Shui-Long Shen

Generalized Approach for Prediction of Jet Grout Column Diameter

AbstractThis paper presents a generalized approach for predicting the diameter of jet grout columns based on the theoretical framework of turbulent kinematic flow and soil erosion. The proposed calculation method is applicable to all conventional jet-grouting systems and takes into account the full range of operational parameters, fluid properties, soil strength, and particle size distribution, including the effect of the injection time on erosion distance. It was demonstrated that the increase in the jet grout column diameter arising from the use of a compressed air shroud in the double and triple fluid systems is approximately 27–81% for the typical range of air pressure of 0.5–1.5 MPa. The proposed method was applied to four case histories involving four variants of jet-grouting systems, i.e., single fluid, double fluid, triple fluid, and an enhanced triple fluid system. Comparison between the calculated and the measured jet grout column diameters indicated that the proposed method can produce reasonab...

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 Strength Increase of Marine Clay under Staged Embankment Loading: A Case Study

This article presents a case history of the performance of a full-scale test embankment constructed on a marine soft clay deposit improved by prefabricated vertical drains (PVDs) in the east of China. For analyzing the subsoil behavior, a 2D FEM model is established, in which the PVD-improved effect is considered by a simplified method of equivalent vertical hydraulic conductivity. The calculated results can predict the settlement behavior well; however, the FEM gives an underestimate for the value of excess pore pressures and it predicts similar values for the dissipation rate of excess pore pressures. The measured undrained shear strength of subsoil, Cu, is compared with the predicted value based on Ladd’s empirical equation and the Modified Cam-Clay model (MCC). The shear strength predicted by Ladd’s equation agrees well with the measured value, whereas the MCC overestimates the ability to improve subsoil shear strength during consolidation. The undrained shear strength of subsoil, Cu, increased as the construction progressed, and the shear strength incremental ratio, ΔCu/Δp′, decreased slightly with the degree of consolidation, U.

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.

Evaluation of allowable withdrawn volume of groundwater based on observed data

To control land subsidence due to groundwater withdrawal, it is important to estimate allowable withdrawn volume of groundwater in a soft deposit. This technical note presents a simple approach for estimating the allowable withdrawn volume of a deposit. A regression analysis method was used based on measured land subsidence and recorded net withdrawn volume. This approach was proposed based on the principle of soil compression at different effective stresses, i.e. the soil compression is small when the consolidation stress is lower than the yield stress of the deposit, but large when the consolidation stress is higher than the yield stress. Two case studies are presented in this technical paper to demonstrate how to use the simple approach to estimate the allowable withdrawn volume.

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.

Laboratory Studies on Property Changes in Surrounding Clays Due to Installation of Deep Mixing Columns

This paper has identified six major factors causing property changes in surrounding soils during and after installation of deep mixing columns: soil thixotropy, soil fracturing, cement penetration and diffusion, cementation, consolidation, and heating. Laboratory tests were performed to investigate the effects of soil thixotropy, soil fracturing, and cementation in a soft marine clay, Ariake clay. Laboratory tests were conducted to evaluate property changes in surrounding clays due to installation of deep mixing columns. Test results showed that an influential zone of property changes existed in surrounding clay ranging from the edge of the columns to the distance of about twice the radius of the columns. Within this influential zone, water content decreased as samples neared the columns, while pH values and electric conductivity increased. Test results also showed that undrained shear strengths of the surrounding clays decreased during mixing but regained after a 7-day curing period and continued increasing during 28 days in this study.

Tornado hazards on June 23 in Jiangsu Province, China: preliminary investigation and analysis

This paper reported a tornado hazard happened on June 23, 2016, in Yancheng city, Jiangsu Province. The moving footprint of this huge tornado was from west to east. Shuoji, Chenliang, Goudun, Banhu, Xingou, Wutan towns in Funing district and Sheyang town in Sheyang district were severely hit by this tornado. This tornado along with rainstorm and hailstorm had claimed 99 lives and caused more than 3800 flats to collapse as well as damaged 48 high-voltage circuits. As the cold air from northwest met the subtropical high pressure system that forms over relatively cool water bodies (i.e., Indian and Pacific Oceans), such a powerful meteorological phenomenon was initiated. The strong connective airflow intensified the development of this tornado. Based on the preliminary investigation and analysis of this tornado, cost-effective timber structures with adequate anchorage of the framing to foundations and adequate connection between walls and roofs may be recommended to ensure occupants safety and reduce potential damage in these extreme wind events. Additionally, it is suggested to utilize early warning system along with geographical information system (GIS), Global Positioning System (GPS), and remote sensing (RS) (3S) to monitor and precast the occurrence of rainfall, hailstorm, and tornado hazards in future.

Identification of Tunnel Settlement Caused by Land Subsidence in Soft Deposit of Shanghai

AbstractThis paper provides a straightforward way to evaluate tunnel settlement caused by land subsidence in the soft deposits of Shanghai. By analyzing field measurements of tunnel settlement and ...