HXYFYX-Current Issue

Some problems in design of geosynthetic-reinforced soil structures

LI Guang-xin

During the past three decades, geosynthetic reinforcement has been used extensively in geotechnical structures, but the mechanism of interaction between reinforcement and soil is not fully understood, as a result, the conservatireness in the design is found frequently. The conservatireness of design method for geosynthetic-reinforced soil structures is examined. Some discussions on the design of geosynthetic retaining walls, reinforced fill slopes, reinforced embankments over soft soil foundations and reinforced piled embankments are presented. On the basis of the recorded strain and stress in a number of published field case histories, the conservatism of design is further pointed out. It is indicated that the research on the interaction between reinforcement and soil is very important, and that the design method on basis of deformation coordination is necessary.

Comparative analysis of model tests on different types of composite foundations

ZHAO Ming-hua, LONG Jun, ZHANG Ling, MA Bing-hui, HE La-ping

Nine groups of model tests on the geocell-reinforced cushion, bag-sand well, gravel pile and flexible pile composite foundation are completed to analyze the consolidation effect of different types of composite foundations based on the similarity theory. Some conclusions are drawn as follows: (1) the inclusion of horizontal reinforcement can spread the upper load and raise the bearing capacity, and the reinforced effect of geocell is superior to that of geogrid; (2) the effect of pile groups cannot be neglected when analyzing composite foundation with piles and their bearing capacity is superior to that of horizontal reinforcement composite foundation obviously; (3) the volatility of bearing capacity of piles and soil changes under different loading ranges, and that under single-pile loading range is higher than that under three piles and seven piles. The pile-soil stress ratio of the pile head of gravel pile composite foundation is larger than that of gravel pile + geogrid composite foundation. The smallest is gravel pile + geocell composite foundation, but for the foundation of flexible pile types is flexible pile + geocell composite foundation larger than flexible pile + geogrid composite foundation, and the smallest is flexible pile composite foundation; (4) the pile-soil stress ratio of bottom pile of bag-sand well and gravel pile composite foundation is about 1 and for the flexible pile composite foundation it is much larger and the largest value is about 24; the pore-water pressure is larger and the velocity of dispersion is faster in shallow foundation than those in deep foundation of pile. The pore-water pressure, and that is bigger of piles composite foundation is larger than that of soft soil foundation and geocell-reinforced composite foundation, and the outflow velocity of bag-sand well and gravel pile composite foundation is faster than that of flexible pile composite foundation.

Evaluation of double-shearing type kinematic models for granular flows by use of distinct element methods for non-circular particles

JIANG Ming-jing, LI Li-qing, SHEN Zhi-fu

The double-sliding free-rotating model, double-shearing model and double-slip and rotation rate model (DSR² model) are three types of double-shearing kinematic models, which formulate the plastic flows of granular materials. These models incorporate different physical interpretations of angular velocity. A developed distinct element method program NS2D is used to generate assemblages which are composed of elliptical particles with aspect ratios of 1.4 and 1.7, respectively. The assemblages are then subjected to undrained simple shear tests to validate the above-mentioned models. The results show that: (1) the postulation in the double-sliding free-rotating model seems to be unduly restrictive for not considering the effect of particle rotation on energy dissipation; (2) a quantitative and qualitative difference between the observed rotation rate of the major principal stress axes and the theoretic angular velocity does not support the double-shearing model; (3) the DSR² model presents a successful prediction of the angular velocity by means of the averaged micro-pure rotation rate (APR), and it can be used to study the non-coaxiality of granular materials; and (4) the APR is a rational and important variable which considers the effect of particle rotation in the energy dissipation process, and bridges discrete and continuum granular mechanics.

