Triaxial Apparatus Research Articles

Estimating multidirectional stiffness of soils using planar piezoelectric transducers in a large triaxial apparatus

Cohesionless granular soils display stiffness anisotropy, and accurately characterizing it is essential for numerous geotechnical applications. The inherent anisotropic behaviour of geomaterials is not comprehensively understood; hence, research on the development of novel geophysical testing methods for assessing multidirectional stiffness is important. This contribution presents an innovative arrangement of vertical and horizontal planar piezoelectric transducers to describe the stiffness anisotropy of soils. The transducer assembly can be used to measure nine different elastic wave velocities of a soil specimen in three directions of principal axes having a wide range of median particle sizes. To describe the performance of the developed transducer assembly, four different types of cohesionless granular materials: Toyoura sand, Kashima river sand, glass beads, and Oiso gravel were tested in a large-sized triaxial apparatus having a rectangular specimen of size 235 × 235 × 500 mm. All the tested materials displayed considerably stiffer responses in the horizontal direction as compared to the vertical direction. Except for the coarse gravel, natural sands and glass beads demonstrated isotropic behaviour in the horizontal direction. No significant influence of specimen dimension on the measured vertical wave velocities was detected when the large triaxial test results and medium-sized triaxial test results were compared. The paper also provides a comparison of the vertical stiffness evaluated using planar piezoelectric transducers with those measured statically by applying small-amplitude vertical cyclic loading.

Hydraulic and thermal fracturing techniques for successful stimulation in unconventional geothermal energy recovery

Unconventional or enhanced geothermal systems (EGSs) have been recently identified as potential geothermal resources which can be utilized to extract the heat trapped in the deep geological formations. However, due to the low formation porosity in these systems, an underground heat exchanger must be artificially created to enhance reservoir permeability. A number of reservoir stimulation techniques are adopted in EGSs to enhance their permeability, including hydraulic fracturing and thermal fracturing. The aim of the present work is to provide an in-depth understanding of these reservoir stimulation techniques based on our current laboratory experimental work. Recent literature on the topic has been comprehensively reviewed, and advanced laboratory tests have been conducted to understand hydraulic and thermal fracturing techniques under reservoir conditions. Experiments were conducted utilizing the high-temperature high-pressure rock tri-axial apparatus and quenching treatments were performed by injecting cold water into granite rocks heated to different temperatures. Flow-through experiments were also conducted on intact and fractured granite rocks and the results were compared to predict production enhancement upon stimulation. CT scanning technology was employed to determine micro-scale characteristics following stimulation. Experimental work revealed that the propagation paths and apertures of hydraulic and thermal fractures are controlled by the in-situ stress and temperature and the heterogeneity of the rock matrix. Although the induced fractures contributed substantial enhancement of reservoir permeability, they were sensitive to stress and temperature changes due to the larger effective stresses and thermally induced volumetric expansion.

An alternative method in triaxial tests to obtain cross-anisotropic elastic parameters

A new, alternative approach in a triaxial set-up to fully determine small-strain cross-anisotropic elastic parameters is proposed, which requires neither radial strain sensors nor a radial loading process. This was made possible by additionally measuring pore water pressure responses during undrained axial loading, and inputting them into theoretical expressions derived based on cross-anisotropic elasticity theory. Although the method is intended for saturated soils, slight undersaturation in the system is accounted for by incorporating the pore water pressure coefficient B into the parameter deduction process. Being free from the cumbersome radial sensors and ultra-precise cell pressure controlling devices/operations, the proposed method is applicable even in less specialised laboratories. Four different deduction routes of elastic parameters were applied to results from tests on two reconstituted fine-grained soils. The horizontal Young's modulus obtained with the newly proposed method is consistent not only with those based on radial measurements, but also with those directly measured by axial loading on horizontally cut specimens. A challenge is still open for precisely determining Poisson's ratios, which exhibited only partial matching between different measurement methods. Overall, however, the proposed method is consistent with those adopting full local instrumentation.

