Ground improvement by small-diameter timber piles

Geotechnical engineers often encounter problems in designing foundations of structures on soft clayey soil. There may be a need for ground treatment to improve the bearing capacity of the soil. In the case of mediumrise buildings in and around Kolkata, a common practice has been to provide closely spaced 150–200 mm diameter timber piles of 5–6 m length. These timber piles have helped the structures to stand on soft clayey soil with little or negligible distress over the years. The present investigation includes the study of bearing capacity and settlement behaviour of model footings resting on small-diameter timber piles. Model footings were tested on an artificially consolidated kaolin bed with small-diameter timber piles inserted into the clay bed. To supplement the results of laboratory model tests, prototype tests were also carried out in the field. In order to obtain a parametric study, the length and spacing of timber piles as well as the extent of piles around and below the footings were varied. The study shows that the use of small-diameter timber piles produced beneficial effects for pile lengths greater than the width of the footing. An appreciable effect on bearing capacity is observed when area ratio (i.e. pile area/soil influence area) becomes more than 10%. The extent of these timber piles beyond twice the width of the footing results in a marginal increase in the bearing capacity. Considerable reduction in settlement occurred when timber piles were used. A semi-empirical method of calculating settlement has been presented for foundations with small-diameter timber piles.