Selection and Timing of Load Tests

EI 02G001

CEMP-E

01 July 1997

(1) Categories of Load Tests. Load tests that are frequently performed are proof tests without

internal instrumentation which are conducted to only twice the design load. Load tests are also performed to

failure with or without internal instrumentation.

(a) Proof tests do not indicate the ultimate pile capacity and, consequently, the pile can be

designed to support a higher load than necessary. Proof tests are not adequate when the soil strength may

deteriorate over time such as from frequent cyclic loads in some soils. Coral sands, for example, can cause

cementation that can degrade with cyclic loads (see chapter 5).

(b) Piles tested to failure allow the reserve capacity and the allowable load to be evaluated.

The allowable load when a load test is performed is one-half of the ultimate pile capacity.

(c) Internal instrumentation allows the skin resistance to be measured in the different soil strata

through which the pile is driven and the end bearing capacity to be measured in the bearing stratum. The

load test results can be used at other locations where the depth of the different soil strata and the bearing

stratum varies, thereby reducing the need for additional load tests.

(2) Selection and Timing of Load Tests. Pile tests are not always practical or economically feasible

under certain circumstances, particularly for deep foundations or remote locations; but, they are technically

desirable for sound and complete engineering practice.

(a) Pile load tests are relatively expensive and may not be justified if the structure has a small

number of piles. An important or complex structure could compensate for the added cost of a pile test

program when the consequences of a failure will be disastrous. Pile tests are necessary and justifiable

when the subsurface data indicate uncertain foundation soils that would compromise the integrity of the

design.

(b) Consideration will be given to the timing of a pile test program in relation to the construction

schedule. Cost estimates and schedules will include resources for site investigation, test data collection, and

design verification. A pile test program by separate contract may be performed for desired subsurface data

and to allow completion of the foundation design prior to award of the construction contract if time and funds

permit.

(c) Test piles should be located at strategic positions such as near boundaries of the foundation

and at interior locations to determine the distribution of the driving resistance. Adequate subsurface

investigations and a pile test program should be performed to confirm the design assumptions. The static

load test should be conducted on the pile with the lowest driving resistance. The use of PDA equipment

permits calibration of wave equation analysis and can reduce the number of load tests required to determine

the pile capacity.

(d) The influence of time-dependent strength gain from soil freeze or loss from soil relaxation is

frequently important. Provisions should be considered in contract documents for delays during load testing

and pile driving operations to allow for soil freeze or relaxation effects so that the true pile capacity can be

determined. This requires careful planing and coordination between designers and construction personnel

to prevent any undue impacts and delays.

(3) Axial Load Tests. Axial compressive load tests should be conducted and recorded according to

ASTM D 1143 but can be modified to satisfy project requirements. The Osterberg load test, where the load

cell is placed at the tip of the pile, is useful in situations where a loading frame cannot be constructed.

(a) The standard load test takes the most time and will measure the most consolidation

settlement of any of the load tests; but, none of the load tests will measure all of the consolidation

settlement.

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