79 research outputs found
Experimental investigation of the time- and rate-dependent behaviour of unsaturated clays
No full textWith the aim to achieve a better understanding of the time- and rate-dependent behaviour of unsaturated clays, particularly stiff natural clays, a critical review of the literature and a program of laboratory testing were carried out as part of this research. With emphasis on examining the effect of suction on viscous response, mainly primary and secondary consolidation (creep), stress-relaxation, and rate-dependency, a series of conventional multi-stage loading (MSL) oedometer tests, long-term single-stage loading (SSL) oedometer tests, and advanced constant-rate-of-strain (CRS) oedometer tests were carried out on natural and corresponding reconstituted specimens of a stiff clay namely, London Clay, retrieved from a site in the Isle of Sheppey, UK. The results obtained provide a significant contribution to the existing database and address important knowledge gaps highlighted from the literature review. The experimental studies performed included;
a) An investigation of the effect of water reservoir surface roughness on the cavitation mechanism and performance of a newly designed high-capacity tensiometer (HCT), with regard maximum sustainable suction (smax) and maximum duration of measurements (tmax).
b) An examination of the influence of soil suction on the primary and secondary consolidation of natural and reconstituted London Clay from a set of MSL and SSL oedometer tests.
c) Characterisation of the stress-relaxation behaviour of the reconstituted London Clay subjected to suction and strain-rate variations from a set of CRS oedometer tests in an in-house designed suction-monitored cell.
d) An examination of the coupled effects of suction and strain-rate on one-dimensional (1D) stress–strain response of reconstituted London Clay.
Based on the experimental findings, the validity of the empirical relationships proposed in the literature for correlating time- and rate-dependency coefficients was examined and necessary modifications were proposed. The main experimental findings can be outlined as:
- The mechanism of cavitation in tensiometers is indeed influenced by the surface roughness of the water reservoir. The new approach for expanding the range and duration of suction measurements for a newly designed HCT, based on hydrophilic coating of the reservoir walls, sounds promising.
- The compression index (Cc) and creep index (Cαe) and the ratio α = Cαe/Cc for Sheppey London Clay are stress- and suction-dependent. The hypothesis of constant α for natural clays, as suggested by Mesri et al. (1994), is therefore rejected.
- The volumetric creep deformations of unsaturated stiff clays appear to be relatively simply modelled by a creep law based on a power function.
- A coupled effect of suction and strain-rate on 1D compression response was identified. At a constant suction, the higher the strain-rate, the higher the apparent preconsolidation pressure (σ′p). Similarly, at a constant strain-rate, the higher the suction, the higher the σ′p.
- The values of relaxation coefficient (Rα) differ in saturated and unsaturated states. However, a clear relationship between Rα and suction was not found. The Rα = Cαe/Cc relationship, suggested by Yin et al. (2014) for saturated soft clays, appears, with an approximation, to be valid for the unsaturated stiff London Clay in the range of applied vertical stresses and soil suctions in this study.
- The process of stress-relaxation in unsaturated clays is highly influenced by factors such as soil suction, pre-relaxation strain, stress, and strain-rate
Experimental investigation of the time- and rate-dependent behaviour of unsaturated clays
No full textWith the aim to achieve a better understanding of the time- and rate-dependent behaviour of unsaturated clays, particularly stiff natural clays, a critical review of the literature and a program of laboratory testing were carried out as part of this research. With emphasis on examining the effect of suction on viscous response, mainly primary and secondary consolidation (creep), stress-relaxation, and rate-dependency, a series of conventional multi-stage loading (MSL) oedometer tests, long-term single-stage loading (SSL) oedometer tests, and advanced constant-rate-of-strain (CRS) oedometer tests were carried out on natural and corresponding reconstituted specimens of a stiff clay namely, London Clay, retrieved from a site in the Isle of Sheppey, UK. The results obtained provide a significant contribution to the existing database and address important knowledge gaps highlighted from the literature review. The experimental studies performed included;
a) An investigation of the effect of water reservoir surface roughness on the cavitation mechanism and performance of a newly designed high-capacity tensiometer (HCT), with regard maximum sustainable suction (smax) and maximum duration of measurements (tmax).
