Based on the framework of critical state soil mechanics，a new bounding surface constitutive model for sand, being applicable to cyclic loading, is proposed. No pure elastic region hypothesis is given in this model. Elastic region of sand reduces to a point. Reverse bullet-shaped yielding surface is adopted. No unique relationship between void ratio and the mean normal stress for sand prevents the direct coupling of yield surface size to void ratio. Incremental deviatoric strain is used as a hardening parameter, instead of adopted plastic volumetric strain as hardening parameter in Original Cam-clay model. The model combines the concept of state-dependent dilatancy by incorporating state parameter concept in Rowe’s stress dilatancy equation. Bounding surface magnitude ratio ? to loading surface gives plastic modulus evolvement. Formulation of d? is deducted. A single set of model constants, once calibrated, can simulate cyclic stress-strain response under different initial void ratios and different confining pressures.

Because ceramics is similar to rock in mechanism, ceramics is selected to precut the test specimens with artificial double initial disk-like cracks. The CT real-time scanning tests of the specimens are carried out to study the propagation and damage mechanism of the 3D crack under uniaxial compression. From the comparative analysis of CT values, CT variances and CT images in three different manifold areas, the different propagation laws in areas of up-crack, down-crack and rock bridge are obtained. Firstly, in the up-crack areas, compression and densification are dominant and growth is subordinate; so up-crack has little influence on the damage of the specimen. Secondly, the down-crack propagates greatly, visible wings appear and macro-cracks join together finally, which has great influence on the failure of the specimen. Thirdly, when the center distance between two cracks equals to the 4 times of the crack radius, the propagation process of the crack is related to its parameters; and the interaction between the dual cracks can be omitted.

Loess structure is an important physical property, which influences mechanical properties of loess with its particle size, density and humidity. Based on the structure potential of loess released by loading, remolding and saturating, the unconfined compressive strength of intact, remolded and saturated loess were measured by uniaxial compression test; and the structural index of loess was defined by comparing the stability and changeability of loess structure which determined respectively by the ratio of confined compression strength of loess and remolding soil and the ratio of confined compressive strength of saturating loess and loess. Through the unconfined uniaxial compression test of four kinds of loess adopted from different sites, the unconfined uniaxial compression stress-strain curves of intact, remolded and saturated loesses with different water contents were tested to reflect the uniaxial compression stress-strain property, as well as the law of unconfined compressive strength and loess structural index changing with the water content was analyzed for different kinds of loesses. When the particle size, density and humidity of loess being similar, the greater the structure index of loess, the smaller the compressibility and the greater the shear strength of loess. So the structure Index structure together with the particle size, density and humidity can be used to describe reasonably the fundamental physico-mechanical properties of intact soils.

The theory of isotropic double scalar damage variables was presented and developed based on the research work done by the predecessors firstly. Then, the elastic damage evolution equation described by the double scalar damage variables was deduced; and the finite element program for the coupled seepage and damage was written using the computer language Fortran90 based on the modified Biot’s theory. Finally, a simple roadbed foundation model was analyzed using the computer program written personally; and the typical node’s settlement, pore pressure, damage variables were analyzed and discussed. The results show that the double scalar damage variables model is more reasonable than the single scalar damage variable model in describing the constitutive relation of soils.

A series of triaxial shear tests on intact unsaturated Q2 loess were conducted using computerized tomography (CT)-multi-function triaxial apparatus developed in Logistical Engineering University. The distinct CT images and detailed CT data of the evolution of meso-structure were obtained during loading. A new method is proposed to identify the yield stress of intact Q2 loess using curves of CT-number versus deviatoric stress. A structure parameter and a structure evolution variable are defined based on CT data. The variable of structure evolution increases with the increase in net confining pressure under the same suction; while the variable of structure evolution decreases with the increase in suction under the same net confining pressure. An equation describing the evolution of structure is developed. In the equation, deviatoric strain and volumetric strain are included. The equation reflects the influences of net confining pressure and suction on the evolution of meso-structure during loading. The structure evolution equation successfully simulates the laboratory tests of unsaturated intact Q2 loess.