A New Suction-Controlled True Triaxial Apparatus for Unsaturated Soil Testing

A new suction-controlled true triaxial apparatus was developed to investigate unsaturated soil behavior under multiaxial stress paths that are not readily achievable in a cylindrical cell. This apparatus is a mixed-boundary type device with a rigid-flexible-flexible boundary that can apply three principal stresses to a cubic specimen via two rigid platens along the vertical direction and four flexible hydraulic bags on the sides. The apparatus was modified based on a previous true triaxial apparatus designed at Xi’an University of Technology, with the addition of four division strips to avoid loading boundary interference. The new apparatus is capable of testing specimens with dimensions of 7 by 7 by 14 cm. The present apparatus was equipped with an independent pore-air pressure control system and pore-water pressure measurement system. Based on the axis-translation technique, a 5-bar high air entry value ceramic disk is seated on the pedestal, and a matric suction state was achieved that remains constant throughout the laboratory tests. Hence, this true triaxial apparatus can be used to conduct suction-controlled experiments. The suitability of this device was validated via two sets of three repeated trial tests and several sets of trial tests performed with remolded loess for different intermediate principal stress parameters under 50, 100, and 200 kPa matric suction values and different net mean pressure conditions. The results presented the stress-strain characteristics and failure loci of remolded loess.

Fractional-order creep model for soft clay under true triaxial stress conditions

To investigate the creep characteristics of soft clay under true triaxial stress conditions, multistage loading creep tests using TSW-40 true triaxial apparatus were performed. The creep deformation characteristics, creep rate, and long-term strength of soft clay under a true triaxial stress path were studied. The results showed that the creep deformation and creep rate of soft clay increase with increasing major principal stress; the axial deformation is larger than the lateral deformation. The long-term strength and the equation for predicting the starting point of accelerated creep were obtained using the generalized shear stress. A four-element fractional-order creep (FFC) model based on the fractional Burgers creep model was established for soft clay and then extended to three dimensions. The model calculation results based on the parameters obtained using MATLAB’s Curve Fitting Toolbox were in good agreement with the experimental results. The comparison results showed that the 3D FFC model was more accurate at describing the entire creep curve of soft clay than the fractional Burgers model and the Burgers model, particularly regarding the accelerated creep characteristics. In conclusion, the results presented herein are expected to provide the scientific basis for determining soft clay creep instability under true triaxial stress conditions.

Microstructure and Reinforcement Mechanism of Lignin-Modified Loess

To develop a new modification method with environmentally friendly and seismically resistant characteristics for saturated loess foundations, lignin extracted from paper industrial wastes was selected for mixing into the loess. Mixtures with different lignin contents were prepared under static pressure. A series of dynamic triaxial tests were performed by applying sine loads after the lignin-modified loess specimens were saturated and consolidated on the dynamic triaxial apparatus. The microstructural images and the mineral components of the modified loess with different lignin contents were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Based on the test results, the variation regularity of the dynamic residual deformation and the dynamic pore water pressure of the lignin-modified loess was analyzed. The microstructural characteristic parameters of the lignin-modified loess were extracted, and the relationship between these parameters and the lignin contents was obtained. Based on the dynamic triaxial test, the SEM, and the XRD results, the mechanical mechanism of the liquefaction resistance of the lignin-modified loess is discussed. The results show that lignin can significantly improve the resistance of cyclic shear deformation of the saturated loess and can effectively control the increase of the pore water pressure. When the lignin content was 4%, the liquefaction resistance of the lignin-modified loess was the most significant. Compared with the unsaturated compacted loess, the number of small and micropores in the lignin-modified loess increased significantly, and the apparent pore ratio decreased. Additionally, the arrangement sequence of pores deteriorated, the pore structure became more complex, and the pores were more compactly arranged. However, when the lignin content was higher than 4%, the microstructural characteristic parameters reflected a reduction in compactness of the lignin-modified loess. No other new minerals were formed in the lignin-modified loess except for a small amount of albite (NaAlSi3O8), which was generated by the ion exchange between the lignin and loess. Furthermore, the liquefaction resistance mechanism of the lignin-modified saturated loess mainly included the filling and cementation of the lignin, thinning of the electric double layer in the modified loess, reinforcement of the lignin fiber, adsorption of fine particles and water by the fiber, and ion exchange between the lignin and loess.

