Ye-Shuang Xu

Modelling the cutoff behavior of underground structure in multi-aquifer-aquitard groundwater system

The quaternary deposit of Shanghai is composed of an alternated multi-aquifer-aquitard system (MAAS) consisting of a sequence of aquitards laid over aquifers one by one. In the MAAS, any drawdown of groundwater head in an aquifer may cause consolidation of the overburden aquitard. When underground structures penetrate those aquifers, groundwater seepage path changes and drawdown occurs at the side characterized by the lower hydraulic potential along the flow direction (hereafter refers as to the lower side). This drawdown may cause additional subsidence at the lower side and unbalanced load between the two sides of the underground structure. In order to evaluate the cutoff effect of an underground structure on groundwater seepage in a MAAS representative of the underground of the city of Shanghai, a numerical analysis based on a groundwater flow model has been carried out. The simulated results have shown that underground structures which cut off groundwater flow locally change both magnitude and direction of the flow velocity field. The induced changes in the groundwater field are highly sensitive to the penetration depth and width of the underground structure. Design recommendations for underground structures in aquifers belonging to a MAAS are also presented, which has not yet been considered in the engineering practice of Shanghai.

Evaluation of the hydraulic conductivity of aquifers with piles

Distributed piles are often installed in the foundation pit in sandy soil before dewatering. Hydraulic conductivity should be estimated considering the block effect of piles on groundwater seepage. Research shows that the effective medium theory (EMT) could be used to calculate the equivalent hydraulic conductivity (keq) of a heterogeneous medium with other material inclusions. In order to verify the applicability of EMT in an aquifer with piles, an experimental investigation is conducted. In this experiment, a sandy soil is considered as an aquifer and polyvinylchloride pipes are adopted as piles. Piles are distributed in rectangular and triangular layouts. The relationship between keq of the aquifer and the volume replacement ratio is plotted for these two layouts. The results indicate that EMT could well predict keq for the cases with the rectangular layout; however, for triangular layouts, the prediction becomes poor. To provide a better prediction of keq of the aquifer with piles distributed in a triangular layout, a modified EMT (MEMT) is proposed, in which a correction factor is adopted considering different layouts of piles. The MEMT can provide better keq predictions for an aquifer with piles in the triangular layout pattern than those obtained using the EMT.RésuméDes pieux distribués sont souvent installés dans une fosse de fondation dans des sols sableux avant dénoyage. La conductivité hydraulique doit être estimée en considérant l’effet de bloc des pieux sur le suintement des eaux souterraines. Les travaux de recherche montrent que la théorie du milieu effectif (TME) pourrait être utilisée pour calculer la conductivité hydraulique équivalente (keq) d’un milieu hétérogène comprenant des inclusions d’autres matériaux. Afin de vérifier l’applicabilité de la TME dans un aquifère avec des pieux, une expérimentation a été menée. Dans cette expérience, un sol sableux est considéré comme aquifère et des pieux de polyvinylchloride sont retenus comme pieux. Les pieux sont distribués selon une configuration rectangulaire et triangulaire. La relation entre keq de l’aquifère et le rapport du volume de remplacement est représentée sur un graphique pour les deux configurations. Les résultats indiquent que le TME permet de bien prédire keq pour les cas de