PILE FOUNDATIONS CONTENTS: 1.0 Introduction 1.1 Choice of pile type 1.1.1 Driven (displacement) piles 1.1.2 Bored (replacement) piles 2.0 Analysis 2.0.1 Driving formulae 2.0.2 Soil mechanics 2.1 Piles in cohesive soil 2.1.1 Bored piles 2.1.2 Driven piles 2.1.3 Under-reamed piles 2.2 Piles in non-cohesive soil 2.2.1 Driven piles 2.2.2 Bored piles 3.0 Negative Skin Friction 4.0 Working Load 5.0 Summary REFERENCES
1.0 INTRODUCTION Piles are used where a structure cannot be supported satisfactorily on a shallow foundation. A single pile can be defined as a long slender, structural member used to transmit loads applied at its top to the ground at lower levels. Examples of where piled foundations may provide a solution are: Where a soil layer of adequate bearing capacity lies too deep for the economic use of conventional footings. Where the soil layer(s) immediately underlying a structure are soft or poorly compacted. Where the soil layer(s) immediately underlying a structure are moderately or highly variable in nature. On sites where the soil strata, and in some cases the ground surface are steeply inclined. On river or shoreline sites where tidal or wave action or scouring may vary the amount of material near the surface. For structures transmitting very high concentrated loads. For structures transmitting significant horizontal or inclined loads. For structures which structurally or functionally may be sensitive to differential settlement. For more detailed treatment of piling methods. pile types and design, refer to the books by Tomlinson (1987), Poulos (1980), Fleming (1985) and Whitaker (1970). A pile carries the applied load via: 1. A shear stress mobilised (developed) on the surface of the shaft of the pile. This is called skin friction in sands and adhesion in clays. 2. Bearing capacity at the base of the pile, called end bearing. From the point of view of both design and construction, piles are classified into two types: a) Driven or displacement piles which are usually preformed before being driven, jacked, screwed or hammered into the ground. b) Bored or replacement piles which first require a hole to be bored into which the pile is then formed, usually of reinforced concrete. -2-
Piles may also be classified according to how they achieve their load carrying capacity; end bearing piles or friction piles. In the majority of cases however, the load carrying capacity is dependent on both the end bearing and shaft friction. NOTE: Pile design must be accompanied by in situ load testing. Eurocode 7 emphasises that pile design must be based on static load tests or on calculations that have been validated by these tests. 1.1 Choice of pile type 1.1.1 Driven or Displacement piles Types of pile foundations a) Preformed piles: Advantages:- - may be inspected for quality and soundness before driving - not liable to squeezing or necking - construction not affected by ground water - can be left protruding above G.L. (useful in marine structures) - can withstand high bending and tensile stresses -can be driven in long lengths Disadvantages: - unjointed types cannot easily be varied in length - may break during driving - uneconomic if the design is governed by driving stresses rather than working stresses - noise and vibration during driving - displacement of soil may affect adjacent structures - cannot be driven in situations of low head room -3-
b) Cast in place piles Advantages: - length can easily be adjusted - ground water can be excluded by driving with a closed end - enlarged base possible - design governed by working conditions - noise and vibration reduced by internal drop hammer Disadvantages: - necking is possible where temporary tubes are used - concrete cannot be inspected after installation - length may be limited if tubes are to be extracted - displacement may damage adjacent structures - noise and vibration may be unacceptable 1.1.2 Bored or replacement piles a) Cast in place piles: Advantages: - 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 2.0 ANALYSIS OF PILES Disadvantages: - 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 collapseable soil - cannot be easily extended above ground - boring may cause instability and settlement of adjacent structures Analysis of piles is quite complex and there are two main approaches: 1. Estimate the carrying capacity from driving formulae and load tests (only suitable for sands/gravels or stiff clay) 2. Calculate the carrying capacity from soil mechanics expressions. -4-
