As the concrete piles are driven to the desired depth, significant excess pore water pressures will build up in these potclay layers. These excess pore pressures typically take a long time to dissipate which, together with the overall ‘’set-up time’' of the piles, might lead to the piles having insufficient bearing capacity at the moment the turbine is being put into service.
Overall, practical experience in designing large driven foundations elements in these soil conditions is limited. Within the project scope Windbase has therefore, in collaboration with Wiertsema & Partners and VHB, advised a pile driving test and provided the monitoring plan for the assessment of both the drivability and the so-called set-up process of the large concrete piles.
In order to measure the set-up of the piles in the clay, so called piezo cone penetration test (CPTU’s) are carried out before, and a certain period after, the test piles are driven. Furthermore, several piezometers have been placed at various distances from the test piles in order to measure the decrease of excess pore pressures in time and space. The test set-up is shown the figure below. The piles driven for this test are the piles 19 to 22.
The CPTU’s before driving (DKMP 801, 802 and 308) are used to determine the intact strength of the potclay. The verification CPTU’s after the piles are driven (DKMP 804, 805 and 806) are used to determine the remolded strength of the clay after a certain set-up time. These CTPU’s are situated between the driven test piles at maximum distances of 0.45 m from the pile shafts. The difference in net conus strength between the two measurements provide an indication of the set-up process in time.
The piezometers are used to measure the dissipation of the excess pore pressures in time at the two particular depths. WSP812 and 821 are placed in the potclay at a depth of NAP – 21,0 m, 811 and 822 at a depth of NAP -15,0 m. These piezometers are used to determine if the excess pore pressures have decreased sufficiently before driving the final verification CPTU’s.