Interstitial Water 



261 



the fact that much sediments mantle slopes 

 at least as steep as 20°. Evidently grain-by- 

 grain accumulation can occur on slopes un- 

 til some disturbance converts the mass into 

 a fluid-like suspension that must then flow 

 downslope, incorporating more and more 

 similar sediment as it moves along and per- 

 haps finally developing into a turbidity cur- 

 rent. The property of remaining stiff until 

 jarred is called thixotropy. In oil well driU- 

 ing muds thixotropy is promoted by addition 

 of montmorillonite clays which have this 

 gel-like property to a high degree; and the 

 basin muds consist of about one-third mont- 

 morillonite. The property is difficult to 

 measure precisely for muds other than wa- 

 tery suspensions, but a wire cutting method 

 devised by W. R. Heiner and described by 

 Emery and Rittenberg (1952) yielded semi- 

 quantitative results. A weighted 0.5-mm 

 diameter wire was able to cut the same cross- 

 sectional area of sediment at the top of a 

 core from San Nicolas Basin nearly 400 times 

 as fast after the sample was kneaded as be- 

 fore. For a section near the bottom of the 

 core, 3 meters, the rate after kneading was 

 about 200 times as great as before. After 24 

 hours the thixotropic strength had largely 

 returned. 



Shear strength of sediments can be meas- 

 ured by devices such as shear boxes (Terzaghi 

 and Peck, 1948, p. 79), torsion plummets 

 (Romanovsky, 1948), or torsion vanes 

 (Moore, 1956). Each of these devices, and 

 especially the latter two, is capable of meas- 

 uring the shear strength of samples before 

 and after kneading so as to show the eff"ects 

 of thixotropy. The results of such shear 

 tests provide direct data for computing the 

 angles of rest of sediments and the depths 

 to which objects can sink in sediments. An- 

 other method of obtaining shear strength is 

 that of measuring directly the depth to which 

 coring devices penetrate diff'erent kinds of 

 sediment. A summary of coring data by 

 Emery and Dietz (1941), supplemented by 

 more recent measurements, shows that a 

 corer which weighs 270 kg in water when 

 dropped into the bottom at a speed of about 

 5 meters/sec penetrates as far as 8 meters in 

 muds as porous as those of Santa Barbara 



Basin but only about 1 meter in sands of 

 the shelves (Fig. 210). 



The velocity of sound in sediments is also 

 controlled by the percentage content of wa- 

 ter. As pointed out by Hamilton (1956) and 

 others, although the velocity of sound is 

 greater in sohd mineral grains than in water, 

 a sediment containing more than about 40 

 per cent by dry weight (54 per cent porosity) 

 has a sound velocity lower than either water 

 or mineral grains alone (Fig. 210). This dip 

 in the porosity-velocity curve comes about 

 because the velocity varies inversely with the 

 square root of the product of bulk density 

 and compressibility of the mixture, and the 

 bulk density of a mixture increases faster 

 than the compressibiUty decreases within the 

 porosity range between 100 and about 75 per 

 cent. Knowledge of the sound velocity in 

 sediments is necessary for converting the 

 data on subbottom reflections obtained by 

 strong echo sounders from lapsed time to 

 sediment thickness. 



Permeability and Natural Compaction 



Permeability of granular materials is de- 

 fined as i^ = QLji/TAH, where K is per- 

 meabihty in darcys, Q is the quantity of 

 water of viscosity /a passed in time T through 

 a sample of length L and cross-sectional area 

 A with a hydrostatic head of /f atmospheres, 

 all dimensions being in cgs units. Samples 

 of sediments of nearly uniform grain size 

 from various depths in a core from Santa 

 Cruz Basin were introduced into the bottom 

 5 cm of a 120-cm-long glass tube which was 

 then filled with sea water and stood on end. 

 As described more fully by Emery and Rit- 

 tenberg (1952), the resulting measurements 

 showed that the permeability ranged be- 

 tween 6 X 10~^ and 30 x 10"^ darcy (Fig. 

 212). 



Under steady-state conditions the curve of 

 water content versus depth should be the 

 same today as it was a thousand years ago 

 or will be a thousand years hence. The rate 

 of deposition of sediments is the same from 

 year to year, and the characteristics of the 

 sediment must also be the same. Such an 

 assumption of steady state is probably justi- 