2.0.1 Driving Formulae There are many different expressions all try to relate the energy needed to drive the pile to the penetration of the pile (for which there is no theoretical justification). e.g. Hiley Formula; Where; R u = W h n s + c/2 R u = ultimate driving resistance W = weight of hammer h = fall of hammer n = efficiency of blow, found from graph s = set or penetration/blow c = total temporary compression of pile Driving formulae take no account of soil type or conditions and are therefore generally disapproved of by foundation engineers. The only sure way is to drive some test piles and then carry out load tests thereby finding the carrying capacity time and cost are big disadvantages. 2.0.2 Analysis using soil mechanics Load capacity of single piles There are two forms of resistance provide by the pile to the applied vertical loads: shaft resistance base resistance At failure the ultimate values of both these resistances are mobilised to give: Q u = Q s + Q b where : Q u = ultimate pile capacity Q s = ultimate shaft resistance Q b = ultimate base resistance and Q b = q b x A b = base bearing capacity x area of base Q s = surface area of shaft in contact with the soil x shear strength of the soil Q s = c a d L (clays) ; where c a = adhesion Q s = f s d L (sands) ; where f s = skin friction where d = diameter of pile L = length of pile in contact with the soil -5-
Piles usually penetrate several different soil types, each providing different shaft resistances and the total shaft resistance is the summation of the individual values. The weight of the pile is usually ignored in the above equations, since it is approximately equal to the weight of soil removed or displaced. 2.1 Piles in cohesive soil (clay/silt ; = 0 o ) Q u Ultimate pile capacity, Q u = Q b + Q s Q s Q s 2.1.1 Bored piles Q b Base resistance, Q b (kn): Q b = q b A b = c u N c A b Where q b = base bearing capacity = c u N c A b = cross sectional area of pile base (m 2 ) c u = undrained shear strength at base of pile N c = bearing capacity factor = 9.0 (intact clays) or = 6.75 (fissured) clays Shaft resistance, Q s (kn): Q s = c a A s Where c a = adhesion = c u = adhesion factor [usually taken as 0.45, but may vary from 1.0 for soft clays to 0.3 for overconsolidated clays] c u = average undrained shear strength over length of pile, L d = diameter of pile L = length of pile in contact with soil stratum -6-
Class example 1 A bored pile, 750mm diameter and 12.0m long, is to be installed on a site where two layers of clay exist: Upper firm clay; 8.0m thick; undrained shear strength = 50.0 kn/m 2. Lower stiff clay; 12.0m thick; undrained shear strength = 120.0kN/m 2. Determine the working load the pile could support assuming the following: i) = 0.7 for firm clay and 0.5 for stiff clay ; N c = 9 ii) Factors of safety of 1.5 and 3.0 are applied to the shaft load and base load respectively iii) The top 1.0m of the firm clay is ignored due to clay/concrete shrinkage. [921 kn] Class example 2 For the ground conditions and assumptions described in Example 1, determine the length of pile required to support a working load of 1200 kn. [14.96m, say 15m] 2.1.2 Under-reamed piles Often used in cohesive soils to increase the base area of the pile, thereby increasing the base resistance. For under-reamed piles the adhesion should be ignored over the: a) height of the under-ream, b) main shaft of the pile up to 2 shaft diameters above the top of the under-ream and c) top 1m of the pile (zone of seasonal shrinkage). Class example 3 A large under-reamed bored pile is to be installed in stiff clay with undrained shear strength of 125kN/m 2. The main shaft of the pile is 1.5m diameter and the base of the under ream is 4.5m diameter with a height of 3.0m and the total length of the pile from the ground level to the base of the under ream is 27m. Determine the working load of the pile in MN, assuming the following: a) = 0.3 ; N c = 9 b) A factor of safety of 3.0 should be applied to the base load but full mobilisation of shaft adhesion can be assumed. [9.498MN] -7-
2.1.3 Driven piles Base resistance Q b : Q b = c u N c A b (as above) Shaft resistance Q s : Q s = c u A s = c u d L where; = adhesion factor dependent on depth of penetration and type of overburden, value found from graph (see next page) c u = average undrained shear strength over pile length L d = diameter of pile L = length of pile in contact with soil stratum Class example 4 A closed end pipe pile, 600mm diameter is driven to a depth of 15.0m into a stiff clay. The undrained shear strength of the clay is 140.0kN/m 2. Assume = 0.43 Determine the working load (kn) the pile could support with an overall factor of safety of 2.5. [778.0 kn] -8-
Adhesion factors for short piles(l<10d) driven into stiff clay Adhesion factors for long piles(l>20 to 40d) driven into stiff clay (Tomlinson, 1987) -9-
2.2 Piles in non-cohesive soil (sand/gravel ; c = 0) Q u Ultimate pile capacity, Q u = Q b + Q s 2.2.1 Driven piles Base resistance Q b : Q b = q b. A b Where; A b = cross sectional area of pile base q b = base bearing capacity = N q v and N q = bearing capacity factor, see chart below v = vertical effective stress at the base of the pile Q s Q b Q s Q b = N q v A b (From Berezantsev et al 1961) -10-
