densities. Two different clean, dry, un-cemented granular soils will have 

 nearly the same modulus and friction angle if they are at the same relative 

 density. As expected, as the packing becomes tighter, volume decreases and 

 strength increases. A way to characterize the density of this packing is with 

 the relative density D r . Relative density is defined as, 



-. _ T d mm ' d » d min C12-D 



• d i ti min T^min 



where y d is the dry in-place unit weight of the soil, y dna is the dry unit 

 weight of the soil in its most dense condition, and y dmin is the dry unit weight 

 of the soil in its loosest condition. Relative density can also be expressed in 

 terms of either void ratio e or porosity n as the following, 



£_ .- ~~ £ w„„ - n 1 - b„j_ .._ _. 



D = "»* = Eg 5?E (12-2) 



r e - e_„ «_„ - n__ 1 - n 



in which e^ is the void ratio of the soil in its densest condition, e mat is the 

 void ratio of the soil in its loosest condition, and e is the in-place void ratio. 

 Void ratio is the volume of voids to volume of the solids and is a common 

 parameter used in geotechnical engineering. In sediment transport, it is more 

 common to use porosity n, which is the ratio of the volume of the voids to the 

 total volume. 



Wave-induced pore pressures may lead to a less stable soil matrix and 

 enhance sediment erosion at the seabed. Cyclic loading of fine sands can lead 

 to volume reduction. An accumulation of pore pressure can result if drainage 

 occurs more slowly than the volume reduction rate. If pore pressure increases 

 to equal the overburden pressure, then the sediment grain effective stress 

 approaches zero and the sediment begins to behave like a liquid. Liquids are 

 not capable of supporting significant shear stresses; thus, liquefied sediments 

 will be more sensitive to shear-induced soil failures (Seed and Rahman 1978). 

 This sensitivity is enhanced by both dynamic and mean excess pore water 

 pressure. 



Several models exist to predict wave-induced pore pressure in sediments. 

 A potential pressure model is appropriate if the soil matrix is assumed to be 

 rigid and the fluid incompressible (Reid and Kajura 1957). A linearly elastic 

 model of the soil was developed by Biot (1941). This model has been applied 

 to horizontal seabed models by many investigators (Yamamoto 1977, Madsen 

 1978, McDougal and Sollitt 1984). Some of these models have been validated 

 with large-scale experiments (Yamamoto et al. 1978; Sollitt and McDougal 

 1984). However, large-scale laboratory studies of these models applied to 

 sloping beaches are not reported in the literature. SUPERTANK provides the 

 data sets for validation of pore pressure models applied to realistic, erodible 

 beaches. 



Chapter 1 2 Pore Pressure and Sediment Density Measurements 



235 