configuration disposition rectangulaire alors que pour la configuration triangulaire la prévision devient de moindre qualité. Afin d’obtenir une meilleure prévision de keq de l’aquifère avec des pieux distribués selon une disposition triangulaire, un TME modifié (TMEM) est proposé, dans lequel un facteur de correction est adopté considérant les différentes configurations des pieux. La TMEM peut fournir de meilleures prévisions de keq pour un aquifère avec des pieux selon une configuration triangulaire que pour les valeurs obtenues en utilisant le TME.ResumenEn los pozos de cimientos en suelos arenosos se instalan pilotes distribuidos antes del drenaje, La conductividad hidráulica debe ser estimada considerando el efecto bloque de los pilotes en la filtración del agua subterránea. La investigación muestra que la teoría del medio efectivo (EMT) puede ser usada para calcular la conductividad hidráulica equivalente (keq) de un medio heterogéneo con las inclusiones de otro material. Se realizó una investigación experimental para verificar la aplicabilidad de EMT en un acuífero con pilotes. En este experimento, un suelo arenoso se considera como un acuífero y se adoptan como pilotes caños de polivinilcloruro. Los pilotes están distribuidos en disposición rectangular y triangular. Se graficó la relación entre keq del acuífero y el volumen de reemplazo para estas dos disposiciones. Los resultados indican que EMT puede predecir bien keq para los casos con una disposición rectangular pero para la disposición triangular, la predicción se convierte en pobre. Se propone un EMT (MEMT) modificado para proveer una mejor predicción de keq del acuífero con pilotes distribuidos en una disposición triangular, en el cual se adopta un factor de corrección considerando las diferentes disposiciones de los pilotes. El MEMT puede proveer mejores predicciones de keq para un acuífero con pilotes en el diseño de la disposición triangular que aquellos obtenidos usando EMT.摘 要在进行基坑降水之前分布式桩基础经常会被打入到基坑的砂性土地层中。估算含有桩基础的含水层的渗透系数时需要考虑桩基础对地下水渗流的阻挡作用。发表的研究成果表明,有效介质理论(EMT)可以用来计算含有块状体的不均匀介质的等效渗透系数(keq)。为了验证有效介质理论评估带有桩基础的含水层等效渗透性的有效性,本文应用室内试验进行了研究。试验中将颗粒较均匀的砂土作为含水层试验土体,采用PVC管模拟基桩。桩基的配置分别采用矩形布置与梅花形布置两种形式。试验结果给出了两种桩基布置方案中等效渗透系数(keq)与体积置换率的相互关系。试验结果表明,考虑砂土体积置换率的有效介质理论可以有效地用来分析矩形布置的基桩础含水层的等效渗透系数,但对梅花形布置的基桩础含水层应用有效介质理论计算所得的等效渗透系数的结果有较大的偏差。基于对渗流路径的分析,提出了考虑有梅花形布置的桩基础存在时含水层等效渗透系数计算的修正方法(MEMT),给出了修正系数的计算公式。修正方法可以较好地预测有梅花形布置桩基础含水层的等效渗透系数。ResumoEstacas cravadas distribuídas são frequentemente instaladas nas escavações de fundações, em solos arenosos, antes da extração de água subterrânea. A condutividade hidráulica deve ser estimada considerando o efeito do bloco de estacas no escoamento da água subterrânea. A investigação mostra que a teoria do meio efetivo (EMT) poderia ser usada para calcular a condutividade hidráulica equivalente (keq) de um meio heterogéneo com inclusões de outro material. A fim de verificar a aplicabilidade da EMT num aquífero com estacas cravadas, foi realizada uma investigação experimental. Nesta experiência, um solo arenoso é considerado como um aquífero e tubos de policloreto de vinil são adotados como estacas. As estacas são distribuídas em redes retangulares e triangulares. A relação entre a keq do aquífero e a taxa de substituição do volume é representada para esses duas tipologias. Os resultados indicam que EMT poderia prever bem a keq para os casos com o formato retangular, mas, para esquemas triangulares, a previsão torna-se pouco fiável. Para proporcionar uma melhor previsão da keq do aquífero com estacas distribuídas num esquema triangular, é proposto um EMT modificado (MEMT), em que é adotado um fator de correção considerando diferentes esquemas de estacas. O MEMT pode fornecer melhores previsões de keq, para um aquífero com estacas no padrão triangular, do que as obtidas com o EMT.

