Pore Water Expansion Disturbance 



As discussed previously, water expands by about 1.5 percent for a 

 10,000-foot change in water depth. Since most of the seafloor has a 

 depth less than 20,000 feet, this analysis will consider what happens 

 to a sample whose pore water expands by three percent during retrieval. 



Almost all soils have a tendency to expand upon the release of 

 in-situ stress, and it is this tendency to expand that creates residual 

 negative pore pressures. If soil has free access to water, the soi] 

 actually does expand. The expansion of pore water is almost exactly 

 analogous to exposing a sample to free water. The only difference is 

 that when a sample is exposed to water, the water must flow into the 

 sample, and there is a time lag. With pore water expansion, the response 

 is immediate. 



The influence of pore water expansion may then be evaluated using 

 experience gained through standard consolidation testing. In a consoli- 

 dation test, samples are first loaded to a particular stress level. The 

 stresses are then reduced incrementally, and the expansion of the sample 

 is measured. The expansibility of the sample is expressed in terms of 

 the swell index, defined as the change in void ratio resulting from a 

 log cycle decrease in vertical stress. Unpublished NCEL data derived 

 from a variety of shallow and deep sea soils indicate that the swell 

 index is usually equal to about 0.1, with very little variation. 



Considering the water expansion problem, if the soil void ratio 

 in-situ were 2.0 (a typical value), a 3 percent change would cause the 

 void ratio to be 2.06 after retrieval, a net change of 0.06. With a 

 swell index of 0.1, this expansion is the equivalent of a 0.6 log cycle 

 reduction in vertical effective stress or about 75 percent. The cause- 

 effect relationship is reversible; if a change in stress can cause a 

 change in void ratio, then a change in void ratio will cause a corre- 

 sponding change in the locked-in effective stresses, as expressed by 

 the residual negative pore pressure. It may be concluded, therefore, 

 that sampling and retrieving a soil under the conditions that have been 

 assumed leads to a reduction in residual pore pressure of 75 percent by 

 virtue of the water expansion. 



It is felt that the disturbance effects (reductions in residual 

 negative pore pressure and strength) produced by pore water expansion 

 are very similar to those produced by the particle reorientations caused 

 by mechanical disturbance at constant water content. Thus, residual 

 negative pore pressure changes produced by pore water expansion would 

 be treated in essentially the same way as those produced by mechanical 

 disturbance. From Figure 6 , a 75 percent reduction in residual negative 

 pore pressure indicates a strength reduction of about 40 percent for 

 soil at constant void ratio. Other forms of disturbance also occurring 

 during pore water expansion, including a slight increase in void ratio 

 and an elongation of the sample in the core liner, should lead to a 

 somewhat greater amount of strength reduction. 



In general, pore water expansion can produce a relatively large 



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