The internal angle of friction, before the installation of the pile, is not easy to determine since disturbance will occur during piling. The value used is obtained from correlations with the SPT N values as shown below: Critical depth, z c As the depth of pile penetration increases, the vertical effective stress increases and therefore the end bearing should increase. Field stress have shown, however, that end bearing does not increase continually with depth. A possible explanation is that as increases the bearing capacity factor decreases. This has lead to the concept of critical depth z c, below which shaft and base resistance are considered to be constant (i.e. the values for z c and below). The value of z c is determined from charts relating depth to - these are somewhat tentative. Shaft resistance Q s : Q s where f s and A s K s = f s A s = skin friction on pile surface = K s tan v = area of pile in contact with the soil = d L (cylindrical pile) = coefficient of horizontal effective stress = angle of friction between pile surface and soil v = average effective vertical stress Q s = K s tan v d L The method of installation affects the values of K s and presented as one factor as shown below; and they are usually -11-
Class example 5 A 10.5m long concrete pile, 400mm square, is to be driven into a thick deposit of medium dense sand, with an SPT N value of 25 and a bulk unit weight of 20.0 kn/m 2. The water table lies at 2.5m below ground level. Estimate the working load this length of pile will support assuming an overall factor of safety of 2.5 and the sand has a saturated unit weight of 20.0kN/m 3 [949.2kN] 2.2.2 Bored piles Boring holes in sands loosens an annulus of soil around the hole and reduces horizontal stresses. Consequently bored piles in dense sands can be expected to have low bearing capacity. Casting concrete in situ will produce rough surfaces but this effect is diminished by the loosening of the sand. Poulus(1980) suggests analysing as if for a driven pile but using reduced values of v. Meyerhof (1976) suggests designing as if for a driven pile, but using one third of the base resistance and one half of the shaft resistance. -12-
3.0 NEGATIVE SKIN FRICTION This term refers to the action (friction or adhesion) of soil layer/s acting with the applied loading i.e. against the pile resistance. It is usually caused by either; Clay soil undergoing consolidation settlement or Fill material compacting over time Negative skin friction is caused by a dragging down effect by the consolidating / compacting layer plus any overlying strata, see diagrams below. Consequently the values of friction or adhesion for the consolidating soil must be added to the applied load. Treat skin friction values as load on the pile and are not factored. FILL (recently placed) Compresses under own weight. Dense GRAVEL Does not compress FILL Soft CLAY Dense GRAVEL (Recently placed) Compresses under own weight Consolidates due to weight of fill. Does not compress Class example 6 A 300m square concrete driven pile driven 12.0m into a layered soils as follows; Fill (recent) 2.5m thick ( = 26.0 kn/m 3 ; = 37 o ) Medium SAND 3.0m thick ( = 17.0 kn/m 3 ; N = 18) Soft CLAY 2.0m thick ( sat = 22.0 kn/m 3 ) Compact SAND 9.0m thick ( sat = 22.0 kn/m 3 ; N = 33) The strength of the soft clay increases linearly from 18.0 kn/m 2 at 5.5m below ground level to 36.0 kn/m 2 at a depth of 7.5m. A water table is present at a constant depth of 5.5m below ground level. Determine the safe working load of this pile by adopting factors of safety of 1.5 and 2.5 for the shaft and end bearing resistance respectively. [1256.3 kn] 4.0 WORKING LOAD OF PILES In order to determine the working or safe load that a pile can carry, it is necessary to apply factors of safety in order to limit the settlement to a permissible value. -13-
Different authors apply various factors of safety to different pile conditions. However the following values are generally accepted. For piles up to 600mm diameter An overall factor of safety of 2.5 should be adopted, to give a settlement which is unlikely to exceed 10mm. working load = ultimate load 2.5 For piles larger than 600mm diameter It is necessary to apply partial factors of safety to the ultimate base and shaft resistance values For London Clay, Burland (1966) suggests that providing an overall factor of safety of 2 is obtained, partial factors on the shaft and base of 1 and 3 respectively should be applied, so that the working load, Q a is the smaller of : Q a = Q s + Q b OR Q a = Q s Q + b 2 1 3 The first expression governs the design of