An investigation on hydraulic fracturing characteristics in granite geothermal reservoir

Geothermal energy has been widely proposed as a potential renewable energy to replace traditional fossil fuel energy. Deep buried geothermal reservoirs are usually called Hot Dry Rock (HDR) or Enhanced Geothermal Systems (EGS). Hot dry rock (HDR) reservoirs are the main geothermal energy resources, and usually consist of low-permeability hard granite without fluid. Developing HDR requires water cyclically flowing between injection and production wells to extract heat energy. Hydraulic fracturing, as a key reservoir stimulation technology, can create the path of fluid cyclically flowing. Although hydraulic fracturing in HDR formations has attracted more and more attention these years, large-size HDR formation environment is difficult to establish in laboratory and large granite samples are difficult to crack because of their high toughness and strength, the reservoir stimulation characteristics in this hard granite have not been thoroughly addressed by the researchers and the hydraulic induced fracture morphology has yet remained unnoticed. To advance this technology development, a true triaxial hydraulic fracturing apparatus with high temperature heated system has been developed by Jilin University to simulate the real HDR environment, and this paper investigated HDR geothermal reservoir stimulation characteristics in large-size granite under different injection flow rates, temperatures and confining stresses conditions. The granite samples were collected form Gonghe Basin where the Chinese first EGS field operation will be built. Results showed that these laboratorial conditions affected hydraulic fracturing characteristics, breakdown pressures increased with the increase of injection flow rate and confining stresses, decreased with the increase of temperatures. Numerical simulations were conducted to model the laboratory tests. These results can provide some reasonable advice for implementing reservoir stimulations on the application of field-scale HDR operation.

Influence of Initial Anisotropy, Stress Path and Principal Stress Rotation on Monotonic Behavior of Clean and Mixed Sands

It is widely accepted that soil behavior is complicated taking into account soil anisotropy owing to the fact that this phenomenon arises from oriented soil fabric or structure forged in the deposition stage. In this study, a review of major findings of authors’ previous studies are presented with the main focus on soil anisotropy using extensive experimental results incuding Triaxial (TXT), Simple Shear (SSA), and Hollow Cylinder (HCA) apparatus. Effects of initial anisotropy, fabric evolution, stress path, principal stress rotation and intermediate stress state are evaluated for a crushed silica sand. In addition, the effects of Portland cement content and granulated rubber contents on anisotropic behavior of the sand are investigated. Bender elments are mounted on triaxial specimens both in vertical and horizontal directions to measure the shear wave velocity and hence maximum shear modulus at the end of consolidation as well as during shearing up to large strains at critical state condition, as an index of evaluating the fabric evolution. The effects of principal stress rotation and stress paths reveals the crucial role of soil anisotropy on the behavior of clean sand. However, adding either cement or granulated rubber to the sand has considerably decreased anisotropy.

Post-liquefaction shearing behaviour of saturated gravelly soils: Experimental study and discrete element simulation

Abstract To investigate the post-liquefaction shearing behaviour of saturated gravelly soil, laboratory tests were conducted using a static-dynamic multi-purpose triaxial apparatus. In addition, numerical simulations using the discrete element method (DEM) were performed to preliminarily understand the micro-mechanism of gravelly soil in monotonic loading after liquefaction. The influences of dry density, initial confining stress and degree of liquefaction on the post-liquefaction shearing behaviour of gravelly soil were discussed, and the evolution of the micro-parameters of the granular system was also analysed. The results show that the stress-strain responses of gravelly soil after liquefaction can be divided into three stages: (1) low strength stage, (2) super-linear strength recovery stage, and (3) sublinear strength recovery stage, which are distinctly different from those of the general saturated gravelly soil without previous cyclic loading. The initial state and prior dynamic stress history have significant influences on the post-liquefaction shearing behaviour of gravelly soil. The DEM simulation revealed that the average coordination number sharply increases, the contact normal shows an obvious orientation distribution, and the destroyed force chain backbones are reconstructed in the monotonic reloading process after liquefaction. The evolution of the micro-parameters of the granular system clearly reflects the interior interaction process and micro-mechanisms in the particles during the three different stages of the macro-mechanical behaviour of gravelly soil.

