Analysis of preliminary pile load tests in chalk at Basingstoke

In order to confirm and enhance the design parameters used for contract piles, six preliminary pile load tests (four in compression, two in tension) were carried out for the Festival Place development in Basingstoke. This paper reports the results of the tests, analyses them and compares the derived parameters with those commonly used for pile design in chalk. Five of the tests were conducted in one area of the site and the sixth in another. The ground conditions at the main site comprised Made Ground underlain by White Chalk Subgroup, initially structureless to around 11 mbgl and then structured. At the second site the Made Ground was removed, revealing structureless chalk to around 9 mbgl and then structured chalk. The chalk was found to improve from CIRIA Grade Dm to Grade B3 over the length of the piles and was of low to medium density. All the piles were installed by CFA techniques and were 600mm diameter. Their lengths ranged from 8.5 m to 16 m, thus some terminated in structureless chalk whilst others penetrated this layer to found in structured chalk. They were tested at different times after construction, ranging from 14 to 71 days. The results found the β method of analysis to be a suitable means of calculating unit shaft resistance. The β value was found to be consistently over 0.9 and to increase with time between constructing and testing the pile. The results found the unit base resistance could be related to SPT N value and a ratio of 500 was found to be conservative. An alternative method of treating the chalk as a drained frictional material was investigated and tentative design friction angles of 32° for structureless chalk and 37° for structured chalk have been proposed, though with an anticipated gradual increase in  with increasing SPT N value.