One may have wondered why piles are most times placed in groups to bear loads instead of singly. Is it not possible to use single piles to carry column loads? The reasons for these are somewhat interwoven. Pile groups are commonly used for driven piles more than bored piles because driving errors could produce eccentricity on the pile which could affect the pile bearing capacity. When piles are placed in group, the piles would complement each other and offset this weakness. On the other hand, when piles are placed in group, the individual capacities of the piles are reduced due to certain reasons. This effect could possibly affect friction piles more because they bear load by friction along the surface of piles where the region of overlap occur.
First, each individual piles produce pressure region or bulb which overlap and produce pressure on the overlapping zones which reduces the individual capacities of the single piles. Thus in the group, this weakness is offset by multiple piles.
Secondly, piles driven through cohesionless soils may cause densification of the soil, thus increasing the load capacity of the pile more than the estimated value. Figure 1 shows pressure overlap in a group of pile.
Having seen the reason why piles are put in groups and the effect of grouping of piles, if one have a knowledge of the bearing capacity of individual piles, when these piles are placed in group, it becomes obvious the individual capacities of these piles when summed together would not give the same as the piles in a group. Owing to this, it becomes imperative that the efficiency of the piles in the group would be determined. The efficiency of piles in a group depends on:
a. Type of supporting medium: Usually efficiency of 1.0 may be assigned to piles resting on bedrock or layer of sand and gravel overlying bedrock for end-bearing piles. Otherwise the value may be lower. Friction piles driven in cohesionless soils may also achieve efficiency of 1.0.
b. The number of piles in rows and columns
c. The diameter of piles and
d. The spacing of piles
Determination of pile group efficiency
The Converse-Laborre equation have popularly been used to determine pile group efficiency
The equation state thus:
Where
Eg = pile group efficiency
0 = arctan of the ratio of pile diameter to pile spacing, that is tan-1(d/s)
d = pile diameter
s = centre – to – centre spacing of piles
n = number of piles in a row
m = number of rows of piles
Example
Using the figure below, determine the pile group efficiency
From the data above,
Piles spacing, s = 2000 mm
Pile diameter, d = 400 mm
Number of piles in a row, n = 3
Number of rows of piles, m = 4
0 = tan-1 (400/2000) = 11.310
Pile efficiency,
Eg = 1 – 0.178 = 0.822
Thus the efficiency of the pile group is 82.2 %
