X-ray tomography is originally used in medical radiography for obtaining a slice through the body (Hounsfield, 1973; Swindell & Barrett, 1977). X-ray microtomography (or X-ray CT) is a radiographic imaging technique that can produce 3D images of the internal structure at a spatial resolution, which process is schematically illustrated in Figure 1.25. The specimen is rotated about a single axis and is scanned by computed tomography, thereafter, a series of 2D X-ray absorption images are obtained. A 3D digital image is produced using mathematical principles of tomography on this series of images, displayed as a series of 2D slices. The 3D internal structure can be inferred from the images based on the relationship between X-ray absorption and material density, thus a virtual model is recreated without destroying the original sample.
Pore size distribution
The pore size distribution of soil relates to soil properties in a complex and useful way, it indicates complexity of structure far more than porosity itself. Large pores can be associated with both large particles but also clays aggregation (Nimmo, 2013). The pore size distribution (PSD) analysis can give supplementary pore volume information in addition to 2D mage information of soil, and has been under numerous investigations on clays. Moreover, the PSD curves were founded to be related to the states of clay, for example, natural or remolded, saturated or unsaturated. Oualmakran et al. (2016) found that the under saturated conditions, the pore size distribution varied more significant with loading. The unimodal PSD curves were identified on some remolded clays by Penumadu & Dean (2000) and Hammad (2010). However, the bimodal PSD curves were obtained in natural sediments and structured clays (Sun et al., 2016; Oualmakran et al., 2016).
Evolution of clay microstructure under different stresses
The most widely used investigations on particle orientation of clay is by means of SEM images. Hicher et al. (2000) assumed an ellipsoidal shape of the particle or aggregate based on the analysis of SEM images of a kaolinite and a bentonite, and proposed three parameters to describe the microstructural properties of clay: – Shape index (or sometimes called aspect ratio), which represents the ratio between the length and width of the individual particles or aggregates element. – The mean surface, assuming the element an ellipse. – Particle concentration represents the number of individual particles by surface unity. The orientation of the particles can be generally considered as the orientation of the main axis of the ellipse or particles with respect to the horizontal.
The general organized terms of clay particles are “flocculated” and “dispersed” (Collins & McGown, 1974; Prashant & Penumadu, 2007; Sachan & Penumadu, 2007). The “flocculated” describes the soil that has an open-ended structure with edge-to-face contacts or random orientation of particle platelets (Figure 1.27a); “dispersed” means a soil with closed structure which consists of face-to-face contacts or preferred orientation of particles (Figure 1.27b).
The slurry was manually filled layer by layer into a double drainage consolidometer with diameter of 95 mm (Figure 2.4a). The material was preconsolidated under one-dimensional compression by steps until a final stress of 120 kPa for at least 3 weeks for a good preconsolidation quality. A cylindrical section can then be cut (Figure 2.4b) from the obtained core for triaxial tests.
Then the soil section was trimmed gently into cylindrical specimen of 75 mm in height and 50 mm in diameter by coulter knife on a rotary cutting base (Figure 2.5a), and then was moved into the triaxial cell through two cylindrical molds quickly (Figure 2.5b).
Axisymmetric triaxial tests
Focusing on the stress-strain relationship and the measurements of volumetric change, a reliable experimental apparatus with automatic recording and controlling is required. The GDS triaxial testing system is a fully automated triaxial testing system, based on the classic Bishop’s & Wesley’s (1975) hydraulic triaxial cell. All triaxial loading following stress paths, and creep tests maintaining a given stress level, in this thesis performed on this GDS system.
The GDS Triaxial testing system and measurements
The fundamental system hardware elements of the used GDS Triaxial Testing System (GDSTTS) shown in Figure 2.6. The whole system includes one triaxial cell, three automatic controllers, one data acquisition pad and PC software control terminal, connected by a closed loop.
The GDSLAB control and acquisition software starts with the Kernel module and Version 184.108.40.206 is used to perform the triaxial tests, which is connected with PC resident via IEEE card. The standard GDS data acquisition device, known as the “serial data pad”, has 8 computer controlled gain ranges, specifically designed to suit the pressure and displacement transducers.
The GDS Advanced Pressure/Volume Controller (ADVDPC) is a microprocessor-controlled screw pump for the precise regulation and measurement of fluid pressure and volume change, which offers the highest level of accuracy, resolution and control. The optional volumetric capacity is 1000cc (for pressure ranges < 2MPa); the resolution of measurement and control is pressure ≤0.1% full range, volume = 0.5 mm3 (<8MPa); the ambient temperature range is 10°C to 30°C. After filling with de-aerated water into the cylinder of ADVDPC, the de-aerated water is pressurized and displaced by a piston moving in the cylinder. The piston is actuated by a ball screw turned in a captive ball nut by an electric motor and gearbox that move rectilinearly on a ball slide (Figure 2.7). An integral solid-state transducer measures pressure. Control algorithms are built into the onboard microprocessor to cause the controller to seek to a target pressure or step to a target volume change. Volume change is measured by counting the steps of the incremental motor.
