LOAD TEST ON PILES
IS: 2911 - Part 4
Pile load test is the most direct method for determining the safe loads on piles including its structural capacity with respect to soil in which it is installed. It is considered more reliable on account of its being in-situ test than the capacities computed by other methods, such as static formula, dynamic formulae and penetration test data.
a) Pile load capacity by high strain load test
In this procedure, the force and velocity response of the pile to an impact force applied axially by drop of hammer causing large strain at the top of the pile are measured to arrive at the ultimate capacity of the pile. The two pairs of strain transducers and accelerometers are fixed at diagonally opposite sides of the pile, built up to 1.5 times the pile diameter. The strain induced under the impact of a heavy falling hammer at a pre-determined height is measured by the strain transducers. The accelerometers record the acceleration in the pile. The pile driving analyser converts the strain into force and acceleration into velocity. It is necessary to calibrate the results of the dynamic analyser with those of a static pile load test carried out according to IS: 2911. The details of the dynamic test are available in IRC: 78-20144, Appendix 7- Part 1.
b) Pile integrity test (PIT):
It is a non-destructive integrity test method for foundation piles. It is also known as “Low Strain” method, standardised by ASTM D 5882. Various methods are adopted to evaluate the integrity of piles:
(i) Pile-Echo method.
(ii) Force-Velocity method.
(iii) Transient Response procedure
(iv) Cross hole sonic logging. Pulse-Echo method is most commonly used. In this method, the impact on the pile with a small hand held hammer and the resultant strain of extremely low magnitude are measured using an accelerometer attached to the top of the pile. The low strain test method is an indication of the quality of work and the pile capacity cannot be derived by this method. The various aspects of load test on piles as given in IS: 2911 are included in this section. There are two types of tests for each type of loading viz., initial and routine. Initial test may be one or more depending on the number of piles. It is carried out at 2 to 2.5 times the estimated safe load and up to the pile destruction. Routine test is carried out on about 1.5% to 2% of the total number of piles. It is carried out to 1.5 times the estimated safe load. Pile load testing is performed when the concrete has reached 28 days strength. Mainly three types of loading tests are carried out on piles.
(i) Vertical or axial load test.
(ii) Lateral load test.
(iii) Pull out test.
2. General requirements for pile load testing
(i) Pile test may be carried out on a single pile or on a group of piles. In group testing, caps will be provided such that the required conditions of actual use are achieved.
(ii) Load application and measurement of deflection are generally made at the top.
(iii) In cases where top portion of piles are likely to be exposed due to scour or dredging, necessary allowance for the length of pile exposed shall be included. In such cases, the test shall be done on the pile cap without resting on the ground.
(iv) The tests shall be carried out at cut-off level as far as possible. Otherwise suitable allowance shall be made in the interpretation of the test results.
3. Vertical load test (Compression)
a) In this method, compressive load is applied to the pile top by means of a hydraulic jack against rolled steel joist or suitable frame capable of providing reaction and the settlement is recorded by suitably positioned dial gauges.
b) The pile top shall be chipped to get a hard, natural plane. The projecting reinforcement shall be cut off or bend. The surface of concrete shall be finished smooth and level with plaster of paris of similar synthetic material. A bearing plate with hole at the centre should be placed on the head of the pile for the jacks to rest.
c) The test is carried out by applying a series of vertical downward load increments of about 20% of the safe load on the pile.
d) The reaction to be made available for the test should be at least 25% more than the final test load proposed to be applied.
e) The reaction may be obtained from the following:
(i) Kentledge supported on a platform supported clear of the test pile. The centre of gravity of the kentledge should be on the axis of the pile and the load applied by the jack should be co-axial with the pile.
(ii) Anchor piles located at 3 times the diameter of the test pile or at 2.0 metres minimum centre-to-centre. If the anchor piles are permanent working piles, it shall be ensured that their residual uplift is within the limits. The datum bar supports should not be affected by the heaving of the surrounding soil.