b) An examination of the influence of soil suction on the primary and secondary consolidation of natural and reconstituted London Clay from a set of MSL and SSL oedometer tests.
c) Characterisation of the stress-relaxation behaviour of the reconstituted London Clay subjected to suction and strain-rate variations from a set of CRS oedometer tests in an in-house designed suction-monitored cell.
d) An examination of the coupled effects of suction and strain-rate on one-dimensional (1D) stress–strain response of reconstituted London Clay.
Based on the experimental findings, the validity of the empirical relationships proposed in the literature for correlating time- and rate-dependency coefficients was examined and necessary modifications were proposed. The main experimental findings can be outlined as:
- The mechanism of cavitation in tensiometers is indeed influenced by the surface roughness of the water reservoir. The new approach for expanding the range and duration of suction measurements for a newly designed HCT, based on hydrophilic coating of the reservoir walls, sounds promising.
- The compression index (Cc) and creep index (Cαe) and the ratio α = Cαe/Cc for Sheppey London Clay are stress- and suction-dependent. The hypothesis of constant α for natural clays, as suggested by Mesri et al. (1994), is therefore rejected.
- The volumetric creep deformations of unsaturated stiff clays appear to be relatively simply modelled by a creep law based on a power function.
- A coupled effect of suction and strain-rate on 1D compression response was identified. At a constant suction, the higher the strain-rate, the higher the apparent preconsolidation pressure (σ′p). Similarly, at a constant strain-rate, the higher the suction, the higher the σ′p.
- The values of relaxation coefficient (Rα) differ in saturated and unsaturated states. However, a clear relationship between Rα and suction was not found. The Rα = Cαe/Cc relationship, suggested by Yin et al. (2014) for saturated soft clays, appears, with an approximation, to be valid for the unsaturated stiff London Clay in the range of applied vertical stresses and soil suctions in this study.
- The process of stress-relaxation in unsaturated clays is highly influenced by factors such as soil suction, pre-relaxation strain, stress, and strain-rate
Effect of soil moisture evaporation rate on dynamic measurement of water retention curve with high-capacity tensiometer
No full textThis paper investigates the effect of the soil moisture evaporation rate on the soil water retention curve (SWRC) of clays obtained using the high-capacity tensiometer (HCT) technique and following the continuous drying (dynamic) method. SWRC measurements, with and without soil moisture evaporation rate control, were carried out on reconstituted London clay specimens using 12 performance-improved HCTs recently developed at the University of Warwick. Furthermore, the HCTs' performance in terms of the maximum attainable suction (smax) and maximum measurement duration (tmax) was evaluated. Moreover, the suitability of a curve-fitting-based model, available in the literature, for attaining the entire retention curve (beyond the capacity of HCTs) was evaluated. The SWRCs for the tests with a controlled evaporation rate were found to generally exhibit higher suction at a given water content, hence inducing air-entry values that were on average 16% higher than those obtained from tests without evaporation rate control. It was also found that for suction in excess of 2 MPa, the curve-fitting-based model predictions of data obtained from tests with a controlled evaporation rate exhibited significantly lower suction at a given water content than those without evaporation rate control, suggesting that such curve fitting correlations should be used with caution.</p
Geological and Geotechnical Characteristics of London Clay from the Isle of Sheppey
No full textThe paper describes a series of experimental testing on natural stiff London clay samples retrieved from the New Hook Farm in the Isle of Sheppey, UK. The experimental program includes determination of macroscopic and microscopic properties, chemical compositions, Atterberg limits, permeability, and compressibility parameters in both intact and reconstituted states. The paper integrates the earlier studies, the new findings, and the commercial investigation results with the aim to extend the current knowledge of the geological and geotechnical characteristics of this stiff clay from the east of the London basin. Some comparisons are also made with shallow depth London clay from Heathrow Terminal 5 site
The combined effect of clay and moisture content on very small strain stiffness of compacted sand-clay mixture