The rigid serve-controlled test machine and acoustic emission equipment are used to study the influence of space between 3D-cracks on the mechanical feature of mortar material; and the failure process of samples with pre-existing cracks is concluded. The analysis of complete stress-strain curves and AE data indicates that sample failure is due to the propagation and coalescence of cracks. Failure process of samples with pre-existing cracks could be divided into the following stages: microcrack initiation, initiation of pre-existing cracks, crack propagation and crack coalescence. The failure pattern is shear failure along single inclined plane. When the ratio of cracks space d and length of elliptic crack long axis 2c ranging from 0-1, as the ratio grows, the initiation stress and maximum stress of samples reduce. With the ratio d/2c growing from 1/3 to 2/3, the initiation stress and maximum stress fall suddenly, showing that this space scope is the sensitive range. When the ratio d/2c =1, the initiation stress and maximum stress of sample fall 22.48 % and 18.60 % respectively corresponding to the sample without pre-existing cracks. The ratio of initiation stress to maximum stress of all different samples ranges from 83.1 % to 78.9 %. As the ratio d/2c grows, the ratio of initiation stress to maximum stress declines, which indicates that as space d grows, the loading range between initiation stress and maximum stress increase.

The compression tests are conducted on Yangling and Qinghai structural loesses under different water contents. Based on the principle of equivalent deformation, the equivalent stress of structure fluid is defined by using compression stress difference between unsaturated and saturated loesses under the same consolidation void ratio . The effective stress principle of unsaturated structural loess is established under compression stress. Relations between equivalent stress of structure fluid and degree of saturation as well as stress are investigated. The relation between equivalent stress of structure fluid and stress is linear as the water content is lower. Based on the equivalent stress of structure fluid under lower water content, the formula of equivalent stress of structure fluid is proposed which can reflect coupling of stress and moisture. According to effective stress principle of unsaturated structural loess under compression stress and introducing equivalent stress of structure fluid reflecting the structure change, the formula of compression of structural loess is established. The compression curves of total stress and that of effective stress can be calculated under the coupling of stress and moisture by the formula. The calculating results are accord with the test results, so as to offer a new approach to evaluating unsaturated collapsibility.

The existence of Kaiser effect was verified under Brazilian tests conducted on coarse-grained granite. Acoustic emission tests were respectively carried out after 2, 15 and 45 days time-delay since the preloading finished. Influence of time-delay on Kaiser effect were then discussed. The test results show that the closer the preloading stress to the ultimate strength, the less-pronounced the Kaiser effect is during reloading; and that the larger the time-delay, the faster the Kaiser effect decay. It is found from the Brazilain tests on granite specimens that Kaiser effect is still distinct after 15 days delay; and the effect disappears completely after time-delays of up to 45 days. The influence of friction contact between the specimen and apparatus on rock’s AE test results is also analyzed qualitatively.

The electrical resistivity of various lime-soil samples are studied indoor with four electrode resistivity measurement, the effects of different factors such as lime-soil ratio, water content, water saturation, soil structure and particle size on lime-soil samples were revealed. Under different water contents (about 15 %, 20 %, 25 % and 30 %), soil were mixed with lime at different lime-soil ratios (about pure soil, 1:10, 1:15 and 1:30). With light compaction test, the traverse and vertical resistivity of mixed soil samples were measured and the relationships between resistivity and physical properties of mixed soil samples with different lime-soil ratios are studied too. The electrical resistivity of mixed soil samples drops greatly with 15 %-20 % water contents, increases with about 20 % water content; and the resistivity reduces as the water content increased continuously. The electrical resistivity of lime-mixed soil sample larger than electrical resistivity of pure soil with 15 % water content, but the resistivity of lime-mixed soil samples is smaller than pure soil samples with 30 % water content. The resistivity is more lower as the lime mixing amount is larger. The electrical resistivity at transverse section is larger than at vertical section. The electrical resistivity increases from 0 to 50 ?•m as the unconfined compression strength increases. When the electrical resistivity larger than 50 ?•m, the resistivity decreases as unconfined compression strength increases. The relationship between water saturation and dry density is negative exponent; the relationship between porosity and void ratio is exponent reverse to the dry density. The resistivity is sensitive to the water content.

Experimental research on water retention characteristics of sand with different fine grains is carried out by pressure plate test to analyze the effect of fine grains on the soil water characteristic curve (SWCC). The results show that the water retention capacity of unsaturated sand enhanced with fine grains increase; while, the effect is varied in the regime of SWCC. The water retention capacity enhanced significantly when suction beyond air-entry value. is advanced to describe the SWCC of silty sand. Then, the effect of fine grain content of silty sand on the parameters of SWCC equation is analyzed; and the relationship of fine grain, suction and water content is described by .

