The principle behind piling is to transfer loads of a structure through strata of low bearing capacity to deeper soil or rock strata that has a higher bearing capacity.
The loads can be transferred into underlying strata by either end bearing pile – (The strength of the pile is based on the bearing of the pile into a stiff strata), or friction base pile – (The strength of the pile is based on the shear interaction between pile and soil), or a mixture of end bearing pile and friction-based pile.
Piles can also be used to resist uplift and/or horizontal loads.
The principle behind piling is to transfer loads of a structure through weak soil to either a solid strata level and/or to rely on the friction between the long and slender piles and the soil they are embedded into.
There are several different types of pile that all follow the same principle. They can be made from a variety of materials including timber, steel and reinforced concrete; the last of which being the most common.
Piling methods can be placed into two categories:
Displacement piles shift soil away from the pile itself and their installation typically generates significant noise and vibration. The exceptions to this are displacement augers and bottom driven steel tubes.
Replacement piles extract the soil to form a shaft, which is then filled with a material that creates the pile. This method of piling does not generate as much noise or vibration.
Piling methods can appear in the following forms:
Bored and cast in situ – A shaft is created within the soil; concrete is poured into the shaft either during or after boring and a reinforcement cage is cast into it. This can be done simultaneously via a continuous flight auger (CFA) piling rig that places the concrete within the pile shaft as it is being dug.
Driven piles – A pre-assembled pile is driven into the soil. The pile is made up of sections and is linked together via couplings.
Cast in place – In cohesionless soils a tube is driven into the soil and the resulting void is filled with concrete. For cohesive soils such as clay, the pile shaft is bored, and the void is filled without the use of a tube.
Jacked piles – Piles that are pressed into the soil via a jack.
The selection of pile type is dependent on several factors including:
- Ground conditions, both geotechnical and environmental
- Durability requirements
- The loads to be applied
- Site access
- Construction programme
- Site environmental constraints (e.g. noise)
For instance, five storeys building in a residential area that is to be built on a clay soil with layers of silt in it will likely have concrete piles placed via a bored process. This reduces the risk of piles failing as the soil collapses into the dug shaft, as a result of the concrete being installed while the pile shaft is excavated. This method does not generate much in the way of noise or vibration either, which is important for a site located within a residential area.
Pile tests need to be carried out when the following conditions arise:
- A method of pile installation and/or type of pile is being used where there is no experience of it being employed
- Where the piles have not been tested in similar soil and loading conditions
- When piles are subjected to loading, that theory and experience do not give confidence in the pile design. Load testing will simulate the design loading condition
- During installation the behaviour of the piles is different from what was expected, i.e. from the assumptions made from the soil property data gleaned from the earlier site investigations
In addition to mandatory testing, specific tests may form part of the pile design and installation process. Testing can inform the suitability of the chosen pile type, how the soil is interacting with the pile and the overall design of the sub-structure.
The extent of testing does have an impact on the design factor of safety of the piles. If a significant amount of testing is to be carried out during construction, then the factor of safety can be reduced, while the opposite is also true.
There are two forms of testing:
With respect to load testing, piles that are primarily subjected to axial compressive loads can be tested via two methods. One is the constant rate penetration test (CRP). This sees a force placed upon the pile that is slowly increased until the pile fails as it penetrates the soil. The other method is the maintained load (ML) test. This has a pile loaded to up to twice the design load and a time vs. settlement chart is plotted every time the load is applied to the test pile and removed.
CRP testing is destructive and is therefore reserved for piles that are installed for that purpose only. ML testing on the other hand can be carried out on piles that will ultimately form part of the foundations, although it is possible to test piles to destruction in this manner.
Integrity tests are typically not destructive and set out to determine the overall quality of the pile along its length. The most used testing methods are:
- Acoustic test
- Radiometric test
- Seismic test
- Stress wave test
- Dynamic response test
- Electrical test
Designing a pile-cap
Pile-caps are elements that transfer the actions from the superstructure of a building, bridge etc. into piles. They can be a form of pyramid truss that spreads the axial and bending forces from a vertical element into the piles on which the pile caps sit. Another way to describe them would be as a transfer structure that accommodates tolerances of the piles and spreads the axial force from a concentrated column or wall into one or more piles.
