The pile can then be tested by one of two static load tests:
1. Maintained load test
2. Constant Rate of Penetration (CRP) test.
- the full test load on pile shall be twice the design load noted on the Drawings and it shall be applied in twelve equal increments. At least two hours shall elapse between the addition of each load increment, i.e. until the rate of settlement is reduced to less than 0.25mm/hour and slowing down;
- the full test load shall be maintained on the pile for at least 48 hours and settlements shall be recorded at intervals of not more than 12 hours. The test pile shall then be unloaded in four equal increments at one hour intervals until the full load is removed. Settlement readings shall be made immediately after and before every load increment is applied or removed.
Constant Rate of Penetration (CRP) Test
For each pile load test, three cycles of pile loading test at a constant rate of penetration shall be carried out to a full test load equal to twice the design load.
The rate of loading shall be such that a constant rate of penetration is maintained throughout the rest insofar as is practicable. The rate of movement of each pile to be tested shall be agreed upon with the S.O prior to the start of the test. At least twelve readings of settlements and their corresponding loads shall be made in the loading process.
After attaining a test load equal to twice the design load, the load shall gradually be released and at least four readings of settlement and their corresponding loads shall be made during the unloading process. The settlement obtained when the load has been completely released shall also be recorded. An interval of at least 15 minutes shall elapse before the next CRP test is commenced.
Interpretation of Test Results
The S.O's interpretation and conclusions on the test results shall be final. Unless otherwise specified, the pile so tested shall be deemed to have failed if:-
- the residual settlement after removal of the test load exceeds 6.5mm; or
- the total settlement under the Working Load exceeds 12.50mm; or
- the total settlement under twice the Working Load exceeds 38.0mm. or 10% of pile diameter/width whichever is the lower value.
Hong Kong Practice
Maintained Load Test
At present, this is the most popular pile test used in Hong Kong. The equipment required for loading the pile is assembled across the top of the pile. The hydraulic jack and the load cell are placed on top of the pile. The ram of the jack is extended until the load cell touches the underside of the main RSJ of the loading frame. Equipment to measure pile settlement is then set up, and the zero reading s are noted.
Before carrying out the load test, the engineer will state the working load and the test load, and he will describe the different stages of loading in the test. Most specification give the test load as twice the working load. The test load is applied in increments of about 25 percent of the working load, until the working load is reached. Smaller increments are added thereafter until the specified limit is reached.
Loading tests are usually carried out in a cycle. Each stage of the loading test or increment of load is applied as smoothly and quickly as possible. Readings of load, time and settlement are taken when loading commences, and at intervals as the load increases. Each increment of load is allowed to remain until settlement has ceased. Similarly, unloading of each incremental load must not commence until recovery has ceased. In general, settlement or recovery is assumed to have ceased when the rate of movement is 0.25 mm per hour.
When the maximum test load is reached, the applied load is unload progressively to zero load. Measurements are recorded again during the unloading process.
Some specifications may require the load to be unloaded at intermediate stages, either progressively or in one operation. In such cases, the load is then re-applied either in increments or in one operation.
Most Hong Kong specifications require that after unloading, the maximum test load be applied again in one operation, and be maintained for a minimum period of 72 hours. It is the step unloaded to zero load.
Graphs of load against time, load against settlement, and settlement against time are then plotted. These graph s are used to determine if the pile has satisfied the requirements of the specification.
The pile is deemed to have failed the test if:
- the total pile settlement > Z + D/120 + 4 mm
- and residual pile settlement > D/120 + 4 mm
where D = diameter of circular or least lateral nominal dimension of other piles in mm.
Z = L (2P+Pt)/3AE mm
P = test load ( kN )
L = length of pile ( m )
A = cross-section of pile ( m² )
E = Young's modulus of elasticity of pile ( kN/m² ).
Pt = resistance at the tip of the pile
= 2APb( kN ) where
Pb =safe bearing capacity of sub-soil strata at the tip of the pile ( kN/m² ). This can be calculated from the appropriate soil parameters obtained by site investigation.
The equipment used in the constant rate of penetration (CRP) test is the same as that used in the maintained load test. However, the method of loading is different.
- for a very long pile, the elastic shortening of the pile during the test may reach 10 percent of the base diameter: and
- for a large pile, there may be difficulty in loading the pile to a settlement as great as 10 percent of its base diameter.
When either of the above two cases is encountered, the ultimate bearing capacity is usually estimated roughly from the load settlement curve.