Considering the practice of embankment construction ,an experimental method used to study moisture migration in unsaturated soil considering influence of the heat radiation of earth surface and rainfall is designed. According to the measurements of water content change in different depths and time under corresponding boundary condition, the rule of moisture migration of unsaturated soil is revealed due to influenced by different surface temperatures, compactness and rainfall intensities. Forthermore, the rule of moisture migration in embankment is discussed; and the conclusion that the higher compactness will reduce moisture migration rate greatly, is drawn.

Based on the true triaxial apparatus of Xi’an University of Technology, the failure modes, intermediate principal stress influence on shear strength parameters and strength surface characteristics on π plane of Q2 loess with different water contents was analyzed under the condition of different intermediate principal stress ratio b and different meaning normal stress. It was shown that the failure modes of Q2 loess cubic samples behaved four kinds of failure formation, including of lateral bulge failure, taper failure, trapezoid failure, irregular hexagon failure by true triaxial tests. The shear strength parameters ratio could be got by comparing the strength parameters measured by true triaxial tests with the results of triaxial tests. The coherence ratio changes between 0.9 and 1.9. The friction angle ratio varies between 0.7 and 1.5. The structure property and failure formation of Q2 loess influence the shape of failure criteria on π plane. The consolidation pressure influence the value of failure surface in π plane. The shapes of Q2 loess failure criteria on π plane under different water contents and different consolidation pressures include pear-shape and irregular hexagon. With the structure property of Q2 loess reducing, the shapes of failure surface on π plane changes from pear-shape into irregular hexagon.

The improved static and dynamic universal triaxial and torsional shear apparatus for unsaturated silty is employed to perform matric suction controlled cyclic triaxial tests under consolidated drained condition. Through a series of static and dynamic triaxial tests on unsaturated remoulded slity clay. The characteristics of static and dynamic strength of unsaturated silty clay are studied. The rules that static strength of silty clay changes with confining pressure and matric suction, and dynamic strength changes with confining pressure and matric suction and number of cycles are also analyzed. Moreover, it is verified that the correlation of strength between static and dynamic test of unsaturated silty clay is remarkable. Finally, some experience formulas of ascertaining dynamic strength curve according to static strength are presented by normalizating test data, so as to reduce the workload for analyzing of stability on unsaturated foundations.

The loading model of the static load and dynamic load being acted at the same time was established according to the superposition theory. The concepts, for example, combinatory load, combinatory stress and combinatory deformation, were put forward. At he same time, it provides the basis for continuing study the mechanical characteristics of layered subgrade under combinatory loads. Based on the similarity theory, through the resemble model test, the variation of the layered subgrade soil stress and deformation is discussed when the static load and dynamic load are acted at the same time. The variation of stress and deformation of the layered subgrade soil is discussed under keeping the dynamic invariable and changing the static load. The results show that: the combinatory stress and the combinatory deformation of layered subgrade linearly increase with the increasing amount of the combinatory load, the combinatory stress and the combinatory deformation of layered subgrade nonlinearly decrease with the increasing amount of the depth; the rule is exponential function. In conclusion, these results are instructive for the design, construction, operation and maintenance of layered subgrades.

Combining large number of indoor consolidation and odeometer test data of Ningbo soft soil, using partial differential finite element software FLEXPDE, and taking into account complicated geometry and material nonlinearity of soil, the characteristic of soil consolidation is investigated. Under the conditions of different ratios of compression index to permeability index, analytical solutions and numerical solution as well as the distribution of effective stress and void ratio with height are obtained. The results showed that numerical solution fully taken into account the complex nonlinearity of soils; and results more in line with actual conditions when the ratio of compression index to permeability index is not equal to 1; the results are consistent with the results of Gibson’s in 1981 when ratio is 1; it’s more safe to forecast the effective stress and the consolidation degree when considering the complex nonlinearity of geometry and materials of soil.

he soils under the foundation of building or within the slope are usually in complex initial stress state before subjected to cyclic loading induced by earthquake or other dynamic loadings; after reforming the DTC-199 torsional cyclic load triaxial apparatus, the dynamic strength of compacted loess can be tested under this consolidation stress conditions. Results show that the initial angle α0 of principal stress has a considerable influence on the dynamic strength of compacted loess when other initial consolidation stress conditions are fixed; the initial coefficient b0 of intermediate principal stress has little influence on the dynamic strength of compacted loess; with the increase of α0, dynamic strength reduces gradually and it reaches minimum when α0 is 90o; maximum reduction range is about 40 % . The initial coefficient of intermediate principal stress has interval effects and amplification effect on the dynamic strength of compacted loess. When b0 increases from 0 to 0.5, the dynamic strength increases; and when b0 increases from 0.5 to 1, the dynamic strength decreases; the rate of increasing is less than rate reduction about 5 %.