Pile-caps spread the axial force N from the superstructure through to piles at an angle, which determines the depth of the pile-cap. This angle is typically 45º but can be no shallower than 21.8º and is from the edge of the element that is being supported by the pile-cap, such as a column.
Spread of axial forces N through 3 pile caps
The layout of piles is largely determined from the magnitude and location of actions they are to support from the superstructure.
They are grouped together based on the pile’s capacity to support axial forces that can be either tension or compression, depending on the direction of the axial forces induced into the pile-cap from actions generated by the superstructure.
The location of piles with reference to the point of axial force application should be symmetrical. The proximity of piles to one another is at least 3 × diameter of the pile. For pile-caps with 1 or 2 piles, some restraint needs to be provided orthogonally to the pile(s), which is usually achieved via ground
S spacing between piles
Due to the setting out tolerances for the placement of piles, the edge of a pile cap should be no less than 150mm from the edge of a pile. This allows for the variances for the as-built location of the pile over the idealised one, plus cover to reinforcement.
Once cut down, the head of the pile penetrates the soffit of the cap by at least 75mm. This affects the placement of the bottom reinforcement as it needs to over-sail the heads of the piles
Another factor that determines the on-plan size of a pile-cap with respect to the pile location, is the requirement to ensure that all tension reinforcement in the base of the pile-cap is sufficiently anchored. The fact when the pile-cap is subjected to compression forces, the area immediately above the pile is in compression. This form of stress can be considered when determining the required anchorage lengths for the tension reinforcement.
When a pile-cap is supporting an axial force N that is placed within the centroid of the pile group, the axial force in each pile is defined as:
N/n where n is the number of piles.
This only applies to pile groups that have a maximum of 5 piles. Larger pile groups are influenced by differential displacement of the piles. This results in an increase in magnitude of axial force in the piles in the extremity of the pile-cap when compared to the piles that are closer to the centroid of the group. In
some cases, the layout of the piles and the stiff ness of the pile-cap can result in the piles closer to the centroid being more exposed to higher forces than those at the perimeter.
When designing a pile-cap, STR partial factors apply (as opposed to GEO) as the pile-cap does not have any direct interaction with the soil. The partial factors for STR as follows:
Gk,j is the partial factor for permanent actions (e.g. self-weight of the structure) and has a value of 1.35
Qk,1 is the leading frequent variable action (e.g. occupancy and furniture) and has a value of 1.5
Qk,2 is the accompanying quasi-permanent variable action (e.g. wind)
ψ0 is the factor for the accompanying quasi-permanent value of a variable action.
The effect of design action on a pile-cap foundation N or Ed is defined as:
N=Ed =Gk, j+Qk,1+ ψ0Qk,2
There are two analysis methods used when designing pile-caps. One is the strut and tie system that assumes the reinforcement within pile-cap is acting as if it were part of a truss, with the compression stresses being withstood by the concrete and the tension by the steel reinforcement. This method is applicable for pile-caps with less than 6 piles.
The forces that pass through a pile-cap into the piles and how the strut and tie method is applied to it. It indicates how the depth of the pile-cap determines the magnitude of the forces within the concrete and the tension reinforcement.
Typically, the angle of the truss is set at 45º, which is then used to determine the depth of the pile-cap. Further iterations of this angle may be necessary as the size of the pile-cap is altered and/or the element it is supporting is modified to overcome geometry constraints and other extraneous design criteria. This can result in having a shallower angle that increases the tension in the reinforcement – as does the compression stress in the struts within the pile-cap.
When the location of the axial force is not eccentric, the applied tension (tensile force) between each pile can be calculated using the following
l is half the distance between centroid of the pile group and the centre of vertical element the pile-cap is supporting
d is the distance between top of the pile-cap and top of the piles
The tension reinforcement As required in the pile-cap is defined as:
There is a need to check the strength of the struts within pile-caps as the vertical action is applied to them. These checks are applicable to all strut and tie models, but they are especially relevant to pile-caps.