straight shafted piles and the second governs the design of large under reamed piles. For soils other than London Clay, e.g. Glacial Till (boulder clay), where there is uncertainty about the effects of installation, ground conditions etc, higher factors of safety should be used so that the working load Q a is smaller of : Q a = Q s + Q b OR Q a = Q s + Q b 2.5 1.5 3.5 Class example 7 Determine the length of a pile, 1200mm diameter, to support a working load of 4500kN in a thick deposit of clay with an undrained shear strength increasing linearly with depth from 55.0kN/m 2 at ground level and at 5.0kN/m 2 per metre depth. Assume; a. the top 1.0m of the pile does not support load due to clay/concrete shrinkage b. an adhesion factor, = 0.5; N c = 9.0 c. factors of safety of 1.5 and 3.0 on the shaft load and base load respectively. [29.5m, say 30m] -14-
5.0 SUMMARY Types of pile: Driven or displacement piles Bored or replacement piles Piles in cohesive soil (clay/silt; Φ = 0 o ) BORED PILES Base resistance; Q b = c u N c A b where, A b = cross sectional area of pile base c u = undrained shear strength at the base of the pile N c = bearing capacity factor = 9.0 for intact clays or = 6.75 for fissured clays Shaft resistance; Qs = α c u As where, α = adhesion factor, usually taken as 0.45, but may vary from; 1.0 for soft clays to 0.3 for overconsolidated clays c u = average undrained shear strength over length of pile A s = surface area of pile in contact with soil stratum DRIVEN PILES Base resistance; Q b = c u N c A b as above Shaft resistance; Q s = α c u A s where, α c u A s = adhesion factor dependent on depth of penetration and type of overburden, value found from graph = average undrained shear strength over pile length = surface area of pile in contact with soil stratum Under-reamed piles Increase of the base area of the pile, thereby increasing the base resistance. The adhesion should be ignored for a distance of two diameters above the top of the under ream. Piles in non-cohesive soil (sand/gravel; c = 0) DRIVEN PILES Base resistance; Q b Shaft resistance; Q s = N q σ v ' A b where, A b N q σ v ' = cross sectional area of pile base = bearing capacity factor, found from graph = vertical effective stress at the base of the pile = K s tanδ σ v ' As where, K s tanδ = installation factor from graph σ v ' = average effective vertical stress -15-
A S = surface area of pile in contact with the soil BORED PILES Boring holes in sands loosens an annulus of soil around the borehole, hence low bearing capacity. Analyse as if for a driven pile but using reduced values of σv', or use 1/3 of the base resistance and 1/2 of the shaft resistance. Negative skin friction The action of fiction or adhesion acts WITH the applied loading i.e. against the pile resistance. Consequently the values of friction or adhesion for the consolidating soil must be added to the applied load. Do NOT factor down skin friction values. Working load of piles Apply factors of safety in order to limit the settlement to a permissible value. For piles =<600mm diameter Use an overall F of S of to give a settlement of <10mm. For piles >600mm diameter Apply partial factors of safety to the base resistance and the shaft resistance. For London Clay, an overall F of S of 2.0 is obtained, with partial factors on the shaft and base of 1 and 3 respectively, so that the working load, Q a is the smaller of: Q a = Q s + Q b OR Q a = Q s Q + b 2 1 3 The first expression governs the design of straight shafted piles and the second governs the design of large under reamed piles. For soils other than London Clay, where there is uncertainty about the effects of installation, ground conditions etc, higher factors of safety should be used Qa is the smaller of: Q a = Q s + Q b OR Q a = Q s + Q b 2.5 1.5 3.5 Note: For negative skin friction, the above factors of safety are NOT applied to the element of load acting against the pile resistance. -16-
REFERENCES Berezantsev et al (1961) Load bearing capacity and deformation of piled foundations Proc. 5th Int Conf Soil Mechanics and Foundation Engineering, Paris, vol.2 pp.11-12 Burland, J B et al (1966) The behaviour and design of large-diameter bored piles in stiff clay Proceedings, Symposium on large bored piles ICE, London Fleming, W G K et al (1985) Piling engineering Surrey University Press / Halstead Press Meyerhof, G G (1976) Bearing capacity and settlement of pile foundations, Proceedings, American Society of Civil Engineers 102(GT3), pp 195-228 Poulos H G and Davis, E H (1980) Pile foundation analysis and design John Wiley & Sons, New York. Tomlinson, M J (1987) Pile design and construction practice 3rd Ed, Viewpoint Publications, Palladian Publications Ltd. Whitaker, T (1970) The design of piled foundations Oxford : Pergamon -17-