Earthquake-induced permanent deformation of rockfill dams based on cumulative damage theory

FANG Huo-lang, GAO Yuan, YIN Ge, YAMAGUCHI Yoshikazu, SATOH Hiroyuki

Based on the Tatsuoka's cumulative damage theory of soil strains under irregular loadings, a numerical analysis method for the earthquake-induced permanent deformation of rockfill dams is proposed. The mechanical experiments of the rockfill materials are carried out by use of the large static and dynamic triaxial apparatus, and the material parameters related to the stress-strain relationship and cumulative strain model are determined. By use of the proposed method, the earthquake-induced permanent deformation characteristics of the rockfill dam under construction and after completion are studied respectively for the Iwate-Miyagi Nairiku earthquake of M7.2 on 14 June 2008 in Japan. The results show that the earthquake-induced settlement computed by the proposed method is basically consistent with the measured one under the construction of the dam. Moreover, the predicted value of the earthquake-induced settlement at the crest after the completion of the dam is within the reasonable range by comparison of the measured data of the other similar dams.

Layerwise summation method for ground foundation settlement based on Duncan-Chang constitutive model

CAO Wen-gui, DENG Xiang-jun, ZHANG Chao

Ground foundation settlement analysis is an important content of soil mechanics. Based on the discussion on ground foundation deformation mechanism, firstly, the effects of the buried depth or the initial ground stress, i.e., the preconsolidation stress, on the deformation modulus of foundation soils are considered, and they are divided into two parts: volume stress and partial stress of the initial stress. An idea of step-loading is introduced, and a determinstic method for deformation modulus of ground soils at different depths is established. Secondly, the ground foundation settlement is divided into two parts: settlement caused by additional volume stress and that by additional partial stress, and a ground foundation settlement analysis model is developed. Thirdly, the nonlinear variation characteristics of the ground foundation deformation are considered, the incremental analysis, the Hooke's law and the Duncan-Chang constitutive model are introduced, and the relevant methods for deformation of ground foundation compressed layers caused by the additional volume stress and additional partial stress are put forward. Also, a new ground foundation settlement analysis method based on the Duncan-Chang constitutive model is proposed by adopting the layerwise summation theory. This method can reflect the effects of the initial ground stress and the additional stress on the deformation modulus of the ground soils and avoid the use of the compression curves of ground soils and the static load test curves of ground foundation. Finally, through practical calculation and analysis as well as comparison with the measured settlements and the calculated values of other existing analytical methods, the proposed method meets the requirements of the calculation accuracy and is reasonable and feasible.

Distribution rules of axial stress of reinforcement in reinforced earth retaining wall

YANG Guang-qing, ZHOU Yi-tao, ZHOU Qiao-yong

Based on the shear-lag model, the interaction mechanism between reinforcement and soil matrix in the reinforced earth retaining wall is analyzed. A hypothesis is proposed that the axial stress in the reinforcement comes from the shear displacement of the surrounding soil matrix and that the surrounding soil matrix will bear the shear stress only. The soil matrix in reinforced soil unit is separated into outer layer and inner layer. The stress equilibrium differential equation for the reinforcement is established, and a formula which describes the distribution rules of the axial stress in reinforcement is brought forward. According to the theoretical analysis, the proposed formula can show non-linear distribution, and a maximum value point xof the axial stress in the reinforcement is x≤ L/2 (Lmeans the reinforcement length). The axial stress in the reinforcement of reinforced earth retaining wall will have only one maximum value when the reinforcement is horizontal, and multi-maximum values will arise when the reinforcement is concave or convex along the reinforcement length. The reasons of the occurrence of multi-maximum values of reinforcement axial stress in reinforced earth retaining wall and the phenomenon of the potential rupture surface close to the wall panel near bottom wall can be explained according to the research results.

Laboratory simulation and theoretical analysis of piping mechanism under unsteady flows

CHEN Liang, LEI Wen, ZHANG Hong-yu, ZHAO Jing-chuan, LI Jia

The problem of piping has gained widespread concern in hydraulic engineering. Usually, the researches on the piping are based on the steady flows. Under the function of unsteady flows, the variation of water level is different from that in the case of steady flows. The transformation rate of water potential that active momentum imposes on soil particles differs from that in steady flows, leading to the difference of the law of soil piping from the situation of unsteady flows. According to the calculation of theoretical formula, the hypothesis that water head of soil piping destruction under unsteady flows is lower than that under the steady flows is confirmed, and it is also pointed out that the migration law of soil particles has great randomness in a small area during the experiment. The experiment is designed based on the theoretical studies. By controlling water head, the piping tests on the soil samples under the situation that the water head changes in approximate sine curve are carried out. The correctness of theoretical analysis is verified by monitoring permeability changes of each part within the samples.