Estimation of Land Subsidence Based on Groundwater Flow Model

This article presents an approach for estimating land subsidence due to withdrawal of groundwater. The proposed method calculates the groundwater seepage in 3-D-condition and calculates the land subsidence one-dimensionally. The governing equation on groundwater seepage is based on the three-dimensional mass conservation law and the principle of effective stress. The land subsidence calculation method is derived based on the following assumptions: (1) displacements occur only in the vertical direction, and (2) in vertical direction the total stresses do not change. The governing equation is solved by numerical method, i.e., finite element method (FEM) in spatial discretization and finite difference method (FDM) in time series discretization. In FEM Galerkin method is adopted and in FDM, lumped matrix method is employed. The proposed method is calibrated via analyzing 1-D consolidation problem and the results are compared with those from Terzaghis one-dimensional consolidation theory and oedometer test. The proposed method is employed to analyze the consolidation of a soft layer due to withdrawal of groundwater from an aquifer under it. Moreover, this method is also applied to a field case of land subsidence due to groundwater pumping in a gas production field in Japan. The analytical results are compared with the field observed data. The results show that this approach simulates the field case well.

Swelling characteristics of fractal-textured bentonite and its mixtures

Swelling deformation and swelling pressure tests of Tsukinuno bentonite and its mixtures with Toyoura sand were performed on a modified odometer. The swelling strain and swelling pressure of bentonite and its mixtures are dependent on the dry density, the increment of water content and the surface fractality of bentonite particle aggregates in voids. The water volume absorbed by bentonite is related to the surface fractal dimension (Ds) of bentonite. The correlation of the water volume to vertical overburden pressure (p) is obtained as Vw/Vm=Kp Ds� 3 for Tsukinuno bentonite with fractal-textured surface. The maximum swelling strain is predicted according to the correlation of the water volume to vertical overburden pressure. The predictions of the maximum swelling strain are in satisfactory agreement with the experimental data. Furthermore, the good relationship between swelling strain and elapsed time is also proposed in this paper. D 2003 Elsevier Science B.V. All rights reserved.

Collapse Behavior of Compacted Clays in Suction-Controlled Triaxial Tests

Using a suction-controlled triaxial apparatus for unsaturated soils, a series of tests on a compacted clay was performed to investigate the influences of stress states, water content, void ratio, and matric suction on the collapse behavior. The triaxial tests were carried out under the conditions of (1) different stress ratios and mean stresses, (2) triaxial compression and extension, (3) different initial void ratios of specimens with the same water content, (4) different initial water contents with the same degree of compaction, and (5) different controlled matric suctions. The main conclusions were obtained as follows: (1) The volume change induced by the collapse mainly depends on the initial void ratio and mean net stress under which the collapse occurs, irrespective of imposed matric suction; (2) the amount of collapse is small at both low and high confining stresses, and there exists a maximum value of collapse at a particular mean stress; (3) the shear strain increment induced by collapse depends on stress ratio, triaxial compression, or extension stress; (4) when imposed suction is decreased, large collapse deformation takes place in the samples compacted dry-of-optimum, while little collapse deformation takes place in the samples compacted wet-of-optimum; and (5) the collapse behavior can be explained by the elastoplastic theory for unsaturated soils.

Investigation into subsidence hazards due to groundwater pumping from Aquifer II in Changzhou, China

This paper presents an investigation into increased deformation of Aquifer II caused by groundwater pumping from the aquifer in Changzhou, China. As groundwater levels of aquifers have been decreasing in recent decades due to uncontrolled water pumping, land subsidence is becoming a serious geohazard in Changzhou. Based on recently reported field data, the compression of aquitards has not increased compared to that of aquifers with the same scale of layer thickness. The Cosserat continuum model was adopted to analyse the observed phenomenon in this study. A classic Cauchy continuum model is also used for comparison. The comparison between these two models indicates that the proposed approach can interpret the increased deformation well, and the classic Cauchy continuum model underestimates the aquifer deformation as it does not consider shear displacement and macro-rotation. A discussion on the relationship between the groundwater level in the aquifer and subsidence is then undertaken. The results show that the severity of the annual subsidence is correlated with the variation in groundwater level in Aquifer II. To mitigate the subsidence hazards, countermeasures should be adopted to avoid the shear stress in aquifers which results from the high hydraulic gradient, by the appropriate allocation of pumping wells and by restricting groundwater withdrawal volume from each pumping operation.

