A field study of stress-dependent soil–water characteristic curves and permeability of a saprolitic slope in Hong Kong

Abstract

It is well recognised that field measurements of stressdependent soil-water characteristic curves (SDSWCCs) and the permeability function (coefficient of permeability with respect to the water phase) are vital for assessing transient seepage, pore water pressure changes and the stability of unsaturated soil slopes. In this study, SDSWCCs and the permeability function were measured by using the instantaneous profile method on a saprolitic hillside in Tung Chung, Hong Kong. The ground profile comprises colluvium and completely decomposed tuff (CDT). A 3 m diameter circular plot was instrumented and subjected to two wetting-drying cycles. Throughout a 48-day monitoring period, variations of volumetric water content and matrix suction in the uppermost 3 m of soil were measured continuously using time-domain reflectometry moisture probes and jet-fill tensiometers, respectively. The field observations reveal that there is a clear trend of decreasing sizes of hysteretic loops of SDSWCCs with depth. In a comparison of the field-and laboratory-measured SDSWCCs, general agreement can be seen in the first wetting-drying cycle but not in the second cycle. The field-observed unsaturated permeability function is highly dependent on suction history (wettingdrying cycles). The field-measured permeability ranges from 4 3 10 27 m/s to 3 3 10 24 m/s in the colluvium, whereas the corresponding measured values vary from 4 3 10 27 m/s to 1 3 10 24 m/s in the CDT. Predictions of SWCC and permeability function using existing empirical approaches based on particle size distributions (PSDs) should be treated with caution. There is a clear qualitative and quantitative inconsistency between predictions of permeability function from PSD and field measurements.

New transient method for determining soil hydraulic conductivity function

“…Test data from three laboratory (Watson, 1966;McCartney 2007;Ng and Leung, 2012) and two field (Hillel et al, 1976;Ng et al, 2011) studies are selected for re-interpretation using the new method.…”

“…Three laboratory (Watson, 1966;McCartney, 2007;Ng and Leung, 2012) and two field (Hillel et al, 1972;Ng et al, 2011) studies are selected for evaluation. The tests reported by Watson (1966), McCartney (2007) and Hillel et al (1972) are internal drainage tests, while those by Ng and Leung (2012) and Ng et al (2011) are evaporation test.…”

“…The tests reported by Watson (1966), McCartney (2007) and Hillel et al (1972) are internal drainage tests, while those by Ng and Leung (2012) and Ng et al (2011) are evaporation test. These five case studies are chosen because they document comprehensive test datasets and most of the necessary information that can be used to evaluate both the old and the new methods.…”

“…This test method hence created no-flux top boundary condition for the old method to determine SHCF. Ng et al (2011) carried out a similar field SHCF tests as Hillel et al (1972) but on a test site consisting of a 1 m-thick colluvium (sandy silt) layer overlying a 3-m thick decomposed silty clay stratum. In the site studied by Ng et al (2011), a bedrock surface was encountered below 4 m depth.…”

“…Due to the slow process of internal drainage, another common test method is to not cover the soil surface and to allow for surface evaporation (Plagge et al, 1990;Wendroth et al, 1993;Tse, 2008;Krisdani et al, 2009;Ng et al, 2011;Ng and Leung, 2012). In order to avoid any measurements of actual evaporation rate which is known to be complicated due to the highly-nonlinear soil-atmosphere interaction at the soil surface (Wilson et al, 1997), zero flux is commonly assumed at the bottom boundary (Wendroth et al, 1993;Krisdani et al, 2009;Ng et al, 2011). Such assumption can be unrealistic, and not able to be justified, as no-flux boundary is not well-defined in the field.…”