EVALUATION OF DYNAMIC COMPACTION

Abstract

Deep Dynamic Compaction (DDC) Technique was used to improve soil bearing capacity in an area located some 65 km west of Islamabad, Pakistan. It was first project in the country where DDC was used for soil improvement. Geotechnical site investigation revealed that the soil strata at construction site are composed of alluvial deposits of Indus River containing varying degree of fines in different layers. The upper 1 m to 2 m layer was compacted fill and contained 75 percent fines. The water table was located at 12 m. The proposed RCC framed structure had a covered area of 25000 m² and individual column loads of structure varied from 1200 kN to 2300 kN. An allowable soil bearing capacity of 150 kPa was required as against 100 kPa available at the site. DDC technique due to its cost effectiveness was selected to enhance the bearing capacity. The compaction program was designed basing on empirical correlations. It comprised two high energy passes and a low energy ironing pass; each high energy pass comprised 10 blows per impact point from a height of 16 m and the low energy pass comprised 2 blows per impact point from a height of 5 m. Grid spacing of primary and secondary pass was 6 m and 3 m respectively. The ironing pass was performed with overlapping grid pattern with overlap of one-third of the tamper dia. The circular tamper, 1.5 m high, 2.4 m in dia and weighed 20 ton, was made of concrete with steel casing. SPT and Electrical Resistivity Test were used to evaluate ground improvement under point of impact and in the lateral direction (away from centre of impact point). Post-compaction boreholes for SPT were drilled within 2 m distance of pre-compaction boreholes. Soil improvement was evaluated after 5 blows, 10 blows and full scale compaction on nine test craters. Total of 48 boreholes were drilled each upto 8 m depth and 384 SPT were performed (one SPT at each 1 m depth interval). SPT test results indicated an appreciable improvement upto 5 m depth only. Slight improvement was observed from 5 m to 6 m depth. Pre-to-post compaction comparison of electrical resistivity of soil indicated an increase in soil resistivity after compaction. The depth of improved zone is less than the desired and estimated from empirical correlations. Research has been carried out to study various aspects of evaluation of improvement and identify causes of low depth of improvement to enhance understanding of DDC technique for future applications.