundation Design Primary objectives of engineering design are : • Safety • Serviceability • Economy Safety and serviceability can be improved by increasing the design margins or factors of safety (FoS) with the aim of reducing the probability of ‘failure’. To assist the engineer and to ensure compliance with a minimum specified level of technical quality we have: • Codes of practice such as BS:8004 and Eurocode 7 (EC7) • Formulation of the geological and geotechnical models • Evaluation of geotechnical design parameters and • Choice of appropriate design methods.
Foundation Design Pile Foundations – Factors of Safety (FoS) London District Surveyors Association guidance note for piles in London Clay, 2000.
* Assumes: • mean cu line based on metal U100 samples) • piles concreted within 12 hours • independent supervision • not for bentonite or CFA piles
Foundation Design Pile Foundations – Negative Skin Friction • Occurs when adjacent ground settles by more than pile Relative movement between fill and pile shaft Relative movement between underlying compressible stratum Consolidation of compressible layers Dewatering • All soil layers above settling layer impose negative skin friction. • Measures to counteract Tolerate additional pile settlement Sleeve piles Accelerate process so as to be complete by time pile is installed E.g. Pre-loading, ground improvement
Install longer piles to resist
Foundation Design Pile Foundations – Specification • ICE 1996 Specification for piling and embedded retaining walls • Particular specifications only to be filled out • Need to agree concrete and reinforcement requirements
Pile Foundations – Tolerances
• 75mm in plan normal • Out-of-position piles have moment induced in head • M=Vd • therefore more reinforcement required • 1 in 75 verticality normal • Out-of-vertical piles have lateral force on head • H = V tan θ • Effects reduce with depth (lateral pile analysis)
Foundation Design Pile Foundations – Testing • ICE 1999 Specification for piling and embedded retaining walls, section 10.0 “Static Load Testing of Piles” Provides requirements for: • Testing / monitoring equipment and procedures • Data to be reported by contractor
Foundation Design Pile Foundations – Load Testing Key terms: • SWL - the specified working load of the pile (for design conditions) • DVL - design verification load of the pile is a substitute SWL for the conditions at the time of the pile test e.g additional pile length, variable groundwater conditions • MLT - maintained load test, with hold points at each load increment • CRP - constant rate of penetration test, pile loaded continuously at a prescribed rate to failure
Foundation Design Pile Foundations – Load Testing
• Proof load test (or contract pile test) - on working piles to prove settlement for design method chosen Proof Load = DVL + 50% SWL • Extended proof test (preliminary pile test) - to failure in order to establish pile design parameters Proof load + increments of 25% SWL to failure
Foundation Design Pile Foundations – Integrity Testing • Impulse testing • Does not require any cast in materials • Detects cracks • Good for near surface defects only (has limited depth range)
Foundation Design Pile Foundations – Integrity Testing • Sonic echo testing (Sonic logging) • can pick up voids, reduced concrete strength • requires tubes cast into pile (not possible for CFA piles) • increase tubes gives better coverage • does not give information on material in cover zone (i.e. outside of tubes)
Foundation Design Pile Foundations – Design Responsibility • Consultant or contractor ? • Design responsibility must be clearly allocated • For contractor designed piles, there will be a performance requirement e.g. settlement at working load, therefore pile testing should strongly be considered • ICE 1996 Specification for piling and embedded retaining walls, Clause 1.2(j)
Foundation Design EC7 • First published by the BSi in 1995 as DD ENV 1997-1 : 1995 • Superseded BS 5930, 6031,8002, 8004, 8006, 8081 • BSi is not updating or maintaining superseded standards • BS8004 is referred to and in use in UAE
Foundation Design EC7 Philosophy • Adoption of limit-state design principals • Compatibility with EN1990 (Eurocode - Basis for Structural Design) • Partial factors applied to material strength, actions and resistances • National Application Standards for the individual member states to include local practices
Foundation Design - Summary
Excavations Open Cut • Slope stability in fine grained soils Critical height,
• BUT pore water changes cause softening in clay => cu reduces and allowable height reduces. • Benching to avoid slope for material to fall into excavation • Material properties are very important in slopes and can have significant effects on stability esp. with pore water present.
