Geotechnical engineering of large pile foundations in challenging conditions; what practical aspects do you need to take into account? In this article Thomas, Niek and Roy explain how they deal with these aspects at Windbase, based on the project wind farm Geefsweer-Oosterhorn (Geefsweer). This includes the interpretation of pile drive tests and in-situ measurements of the set-up process of piles in potclay to validate the design.

Wind farm Geefsweer

Wind farm ‘Geefsweer’, situated in the north-east of the Netherlands (Groningen), is a project of several stakeholders (Geefsweermolen BV, Eneco Geefsweer and Mauve Geefsweer). The wind farm consists of 14 turbines of the Vestas V136 type with a capacity of 4.3 MW and hub heights of 145 meters. Windbase was responsible for the overall foundation design and the groundwork of the crane hardstands.

The foundations consist of circular concrete bases with a diameter of 19.0 meters. These concrete bases are founded on 42 precast 450×450 mm concrete piles with placed 8 to 1 (vertical to horizontal). The crane hardstands consisted of both shallow and piled foundation.

Soil and site specifics

Typically to this part of the country is the variability in soil stratigraphy dating back to glacial periods. Geological processes created a landscape with huge gullies, which can be several kilometres wide and up to a hundred meters deep and which are usually filled with clay sediments. This preglacial clay is a very stiff impermeable type of clay, called potclay or “potklei” in Dutch. The wind farm is located in such a gully. Foundation designs for large wind turbines in this part of the Netherlands are challenging as piles of up to almost 30 meters will have to be driven deep into this clay. A typical cone penetration test from this area is shown in the figure below where the potklei is clearly visible and extends to the maximum explored depth of the CPTu.

Problem description

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.

Test set-up

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.

The driving equipment preferred by the contractor for the test was a standard diesel fall block Delmag D62. If the test driving proved to be successful, the block would be used for the remainder of the pile driving work.

To prevent delays of the planning of the actual construction works, a measurement period of about 30 days for GW08 was possible.

Analyses of results

The dissipation of the excess pore pressures over time is shown in the figure below. From the figure it can be deduced that the dissipation of the excess pore water took roughly only one month (from 8 October to the beginning of November) to stabilize after pile driving. This meant that after 30 days it was possible to drive the verification CPTs to determine the remolded cone strength. A possible explanation of the unexpected fast consolidation period is the fact that the many fine sand meanderings through the potclay might allow for natural drainage.

A comparison between the net strength of the soil before and after pile driving based on the CPTU’s is shown in the figure below. The CPTU’s measurements showed no significant decrease of the soil strength after one month. In fact, even some densification effects in local sand lenses were observed. The computed bearing capacities according to the Dutch standards based on CPTU’s taken before and after are therefore almost equal. Practically this means that at the time the construction of the turbine is completed the piles should have more than sufficient capacity to sustain the working loads.

Furthermore, the pile drivability test showed that the drivability of the concrete piles deep into potclay was possible with blow counts up to 12 blows per 0.25 meters using a Delmag D62 and up to 20 blows per 0.25 meters using a Delmag D46. These were significantly less than expected from driving analyses in which blow counts up to 40 blows / 0.25 meters were calculated with a D62 block. This further underpins the significance of in-situ testing of special soil types like potclay.

Conclusions

Wind farm ‘Windpark Geefsweer’, consisting of 12 Vestas 4.3 MW turbines, is situated in a complex geological location in the north-east of the Netherlands. The 42 prefab concrete piles of the foundation have to be driven deep into the so called potclay layers originating from glacial periods. Because of uncertainties with respect to the time frame in which the piles would reach their designed bearing capacity and the drivability into these layers, Windbase has advised the client to carry out a pile test.

From the in-situ drivability test it was concluded that the piles could be driven to their desired depths at relatively high speeds and low blow counts without problems. Furthermore, the in-situ testing of the potclay strength before and after driving showed that the piles would reach the designed bearing capacity within roughly one month after driving. By combining the theoretical experience of the engineers of Windbase with the local conditions, Windbase was able to advise the client with a practical test set-up to mitigate any risks. As a result, no alterations of the original design were required.