(iii) Rock anchors at a distance of 1.5 m or twice diameter of test pile, whichever is greater.
f) Settlement shall be recorded with minimum 2 dial gauges for single pile and 4 gauges of 0.01 mm sensitivity for groups. Each shall be installed at equal distance around the piles, held by datum bars resting on immovable supports.
g) The safe load on single pile should be least of the following
(i) Two-third of the final load at which the total displacement attains a value of 12 mm or to the stated total displacement.
(ii) 50% of the final load at which the total displacement is equal to 0% of the pile diameter in case of uniform diameter piles and 7.5% in the case of underreamed piles.
NOTE: Routine test shall be carried out for a test load of 1.5 times the working load, the maximum settlement under the test loading not exceeding 12 mm.
4. Lateral load testing
a) This test is carried out by introducing a hydraulic jack of adequate capacity with pressure gauge abutting the pile horizontally and reaching against a suitable system. If it is conducted by jack located between two piles or groups, the full load imposed by the jack shall be taken as the lateral resistance of each pile or group.
b) The lateral load applied on the pile shall be measured by a calibrated pressure gauge mounted on the jack and supported by the datum bar arrangement.
c) The loading shall be applied in increments of about 20% of the estimated safe load. The next increment shall be applied after the rate of displacement is nearer to 0.1 mm per 30 minutes.
d) The next increment should be applied after the rate of displacement is nearer to 0.1 mm per 30 minutes or 0.1 mm per hour or 2 hours whichever is later.
e) Unless failure occurs first, test pile shall be loaded up to 2 times the design load (Initial test) and 1.5 times the design load (Routine test).
f) The maximum test load corresponds to the total displacement increased to 12 mm.
g) The displacement shall be read by at least two dial gauges of 0.01 mm sensitivity spaced at 30 cm and kept horizontally one above the on the test pile and displacement interpolated at cut-off level of the pile. However, one dial gauge placed diametrically opposite to the jack shall directly measure the displacement.
h) The datum bars shall be provided with rigid supports of steel sections, embedded well into the ground. The supports shall be located more than 3 times the diameter of pile
i) The safe lateral load on the pile shall be taken least of the following: (i) 50% of the final load at which the total displacement increases to 12 mm.
(ii) Final load at which the total displacement correspond to 5 mm and
(iii) Load corresponding to any displacement at cut-off level, as specified in the design.
j) Pile groups shall be tested under conditions as per actual use in the structure as far as possible.
5. Pull out test
a) The test shall be conducted on a separate pile installed specifically for this purpose.
b) The test pile shall be built up to the proper length and the head provided with suitable arrangements for anchoring the load applying system
c) Uplift force may be applied by means of a hydraulic jack system resting on rolled steel joists supported on the ground. The supports shall be located at least 2.5 times diameter of the test pile.
d) The jack reacts against a frame attached to the top of the test pile such that when the jack is operated, the pile gets pulled up and the reaction is transferred to the ground. The frame work is normally attached to the reinforcing bars. A central rod designed to take the pile load and embedded into the concrete with sufficient bond length is sometimes preferred.
e) The test pile shall have adequate steel to withstand pulling.
f) The pull out load increments and consequent displacement readings shall be read as in the case of vertical load test.
g) The pull out load increments and consequent displacement readings shall be as in the case of vertical load test.
h) The safe load shall be taken least of the following:
(i) Two-third of the final load at which the total displacement is 12 mm or the load corresponding to the specified permissible uplift.
(ii) Half of the load at which the load-displacement curve shows a clear break (downward trend).
i) The initial test shall be carried out to 2.0 times the estimated safe load or till the loaddisplacement curve shows a clear break (downward trend).
j) Routine test shall be carried out to 1.5 times the estimated safe load or 12 mm total displacement whichever is earlier.
The test report shall include the following:
(i) Tabulation of test data - Load and movement of test pile head, reference beam and reaction piles.
(ii) Load - settlement curve.
(iii) Time load curve.
(iv) Limit load analysis if the pile has failed within the maximum test load.