Field Evaluation on the Strength Increase of Marine Clay under Staged Construction of Embankment

This article presents a case history of an embankment constructed on the soft Ariake clay on the bank of Rokkaku River, Saga, Japan. The staged method was used in the construction of embankment. To shorten the time of construction, the soft subsoil was treated by a prefabricated vertical drain (PVD). The physical and mechanical properties of soft subsoil, such as water content, unit weight, and unconfined compressive strength, were investigated before and during embankment construction. Based on the field data, the strength increase of Ariake clay during construction was revealed. A nonlinear relationship between strength increment and the degree of consolidation was found. Finite element method (FEM) was employed to investigate the strength increase behavior. The results showed that it is difficult to predict the filed behavior of strength increase using Cam-clay model. Based on the concept of structure loss of structured clay, the strength variation of Ariake clay during consolidation is discussed.

An Approach to Evaluate Hydraulic Conductivity of Soil Based on CPTU Test

This article presents a new approach to estimate hydraulic conductivity of soil from cone penetration test with pore water pressure measurement (referred to as CPTU hereafter). The proposed approach is based on the test result of the spherical cavity expansion of the soil at the tip of a pile. During the piezocone penetration, the flow shape of pore water around the tip of the cone is assumed to be a spherical crown and induced excess pore water is assumed to dissipate from the crown surface. Based on this assumption, a bi-linear relation between the piezocone sounding metric (which is the product of the pore water pressure ratio Bq and the tip resistance Qt) and the hydraulic conductivity index KD is derived to estimate the hydraulic conductivity of the soil layer. The derived approach expands the applicable range of existing approaches in the literature. It is demonstrated that the proposed approach can cover the entire tip angles of the cone and the modified equation can fit the CPTU test data well.

Evaluation of Effective Depth of PVD Improvement in Soft Clay Deposit: A Field Case Study

This article presents a case history of determination of effective depth of prefabricated vertical drains (PVDs) under embankment loading on a very soft clay deposit in central China, near Jiujiang, Jiangxi Province. The height of the embankment was 5.3 m and construction time was about one year. The PVDs were installed to a depth of 8.5 m at a spacing of 1.5 m in a triangular pattern. Field observations and the finite element method (FEM) were employed to analyze the performance of the soft deposit during embankment construction. The influential depth of the embankment loading was evaluated based on settlement, excess pore pressure, and stress increase in subsoil, both from the observed data and FEM analysis. The effective PVD depth was determined in the following ways: (1) the depth of 5% subsoil settlement of surface settlement; (2) vertical stress increase in subsoil of 25% in-situ stress; and (3) consolidation time/PVD depth relation by FEM. Based on the analysis, the effective depth of PVDs was determined to be between 10 and 12.8 m for this field case.

Environmental impacts caused by phosphate mining and ecological restoration: a case history in Kunming, China

Abstract This paper presents the environmental impacts caused by surface mining and the ecological rehabilitation of Haikou phosphate deposits, Kunming, China. Surface mining entails the removal of the overburden to expose bare rock surfaces, not only causing destruction of pre-existing vegetation, but also occupying large areas of land for dumping the spoil. Severe environmental impacts are caused, e.g. rock desertification, poor forest stand structure, loss of biodiversity, aesthetic depreciation of the landscape, and the potential hazard of landslide and ground erosion. Ecological restoration has been conducted in a demonstration area since 1989 by means of control of geological hazards and revegetation of the disturbed areas. On-site dumping of waste rock for restoration of the abandoned mine area was adopted, providing a new mode of mining reclamation with high economic, environmental, and social value. Countermeasures for prevention and control of landslides included cut and fill technology, drainage, safety netting protection, retaining wall construction, and vegetation cover. By recruiting native plant species to the mine site and planting a forest imitating a natural mixed forest, the mine area was successfully revegetated.