Excavations Open Cut • Slope stability in coarse grained soils • If soil is dry, soil can be angled at natural angle of repose • Pore water will have major impact on slope stability • If shallow, water bearing soils, sheet piling may be used • Nb flexible wall – requires many anchors at increasing depths to minimise ground movements
Retaining walls Cantilever (Pile) wall • King post • • •
Basic, cheap Timber/concrete lagging Watch passive resistance
• Contiguous pile • •
Tangent pile Shotcrete finishing
Reducing pile spacing
Reducing wall flexibility (=> reduced ground movements)
• Secant pile • • •
Ground water cutoff Hard-soft (soft is unreinforced Hard-hard (all reinforced) I beam or rectangular rebar cage in primary •
Retaining walls Diaphragm Wall • Ground water cutoff • Can use as permanent wall • Grab or Hydrofraise/Mill • Better verticality with hydrofraise • Panels installed in threes • Primary, secondary and closing
Retaining walls Propping • Raked struts • Horizontal Struts • Effect on construction access • Top Down Construction • Use floor slab as strut to control ground movements • Detail temporary openings in slabs • E.g. urban environments (metros, towers, etc)
Retaining walls Anchoring • Ease of construction • Need to consider obstructions behind wall (e.g. utilities, basements, other retaining walls) • Prestressed anchors can act to minimise ground movements • Nb effect of reverse curvature on structure
Part 4 Workshop Using Previous Exam questions • • • • •
Quickly review each question. Identify main geotechnical elements. Interpret geotechnical data. Assess structural implications due to the geotechnical setting. Undertake some simple geotechnical assessments.
Q1. Office Building Next to Existing Stone Tower Client’s requirements 1. An existing stone tower is to be used in a new office development; see Figure Q1. 2. The tower is made from stone set in mortar and cannot be used to support the new office structure in any form. The new building is to be set partially into the interior of the tower as shown on Figure Q1. 3. The Architect wishes to retain the smallest floor depth possible and to have the building clad entirely in glass. The Architect has also stipulated that there is to be no visible structure around the glazed perimeter other than columns and the floor plate. Columns are to be spaced at least 8.0m apart. 4. The building is to have a 3.1m clear height between each floor and ceiling and is to be 4 storeys high. The height of the tower is 16.5m. The Architect has requested that the maximum level of the roof line of the building matches the height of the tower. 5. The existing stone tower is founded at a constant depth of 1.0m below ground level. The foundation of the tower does not extend beyond its plan area. Imposed loading 6. Roof 2.5kN/m2, Floor loading 6.0kN/m2, Loadings include an allowance for partitions, finishes, services and ceilings. Site conditions 7. The site is level and located in a park in the centre of a town. Basic wind speed is 40m/s based on a 3 second gust; the equivalent mean hourly wind speed is 20m/s.
Q1. Office Building Next to Existing Stone Tower Borehole 1 • 0 – 1.0m: Made ground • 1.0m to depth: Rock. Allowable bearing pressure = 1000kPa Borehole 2 • 0 – 5.0m Made ground • 5.0m - 8.0m Stiff clay – C = 80kPa • Below 8.0m Rock. –Allowable bearing pressure = 1000kPa
Q1. Office Building Next to Existing Stone Tower • • • •
• • • • • •
Stone tower is settlement sensitive. Glass cladding is movement sensitive façade. New foundation could cause existing stone tower to settle. Variable ground conditions! Rock is at different depth between boreholes. Additional SI required to understand variability (i.e. what about third dimension?) . Make clear assumption but state further SI required. Plan for new foundations to be below level of existing. Raft would require too much excavation. Placing foundation on clay gives SABP = 2*cu = 160kPa. E = 150*cu = 12MPa. Nb can use benefit of surcharge to improve bearing capacity assessment. Loads are roughly 50kPa and assuming 8m c/c spacing gives 3200kN column load i.e. 4.5m square pad. Settlement of a 4.5m pad = qB/E = 50kPa*4.5m/12MPa = 19mm. If bearing capacity not enough or settlement too large use piled foundations.