Interpolation algorithm for shallow foundations settlement based on compression and load-settlement curves

WANG Hong-xin

There is a large error in calculating foundation settlement of small size by using the layerwise summation method based on consolidation tests, while the calculation of foundation settlement based on the plate load tests can not reflect the deformation characteristics of the foundations of large size. Considering the theoretical and experimental basis in calculating the foundations settlement under large-area loads by means of the layerwise summation method based on the e-pcompression curve, which can be deemed as the upper limit of the practical foundation settlement with the same base pressure, while p-ssettlement curve can reflect the deformation characteristics of the foundations of small size, which can be deemed as the lower limit of the practical foundation settlement with the same base pressure. Based on the above theory, an interpolation algorithm for the foundation settlement is proposed, and the issues of determining the modulus of compressibility are transformed into those of finding the interpolation functions. This algorithm can reduce the absolute error produced by the traditional algorithm to the relative error within a certain range so as to improve the calculation accuracy of foundation settlement. Furthermore, a method to predict the foundation settlement is suggested by adopting two load test results under different load plate sizes, whose rationality is verified through four-group plate load tests on the same foundation. And the interpolation function of a circle foundation is derived and analyzed to show the process of foundation settlement calculation through the interpolation method. The analysis results show that the foundation settlement curves gradually change from concave to convex when increasing the size of foundations. The proposed method combines the results of the consolidation tests and plate load tests, which can reflect the settlement characteristics of the foundations of different sizes. It is suitable for promotion in engineering practices with its clear theoretical and experimental basis and simple and convenient calculation process.

Long-term field observation of sediment consolidation process in Yellow River Delta, China

YANG Xiu-juan, JIA Yong-gang

Based on the field tests on the tidal flat of Diaokou delta-lobe from April 2004 to June 2009, the consolidation process of rapidly deposited sediments in the Yellow River Delta is studied. It is found that after the sediment is deposited, its strength increases linearly, and its pore water pressure dissipation curve accords with the index model, and in 5 days, the soil is over-consolidated. Along the depth direction, strength layering phenomenon appeares and comes to be heteropical gradually. And then under the action of the hydrodynamics and the self-weight stress, the soil strength exhibits a trend of growth-reduction-growth, and along the depth, heterogeneity is improved. This study, for the first time, reveals the rapid consolidation properties of the Yellow River sediment into the sea as well as the changing rules of sediment strength with the time and space inhomogeneity.

Numerical analysis and fluid-solid coupling model tests of coal mining under loose confined aquifer

To study the stability of coal mining under loose confined aquifer, fluid-solid coupling model technology is newly developed, and an extensible geomechanical model system under stress and seepage loading is established. Major improvements in the tests are made in terms of similar materials of fluid-solid and advanced measurement methods/techniques. By using the new similar materials of fluid-solid, true reappearance of catastrophe evolution process for the water inrush induced by mining disturbance is obtained, and multivariate precursor information for real time water inrush is collected by means of the fiber bragg grating technology, acoustic emission technique and resistivity tomography technology. A numerical method considering the coupling effect of multi-physics field is adopted in order to know the coupling evolution law of stress field, displacement field and seepage field in the catastrophe evolution process of water inrush and the response characteristics of precursor information for multi-physics field of water inrush are analyzed. Based on the results of model test and numerical simulation, rock stress suddenly decreases after sustainable growth when the water-resistant key strata rupture, and rock displacement suddenly jumps after sustainable growth, but there exits a stable surging segment in which precursor information of water inrush is contained before jumping appears, and rock seepage suddenly decreases after continuous decrease, but there exits an abnormal fluctuating segment in which precursor information of water inrush is contained long before water inrush happens, and rock apparent resistivity and acoustic

