Huai-Na Wu

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.

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.

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.

Evaluation of the uplift behavior of plate anchor in structured marine clay

ABSTRACT The uplift behavior of a plate anchor in a structured clay (soft Ariake clay) is investigated through a series of laboratory tests and method of finite element analysis. The tests are adopted to identify the factors influencing the behavior of the anchor, including the thixotropic nature of Ariake clay, consolidation time, and embedment ratio of the anchor. A finite element method (FEM) is used to analyze and predict the uplift behavior of the anchor plate well in the elastic region and the yield load. The results from both the laboratory tests and the FEM analysis suggest that the embedment ratio for a deep anchor in Ariake clay is close to 4. Further increase in embedment ratio improves the capacity to a lesser extent. FEM overestimates the failure load of the uplift anchor in soft Ariake clay by about 20%. This may be ascribed to the hypothesis in the FEM analysis that there is continuous contact between the clay and the anchor until failure. Vesic’s theory for deep anchors, which may be used to predict the ultimate pullout resistance of the plate anchor in reconstituted Ariake clay, is verified to be applicable. In this paper, the plastic flow zone around the anchor is discussed using FEM which makes the behavior of anchor more understandable during the design stage.

Geological characteristics of strata in Chongqing, China, and mitigation of the environmental impacts of tunneling-induced geo-hazards

This study investigated the geological characteristics of strata in Chongqing and the tunneling-induced geo-hazards therein. The urban area of Chongqing contains six syncline–anticline pairs and is bounded by the Huayingshan basement fault to the west and by the Changshou-Zunyi basement fault to the east. The bedrock, containing sandstone and mudstone of the Upper Shaximiao formation resulting from Middle Triassic strata, is occasionally outcropped. During urbanization, many valleys in Chongqing require to be backfilled to create more flat ground. Thus, the extent and thickness of the loose backfill layer varied from place to place. While tunneling in Chongqing, the following geological characteristics may cause geo-hazards: (1) backfill with a loose structure, poor gradation, high permeability, and large void ratio, (2) highly, or moderately weathered mudstone and sandstone that can easily become softened once subjected to groundwater seepage, and (3) syncline folding containing perched water or confined fissure water. The potential geo-hazards include groundwater ingress, ground surface settlement, and strata collapse. While tunneling within backfill synclines, sandstone synclines, or weak fractured rock, significant amount of groundwater may be encountered, thus leading to a high risk of groundwater ingress. Ground surface settlement can be triggered due to the disturbances of tunnel construction to the backfill or consolidation effect after groundwater ingress. Strata collapse can be caused by the blasting-induced disturbances to poor surrounding geology or by inadequate overburden thickness or by an increase in the effective stress resulting from groundwater loss caused by leakage. To mitigate the environmental impacts of the tunneling-induced geo-hazards, a series of countermeasures; that is, advance geological drilling, advance ground improvement including full-face grouting, and grouting of the surrounding geologic media, and advance pipe umbrella use, are proposed in this paper. The results from a case study indicate that the proposed countermeasures can greatly decrease the ground surface settlement and amount of groundwater ingress during tunneling in Chongqing verifying the effectiveness of the proposed countermeasures.

Geotechnical characteristics of weathered granitic gneiss with geo-hazards investigation of pit excavation in Guangzhou, China

This paper presents a comprehensive investigation into weathered granitic gneiss in Guangzhou, China, including its formation, grading, classification, and the geotechnical characteristics of different weathered states. The granitic weathering profiles are graded into five weathered states: fresh rock, slightly weathered, moderately weathered, highly weathered, and completely weathered. The fresh rock and the slightly weathered rock have good mechanical behaviour. The highly weathered granitic gneiss was found to possess a relatively higher permeability than the other weathered states, in which the fissure water is usually stored. Geotechnical characteristics of the weathered granitic gneiss that poses geohazards during excavation are identified, including: (1) uneven grain size distribution, (2) water softening of the residual soils, and 3) spherical weathered boulder. The following potential geohazards are identified: (1) quicksand and piping, (2) water–ground settlement or even collapse due to water softening, (3) trenching difficulties in a diaphragm wall, and (4) instability of the lattice columns. Countermeasures for preventing these geo-hazards are proposed, including (1) rate-controlled drilling of dewatering wells, (2) grouting in both sides of retaining walls, and (3) deep hole blasting of large-size residual gravel boulders. These measures has been demonstrated to be useful in construction of Zhongxin Station.

Factors Influencing the Longitudinal Deformation of a Metro Tunnel in Soft Deposit: An Overview

The Quaternary deposits in coastal regions are multi-aquifer-aquitard system with characteristics of high compressibility, high sensitivity, and low shear strength. The metro tunnels have been widely constructed in coastal regions in recent years. The tunnel settles continuously since the metro tunnel operated. This paper gives a general introduction to influencing factors on the longitudinal deformation of metro tunnel in soft deposit in coastal regions of China: (i) regional land subsidence; (ii) shield tunnel construction; (iii) construction of nearby engineering, such as pit excavation, construction of pile, construction of bypass tunnels, etc; (iv) groundwater leakage; (v) dynamic loading of trains. Influence of land settlement, groundwater leakage and train loading is long-term and influence of tunnel construction and disturbance from the adjacent construction is short-term. Since all of these factors interact with each other, the longitudinal deformation of the tunnel is caused by the comprehensive action of the aforementioned factors.