Q2. Hazardous Liquid Storage Building
Client’s requirements • 1. A waterproof building is required to store two tanks containing hazardous liquids. • 2. The tanks are 2 metres in diameter, 5 metres long and each weigh 400kN. • 3. The tanks must be stored so that the underside of each tank is at an elevation of 5.0m above ground level. No internal columns are permitted either within the building or beneath the tanks. A 6.0 m clear space is required around each tank for inspection and the tanks must have at least 4.0m clear space between them. • 4. The building is to be situated at the centre of an island approximately 100.0m square protected by sheet piling. • 5. The tanks will be delivered by barge at a minimum distance of 5.0m from the edge of the island and then stored in the building for approximately one month. During this period the access doors of the building must be kept shut. • Imposed loading • 6. Roof loading 1.5kN/m2 (including imposed and services)
Q2. Hazardous Liquid Storage Building Site conditions The site is located in a river estuary. Basic wind speed is 46m/s based on a 3 second gust; the equivalent mean hourly wind speed is 23m/s. Mobile crane capacity on the island is limited to 20 tonnes due to the poor ground conditions. No suitable barge mounted cranes are available in this location. Borehole 1 at island edge Ground level – 1.5 m River silt 1.5 m to 6.0 m – Soft clay C = 25 kN/m2 6.0 m - depth – Rock – allowable safe bearing pressure 1000 kN/m2 Water was found at 4.0m depth Borehole 2 at island centre Ground level – 0.5 m – Topsoil 0.5 m – 3.0 m Soft clay C = 25 kN/m2 3.0 m - depth Rock – allowable safe bearing pressure 1000 kN/m2 Water was found at 2.0m depth
Q2. Hazardous Liquid Storage Building
• • • • • • • • •
Varying elevations of ground conditions. River Silt and Topsoil not suitable as founding stratum. Is Soft clay suitable as founding stratum? Placing foundation on clay gives S.A.B.P. = 2*cu = 50kPa. E’ = 150*cu = 3.75MPa. Loads are roughly 400kN and assuming 50kPa bearing capacity gives 2.8m square pad. Settlement of a 2.8m pad = qB/E = 50kPa*2.8m/3.75MPa = 37mm. What about a raft? Settlement of a raft 17m wide loaded to 30kPa = qB/E = 30kPa*17m/3.75MPa = 136mm! If bearing capacity not enough or settlement too large use piled foundations.
Q4. Commercial Building Client’s requirements
1. A seven-storey commercial building on a square site 45.0m x 45.0m: see Fig. Q4. 2. The facade at the south-east corner is to be inclined between level 2 and the roof. All other facades are to be vertical. All facades are required to be fully-glazed between level 2 and the roof. 3. To provide flexibility for building entry points, the clear distance between external columns on level 1 must be a minimum of 8.0m. External columns on level 2 and above, if required, must be evenly-spaced. No column is permitted on any level at the northwest corner of the building. 4. Neither external nor internal structural walls are permitted. A clear distance of at least 7.0m is required between an internal column and any other column or external enclosure. The service cores are to be structurally independent of the main building. 5. No foundations may extend beyond the site boundary. 6. Allowable structural floor zones are: Level 2: 1.7m Other levels and roof: 1.2m 7. A minimum fire resistance of 2 hours is required for all structural elements. Imposed loading 8. Roof 2.5kN/m2 All floors 5.0 kN/m2
Q4. Commercial Building Site conditions 9. The site is level and is located in the suburban area of a town 200km from the sea. Basic wind speed is 40m/s based on a 3 second gust; the equivalent mean hourly wind speed is 20m/s. 10. Ground Conditions • 0.0m – 2.0m Loose fill • 2.0m – 5.0m Sandy gravel. N varies from 10 to 20 • 5.0m – 8.0m Weathered rock. Allowable bearing pressure 500kN/m2 • Below 8.0m Rock. Allowable bearing pressure 1500kN/m2 • Ground water was encountered at 2.5m below ground level.
Q4. Commercial Building • •
Draw cross section. Loose Fill not suitable as founding stratum.
Is Sandy Gravel suitable as founding stratum?
Gravel – N = 10 to 20 so φ = 30 – 33 degrees
• • •
Using Charts - S.A.B.P. = 200 to 250kPa.