Upgrading disk transducer to measure elastic waves on coarse-grained granular materials: Development and performance revelation

This paper describes a recently developed technique of evaluating elastic properties of geomaterials in the large size triaxial apparatus enabling to propagate elastic waves even in coarse-grained geomaterials. A flat surface transducer of 80 mm in diameter has been developed embedding four pairs of compressional and shear wave measuring piezoceramic elements. The assemblage of piezoceramics, interpretation of results, verification of performance and applicability are corroborated. It has been shown that an accumulated strength of numerous piezoceramics in form of a single transducer significantly improved the integrity of the transducer enabling to evaluate elastic wave velocity in a large size the triaxial specimen of 230 mm × 230 mm × 500 mm. An investigation on three sorts of granular materials; fine, (d50 = 0.2 mm), medium, (d50 = 1.7 mm) and coarse, (d50 = 12 mm) is carried out. Small strain stiffness of tested geomaterials is evaluated by newly developed large-size disk transducer method and small-strain cyclic loadings. Stiffness measured by both static and elastic wave measurement is confirmed to be followed similar trends. The large size disk transducer evinces the competent performance showing the stiffness evaluated by this method scattered within 20% of the stiffness achieved by a static method and compared with available previous researches as well.

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates

Abstract. Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide–hold–slide (SHS) friction experiment performed on simulated fault gouges prepared from bituminous coal collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 ∘C, employing sliding velocities of 0.1–100 µm s−1 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ∼ 1–2 mm, from a peak friction coefficient approaching ∼0.5 to (nearly) steady-state values of ∼0.3, regardless of effective normal stress or whether vacuum-dry or flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip-weakening behavior can be attributed to the development of R-, B- and Y-shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ∼0.006. We also determined the rate dependence of steady-state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities >1 µm s−1 in the coal sample under vacuum-dry conditions but at >10 µm s−1 in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the frictional properties of coal–shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear out coal seams may promote unstable, seismogenic slip behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

Acoustic Emission Experimental Research of the Damage Characteristics of Raw Coal under Different Loading and Unloading Rates

In this study, triaxial load failure experiments of coal samples under different strain rates and different confining pressure unloading rates were carried out using an RTX-1000 rock triaxial apparatus, and the acoustic emission characteristic parameters of a Micro-II acoustic emission imaging acquisition instrument were used to study the acoustic emission characteristics and damage deformation law of coals under different conditions. Damage models were constructed on the basis of the characteristic parameters to analyze the damage law of coal samples. Experimental results show that the acoustic emission (AE) counts and AE energy of the coal samples decrease, but the peak AE counts and peak AE energy increase with the increase in strain rates. The cumulative AE counts decrease from 9902 times to 6899 times, the peak counts increase from 209 times to 431 times, the cumulative AE energy decreases from 6986 aJ to 3786 aJ, and the peak AE energy increases from 129 aJ to 312 aJ. The overall level of the AE count rates and the AE energy of the coal samples decrease, but the peak AE counts and peak AE energy increase with the increase in unloading rates. The cumulative AE counts decrease from 18,689 times to 16,842 times, the peak AE count rates increase from 245 times/s to 535 times/s, the cumulative AE energy decreases from 9846 aJ to 7430 aJ, and the peak energy increases from 257 aJ to 587 aJ. The damage models are constructed on the basis of AE counts, and the comparative experimental and theoretical curves are analyzed to obtain a higher fitness close to 1. The damage threshold increases from 0.30 to 0.50 and from 0.34 to 0.55, and the damage amount increases from 0.50 to 0.60 and from 0.34 to 0.62 with the increase in strain rates and unloading rates. The research results have practical significance for revealing the mechanism of disaster occurrence in actual engineering excavation and proposing engineering measures to prevent coal rock damage and disaster occurrence.

