Behavior of Bored Piles in Two Soil Layers, Sand Overlaying Compressible Clay (Case Study)

Abstract

Piles foundations have the function of transferring loads from the superstructure through weak compressible strata onto stiffer or more compact soils or onto rock. This is soil profile is considered the ideal profile for pile foundations. While for soil profile including strong soil strata overlaying compressible soils, the estimation of piles behavior will be more difficult. Forty six residential buildings consisted of twelve stories were constructed at the north of Nile delta in Egypt. Bored piles with lengths equal to or more than 27.0 m were chosen to support the raft foundations of these buildings. Soil investigations concluded that the soil consists of dense sand overlaying compressible clay that extended down to 40.0 m, while the ground water was found near ground surface. Soil properties were determined through drilling four boreholes at the site with 60.0 m depth and disturbed and undisturbed samples were collected. Six cone penetration tests (CPTU) with 25.0 m depth were achieved to estimate soil properties and also to estimate bearing capacity of the piles. Five preliminary pile load tests were carried out on different types of piles. Four rotary drilling bored piles and one contentious flight auger were tested having lengths ranged between 27.0 m and 37.0 m, while its diameter ranged between 0.60 m and 0.7 m. The static load tests continued to load equal to 200 t which is equal to 250% the allowable pile load. Some tests completed within 48 h and some within 7 days to check the time-load-effect. In the light of these measurements, comparisons have been achieved between the bearing capacities of piles estimated by the results of CPTU and those measured by field tests. Moreover, the ultimate bearing capacities of piles estimated by Egyptian and Canadian Codes have been compared with those measured by field tests. Good agreement was noticed between the bearing capacity of piles estimated by methods depended directly on CPTU. While the indirect methods depended on soil parameters derived from CPTU produced lower values than the measured. It was also noticed that the estimated bearing capacities of piles based on soil properties derived from lab tests did not soundly match with the measured values.