Static Load Test (Maintained Load Test)

The most accurate way to determine the load capacity is to install a full-size prototype pile at the site of the proposed production piles and load it to failure. However, load tests also are much more expensive, and thus must be used more judiciously.

Varieties of equipment and procedures have been used to conduct load tests. The differences in the equipment and procedures can influence the results and become the point of debate among engineers. Therefore, there is no single correct capacity for most piles. Nevertheless, engineers judge the accuracy of all other methods by comparing them to full-scale load tests.

2.2.1.1 Test Equipment and Instruments

Test equipments and instruments consist of load application arrangements and the instruments to measure the resulting movements or deformations. Stacking

large weight on top of the pile as imposed load, posed danger because it is difficult to place large weights without creating excessive eccentricities that can cause them to collapse. As an alternative, a hydraulic jack is used to provide the test load. This system is more stable and less prone to collapse.

Figure 2.12 shows the actual setup of static load test using hydraulic jack performed on a bored pile.

Figure 2.12 Static load test in progress

Traditionally, engineers have measured the applied load by calibrating the hydraulic jack and monitoring the pressure of the hydraulic fluid during the test. However, even when done carefully, this method is subject to errors. Therefore, load cells (an instrument that measures force) are developed. The load cell is placed between the jack and the pile and is used to measure the applied load.

Displacement is another measurement that is very important in conducting load test besides the capacity. The displacement is measured by utilizing dial gauges mounted on the reference beams. Surveyor’s level can also be used as a cross reference to the measured records of displacement.

Another measurement employed in load test is the incremental strain measurement along the pile length to determine the distribution of load transfer from pile to the soil. These provide information on pile tip movements or deflections along the pile. In order to obtain this measurement, a pile needs to be instrumented prior to installation. Instruments that can be used are the strain rods (or telltales) and the electric strain gauges (or vibrating wire strain gauges).

Telltales or strain rods (vibrating wire extensometer) normally consist of polyvinyl chloride (PVC) tubing extended to steel end plates embedded inside a concrete pile or welded on the steel pile or housed in sonic logging pipe at various locations along the pile length. Inside the sonic logging pipe tubing, a stainless steel/graphite/fiberglass rod is installed extending from the end plate to the top of the pile.

Both the tubing and the steel rod extend to the top of pile. The steel rod must be allowed to move freely in the tube. The movement of the top of the telltale or strain rod relative to the top of the test pile is measured with a dial gauge or the measurement of the vibrating wire extensometer is logged using a data logger.

Normally, telltale readings are referenced to the top of the pile. By noting the location of the specific telltale rod anchor plate and by measuring the relative movement of the individual rod, elastic shortening of pile at that location can be obtained.

At present, data recoded by the data logger can give the elastic shortening of the pile without having to measure the top measurements. With this information, the load in the pile at the midpoint between two telltale anchor plates separated by a distance L can be obtained by the following relationship:

ra

AΔLE Q =

L where

Qra = load in the pile midway between two anchor plates

A = cross section area of the pile

ΔL = difference in movement between two telltale rods E = modulus of elasticity of the pile material

L = distance along the pile between the two telltale anchor plates

Electric strain gauges or vibrating wire strain gauge can be mounted along the pile length at various locations before the pile is installed. In bored piles, these gauges can be tied up with the reinforcing bars and wires can be brought up through a casing just like the vibrating wire extensometer. Since these gauges are temperature sensitive, additional temperature-compensating gauges should be used for each strain gauge. The strain ε can be determined directly by noting the changes in the strain gauge reading from the unstrained to any desired load. At present the measurements are data logged using a data logger. The load at the point will then be calculated by the following relationship:

ra

Q =AEε

where

Qra = load in the pile at the location of the strain gauge

A = cross section area of the pile

E = modulus of elasticity of the pile material ε = strain gauge reading

2.2.1.2 Test Procedures

There are two categories of load tests: controlled stress tests and controlled strain tests. The former uses predetermined loads (the independent variable) and measured movements (the dependent variable), while the latter uses movement as independent variable and load as dependent variable.

Controlled stress tests are the slow maintained load test and quick maintained load test. In slow maintained load test, the pile is loaded in eight equal increments (i.e. 25%, 50%, 75%, 100%, 125%, 150% 175% and 200%) to two times of the design load. The load is maintained under each increment until the rate of settlement is acceptably small. At two times the design load, the load is maintained for 24 hours. After the required holding time, the loading is removed in decrement of 25% with 1 hour between decrements.

After one cycle of the load, the pile is reloaded to test load in increments of 50% of the design load, allowing 20 minutes between load increments. Then the load is increased in increments of 10% of design load until failure, allowing 20 minutes between load increments. This method is commonly considered as the ASTM Standard test method and is generally used for site investigation prior to installing contract piles and writing specifications. The disadvantage of this test is that it is time consuming.

In quick maintained test the procedure is almost the same as slow maintained test except that each load increment is held for a predetermined time interval regardless of the rate of pile movement at the end of that interval. Pile is loaded in 20 increments to three times of the design load (i.e. each increment is 15% of the

design load). Each load increment is maintained for 5 minutes with readings taken every 2.5 minutes until the test load has been reached.

After 5 minutes interval, the full load is removed in four equal decrements with 5 minutes between decrements. The advantage of this test is that it is fast and economical. The method represents a more nearly undrained condition. This method cannot be used for settlement estimation because it is a quick method.

Controlled strain test is the constant rate of penetration test, which presses the pile into the ground at a constant rate. As the test progresses, the load and settlements are measured to develop a load-settlement curve. In this test, the pile head is forced to settle at typically 0.01 to 0.05 in/min (0.25 to 1.25 mm/min) for clays and 0.03 to 0.10 in/min (0.75 to 2.5 mm/min) for sands. The force required to achieve the penetration rate is recorded. The test is carried out to a total penetration of 2 to 3 in. (50 to 75 mm).

The advantage of this test method is that the test is very fast and economical. This method can be employed for friction piles but less practical for the end bearing piles because of high force requirements to cause penetration through hard bearing stratum.