Adsorption of nitrogen and water vapor by sliding zone soils of Huangtupo landslide

CHEN Qiong, XIANG Wei, CUI De-shan, LIU Qing-bing, ZHANG Qian

In order to study the adsorption of water vapor and nitrogen by sliding zone soils of Huangtupo landslide, the specific surface area and pore size analyzer instrument of Autosorb-iQ of United States and the F-sorb3400 of China are taken. The adsorption tests on water vapor at 293 K and nitrogen at 77 K under drying samples and vacuum freeze-dried samples of sliding zone soils are carried out. The theories of Brunauer-Emmet and Teller (BET) and Frenkel-Halsey-Hill (FHH) are taken to calculate the specific surface area and surface fractal dimension. The test results show that the specific surface area calculated by water vapor adsorption is larger than that by nitrogen. The adsorption volume of water vapor on per unit mass sliding zone soils is more than that of nitrogen. It is found that smaller water vapor molecules can get into the micropore and adsorb on the clay mineral surfaces and intralayer with function of polar, which makes the surface cations of clay mineral hydrate and makes the adsorbed water surface more smooth. On the contrary, the bigger nitrogen molecules cannot get into all the micropore and the surface of nitrogen adsorbed seems to be more uneven. The adsorption capacity of water vapor in dried samples is weaker than that of the freeze-dried samples because of weak hydrogen bonds.

Influence of shallow soil improvement on vertical bearing capacity of inclined piles

HU Wen-hong, ZHENG Gang

The inclined pile under vertical load may yield large horizontal displacement, deflection and bending moment. Consequently, bending failure may happen before the ultimate bearing capacity of vertical pile provided by soils is fully mobilized. Based on the field test results, numerical simulations are carried out to analyze the influence of surface soil improvement on the behavior and capacity of inclined piles. The results indicate that the depth, area and location relative to the pile of the treated soil may affect the behavior and failure mechanism of inclined piles under vertical load. The improvement of shallow soils can provide significant lateral support to piles and thus reduce their horizontal displacement. In addition, the treated soils can reduce the rotation and deflection of piles. Both of the two effects can prevent the inclined piles from bending failure before penetration failure happens. Large elastic modulus of the treated soils can better reduce the horizontal displacement, deflection and bending moment of piles under vertical load. However, there is an optimum value, i.e., when the elastic modulus is larger than it, the reinforcement effect no longer increases.

Development and application of large-scale multi-functional frozen soil-structure interface cycle-shearing system

ZHAO Lian-zhen, YANG Ping, WANG Hai-bo

In the permafrost regions and artificial frozen soil projects, there are many problems of interface between frozen soils and architectural construction, but there is no related test equipment, which can be used for researches on frozen soil-structure interface. Based on this, a new apparatus called large-scale multi-functional cycle direct shear apparatus for frozen soil-structure interface (DDJ — 1) is developed. The shear system is excellent in many performances: (1) the system can precisely realize the temperature control in the range of 0锝�-20鈩�; (2) four normal boundary conditions, including constant stress, constant displacement, constant stiffness and zero stress, can be exerted accurately in the system; (3) various roughnesses of interface can be simulated; (4) two shear forms, circular shear and drab shear, can be provided; (5) the state of failure, thickness and shear displacement of the shear bands in frozen soils can be monitored; and (6) the stress applying, displacement control and temperature regulation can be precisely measured. The test results show that the apparatus can accurately reproduce the mechanical and deformational behaviors of the interface between frozen soils and structures, meanwhile, it can provide a experimental basis and important ideas for carrying out researches on the frozen soil-structure interface.