Evaluation of monotonic and dynamic shear modulus of sand using on-sample transducers in cyclic triaxial apparatus

This paper highlights the importance of the measurement of local strains in triaxial test using on-sample transducers. Specimens of sandy soil, prepared at two different relative densities (30% and 90%), were subjected to different effective confining stresses, and sheared monotonically as well as cyclically. Local strain measurements are obtained using Hall-type on-sample transducers attached at the mid-height of the specimen. The measurements from the on-sample transducers were further used to determine the modulus degradation curve for low-strain (less than 0.01%) to high-strain (greater than 1.0%) levels. Maximum shear modulus estimated from the cyclic tests is found to have an agreeable match with the same obtained from the monotonic tests corresponding to very low strains, as well as with those obtained from standard correlations. The experimental study indicates that for both monotonic and cyclic triaxial tests, the on-sample transducers can be effectively used to measure local strains in the soil specimen and evaluate the corresponding strain-dependent dynamic shear modulus.

Study on the Strength Characteristics of Columnar Jointed Basalt with a True Triaxial Apparatus at the Baihetan Hydropower Station

Columnar jointed basalt is widely distributed in the dam foundation and underground cavern of the Baihetan hydropower station, in which columnar jointing and microfractures are developed, the integrity of the rock mass is poor, and the material easily relaxes after excavation. Therefore, it is important to obtain the real mechanical parameters of columnar jointed basalt in rock mechanics problems. In the test cave at the dam site, six in situ samples with a size of 50 cm × 50 cm × 100 cm were successfully made, and in situ true triaxial test were carried out. This paper presents a method for obtaining the strength parameters of the Mohr–Coulomb criterion and Hoek–Brown criterion by true triaxial testing. The peak strength parameters under three states and yield strength parameters under one state were obtained by setting various stress paths for the undisturbed samples and failure samples. The strength characteristics of columnar joint basalt and the crack characteristics of the specimens are analyzed, and the results of the triaxial test and direct shear test are compared. By means of acoustic emission (AE) techniques and surface crack descriptions, the failure process and failure pattern of the samples were obtained. The abundant test results provide strong support for the selection of engineering rock mechanics parameters.

Soil Densification Effect on the Behaviour of Chlef Sand (Algeria) under Static and Cyclic Loading: A Laboratory Investigation

This paper presents a laboratory investigation on soil densification effect on the behaviour of Chlef sand (Algeria). The study is carried out via a series of monotonic and cyclic tests in drained and undrained conditions using the triaxial apparatus. It’s conducted in three stages: the first consists on the study of the sand behaviour under monotonic loading in drained conditions. Effects of the confining pressure and of the relative density Dr on the sand’s shear strength are observed. In the second step, the undrained behaviour of the studied sand under monotonic loading is studied, and finally the last step deals with the sand undrained behaviour under cyclic loading. In each step, sand samples with different values of relative density that are ranging from loose, medium and dense state are tested (Dr = 10%, 15, 18%, 50%, 65% and 80%). The obtained results show a significant upgrading of the mechanical properties of the sand samples proportionally to the densification rise. The aim of the study is to highlight the improvement that can bring soil densification process on the behaviour of Chlef sand, and therefore its use as a soil reinforcement technique to solve soil deformations in the study area, which is a prone zone to seismic risk.

A four-element fractional creep model of weakly cemented soft rock

The creep model is the main form used to describe the creep behavior of weakly cemented soft rock (WCSR). To investigate the creep behavior of weakly cemented soft rock, multi-stage loading creep tests were performed by using GDS HPTAS creep triaxial apparatus. The creep curves, creep rates, and creep failure modes of weakly cemented soft rock under different water contents were obtained. A novel four-element fractional-order creep model to describe the three-stage creep behavior of weakly cemented soft rock was proposed using the Abel dashpot basing on the creep test results and fractional calculus theory. The formula of this model was developed according to the H-Fox special function. The trust-region method was used to obtain the model parameters, and the parameters were optimized using the ant colony optimization approach. The creep curves of weakly cemented soft rock under different water contents were calculated and compared with the experimental results, verifying the superiority of the four-element fractional-order creep model in describing the creep characteristics of weakly cemented soft rock. The calculated results were also in good agreement with the experimental results, which confirms that the four-element fractional order creep model can accurately reflect the complete creep behavior of weakly cemented soft rock.

