Model tests have been conducted on a single pile and pile group in sand under both compressive and tensile loadings. Axial forces

loads and displacements at the pile head

and pile tip resistance were measured. Experimental results show that the compressive group pile effect coefficient ? is greater than 1. The optimal pile spacing is 4D(D is pile diameter)

at which η is about 1.2. The pile tip resistance coefficient is greater than the lateral friction coefficient . The pile tip resistance coefficient reaches maximum at the spacing of 5D with a value of about 1.3. The lateral friction coefficient is about 1.2 at pile spacing of 3D

but it decreases with the increasing of pile spacing. Different from the results revealed by previous theoretical analysis

the test results show that the pullout group effect coefficient is greater than 1

due to sand densification during pile driving. The optimal pile spacing is between 4D-5D

with a maximum coefficient of about 1.2. The group coefficient is close to 1 when the pile spacing is greater than 7D and the group pile effect can be ignored.