amount of disturbance. However, if residual pore pressure measurements 

 are made, it is possible to compensate for it by using the framework of 

 this report. Also samples from shallower water or with lower void 

 ratios would be less disturbed. 



A few, unique soils may be more troublesome. There are the soils 

 with such rigid interparticle bonding and such inflexible mineral grains 

 that the soil would not expand under free water source conditions as 

 much as the water expands during sample retrieval (very small swell 

 index). The interparticle structure could be destroyed in this case, 

 although a slight amount of water flow out of the sample is equally 

 plausible. The extent of disturbance under these conditions could only 

 be determined through an experimental study involving in-situ and 

 laboratory shear testing. These problem soils should be rare, because 

 an unusually low swell index is required. Also, soils with rigid grains 

 and cemented interparticle bonds usually have relatively high permeabil- 

 ities, making the possibility of water flow more probable. 



Gas Expansion Disturbance 



Gas-expansion-induced disturbance was not investigated directly 

 with the Santa Barbara Channel samples since none of these contained 

 significant amounts of dissolved gas. However, the author, as a 

 participant in Leg 19 of the Deep Sea Drilling Project, observed several 

 Bering Sea and North Pacific Ocean cores which had obviously undergone 

 significant disturbance by this mechanism. Gas bubbles were visible, 

 and several of the cores were protruding from their barrels. Severe 

 distortion of engineering properties was apparent. On one highly gase- 

 ous core from a sediment depth of 2,175 feet, the vane shear strength 

 was 0.13 psi. Using a conservative estimate of the rate of strength 

 increase with depth, the in-situ strength must have been at least 60 psi, 

 or a ratio of in-situ to measured strength of at least 460. It would 

 be impossible to develop curves such as Figure 5 to compensate for such 

 extreme disturbance. Therefore, it is concluded that laboratory testing 

 of samples disturbed by severe gas expansion is meaningless. Either 

 in-situ testing or retention of seafloor pressures during sampling and 

 testing is required. 



It may in some cases be difficult to recognize samples that contain 

 moderate amounts of gas. One indicator is a low degree of saturation. 

 Another, discovered during the Deep Sea Drilling Project testing, is a 

 large reduction in the ability of gaseous sediment to conduct sound 

 waves . 



In samples with a moderate gas content the residual negative pore 

 pressure may be moderately high since only discrete portions of the 

 interparticle fabric are disturbed. Therefore, in order to make a 

 complete estimate of the extent of disturbance in samples, it is recom- 

 mended that the extent of gas disturbance be determined by measuring 

 the degree of saturation or acoustic transmissivity . 



25 



