Predicting seasonal shrink swell cycles within a clay cutting

Abstract

A numerical method for predicting the seasonal pore water pressure changes and consequently the shrinking and swelling of a clay cutting is proposed. The cutting is located on the A34 Newbury bypass and has been extensively monitored by Smethurst et al (Smethurst et al., 2006) since January 2003. A hydrological finite difference model, SHETRAN, is used to accurately predict the pore water pressure changes within the cutting using meteorological data together with soil data and vegetation properties. The daily surface pore water pressure changes are then imported into a geotechnical finite difference program FLAC tp flow which is used to predict the mechanical response of the cutting. This paper details the comparability of the hydraulic simulation with 1 year observed data and further predicts the hydraulic and mechanical response over the next 3 years. Fig. 1 SHETRAN surface model from the canopy. Evapotranspiration, the movement of water from the soil and within plants, is modeled within SHETRAN using the Penman-Montieth equation for actual evapotranspiration (Monteith, 1965). This is calculated as a loss term to describe the uptake of water through plant roots. Overland flow is also calculated within the program. The depth of runoff water is determined from the available water from the interception evapotranspiration component and the rate of infiltration into the subsurface. Flow resistance parameters are then used to model the overland flow using approximations of the St. Venant equations of continuity and momentum. The subsurface is assumed to consist of a variably saturated porous medium. Flow through the medium is calculated by solving the non-linear partial differential Richard’s equation. 1.2 FLAC tp flow The FLAC finite difference code allows the numerical modeling of structures built of soil and rock. The two-phase flow option within the FLAC program is able to model two immiscible fluids within a porous medium. This allows the modeling of an unsaturated soil with the fluids present being water and gas. FLAC tp flow is capable of solving a fluid only calculation, a mechanical only calculation and a fully coupled fluid-mechanical calculation. A fully coupled calculation with a Mohr-Coulomb constitutive model is presented within this paper. FLAC tp flow requires soil properties, water properties and boundary and initial conditions to be specified. Water is able to enter the grid by specifying either a discharge or a pore water pressure at the boundary.