Investigation into performance of deep excavation in sand covered karst: A case report

Performing deep excavations in karst regions is likely lead to geo-hazards, e.g. sinkholes, water ingress, and ground surface settlement. This case report presents the performance of a 16-m deep excavation in sand covered karst in Metro Line no. 9, Guangzhou, China. The performances during deep excavation, including lateral wall deflection, vertical wall movement, ground surface settlement, and settlement of adjoining buildings, were monitored and evaluated. Based upon the interpretation of the field measurements, the following major findings were obtained: (i) the lateral wall deflection and surface settlement measurements were smaller those cited in other case studies with similar geological conditions, (ii) both the grouting of the karst cavern and the use of the socket diaphragm wall panel dominated the performance of the deep excavation; (iii) the type of the founding structure could significantly affect the settlement development of the adjoining buildings. This case report provides insight into the design and construction of deep, narrow excavations in sand covered karst in a metropolitan environment.     

Analysis of shearing effect on tunnel induced by load transfer along longitudinal direction

In order to analyze the behavior of load transfer mechanism of shield-constructed tunnel in longitudinal direction, tunnel is modeled as the cylindrical shell within elastic foundation (CSEF). By applying the theory of elastic cylindrical shell (ECS) with considering shear deformation and assumed displacement functions of trigonometric series, the distribution of stress and deformation in tunnel lining is obtained. In the solution, the stiffness of tunnel lining is decomposed into two components of circumferential and radial stiffness. The effects of both components on the behavior of deformation and internal forces of tunnel lining are discussed in details. By using the proposed solution, more reasonable results on the behavior of tunnel lining are obtained, e.g bending moment in tunnel cross section becomes smaller with the increase of the circumferential shear stiffness. The analytical results are verified by the results of 3D FEM analysis and field measured data. In accordance with the proposed analytical method, the tunnel lining in soft ground should be designed via considering the following aspects: (i) three dimensional effect of tunnel lining; (ii) relatively weaker shear stiffness in radial direction, and (iii) increase the circumferential shear resistance between rings.     

Dynamic brittle fracture with eigenerosion enhanced material point method

This article proposes an approach to resolve the dynamic fracture of brittle materials by incorporating eigenerosion into the material point method (MPM) framework. The eigenerosion approach links the crack propagation to energy conservation based on the variational theory of fracture mechanics. This idea closely resembles the conventional treatment for the phase-field method. The major difference is that the effective energy release rate of each particle that controls the crack propagation is only calculated within its neighborhood domain for the eigenerosion approach. Because evaluation of the material's fracture behavior can be decoupled from the governing equations as a separate solution step, the eigenerosion scheme allows straightforward implementation into any standard MPM solver with minor modifications. In addition, a phantom-node method is employed to handle the preexisting crack. With these settings, the proposed model can capture complex fracture behaviors. Several representative benchmark tests demonstrate the efficiency and validity of the proposed model.     

影响ZTA陶瓷微波烧结的主要工艺过程

通过对ＺＴＡ陶瓷进行微波烧结试验,了解影响陶瓷微波烧结速率的主要工艺过程,探索有关微波烧结机理;采用以ＴＥ４４４为基准模式的微波谐振腔,在混合加热模式的基础上增设辅助加热体,实现了ＺＴＡ陶瓷微波烧结; ＺＴＡ材料中ＺｒＯ_２含量越高,该材料的烧结速率越快;输入功率的提高有助于提高烧结速率;辅助加热体的老化现象降低微波烧结速率;微波烧结过程中应避免出现热剧变现象．     

Geological difficulties and countermeasures for socket diaphragm walls in weathered granite in Shenzhen,China

