Piles are one of the commonest types of deep foundations. A foundation is said to be a deep foundation if the the depth of footing (Df) is greater than 15 times the breadth of the footing. That is Df ˃ 15B. Other common types of deep foundations are pier foundation and caissons/well foundation. Piles can be classified under different basis such as:
Material of construction: Timber piles, concrete piles, steel piles
Functions: End bearing piles, friction piles, compaction plies, tension piles, raker piles
Method of construction: Displacement piles (driven or driven and cast-in-situ), non-displacement or replacement piles (driven and cast-in-situ (bored) piles).
The construction of deep foundations are usually costly and can only been used where shallow foundations which are much cheaper cannot be used. In high-rised buildings for instance, piles are used to transfer the enormous loads from columns to the piles. Sometimes, the piles are arranged in groups of 2 – 5 or more to bear the column loads. Table 1 shows typical pile arrangement for 2 -5 groups of piles
Table 1; Typical Pile Arrangement
Distribution of column loads on individual piles in a group
Most times, it is more ideal to use group of plies instead of single piles to transfer column loads to firm soils. In a uniformly loaded pile cap, it is common practice to distribute the column loads equally to all the piles (Figure 1).
All piles in the group share equal load if the load is central on the pile cap. That is
Where
Qm is axial load on any pile, m,
Qg is the group load coming on pile which could be the column axial load,
n is the number of piles.
However, in some cases due to piling errors or construction errors, column loads on pile caps can be eccentrically loaded about Y-Y axis (Figure 2), X-X axis (Figure 3) or both axis (Figure 4).
If the column load is eccentric or if the column load have moment, the sharing of load is estimated assuming the pile cap is rigid. The pressure distribution is planar, thus the pile reaction vary linearly with distance from the centroid of the pile cap.
If the load is eccentric about Y-Y axis,
If the load is eccentric about X-X axis,
If the load is eccentric about both axis,
Where
My = Qg.ex
Mx = Qg.ey
ex = eccentricity of the load about Y-Y axis
ey = eccentricity of load about X-X axis
Sign convention
Cartesian coordinate sign convention (Figure 5) is used in the distribution of the loads.
If the sign convention above is not followed, the positive sign can be taken for piles on the same side as the eccentricity.
Note: If the load on any pile is negative, it indicates that the pile is in tension.
Example
A set of twelve (12) piles arranged in a rectangular array is expected to bear an eccentric load of 6 MN from a column as shown below (Figure 6). Determine the amount of load that each pile will bear.
Solution
Given, Qg = column loads coming on group of piles =6 MN;
Eccentricity of column load about Y-Y axis, ex = 0.5 m;
Eccentricity of column load about X-X axis, ey = 0.3 m
Moment of column load about Y-Y axis, My = Qg.ex =6 x0.5 = 3 MN
Moment of column load about X-X axis, Mx = Qg.ey = 6 x 0.3 = 1.8 MN
x = distance of each pile from Y-Y axis
y = distance of each pile from X-X axis
Follow the solutions as tabulated below in Table 2 (first column shows the piles as numbered in Figure 6).
Table 2; Solution to the given example
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References
Reynolds, C.E. and Steedman, J.C. (1988): Reinforced Concrete Designer’s Handbook, 10th edition. E & FN Spon, Taylor & Francis Group 11 New Fetter Lane, London.
2 Comments
Thank you for pointing out that it is preferable to transmit column loads to firm soils using groups of piles as opposed to single ones. My mother wanted to buy the pile for her factory. I’ll suggest to her that she use piling professionals to move column loads to firm soils.
Thank you for valuable knowledge.