In typical foundation design, one footing usually has more than one pile. The question that always follows is whether a 4-pile footing carries 4 times the load of a single pile. The engineering answer is: not necessarily, and in soft clay such as that common in Bangkok and its surrounding provinces, it often carries less.
What Is Pile Group Efficiency?
Pile group efficiency is the ratio of the ultimate bearing capacity of the whole pile group to the sum of the ultimate bearing capacities of the individual piles in that group. It can be written as:
Group efficiency = ultimate capacity of the whole group ÷ (number of piles × ultimate capacity of a single pile)
If the value equals 1, the group works fully, as if each pile acted independently. If it is less than 1, the group loses capacity because the piles interfere with each other. If it is greater than 1, installation has improved the surrounding soil so the group performs better than the sum, which does happen in some cases.
Rearranging the formula, group capacity equals group efficiency × number of piles × single-pile capacity. For example, if group efficiency is 0.8 (an assumed value for illustration only), a 9-pile foundation would carry about 7.2 times a single pile, not 9 times.
Why Do Closely Spaced Piles Carry Less Load?
Closely spaced piles carry less load because they rely on the same soil mass. Each pile transfers load to the soil in two ways: skin friction along the pile shaft and end bearing at the pile toe. Both create stress that spreads into the soil mass around the pile, known as the stress bulb.
When two piles are far apart, their stress bulbs barely overlap, so the soil supporting one pile is not disturbed by the other. When piles are close together, the stress bulbs overlap and the same soil mass must carry load from several piles at once. The stress in the soil is therefore higher than a single pile would create. As a result, the soil approaches failure sooner and settles more than calculated from a single pile.
The figure below compares piles spaced about 4 times the cross-section size (S ≈ 4D) with piles spaced about 2.5 times (S ≈ 2.5D). Note how the stress bulbs begin to overlap as spacing decreases.
How Much Does Pile Spacing Affect Group Efficiency?
Spacing is the single most influential variable. The farther apart the piles, the less the stress bulbs overlap and the closer group efficiency gets to 1. In practice, engineers usually specify spacing as a multiple of the pile cross-section size, measured center to center. The spacing most often seen in typical foundation designs is about 2.5–3 times.
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| Spacing (multiple of pile size) | Degree of stress overlap | Group efficiency trend | Site notes |
|---|---|---|---|
| Less than 2.0× | Heavy overlap | Clearly below 1 | Risk of pile collision during installation and of block failure in clay |
| 2.5–3.0× | Moderate overlap | Slightly below 1 to near 1 | Most common range in typical foundation designs |
| 3.0–4.0× | Little overlap | Approaching 1 | Larger pile cap, more concrete and steel |
| More than 4.0× | Almost no overlap | About 1 | Often uneconomical to build, unless there are specific constraints |
This table shows conceptual trends only. The group efficiency used in actual design depends on soil type, installation method, number of piles and the layout of each footing. The design engineer must set this value directly from the project's soil investigation data, not take numbers from the table.
There are several empirical formulas in textbooks for estimating group efficiency from spacing and number of rows, such as the Converse-Labarre formula, which still appears in some specifications and textbooks. Today, however, many guidelines regard these formulas as only rough estimates that do not replace analysis based on actual soil data.
What Is Block Failure in a Pile Group?
Block failure is the case where the whole pile group and the soil between the piles move down together as a single block. It usually occurs in clay when the piles are close enough together that the soil between them is held and moves with the piles. The whole group then behaves like one large pile with a cross-section equal to the outer perimeter of the group, hence the name block failure.
In this failure mode, only two sources of resistance remain: shear of the soil around the outer surface of the block and bearing of the soil at the base of the block. The skin friction of the individual piles inside is not mobilized, because the inner soil already moves together with the piles.
Block Capacity = shear around the outer surface of the block + bearing at the base of the block
Outer-surface shear = block perimeter 2 × (Bg + Lg) × pile length L × average soil shear strength over the length. Base bearing = block base area Bg × Lg × soil bearing capacity at pile-toe level, where Bg and Lg are the width and length of the outer perimeter of the pile group.
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| Issue | Individual Failure | Block Failure |
|---|---|---|
| Movement mode | Piles slip down into the soil; soil between piles stays in place | Piles, soil between piles and pile cap move down together |
| Resisting surfaces | Shaft of every pile + toe of every pile | Outer surface of block + base of block |
| Usually governs when | Wide spacing, or end-bearing piles in a hard layer | Close spacing, in clay, with many piles in the group |
| Effect on design | Group capacity close to the sum of single piles | Group capacity may be significantly lower than the sum |
Engineering practice is to check both cases and use the lower value as the group capacity. Overlooking block failure is a major risk in soft clay, the main soil layer of Bangkok and its surrounding provinces. Read about the characteristics of Bangkok's soft clay in Foundations on Bangkok Soft Clay.