Based on porous medium theory, the longitudinal coupled vibrations of saturated soil-pile is investigated with layer method by regarding soil around of the pile as saturated porous medium. A comparative analysis between the results of the three-dimensional model and Novak layer method is carried out; and validity of the results is shown. The influences of main mechanical parameters of pile and soil on dynamic behaviors of coupled system is analyzed; the results indicate that the length-diameter ratio and modulus ratio of pile and soil have a great effect on dynamic behaviors of the pile in saturated soil; while the influences of damping coefficient and coupling coefficient are smaller.

A great deal of factors evaluating the adjoining rock stability is uncertain. As the set pair system and connection entropy model can dispose the uncertain problem, a connection entropy model for stability evaluation of adjoining rock is presented. By calculating the connection entropy values at different safety gradation，the stability grade of adjoining rock could be obtained. The method could provide a new way to evaluate uncertain system like stability evaluation. The case study shows that the bigger the connection entropy value is, the worse the rock stability is, i.e. the more dangerous adjoining rock is.

Using four consolidation apparatus and triaxial instrument for unsaturated soils, they have advantages that could measure volume change and take into account effect of net mean stress that pressure plate apparatus could not. A seriers of tests with considering and without considering net normal pressure or net mean stress about generalized SWCC, are carried out. Test studies indicate that net normal pressure or net mean stress plays an important role in the SWCC. The test results are consistent with the theoretical formula. Combining unsaturated soils theories with practices, a formula of generalized SWCC under without considering and considering net normal pressure or net mean stress affect. Test results well agree with theoretical formula.

Based on anti-explosion model test, the explosion resisting performance of the tunnel which is reinforced by fully bonded rock bolts is studied under explosive stress waves produced by plane charging. The results show that test data are reliable by analyzing pressure temporal curves of free field; vault displacement of the anchored tunnel is reduced significantly; and the tunnels are damaged when the scale distance is 5.23 cm/g1/2; the surface strains of two tunnels are pressed under plane explosive stress waves，the most of which is on arch foot; with the reducing scale distance, the peak strains increase firstly; and then reduce, the maximal peak strains increase; the acceleration of roof is maximal, which is relatively reduced when anchored; and the acceleration of floor of the anchored tunnel is increased under the condition of small deformation which should be reduced as necessary by adopting some vibration-absorptive ways; the more the tunnel damages severally, the more the acceleration of ribs of the anchored tunnel reduce significantly, with the reducing scale distance, the acceleration of roof and ribs grow, especially when the scale distance is 5.23 cm/g1/2.

At same cement content there existed obviously strength difference between the stabilized soil samples formated from soil samples with different mineral components, which was adjusted manually. In order to find the reason of these experimental results, concentrations of main ions in the pore solutions squeezed from the stabilized soil samples were measured; and the concentrations of Ca(OH)2 in the pore solutions with many ions were calculated by the thermodynamic method. The result shows that ,at same cement content，when the mineral component of soil samples is montmorillonite, the concentration of Ca(OH)2 in the pore liquid of the stabilized soil can not reach to saturation; at this stage, the lower the content of montmorillonite is, the more the hydrates generated by cement hydration; therefore the higher the strength of the stabilized soil is; after the concentration of Ca(OH)2 reach saturation point, the hydrates will fully grow and the strength of the stabilized soil will reach maximum. On the basis of this, the concentration of Ca(OH)2 in the pore liquid of the stabilized soil was adjusted through alkaline matter; The result shows that alkaline matter can increase the concentration of Ca(OH)2 in the pore liquid, heighten the strength of the stabilized soil samples.

