Mechanisms of compression in well-graded saprolitic soils

Abstract

Soils originating from weathering processes present considerable heterogeneity in their composition, which can make it difficult to analyse their behaviour in a systematic way. For the granitic saprolites discussed in this paper, based on a trend between soil density and weathering degree, there appears to be two different domains of behaviour, a granular domain and a clay matrix one, according to the degree of weathering reached. Recognition of these domains can reduce the apparent scatter of data for the engineering behaviour of weathered soils. A number of one-dimensional compression tests are presented for saprolitic soils from Hong Kong having different weathering degrees. In addition, isotropic and one-dimensional compression tests from the literature on other saprolites from Hong Kong and around the world were reanalysed and used to identify possible trends in the mechanisms of compression for these two domains. From practical considerations, the trends considered were between compressibility and common engineering grading descriptors. An attempt was also made to provide the physical explanations behind the behaviour observed, and the particle breakage was investigated in detail, both from a quantitative and qualitative point of view. It was found that the values of relative breakage (Hardin in ASCE J Geotech Geoenviron Eng 111 (10): [1177][1178][1179][1180][1181][1182][1183][1184][1185][1186][1187][1188][1189][1190][1191][1192] 1985), for a same stress level, might be very similar for soils having different compressibility values and different initial gradings. When studying particle breakage in further detail, it can be observed that it is linked to the amount of large particles and their characteristics. The maximum particle size, rather than the amount of fines in a mixture, may be a better predictor for differences in compressibility and breakage.

Influence of fines on the compression behaviour of decomposed volcanic rocks

“…[5]). Rocchi and Coop [21] found the same trend for the tests at the same maximum stress for their study on well graded saprolitic soils. Also, they stated that the particles in a loose state will have lesser contact points between each other and therefore experience larger stresses.…”

“…It has been observed that maximum grain size seems to dominate breakage rather than the amount of fines in a mixture [27]. For the granitic saprolites, the particle breakage has been linked to the quantity of large particles and their characteristics [21]. For the samples presented in this paper, the maximum grain size is the same (Fig.…”

“…Compared to other geomaterials, it is more difficult to characterise weathered soils due to high variability in their particle mineralogy, size, shape and distributions [15,16,21] and more studies are required to shed light on their behaviour. The investigation into the influence of fines on the mechanics of weathered soils in compression is very few and very scarce for soils resulting from volcanic rock.…”

Mineralogy, Micro-fabric and the Behavior of the Completely Decomposed Granite Soils

“…Residual soils such as granitic saprolite soils originated from the chemical and physical weathering processes of granite rock, thus the mineralogy and micro-fabric of the parent rock are considered substantial factors governing the resulted soils behavior [1]. Residual soils are generally well-graded and include a broad domain of grain size distributions [2]. Many studies have shown that the grading of granitic saprolite soil depends on the environmental circumstances of the weathering process, mineralogy and micro-fabric of the parent rock; in which the soil grading curve shifts towards the fine domain as the weathering grade increases [3][4][5][6].…”

“…On account of shortage and insufficient data available about mineral composition of soils in addition to grains morphology, researchers have concluded that grading is considered the dominant factor in the compressive behavior of saprolitic soils [2,7]. Ham, Nakata [8] found that content of gravel had a significant impact on the compression characteristics of CDG soils.…”