The first figure below is a 2D section comparing block failure (panel ①) with the equivalent raft concept used to estimate settlement (panel ②). The second figure is a 3D model of a 3×3 pile group. Look at the red box, which represents the soil block and piles moving down together, and the outer perimeter Bg × Lg used in the formula above.
How Do Pile Groups Differ in Sand and Clay?
They can differ in direction, not just degree. In clay, group efficiency is usually below 1, while driven piles in loose sand may reach 1 or more.
In loose sand with driven piles, pile driving compacts the surrounding sand so it becomes denser. Piles driven later therefore sit in soil in better condition than at the time of investigation, and piles already driven also benefit from the denser soil. In this case group efficiency may equal or exceed 1. This effect comes from compaction during driving, which is a different matter from the change in pile capacity over time after driving (Setup or Relaxation), which can be checked by retesting. Read more in Restrike Test and the Setup Phenomenon.
In clay, pile installation disturbs the soil structure and generates excess pore water pressure. Soil strength gradually recovers over time, but the soil does not become permanently denser as sand does. Combined with the risk of block failure, this means group efficiency in clay is usually below 1, especially at close spacing.
Bored piles, whether in sand or clay, do not densify the surrounding soil as driven piles do, so they gain no benefit from soil compaction. In some cases boring even loosens the soil around the hole slightly. The differences between the two systems are explained in Bored Piles vs. Driven Piles: How Testing Differs.
Why Does a Pile Foundation Settle More Than a Single Pile?
Because stress from a pile group reaches much deeper than the pile toes and loads compressible soil layers that a single pile barely affects. On many projects this matters more than capacity, because many foundations are governed by settlement criteria, not strength criteria.
A single pile creates a small stress bulb in the soil, roughly a few times the pile cross-section in size. In a group of many piles, the bulbs of all the piles merge into one large mass, so the depth at which stress is still significant extends much deeper than the pile toes. If there is a compressible clay layer below the pile toes, that layer is loaded by the group even though a single pile barely affects it.
As a result, a single-pile load test showing settlement of only a few millimeters does not mean the whole foundation will settle only that much. Group settlement must be assessed separately. The usual method is the equivalent raft concept, which assumes the total load is transferred as a spread footing the size of the pile group at a certain depth, then calculates the compression of the soil layers below that level (see panel ② of the 2D figure above).
How to read settlement from a single-pile test result is explained in How to Interpret Static Load Test Results.
How Much Does Testing Individual Piles Tell You About Group Behavior?
The pile tests used on typical projects all test one pile at a time, so they show the capacity and behavior of a single pile, not group efficiency directly. An overview of each test method is on the pile testing services page.
This data is still essential, because every method of analyzing group behavior must start from reliable single-pile capacity and behavior. If the single-pile value has not been confirmed, multiplying or dividing by a group efficiency is meaningless.
- Static Load Test (ASTM D1143 and DPT 1251) gives the load–settlement relationship of a single pile directly, making it the most direct input data for group analysis.
- Dynamic Load Test (PDA) to ASTM D4945 and DPT 1252-51 gives capacity and the distribution of skin friction with depth from CAPWAP analysis, which helps show whether the pile relies mainly on skin friction or end bearing. This matters because friction pile groups are more affected by stress overlap.
- Seismic Integrity Test to DPT 1551-51 checks the integrity of the pile shaft. It does not give capacity, but it helps confirm that no pile in the group is defective and shifting extra load onto its neighbors.
This last point is often overlooked. In a footing with only a few piles, if one pile is defective and carries less load, the load shifts to the remaining piles and may tilt the footing. Random integrity testing therefore has system-level value, not just pile-by-pile quality control. See the sampling criteria in How Many Piles Need to Be Tested.
Summary: How Much Load Can a Pile Group Carry?
The capacity of a pile foundation is not simply the sum of the individual pile capacities; it equals group efficiency × number of piles × single-pile capacity, because closely spaced piles use the same soil mass to transfer load. Spacing is the main variable controlling this. In clay, block failure must also be checked and the lower value used, while for driven piles in loose sand, soil compaction may let the group perform better than the sum. Beyond capacity, group settlement is often larger than estimated from a single pile, so it must be assessed separately.
What project owners can do is provide good input data: soil investigation data covering the depth below the pile toes, and pile test results confirming the actual single-pile capacity on site. These two are the raw materials the design engineer needs to assess group behavior.
This content explains the principles of pile group efficiency and block failure for general understanding only. The group efficiency, pile spacing, number of piles per footing and acceptable settlement criteria of each project must be set by the design engineer from the actual soil data and loads of that project.