DEBONDING OF PILES AND DEEP FOUNDATIONS

6.4.1 Where the Contract requires the DFEs to support loads from adjacent structures then the permanent works shall be designed for the loads imposed by the DFEs unless the DFEs are debonded such that there is no load transfer.

6.4.2 If it is chosen to design the permanent works for the imposed load from the DFEs, then it is required that the assumptions for load transfer along the length and at the base of the DFEs are verified by means of

instrumented load tests. The instrumented load tests must be carried out before the design of the affected permanent works is finalised. The number of instrumented load tests must be sufficient to provide information for all of the types of DFEs and in all of the ground conditions present.

6.4.3 Any imposed loads from the DFEs shall be additional to the loads and surcharges defined in Chapter 3 and elsewhere in this document.

6.4.4 The option to design the permanent works for the imposed loads (rather than debonding), which is identified in Clause 6.4.1, applies only in cases where the DFEs and the permanent works are being designed and constructed under the same Contract.

6.5 LOAD TESTING

6.5.1. General

The designer shall specify the number and type of load tests in accordance with the requirements given in Table 6.2. The number and type of tests shall be indicated on the Drawings.

The Ultimate Limit State (ULS) criterion for a preliminary load test is when the settlement of the pile head has exceeded 10% of the pile diameter.

The designer shall establish Serviceability Limit State (SLS) criteria at 1 and 1.5 times the working load. The designer shall also establish an allowable residual settlement upon final release of test load. These criteria shall be calculated to be consistent with the whole foundation system meeting the settlement requirements under Clause 6.3, and shall be used as the criteria for assessing load tests. These criteria shall be stated on the Drawings

Similar SLS criterion shall also be established for laterally loaded and tension DFEs.

For guidance, some typical failure criteria are given below. The maximum settlement figures may be increased by 2.2% for every metre length of DFE beyond 30 metres. Use of alternative criteria shall be to the acceptance of the Engineer.

For Bored Piles

a) The maximum settlement at working load (second cycle) exceeds 7 mm.

For Driven Piles

a) The maximum settlement at working load (second cycle) exceeds 9 mm.

b) The maximum settlement at 150% working load exceeds 20 mm.

For all DFEs

a) Failure of the DFE materials (due to defects in the DFEs).

b) Failure of concrete to reach the design compressive strength.

c) The residual settlement upon the final release of the load exceeds 5 mm.

6.5.2 Preliminary Load Tests a) Test Loads

The target for the preliminary load testing is to achieve the ULS criterion.

b) Dimensions for Preliminary Load Tests of DFEs

Preliminary load tests on piles can be considered representative for working piles up to twice the diameter of the preliminary test piles, provided that they are installed in similar ground conditions.

For the load testing of barrettes and diaphragm wall, the contractor may propose the testing of piles in place of the barrettes or diaphragm wall. However, the contractor must ensure that the diameter of the pile is at least equal to the minimum dimension of the diaphragm wall or barrette and that the method of construction for the pile is as similar as possible to the diaphragm wall or barrette construction.

c) Ground Conditions

Preliminary test DFEs shall be installed in ground similar to that where the working DFEs are to be installed.

6.5.3 Working Load Tests 6.5.3.1 Test Loads

The test loads in a Working Load Test shall be 150% of the working load of the DFEs.

6.5.3.2 Failure of Working Load Testing

A DFE is considered to have failed a loading test if it does not comply with either of the SLS criteria. Where a pile fails the test the contractor

shall install suitable piles. If, in the opinion of the Engineer, it is impracticable or inadvisable to install substitute piles in place of a failed pile, the Contractor shall submit proposals to rectify the defect. The proposal is subject to the acceptance of the Engineer.

Where a DFE fails the working load test, the Contractor shall carry out two additional working load tests. The Engineer shall select the piles for these tests.

Working load tests shall be in the direction of the design load; i.e., Piles where the major load is in tension shall be tested in tension.

6.5.4 Quantity of Testing

The values in Table 6.2 represent the Authority’s minimum requirement for DFE testing

For the purpose of establishing the number of tests required the percentages given in Table 6.2 relate to the total number of working DFEs on a contract. Calculations of the number of tests as a percentage of the total number of DFEs shall be made to one decimal place. If the result is not a whole number then the value shall be rounded up to the nearest whole number: e.g., if 137 working piles were installed the requirement would be for 3 working load tests.

Where more than 300 piles are installed in consistent ground conditions, using similar methods, on a single contract by the same piling contractor then the number of DPT, Sonic Coring, Sonic Logging and Low Strain tests can be reduced on the following basis:

For the first 300 piles, the percentage shall be as shown in the Table 6.2. For piles in excess of the first 300, the percentage tested may be reduced to half of that shown in the Table 6.2 for the relevant tests.

6.5.5 Selection of DFEs for testing

DFEs to be tested shall be proposed for the acceptance of the Engineer. Piles for testing shall be selected after they have been constructed unless the method of testing requires the piles to be pre-selected.

6.6 PERMANENT GRAVITY AND CANTILEVER RETAINING WALLS