No full textThe very small strain shear modulus (stiffness) of soils, Gmax, is one of the most important parameters for predicting ground movements and dynamic responses of geo-structures. In this study, the combined effect of clay fraction and moisture content on shear stiffness of an unsaturated sand-clay mixture at very small strains was investigated using bender elements. Compacted soil specimens were prepared at three different clay contents of 10, 20, and 30%, and at four different initial moisture contents of 3, 6, 9 and 12%. Bender element tests were carried out under isotropic and constant moisture content conditions and inside a modified triaxial testing system equipped with a pair of piezoelectric bender-extender elements. Gmax was calculated based on the velocity measurement of shear waves propagated through the specimen. The tests results showed that Gmax decreases approximately linearly with an increase in moisture content, and non-linearly with an increase in clay content. A basic empirical equation was derived from an examination of trends in evolution of Gmax with clay and moisture content. Additional empirical correlations were also derived for estimation of moisture content and degree of saturation based on the compression wave velocity measurements
Effect of Soil Moisture Evaporation Rate on Dynamic Measurement of Water Retention Curve with High-Capacity Tensiometer
No full textThis paper investigates the effect of soil moisture evaporation rate on the soil water retention curve (SWRC) of clays obtained using high-capacity tensiometer (HCT) technique and following the continuous drying (dynamic) method. SWRC measurements, with and without soil moisture evaporation rate control, were carried out on reconstituted London clay specimens using 12 performance-improved HCTs recently developed at the University of Warwick. Furthermore, the HCTs’ performance in terms of the maximum attainable suction (sₘₐₓ) and maximum measurement duration (tₘₐₓ) was evaluated. Moreover, the suitability of a curve fitting-based model, available in the literature, for attaining the entire retention curve (beyond the capacity of HCTs) was evaluated. The SWRCs for tests with controlled evaporation rate were found to be generally exhibiting higher suctions at a given water content, hence inducing air-entry values that were on average 16% higher than those obtained from tests without evaporation rate control. It was also found that for suctions beyond 2 MPa, the curve fitting-based model predictions of data obtained from tests with controlled evaporation rate exhibit significantly lower suctions at a given water content than those without evaporation rate control, suggesting that such curve fitting correlations should be used with caution
Evaluation of delays in technical approval of UK Highways Act section 278 projects
No full textThis paper attempts to address the causes of delays to the legal and technical approval processes involved in the creation of agreements authorising works to public highways under s.278 Highways Act (1980) specific to Warwickshire County Council, UK, and whether the type of contract (JCT or NEC) or s.278 agreement (minor or major) has any tangible influence. A series of questionnaires and interviews were carried out on a sample group of individuals including designers, developers, construction lawyers, and council engineers with extensive industry experience in relation to s.278 legal, technical, construction and adoption processes. The results revealed the key causes of delays, and therefore the barriers to prompt and efficient approval processes, as the lack of communication between developer and local authority, inexperienced developers’ engineers, poor quality drawings, and insufficient information in the local authority’s design guide. These key factors are discussed and recommendations are provided to tackle these issues
Geological and Geotechnical Characteristics of London Clay from the Isle of Sheppey
No full textThe paper describes a series of experimental testing on natural stiff London clay samples retrieved from the New Hook Farm in the Isle of Sheppey, UK. The experimental program includes determination of macroscopic and microscopic properties, chemical compositions, Atterberg limits, permeability, and compressibility parameters in both intact and reconstituted states. The paper integrates the earlier studies, the new findings, and the commercial investigation results with the aim to extend the current knowledge of the geological and geotechnical characteristics of this stiff clay from the east of the London basin. Some comparisons are also made with shallow depth London clay from Heathrow Terminal 5 site
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