Abstract: This paper is based upon the results of static load tests on 14 uplift （D=45, 60cm; L = 4.2¹2m） and 2 compression （D=60cm, L = 9, 12m） prototype piles in silty clay and silts. Influences of the variation of pile length on ultimate uplift skin friction and development of shaft resistance in uplift and compression versus pile top displacement are discussed.The ratios of skin friction in uplift to that in compression are presented and found to be varying with depths of penetration.Displacement patterns of ground surface around piles of different penetration during uplift are observed.A group efficiency of 0.9 for 2-piles groups with 2D spacing is also suggested.