Experimental studies of deformation anisotropy of gravel soils

When gravel-pebble soil is laying and compacting into the dam body, particles are reoriented to form a layered body (transversely isotropic soil), in which not only the filtration but also the deformation properties are different in directions along and across the layering. To analyze the differences in the gravel-pebble soil deformations in correspondence with the particles’ orientation we have performed experimental researches on the river pebble gravel using odometer, vacuum triaxial compression machine, and a large scale triaxial apparatus. To obtain the deformation characteristics of the soil in different directions we’ve carried out experiments according to two schemes with gravel particles (fr. 10-60 mm) laid horizontally and vertically. More deformations occur in the cross-layerage axis of the soil rather than in the parallel direction (εy > εx), so for the soil in the dam body Ex > Ey. The experiments showed that for gravel-pebble soil the strain-induced anisotropy coefficient η is dependent on its loading conditions and changes in the range from 1 to 1.8. The change of anisotropy degree is bound with the main tension value σ1 with the increase in which the anisotropy decreases.

Experimental Study on the Mechanical Behaviour of Natural Loess Based on Suction-Controlled True Triaxial Tests

Loess is mostly distributed in an unsaturated state in nature, and the complexity of the engineering properties of unsaturated soil is mainly due to the existence of matric suction. Therefore, matric suction must be considered in investigating the mechanical properties of unsaturated loess. However, soils are most often subjected to three principal stresses with different magnitudes in practical engineering. For the sake of examining and discussing the mechanical behaviour of unsaturated natural loess under a complex stress path, a suction-controlled true triaxial apparatus with a rigid-flexible boundary is used to test unsaturated natural loess under a complicated stress path. Four trials of isotropic consolidation tests are conducted on natural loess under suction-controlled conditions via the true triaxial apparatus. The consolidation yield characteristics of the natural loess under different matric suctions are investigated. Forty-eight trials of consolidated drained true triaxial tests under suction-controlled conditions are conducted on unsaturated natural loess to examine and discuss the influence of the matric suction and intermediate principal stress parameter (b-value). The consolidated drained trials are performed under a constant net mean stress and a constant matric suction with different intermediate principal stress parameters (b-values). Stress-strain curves and failure envelopes of the natural loess are also presented. The results indicate that the stress-strain-strength response of unsaturated natural loess depends on the matric suction and intermediate principal stress parameter under true triaxial conditions.

Experimental Investigation of Creep Behavior of Bayer Red Mud in Guizhou Aluminum Factory's Red Mud Disposal Field

Long-term stability and safety of red mud disposal field is very important for the local residents’ life, which necessitates the knowledge of its creep behavior. Through the improved triaxial creep apparatus, a series of triaxial drained creep tests were conducted. The results indicate that the creep behavior of BRM is significant with confining and deviatoric stresses being critical factors. The soil creep occurs in two stages of transient creep and the steady creep, and a larger deviator stress will lead to a larger creep strain. The main failure mechanism of BRM is plastic shear, accompanied by a significant compression and ductile dilatancy. The Burgers model and the Singh–Mitchell model are used to describe the creep behavior of BRM, and four parameters of these two models are estimated by the nonlinear regression. The deformations predicted by this model are in reasonable agreement with experimental data.

Study on creep characteristics of unsaturated sericite schist residual soils

Sericite schist is an extremely soft rock, which is characterized by obvious rheological properties and large creep deformation. The residual soil of Sericite schist is in unsaturated state for most of the time. When encountering water, it interacts with water, which has a great influence on its strength. In this paper, one dimension and three axis creep tests of Sericite schist residual soil are carried out by using unsaturated quadruple consolidation apparatus and unsaturated triaxial creep apparatus. The results show that the creep process of the residual soil can be divided into three stages: rapid growth stage, slow growth stage and stable stage. The creep deformation of triaxial test is larger than that of one dimension.