Loosening zone and earth pressure around tunnels in sandy soils based on ellipsoid theory of particle flows

WU Jun, LIAO Shao-ming, ZHANG Di

The vertical loosening earth pressure on a tunnel in sandy soils is affected by the development of loosening zone and arching effect that is influenced by the sand particle flows. Based on the ellipsoid theory of particle flows, the method to determine the loosening zone of tunnels in sandy soils is presented, and the traditional Terzaghi loosening theory is improved. The research results show that, unlike the vertical sliding surface assumed by Terzaghi, the loosening zone of tunnels in sandy soils is an ellipse or part of it, which is related to the eccentricity and loosening factor associated with the shape of sand particles, granular composition, relative density and other characters of stratum. The lateral earth pressure coefficient of sliding surface in sandy soils is less than the value suggested by Terzaghi. Finally, the results of proposed method agree quite well with those reported in other literatures based on the Terzaghi theory. It can be used to analyze the vertical earth pressure in underground tunnel and pipe engineering.

Change of pore water pressure in soil as filter cakes formed on excavation face in slurry shield

MIN Fan-lu, ZHU Wei, WEI Dai-wei, XIA Sheng-quan

To ensure the stability of excavation face in slurry shield, a filter cake with low permeability must be formed on the working face. Based on the filter cake, part of the slurry pressure can transform into effective stress. Then the slurry pressure can counteract the soil pressure and water pressure in the soil. The variation of slurry pressure as forming a filter cake and its influencing factors are still unknown at present. With regard to the problem, a slurry infiltration apparatus is developed and a series of pressure filtration tests are carried out on 12 different slurries. By measuring the change of the excess pore water pressure as forming a filter cake and analyzing its influencing factors, the working mechanism of the filter cake is identified. The test results indicate that the slurry pressure is transformed into three different parts: pore pressure to counteract the water pressure, excess pore water pressure, and effective pressure to counteract the soil pressure. The excess pore water pressure rapidly decreases in the filter cake, then reaches a steady value in the deep zone of soil. The density of slurries is one of the primary factors that influence the distribution of the excess pore water pressure, while the viscosity has little effect on it. This study has an important directive significance for setting slurry pressure and adjusting slurry in slurry shield.

Effects of shear rate on shear strength and deformation characteristics of coarse-grained soils in large-scale direct shear tests

XU Xiao-feng, WEI Hou-zhen, MENG Qing-shan, WEI Chang-fu, AI Dong-hai

The strength and deformation of soils play an important role in the stability of slopes and subgrades. In order to analyze the influence of shear rate on shear strength and deformation characteristics of coarse-grained soils, the strength and deformation of coarse-grained soils under different shear rates are investigated by use of the THE-1000 lab large-scale direct shear apparatus. The experimental results indicate that the curves of shear stress versus shear displacement present themselves to be strain-softening ones under different shear rates. While the samples are sheared at the rate lower than 5 mm/min, the correlation coefficient is ideal. With the increase of the shear rate, the internal friction angle tends to decrease and ranges from 27.8° to 22.8°, and the interlocking force changes in the range between 90.30 to110.2 kPa. The breakage ratio and the maximum amount of shrinkage increase with the decrease of the shear rate or the increases of vertical stress. In other words, shrinkage occurs easily under a low shear rate. The essential reason why the shear strength shows a high value under a low shear rate is that the broken pieces of coarse particles fill the pores, which consist of the coarse particles, resulting in the increase of density of the soil samples.

Numerical simulation of electro-osmosis consolidation considering variation of electrical conductivity

WU Hui, HU Li-ming

Electro-osmosis is an effective method for soft ground improvement. During the electro-osmosis process, the electrical conductivity of soil is different at different positions and changes with time. In order to predict the behaviour of soil, a 1D axisymmetric electro-osmosis experiment is carried out to study the variation of the electrical conductivity during the electro-osmosis process. The finite element method software is developed to simulate the electro-osmosis process in which the seepage field is coupled with the stress and strain fields with relationship of the electric conductivity and the void ratio incorporated. The results indicate that the voltage and voltage gradient change with time. The results of the numerical model which considers the variation of electric conductivity agree better with the experimental ones than those of the numerical model which neglects the variation of electric conductivity. During the electro-osmosis process, the settlement at the anode is the largest. However, in the experiment, the largest settlement is observed between the anode and the cathode since the friction between the soil and the test devices limits the settlement at the anode.