Loads assume roughly 40kPa and assuming 8m column spacing gives 2560kN, thus pad size = 2560kN/250kPa = 3.2m square pad.
Settlement – qB/E = 250kPa*3.2m/25MPa = 32mm
E’ = 1 or 2N = 10 – 40MPa. (say 25MPa)
Q5. Art Gallery Client’s requirements 1. A two-storey art gallery is to be constructed on a sloping city-centre site containing a buried culvert: see Fig. Q5. 2. Level 1 is to have plan dimensions of 56.0m x 35.0m with columns at a minimum centre-to-centre spacing of 7.0m in each direction. Level 2 is to have plan dimensions of 50.0m x 12.0m with no internal columns. An allowance for lift and stair cores is included within these plan dimensions. 3. The floor-to-floor height from levels 1 to 2, and the floor-to-eaves height from level 2 to the roof is to be 4.5m. A maximum structural zone of 0.75m is permitted. 4. A flat, level access route of minimum width 3m is to be provided around the perimeter of the building at level 1. 5. A single car park with plan dimensions of 20.0m x 50.0m is required. 6. Access to the site is to be provided at the two locations shown on Fig. Q5, one for vehicles and one for pedestrians. 7. The culvert may be built over but may not be diverted and no additional loads may be applied to it either vertically or laterally. No construction may approach horizontally closer than 4.0m to the centreline of the culvert. Imposed loading 8. Gallery floors, levels 1 and 2 5.0kN/m2 Roof 1.5kN/m2 Car park 2.5kN/m2
Q5. Art Gallery Site conditions 9. The site is located in a city 100km from the sea. Basic wind speed is 46m/s based on a 3 second gust; the equivalent mean hourly wind speed is 23m/s. 10. Ground conditions: • Datum level - 12.0m sandy clay, C =100kN/m2, φ =15 degrees • Below -12.0m rock, allowable bearing capacity = 2000kN/m2 • Groundwater was found at 3.0m below ground level. The soil strata and ground water level may be assumed to follow the slope of the ground.
Q5. Art Gallery • •
How to deal with the culvert? Placing a load bearing structure above it will induce a stress on the culvert which is not allowed. • Placing foundations to the side of the culvert will also impart a horizontal stress if it is too close. • No construction can be placed within 4m of the culvert. • Options are: 1. Excavate for shallow foundations and place them below the level of the culvert. Will have to deal with groundwater (dewatering), and possibly temporary shoring to the culvert using sheet piles. 2. Place shallow foundations above water table and a calculated distance from the edge of the culvert to ensure that no horizontal stress increase on culvert, i.e. use a conservative 45 degree distribution. Structure will have to cantilever or bridge over. 3. Use two rows of sleeved piles either side of the culvert 4m away and bridge over the culvert. The upper portion of the pile above culvert level has a frictionless sleeve to make sure no stresses imparted onto the culvert. Frictionless sleeve is either bitumen coating or an outer casing with a gap between the actual pile.
List of references • • • • • • • • • • •
BS5228 Part 4 (1992). Code of practice for noise and vibration control applicable to piling operations BS 5930 (1999). Code of Practice for Site Investigations BS 6031 (2009). Code of Practice for Earthworks BS 8002 (1994). Code of Practice for Earth Retaining Structures BS 8004 (1986). Code of Practice for Foundations BS 8006 (1995). Code of Practice for Strengthened Reinforced Soils and Other Fills BS 8081 (1989). Code of Practice for Ground Anchorages BS EN 1997-1 (2004). Eurocode (EC)7: Geotechnical Design CIRIA R166 (1997) CDM Regulations work sector guidance for designers CIRIA TN142 (1992) Ground-borne vibrations arising from piling ICE (1996) Specification for piling and embedded retaining walls
• Special thanks to Esad Porovic (Arup) and Imraan Motara (WSP).
What is the active earth pressure for undrained conditions? • Refer to BS 8004 • For active conditions; • Total active thrust normal to the wall = Pan 1 2
=> Pan = γ .z 2 − K ac cu z
Take cw/cu = 0.75 => Kac = 2.6
Assuming σv = γ.z