At present, improving the engineering properties of soft soil by developing and using solidification materials is a worldwide pop topic. A new cement material (ALOFIX-MC) is used to solidify the soft soil; and the mechanism of soil reinforcement is researched based on indoor tests. Different percentages of MC, i.e. 0 %, 1 % , 2 %, 3 %, 4 % and 5 % by weight of cement were randomly put into the soil samples which included 15 % cement content and 2 % gypsum content. Six samples were prepared and subjected to unconfined compressive strength test. The results show that a low MC content can significantly improve the strength and stability of cement soil with 15 % cement content. The microstructural analysis using scanning electron microscopy (SEM) shows that MC can provide new cementations, and MC improves the hydration of cement, pore filling, and water sorption, which contribute to significant improvement on solidification effect. The results have provided a new way for cement soil engineering property improvement and reduction of cement content in conventional cement soil.

In order to research the salt expansion laws and moving patterns of water and solute migration after several freezing and thawing cycles, the crude coarse grain saline soil samples taken from China’s Gansu province were selected for experiment of freezing and thawing cycles. In addition, the effects of the microstructure of crude saline soil after the testing of freezing and thawing cycles was researched by using SEM. It is shown that salt expansion value of crude coarse grain saline soil is accumulated before the seventh freezing and thawing cycles; but soil body could be dented in the process of cooling; and collapsibility value is accumulated with increasing cycles; and one-dimensional soil column exhibits thermal sieve effect from the top to the lower; the moving patterns of moisture content and different ion content in soil column are different under many freezing-thawing cycles.

Strength and stability of reinforced soil depends on intercrossing and tensility of reinforced materials, and friction between reinforced material and soil. By means of SEM photos, it is observed that SH agent is infiltrated through the pore of wheat straw, and adhered to its surface. For studying its tensility, maximum tensile and elongation of wheat straw with one stem node and without stem node, is tested in the condition of original, soaked in sea water, marinated in SH agent, and soaked in sea water after marinating in SH agent. It has been certified that the maximum tensile and elongation of wheat straw without stem node is more than that of wheat straw with one stem node. Respectively comparison on the marinated in SH agent and original, and soaked in sea water after marinating in SH agent and soaked in sea water, the maximum tensile and elongation of wheat straw marinated in SH agent is more than that of original wheat straw. SH agent benefits to increase the tensility of wheat straw.

China has a large area with seasonally and perennially frozen soils. It is necessary to study mechanical properties of frozen soils for the sake of constructions in cold regions, so as to guarantee their stability. In this paper, the distribution of frozen soils in China and the main features of frozen soils differing from unfrozen soils, are briefly described firstly. Generally speaking, frozen soil mechanics consists of two main aspects, i.e. freeze-thaw related problems and mechanical properties of the already-frozen soils. Frost heave, thaw settlement and freeze-thaw induced changes in mechanical properties are freeze-thaw related problems. Frost heave has been extensively studied. Various theories have been put forward to describe the mechanism of ice segregation which is considered as the main cause of frost heave; and some theories have even been applied to quantitative analysis. The study of thaw settlement has a long history, while most of the previous studies still stay in empirical stage. Thaw consolidation has been studied; but it has considerable limitations so far. Therefore, we suggest that the artificial neural network could be used to improve the precision and applicability in estimating thaw settlement on the one side, large strain theory could be used to improve thaw consolidation theory. For the already-frozen soils, this paper summarizes the previous studies on strength, stress-strain relationship and dynamic properties. Study of the strength of frozen soil borrows strength theories for unfrozen soils; however, they can hardly reflect stress melting of frozen soils under high stresses. The previous studies of stress-strain relationship of frozen soils have been focusing on creep; and the empirical relationship directly obtained from testing data is the mainstream. Influence of temperature on dynamic parameters of frozen soils seems to be the only task in frozen soil dynamics for a long time; while influence of frozen layer in dynamic response of a site has attracted attentions in recent years. At last, physico-mechanical processes involved in deformation of constructions in permafrost regions are briefly analyzed.

Although prestressed anchor cables have been used to reinforce slopes widely, they can happen to fail at some time because they have bear high-tension stress in moist environment and been influenced greatly by many external factors. In order to make sure of failure effects of anchorage system well, failure mechanisms are put forward firstly according to monitor data of an actual slope engineering. Then, unloading test of a cable at site was performed to get the influence on tension-force of neighboring cables. By adopting the numerical simulation, the influence on anchorage force of neighboring cables and stability of slope because of cable failure is analyzed in detail. Finally, failure criterion for part slope is put forward.