Leaching behaviors of cement-based solidification/stabilization treated lead contaminated soils under effects of acid rain

JIANG Ning-jun, DU Yan-jun, LIU Song-yu, LI Chen-yang, LI Wen-tao

The effects of acid rain on leaching behaviors of cement solidified/stabilized lead contaminated soils are investigated. Laboratory tests including batch leaching and column infiltration tests are performed. The results of batching leaching tests show that pH and Ca concentration of equilibrium solution drop significantly when the initial pH of artificial acid rain is 2.5. Pb concentration of equilibrium solution otherwise decreases with the decrease of the initial pH of artificial acid rain. Through the column infiltration tests, it is found that the hydraulic conductivity of soils decreases with the increase of PVF. The decreasing rate in the case of the acid rain is lower than that in the case of the distilled water. pH, Ca and Pb concentration of effluent solution decrease with the increase of PVF. Meanwhile, in-situ acid rain infiltration time is correlated with laboratory acid rain infiltration time.

Stochastic analysis method of critical slip surfaces in soil slopes considering spatial variability

QI Xiao-hui, LI Dian-qing, ZHOU Chuang-bing, PHOON Kok-kwang

The effect of spatial variability in soil parameters on the critical slip surfaces has not been investigated substantially. Therefore, this paper aims to propose a stochastic method to determine the critical slip surfaces in soil slopes considering spatial variability of soil strength properties. First, the spectral representation method is adopted to simulate random field of spatially varying soils. Based on this random field model, the critical slip surface is determined by means of the auto-searching method with SIGMA/W and SLOPE/W. Second, the effect of spatial variability in soil parameters on the characteristics of the critical slip surfaces is investigated using a non-intrusive stochastic analysis method. Finally, an illustrative example is presented to demonstrate the validity of the proposed method. The results indicate that the proposed stochastic method can determine the characteristics of the critical slip surfaces effectively. The spatial variability has a significant influence on the position and scale of the critical slip surfaces. The longer the scale of fluctuation is, the wider the critical slip surface distribution is. The possibility of upper-located local sliding increases with the increasing ratio of horizontal fluctuation scale to vertical fluctuation scale. As the coefficients of variation of soil parameters increase, the range of critical slip surface will increase, and small-scale local sliding is more likely to happen.

Development and tests of large-scale inclined direct shear apparatus

SHI Jian-yong, QIAN Xue-de , ZHU Bao-kun

The shearing status for the landfill liner system located on the slope is different from that located on the bottom floor. A large-diameter inclined direct shear apparatus is designed to simulate liner shearing on various slopes. The comparative tests between the inclined and traditional direct shear apparatuses are conducted using the standard sand and the selected clay. It is found that both the displacement behavior and the shear strength of the standard sand and the selected clay obtained from the inclined and traditional direct shear tests are basically the same. More inclined shear tests are conducted for a composite liner interface between smooth HDPE geomembrane and compacted clay. All curves of the ratio of shear to normal stresses vs. the displacement under various vertical pressures show strain-softening behavior in the inclined shear test. The advantage of the inclined shear test is that more detailed shearing characteristics regarding changes of the normal and shear stresses on the shearing plane and the ratio of the shear to normal stresses can be investigated.

Experiment study on dynamic parameters of artificial polycrystalline ice

ZHU Zhan-yuan, CHEN Shi-jun, LING Xian-zhang, WANG Li-na, LI Qiong-lin

Based on the dynamic tri-axial tests at low temperature, the dynamic constitutive model and its parameters of artificial polycrystalline ice are studied under stepped axial cyclic loading. It is concluded that the dynamic constitutive relation of ice can be fitted by use of the linear viscoelastic model through the experiments under various temperatures and frequencies. Dynamic elastic modulus decreases with the rise of temperature, and increases with the rising frequency. Viscosity coefficient increases with the increasing temperature, and declines with the increase of frequency. Furthermore, the influences of temperature and frequency on the dynamic parameters of ice are interpreted rationally.