Radial stress is applied by controlling the water pressure into the triaxial cell through ADVDPC 1 (Figure 2.6). ADVDPC 2 linked to the specimen by a capillary tube can acquire backpressure and volume change. Pore pressure transducer is connected to the base of the pedestal, which shows the local pore pressure at the bottom of the sample, and it can react the whole pore pressure through the soil if the sample is well saturated. Axial displacement/strain is achieved by applying a strain increment on the base of soil specimen in the cell. The measurements obtained directly from GDSTTS are summarized in Table 2.5.
GDS testing techniques
For creep test performed by the triaxial apparatus, the leakage of membrane between the cell water and the specimen can be an important aspect that controls the accuracy of test. Since the volume change is obtained by measuring the amount of pore water that circulated from and to the soil specimen by GDS controllers, the detecting accidental leakage of water through the membrane is highly significant.
To perform the creep test in a long time duration, we need membrane of high quality; meanwhile, to reduce the restraining effect of the membrane, the thickness of the membrane should not be too large. According to the British Standard (BS 1377: Part 8, 1990), the membrane thickness shall not exceed 1 % of the specimen diameter, otherwise the calculated deviator stress need to be corrected to allow the effects of the rubber membrane (Head, 1992; Havel, 2004). After tested with several types of membranes, a customized membrane made of latex which thickness is 0.35 mm, marked by Piercan.fr was used in this research and performed well in the long-term creep tests. In the classical triaxial test, the non-uniformity of the specimen due to end restraint is an important problem. Lanier (1989) pointed out that the end restraints and the slenderness of the sample were the main points that should be taken into account. The end restrains due to the friction between the loading plates and the material becoming too large, meanwhile, the radial displacement of the specimen is restrained at the boundary. Consequently, the stress state is non-homogeneous and the maximum shear strength is usually overestimated. In order to decrease the friction, Rowe & Barden (1964) used greased membranes on the two ends of soil specimen, between the specimen and the platens. Hattab & Hicher (2004) used similar technique; they used a “sandwich” device that was made of two smooth plates lubricated by a thin layer of grease and latex layers. In the research of this thesis, we chose the same “sandwich” anti-frictional device (see Figure 2.8).
Table of contents :
Chapter 1 Literature review
1.1 Macro approach to creep of clay
1.1.1 One-dimensional creep test
1.1.2 Triaxial creep test
1.1.3 Other creep tests
1.1.4 Field creep tests
1.2 Shear dilatancy/contractancy related to creep
1.2.1 Typical shear dilatancy/contractancy equations
1.2.2 Evolution shear dilatancy/contractancy during creep
1.3 Micro approach to creep of clay
1.3.3 Evolution of clay microstructure under different stresses
1.3.4 Evolution of clay microstructure related to creep
1.4.1 Macroscopic study of creep
1.4.2 Microscopic study of creep
Chapter 2 Material and Experimental Techniques
2.1 Material properties and specimen preparation
2.1.1 Mineralogy and kaolinite properties
2.1.2 One-dimensional consolidation and sample preparation
2.2 Macroscopic research techniques
2.2.1 Axisymmetric triaxial tests
2.2.2 Calculation of test parameters
2.3 Microscopic research techniques
2.3.1 Preparation of specimens for MIP and SEM
2.3.2 Mercury intrusion porosimetry (MIP)
2.3.3 Scanning Electron Microscope (SEM) technique and method of analysis
2.3.4 Method of analysis of particles and void
Chapter 3 Creep behavior and strain mechanisms
3.1 Strain mechanisms along purely deviatoric stress path
3.1.1 The 3 domains of deformation on (p′-q) plane
3.1.2 The Kaolin K13 clay behavior
3.2 Identification of creep behavior related to the dilatancy phenomenon
3.2.1 Introduction of the creep phase
3.2.2 Creep analyses in (ε1- εv- q) and (e-log p′) plane
3.3 Influence of stress condition on creep evolution
3.3.1 Case of test P010
3.3.2 Creep evolution in normalized (p′-q) plane
3.3.3 Creep rupture
3.4 Evolution of creep strain rate
3.4.1 Axial strain rate
3.4.2 Volumetric strain rate
3.5 Evolution of deviatoric strain
3.6 Summary and discussion
3.6.1 Dilatancy rate
3.6.2 Viscoplasticity evolution
Chapter 4 Microscopic characterization of clay related to creep
4.1 Quantitative analysis of clay structure evolution
4.1.1 In contractancy domain
4.1.2 In pseudo-elastic domain
4.1. 3 In dilatancy domain
4.1.4 Structure evolution with the variation of stress
4.2 Evolution of the pores
4.2.1 Pore orientation
4.2.2 Pore space by SEM and MIP techniques
4.2.3 Pore shape
Conclusions and perspectives
Summary and conclusions