OCEAN WAVE-SOIL-GEOTEXTILE INTERACTION

Geotextiles are synthetic fabrics which may be substituted for graded aggregate to protect ocean and coastal structures from erosion and soil instability adjacent to the structure. They are commonly used as a filter and a structural membrane between an undisturbed sediment surface below and an erosion resistant coarse aggregate above. Geotextiles provide a cost effective alternative to graded aggregate in marine foundations. The need for rational design procedures has led to a theoretical description of the combined soil-geotextile behavior which quantifies failure potential and facilitates optimum geotextile selection. A two-dimensional analytical model has been developed for a three layered system; two different soils separated by a geotextile. The soil response is modeled by Biot consolidation theory and an unsteady form of Darcy's equation in which each soil is considered homogeneous, isotropic and linearly elastic. The soil layers are coupled through the geotextile which acts as an elastic permeable membrane. Soil displacements and stresses and fluid pressures and flows are determined analytically. Potential failure conditions are identified from the cyclic shear stress ratio and from a Mohr-Coulomb stress analysis.; Two series of laboratory experiments were conducted at the Oregon State University Wave Research Facility to verify the model. The large scale facility includes a wave channel which is 12 feet wide, 15 feet deep and 342 feet long. A test section 36 feet long was constructed in the wave channel and filled with approximately three feet of fine sand, a geotextile and one foot of gravel. The test section was exposed to simple harmonic and random waves with heights up to four and one-half feet and periods to eight seconds in water depths to eight feet. The pore water pressure was monitored continuously at seven to ten soil depths and three to five lateral positions and recorded on magnetic tape along with displacement of the free surface. Four geotextile conditions were tested including woven, impermeable, semi-rigid and no geotextile. Wave-induced liquefaction was observed for a low permeability geotextile.; The experimental results verify the soil-geotextile interaction model and also provide insight into the dynamic response of horizontally layered soils. Results indicate that for the permeabilities of commonly available geotextiles that the hydraulic properties of the geotextile are dominated by the adjacent soil properties. However, clogging of the geotextile increases the potential for soil failure. The pore pressure amplitude response is frequency selective, the higher frequencies being more highly damped. For a given soil condition a "worst" wave period may exist which produces maximum failure potential. Conversely, for a given design wave, there is a "worst" combination of backfill and armor in terms of potential failure.