Experimental study on performance of GRS bridge abutment with flexible face

XIAO Cheng-zhi, LIU He, WANG Rong-xia, CHEN Pei

Based on the experimental study on performance of geogrid-reinforced soil (GRS) bridge abutment with flexible face under static load, the effects of offsets distance, D, between abutment foundation and panel of retaining wall on the ultimate bearing capacity of GRS bridge abutment, deformation characteristics, strain of geogrids and earth pressure are comprehensively and comparatively analyzed. The test results show that the ultimate bearing capacity of GRS abutment exhibits a remarkable increase tendency with the increase of D/H_L(H_L, height of geogrid-reinforced retaining wall) before D/H_L=0.4 for GRS retaining wall with the length of geogrids supposed to be equal to the height of GRS abutment, and the maximum ultimate bearing capacity can be obtained when D/H_L=0.4, which is followed by a significant decrease while Dis greater than 0.4H_L. Before failure happens to GRS abutment, the settlement of abutment foundation and top surface of GRS behind abutment tends to be linear and the differential settlement reaches the lowest level when D/H_L=0.4, and the ratios of horizontal deformation of panel to the height of lower wall are less than 1%. Moreover, horizontal deformations at top of lower walls are significantly greater than those in the middle and at the bottom of lower walls. Additionally, the maximum values of strains of geogrids occur and keep to be away from panel with the increase of D/H_L, and the strain level of geogrids in the lower wall and upper wall is almost the same as that when D/H_L=0.4. Therefore, the optimum performance of GRS bridge abutment can be obtained simultaneously.

Centrifugal model tests on post-construction settlement of high embankment of Hechi Airport

ZHANG Jun-hui, HUANG Xiang-ning, ZHENG Jian-long, WEI Jian-ming, XU Xian-jin

Taking the high embankment of Hechi Airport as an example锛宼he centrifugal model tests are carried out based on an unequal stress model design plan. The filling stones for tests are made by means of the similarity method of trapezoid distribution. The test results of change and settlement of unit fill height are consistent with the field data. The settlement of the shoulder is larger than that in the center of the embankment surface which presents saddle shape. There are no cracks in the embankment. It is shown that the main problem of the high embankment of Hechi Airport is not landslide or failure, but too large settlement. Finally, the exponential model is selected to predict the final post-construction settlement. Regardless of the post-construction settlement, residual settlement, grade and settlement rate, the results show that the gangue filling embankment will have large settlement and long construction period because of high content of flat-elongated particles and low strength of base materials.

Fundamental period formula for horizontal layered soil profiles

QI Wen-hao, BO Jing-shan, LIU Hong-shuai

The current formula can not calculate the fundamental periods of the sites containing soft soil layers accurately. In order to solve the problem, a fundamental period formula for a horizontal layered site, on which the effect of soil layer construction is considered, is deduced based on the vibration theory. According to the rules of natural vibration periods being equal, a single layer site is simplified into a SDOF system by means of the lumped mass method, and the equivalent height coefficient between their heights is deduced. A horizontal multi-layer site can be simplified into a SDOF system based on any one soil layer, and the height of the SDOF system is equal to that of the soil layer multiplied by the equivalent height coefficient. The natural vibration period of the SDOF system is called contributing fundamental period of the soil layer here, and the fundamental period of the site should be equal to the square root of sum square (SRSS) of the contributing fundamental periods of all the soil layers in the site, and its formula is also deduced. The test results show that (a) the proposed formula can reflect the effect of soil layer construction on fundamental periods of sites very well, and (b) by comparing the calculated fundamental periods with the test ones of 5 engineering sites by use of the microtremor method, the proposed formula is of higher accuracy and reliability than the current formula. A conclusion is drawn that the deduced formula can be adopted to estimate the fundamental period of a layered horizontal site.

