Hello Dong Tang

 Bear in mind that compressibility is the opposite of stiffness. Hence, you cannot say that the compressibility becomes stiffer. Instead, you should say that the compressibility increases or decreases.

A measure of compressibilty is the coefficient of compressibility also termed the coefficient of volume change in British literature defined by

Mv = dV/V/dp'  = de/(1+e)/dp', that is the strain per unit change of stress. V = volume, p' = effective mean stress, e = void ratio. In soil mechanics it is common to define a Bulk modulus K given by K = 1/Mv. The response of a soil to stress increment depends on the drainage condition.  A partially saturated soil is much more compressible than a fully saturated soil under undrained conditions. The compressibility of a saturated soil under undrained conditions is very small being almost equal to the compressibility of the pore fluid which is frequently water.  It can  be shown that the undrained bulk modulus of a soil is given by

 Ku = K' + Kw*(1+1/e)    where Kw = bulk modulus of water and K' = bulk modulus of the dry soil. Because K'

Wonderful piece from Anyaegbunam. Please see below link to assist yourself clarify the issue. http://www.google.com.ng/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CCUQFjAC&url=http%3A%2F%2Fwww3.imperial.ac.uk%2Fpls%2Fportallive%2Fdocs%2F1%2F1087899.PDF&ei=xW9oVbqyKozd7QbFwIKYBg&usg=AFQjCNHNmv-xVCLqHiXEyul5kVibMLaahg&bvm=bv.93990622,d.ZGU

Thanks for the answer.

As you said, "Under drained conditions the compressibility of a saturated or partially saturated soil simply equals the compressibility of the dry soil", it seems the test data does not support this. For example, attached picture is for sand in the southeast coast of Cyprus. Usually in the constitutive modelling, people assume the slope of the normal compression line to change with suction or saturation.

I agree that no matter at saturation or dry state, water and air can be viewed as fluid, in this case, the compressibility should not change. But how to explain those I mentioned above?

I took a look at the paper and it must be noted that the compression test in this paper goes up to 10 MPa. Under such stress, the measured compression index Cc is mainly determined by particle breakage and particle re-arrangement associated with breakage events. Water plays an important role here in changing the surface energy of the minerals and hence modify the breakage behavior of the assembly. Fully saturated specimen is more susceptible to particle crushing, while notionally 'dry' specimen are more difficult to crush. That explains why the former has higher compressibility.

Interesting discussions can be found in attached papers. Also, please be clear in the question description that it is 'sand' we are talking about, rather than 'soil' which is very general.

Hello Dong Tang,

My statements apply to fully drained or undrained conditions. In a fully drained test on a soil  the pore pressure Uw = 0  and pore air pressure Ua is also zero and in that case the soil suction s = Ua - Uw = 0. Consequently, suction should not be a parameter when studying the drained behavior of a soil, that is, when a soil is so slowly loaded that the excess pore pressures are dissipated. 

Suction becomes relevant when considering the deformation of partially saturated soil in undrained condition. This occurs when a fine grained partly saturated soil is rapidly loaded. The undrained response of a partly saturated soil is a field of ongoing research in which successful constitutive relations (stress-strain relations) have not yet been established. However, a break through may be achieved in the near future.

 Your test suggests that saturated sand is more compressible than dry sand.     The question is, "does the dry sand possess some cementation that is lost when the sand becomes saturated?"  By this I mean is the sand sample a substantially undisturbed sample probably obtained by a ground freezing technique?   

 Another question is, " was the test on saturated soil being conducted slowly enough to allow the excess pore pressure to remain zero throughout?" 

I believe there is no solid answer to your question. It depends on the soil type, (Sand, silt, clay). Confinement is also another aspect that you have to consider in your comparison

Thanks for all the replies above.  I have better understanding now.