There are the two different types of node that can exist within a pile-cap, relating to the types of forces present: ‘CCT’ (tension) nodes and ‘CCC’ (compression) nodes.
The compressive strength at the pile head needs to be checked for either CCC or CCT nodes, depending on how the action from the superstructure is being applied to the pile-cap.
Compressive strength of CCC nodes are defined as:
vRd,max = k1vfcd
k1 is the factor applied to node compressive strength and is taken to be 1.0
v is the strength reduction factor for concrete cracked in shear and is defined as: 1-(fck/250)
fcd=0.85xfck/1.5 for permanent condition
fcd=0.85xfck/1.2 for accidental condition
vRd,max = k2vfcd
k2 is the factor applied to node compressive strength where a tension force is present and is taken to be 0.85
This node force is compared against the applied force onto the pile. This typically becomes critical in shallow pile caps due to the increase in applied forces.
A critical shear plane adjacent to the vertical element that the pile-cap is supporting needs to be checked to determine whether or not it fails in shear. The plane’s location is based on the distance av, which is the dimension from the face of the vertical element the pile-cap is supporting and the face of a pile plus 0.2 × the pile diameter.
The shear resistance is determined based on the enhanced shear capacity of the pile cap near the point of support i.e. the head of the pile. The applied shear force VEd can be reduced by the ratio of the depth of the pile-cap to the distance to the critical shear plane. Additionally, the shear at the face of
the vertical element that is fixed to the pile cap needs to be checked.
The shear resistance of the pile-cap is defined as:
D is the effective depth of the pile-cap and is expressed in mm
As/bd must be ≤ 0.02
As is the provided area of tensile reinforcement
b is the width of the pile-cap
Provided Ved ≤ VRd,c then no additional reinforcement to resist shear over the minimum is required. The minimum area of shear reinforcement VRd,c(min) is defined as:
VRd,c(min) = 0.035k3/2 fck1/2
An additional check with respect to shear is required at the face of the vertical element the pile-cap supports. The shear resistance VRd,max is defined as:
VRd,max = 0.5v1 fcdpd
v1 is the shear strength of the concrete in N/mm2
fcd is the applied shear at the face of the vertical element the pile-cap is supporting = 0.5v1 (fck/1.5) pd
p is the perimeter length of the vertical element of the superstructure
d is the depth of the pile-cap
There are several unique detailing requirements that are specific to pile caps. The anchorage length of the tension reinforcement is dependent on the bond
conditions between the concrete and the steel. In the case of pile-caps, a good condition bond requires the reinforcement to be located within the 250mm depth of the concrete pour. For anything placed outside of that zone the bond is ‘poor’.
Typical anchorage length of tension bars in pile-caps
Values of fctm vs. concrete strength class
The minimum diameter of reinforcement used in a pile-cap is 8mm.
The minimum area of tension reinforcement in the pile-cap As,min is defined thus: As,min = 0.26btd(fctm/fyk)
bt is the average width of the tension area above the piles
d is the effective depth of the pile-cap
fctm is axial tensile strength of concrete
A 400mm x 400mm concrete column has an axial design action of N=3000 kN. Design a n=3-pile pile-cap to support the column. The piles are D= 350mm diameter cylindrical concrete piles. The pile-cap’s concrete is to be grade fck=32 N/mm2 32/40 with fy= 500 N/mm 2 high tensile steel reinforcement. The column is placed in the centroid of the pile group.
Determine area of tension steel required at base of pile cap:
Pile spacing l=1200mm
The average width of the tension area above the piles bt=800mm
Tensile force T=2*N*s/(9*d) =500000.000N
Therefore Asreq =T/(0.87*fy) =1149.425mm2
These reinforcement bars need to be placed in BOTH directions
Therefore, As required 2* Asreq=2298.851mm2
Check minimum reinforcement
Node Stress Check
Node Strength Rdmax=0.85*v*fcd*1mm2/N=13.440N/mm2
Total compressive [email protected] Cstrength =3.14*D2/4* Rdmax=1292.465kN
Applied force N/n=1000.000kN less than Cstrength =1292.465 kN Therefore OK