Influence of uniaxial tensile strain on filtration characteristics of geotextiles

TANG Lin, TANG Xiao-wu, SHE Wei, GAO Bai-song

Gradient ratio tests are carried out to study the influence of uniaxial tensile strain on the filtration characteristics of geotextiles. Four kinds of geotextiles (two needle-punched nonwoven and two slit-film woven) are employed in the tests. The filtration systems are composed of the geotextiles under designed uniaxial strains and gap-graded soils. The variations of the mean flow rate, the weight of soil passing through the geotextile and the gradient ratio with strains are analyzed based on the filtration criteria. The experimental results illustrate that the permeability and anti-clogging capabilities of woven geotextiles increase with the increase of the uniaxial tensile strain, but the retention capabilities decrease. The influence of tensile strain on nonwoven geotextiles is just the reverse. Moreover, the thicker the geotextiles are, the larger the influence of strain is.

Optimization technology for geogrid-reinforced subgrade widening projects of highways

SHEN Li-sen, YANG Guang-qing, CHENG He-tang, ZHENG Rui-jun, CHEN Jun-chao

The causes, control methods and standards for uncoordinated deformation between new and old subgrades in highway reconstruction and extension projects are analyzed. According to the typical test section of Shijiazhuang to Cixian (demarcation line between Hebei Province and Henan Province) reconstruction and extension project of Beijing-Hongkong- Macao Highway, a finite element model for subgrade widening of highways is established. The additional settlement and horizontal displacement of ground surface as well as transverse slope ratio of pavement are analyzed during the construction period of new subgrade and after construction of 15 years. The geogrid-reinforced technology for the new subgrade is used in this project. The geogrid modulus, reinforced layers and reinforcement length are changed during calculation. The effects of these parameters on surface settlement of the subgrade and transverse slope ratio of pavement are studied. Based on the computational results, the optimization design technology of geogrid-reinforced subgrade widening for highways is presented. The test results can be used as a reference for future studies and design of geogrid-reinforced subgrade in highway reconstruction and extension projects.

Model tests on initial freezing process of column foundation on slope in permafrost regions

SU Kai, ZHANG Jiang-ming, FENG Wen-jie, QU Guang-zhou, ZHANG Hu

The Qinghai-Tibet ±400 kV electrical transmission line traverses more than 550 km in permafrost regions, which may lead to a lot of frozen soil engineering problems for the foundations of transmission tower. Since the column foundation can not only reduce frost heave force but also provide higher bearing capacity, it has been accepted as the first choice of foundation design in permafrost regions. Considering the engineering background of the electrical transmission line, the stability of the column foundation in the initial freezing process is highlighted. In the laboratory, the model tests with a scale of 1:10 are carried out, and both the frost heave and the heave force of the column foundation are monitored. Based on the test results as well as engineering experience, suggestions for the design, construction and maintenance of the project are addressed. The model tests show that the heave force increases with the decreasing temperature in the foundation soil, and the horizontal heave force on the top of the foundation can rise to 130 kPa, while the normal heave force at the bottom can rise to 80 kPa. During the freezing process, the frost heave of the foundation is much greater. The vertical displacement of the foundation can reach 5.5 mm, and the horizontal displacement at the top of the foundation can reach 3.8 mm.

Stress distribution of reinforcement of reinforced soil structures under drawing force

LIU Xu, TANG Xiao-wu, SHEN Hao, GAO Bai-song

The relationship among tension, shear stress and strain of geosynthetics is analyzed by means of pull-out tests. By using the shearing resistance stiffness coefficient G existing at the reinforced soil interface, a pull-out coefficient E_r that relates to the tensile modulus E_rand G, is established, and thus the formulas for stress distribution and the relative displacement along the geosynthetics are derived. The feasibility of the formulas is confirmed by comparing with the existing experimental data. Then the stress transmission way of geosynthetics with the increase of tension and the influence of α on stress distribution of geosynthetics are analyzed. It is shown that α can be better used to reflect the influences of many factors such as stress, friction and soil properties on the tension of geosynthetics. In the structure of reinforced soil, the proposed formulas are better to estimate the tension of geosynthetics under small displacement.