Soft-hard alternant strata slopes formed by the interbedding of sandstone and mudstone or the mudstone imbedded in thick sandstones are distributed widely in the Three Gorges area. Cavities are formed by differential weathering between sandstone and mudstone and they can cause collapse of slopes. Firstly, mechanism of collapse of high slopes is studied from the aspects, such as the topographical features, lithologic characters, combination of structural surfaces, effects of rainfall and the weathering effects. Differential weathering is the main reason for the collapse of high slopes. Secondly, according to field surveys, failure mechanisms of high slope collapse are classified into toppling, sliding, creeping-tension, cantilever-tension and staggered breaking. Sketch maps of failure were generalized according to the failure mechanism. Forming process of collapse is determined by the numerical simulation. The failure process of collapse is mudstone spalling, cavity, sandstone crack opening, tilting of dangerous rock mass, toppling cillapse and rockfall deposite. Lastly, supporting measures were put forward against all the failure modes of high cutting slopes in the Three Gorges area. Research results are significant in the prevention and treatment of geo-hazards of the Three Gorges reservoir area.

The Tianhe Station horizontal frozen construction in Guangzhou metro No.3 line is the longest and biggest artificial ground freezing (AGF) project of the national metro construction currently. In the different phases of the construction; several parameters such as brine temperature, ground temperature, surface settlement/heave, swelling pressure, tunnel liner convergence and so on, are monitored dynamically. According to the data obtained from about 400 days in-situ monitoring, the features of propagation speed of the frozen soil wall and the brine temperature variation are discussed. The characteristics of ground temperature and the swelling pressure of soil are analyzed. The relationship between the ground pressure and the soil temperature is also achieved. Results show that the temperature gradient variation and soil temperature variance ratio decrease with the increasing of freezing time in the active freezing phase. The safety of frozen earth wall is also evaluated by different monitored data. Moreover, the environmental influence of the project is evaluated based on the surface frost heave and settlement data monitored in the process of construction. It is found that the pre-freezing time is too long which induces large surface frost heave and increases the project capital cost. The analytical results show that the pre-freezing time should be reduced.

On the basis of in situ measurements, it is necessary that the embankment only with crushed rock protective slopes in the warm permafrost regions along the Qinghai-Tibet Railway needs proactive mitigative measures to maintain the long-term stability. Thus, an ad hoc technique of the crushed-rock protective slopes and the awning combination (CPS-AC) was proposed. The cooling effect and mechanisms of the CPS-AC were explored on the basis of previous studies on the individual mitigative measures of the awning and the crushed-rock protective slopes, and of the current studies on the temperatures and velocities of the CPS-AC embankment configurations. The CPS-AC is further divided into two types, the closed and open systems. In the closed system, the airs between in the ambient environment and under the awning can not exchange freely. In reverse, the airs can do freely in the open system. The awning in the closed CPS-AC can shade the direct solar radiation on the protective slopes and protect the embankments from blowing sands and snow, as well as winds; and the crushed rock layer in the closed CPS-AC has the “thermal semi-conductor” effect and can shade the secondary radiation from warm awning. In the open CPS-AC system, the shading board shades sunlight and obstructs snow, and the crushed rock layer can shield the secondary radiation of the warmer awning, and protect warmer winds from entering the embankment, and also has a chimney effect for cooling the embankment. The cooling effect of the open channel between the awning and the crushed rock slope can be explained using the siphon, channeling and chimney cooling effects. The measurements and research on the effects and mechanisms have provided important evidences and application techniques for the CPS-AC practices in the Qinghai-Tibet Railway.

The vertical static load test is adopted to investigate 4 super-long bored piles in loess, in which the variation of axial force in piles is tested by vibrating wire strain gauge. Hence, the bearing capacity behavior, load transfer mechanism, the lateral friction and end bearing capacity are researched. With the increasing of vertical static load, the results indicate that: (1) shaft force distribution of bored pile is different from that of original cases; and this difference mainly concentrates on top of 1/3 pile length; (2) enhancing effect of static lateral friction and degradation effect of static lateral friction are obtained in sand soil; (3) the vertical static loading can induce decreasing of axial stiffness of single pile; and (4) axial stiffness of single pile reduces about 40 %-70 %.