Interstitial Water 



259 



the core from Santa Barbara Basin is slightly 

 coarser than those from San Clemente and 

 Santa Catalina Basins, its water content is 

 far greater. Even more extreme is the very 

 high water content of a core from Lake 

 Mead near Hoover Dam (Gould, 1953; 

 1954, Fig. 95), which is very fine-grained 

 and was deposited much faster than were 

 any of the marine cores. Rate of deposi- 

 tion is a reasonable factor if time is required 

 for the grains to become reoriented and de- 

 formed before water can be forced out dur- 

 ing compaction. An empirical formula re- 

 lating water content, grain size, and rate of 

 deposition could not be developed, possibly 

 because of errors in basic data, but more 

 likely because of the presence of other com- 

 plicating factors such as extent of burrow- 

 ing activity by animals and amount and 

 kind of clay minerals. 



An approximate relationship between 

 porosity and median diameter (Fig. 210) 

 shows the range of values that have been 

 determined for several hundred samples. 

 Data for sediments coarser than 30 microns 

 are from measurements on the shelf off 

 San Diego (Hamilton, Shumway, Menard, 

 and Shipek, 1956); those for finer sediments 

 are from basin cores below the depth of 

 marked decrease of water content — at 3 

 meters. Since the water content of sandy 

 sediments does not decrease much with 

 depth, data from surface samples of them 

 are satisfactory. Experimental data for 

 both coarse- and fine-grained sediment re- 

 ported by Trask (1932, p. 82) fall within the 

 range depicted on Figure 210. It is well 

 known that simulated sediments composed 

 of similarly packed spheres of uniform size 

 have identical porosities, regardless of what 

 that size is. However, actual sediment 

 grains are not spherical, nor are they so well 

 sorted as to be all of a single size. The 

 presence of flat grains tends to increase the 

 porosity, and the presence of small grains 

 between large ones tends to reduce the po- 

 rosity. In addition, as the grains become 

 smaller, their total surface area increases 

 markedly, so that for small grains the sur- 

 face film of water may comprise a far larger 

 volume than that in the main interstices. 

 Because clayey sediments have a large sur- 



face area, poor size sorting, and a predom- 

 inance of flat flakes, their water content 

 (and porosity) greatly exceeds that of sands. 



Effects on Physical Properties 



Porosity and bulk density are parameters 

 of the sediment that are closely related to the 

 water content, from which they may easily 

 be computed if the grain specific gravity is 

 known. Values of porosity and bulk density 

 based on the average of several score 



% POROSITY 



40 60 





; — uj ' 

 o>i 



-SEA WATER 



80 100 

 WATER CONTENT-% WET WEIGHT 



Figure 210. Dependence of several physical properties of 

 sediments on porosity. The relationship of porosity to 

 water content and bulk density was computed from an 

 average measured grain specific gravity of 2.65. Data on 

 porosity versus median diameter are from measurements 

 of shelf sands by Hamilton, Shumway, Menard, and 

 Shipek (1956) and from many basin cores collected by 

 the University of Southern California. Penetration of 

 core barrels relative to porosity is from Emery and Dietz 

 (1941) plus many later measurements. Sound velocities 

 are from Hamilton (1956). 