In Shenzhen, a complex series of folds and fractures, and intensive metamorphism took place during the pre-Caledonian, Caledonian, and Hercynian to Indosinian orogenies. Then, Yanshanian orogeny, with the most tectonic movement, occurred. Strong invasive activity of granitic magma, a massive blowout of acidic volcanic rock, and magmatism-related mineralisation are the features of Yanshanian orogeny. Moreover, deep faults were also developed and wide folds were formed. A large amount of granite rock was formed from the Yanshanian orogeny in Shenzhen. The outcrop area of the granite in Shenzhen reaches 760 km², almost covers 50 % of Shenzhen land area. Granite is primarily susceptible to chemical weathering. Though weathered, granite and solitary granite boulders are of high uniaxial compressive strength (UCS). The weathered granite may be covered by Quaternary soil with low shear strength, such as backfill, sand, silty clay, and clay. The thickness of the Quaternary soil ranges from 0 to 40 m. Socket diaphragm walls are usually adopted during deep excavation construction in Shenzhen for their safety and low impact on the surrounding environment. When the excavation depth is increased, the socket diaphragm walls are embedded in weathered granite to a depth of between 3 and 10 m. However, the traditional socket diaphragm wall construction is only viable in soft ground, as it is not practical to penetrate hard weathered granite. As a consequence, difficulties arise during construction in the Shenzhen region, such as difficulty in breaking down the rock, socket diaphragm wall collapse and leakage. To solve these construction problems, countermeasures, such as monitoring, heavy hammer dropping, smooth blasting and a modified slurry, are proposed in this study. It can be concluded that the heavy hammer dropping method should be chosen to deal with weathered granite with a UCS of less than 50 MPa and the smooth blasting method should be adopted to deal with weathered granite with a UCS of more than 50 MPa. A case study of a metro station excavation using the smooth blasting method in Shenzhen is also introduced. The successful construction of the socket diaphragm walls demonstrates the applicability of the smooth blasting method.     

Deep Mixing Induced Property Changes in Surrounding Sensitive Marine Clays

This paper presents a field study of installation effects of deep mixed columns on properties of the sensitive Ariake marine clay. Cone penetration tests were performed in the field to evaluate the change in the strength of the surrounding clay with time. Soil samples were taken before and after column installation to evaluate variations of physical, mechanical, and chemical properties of the surrounding clay. Test results indicated that the water content of the surrounding clay decreased while the concentration of cations increased as sampling locations approached the columns. Shear strength of the surrounding clay decreased during the installation but recovered after a short period of curing. Shear strength continued to increase with time over a period of 70?days. Based on the regression results, the surrounding soil after the installation of the columns took approximately 10?days to recover to the strength value before installation. On average, the shear strength of the surrounding clay increased over the original strength by approximately 23% after 40?days and 50% after 70?days, respectively. Discussion is presented on strength changes and key influence factors including soil disturbance and fracturing, thixotropy, consolidation, and diffusion of cations from deep mixed columns to the surrounding clay.     

Mechanical,flame-retarding,and creep-recovery proprieties of ethylene–propylene–diene monomer enhanced with nano-hydroxide for undersea tunnel sealing gasket

This study investigates the mechanical, flame-retarding, and creep-recovery proprieties of ethylene–propylene–diene monomer (EPDM) composites. The EPDM composites are filled with different contents of nano-Mg(OH)₂ (NMH) or nano-Al(OH)₃ (NATH). To examine the effect of contents of NMH (or NATH) and seawater immersion, mechanical tests are conducted and the limiting oxygen index (LOI) method was applied. Creep-recovery tests are then conducted to obtain the creep resistance property of EPDM composites with contents of NMH (or NATH). The microstructure of EPDM composites is investigated by using scanning electron microscopy (SEM). The results indicate that NMH (or NATH) can enhance the flame-retarding, creep resistance, and hardness properties, whereas tensile strength and elongation at break increase at first and then decrease with increasing content of NMH (or NATH). The Findley power law and the Weibull distribution function show good correspondence with the results of creep-recovery tests for EPDM composites. Furthermore, SEM results show good interfaces between nanoparticles and the matrix implying the mechanism of the above results.     

Simple Method of Modeling PVD-Improved Subsoil

On a macroscale, the effect of installing prefabricated vertical drains (PVDs) in a subsoil is to increase the mass hydraulic conductivity of the subsoil in the vertical direction. Based on this concept, a simple method for modeling PVD improved subsoils is proposed, in which an equivalent vertical hydraulic conductivity kve for the PVD improved subsoil is explicitly derived. With the proposed simple method, analysis of PVD improved subsoil is the same as that of the unimproved case. The theoretical verification of the simple method was made under 1D condition. The calculated average degree of consolidation and excess pore pressure distribution in the vertical direction using the simple method are compared with existing theoretical solutions (combination of Terzaghi's consolidation theory and Hansbo's solution for PVD consolidation). It has been proved theoretically that, in terms of average degree of consolidation, in the case of one layer and ignoring the vertical drainage of natural subsoil, the maximum error of the proposed method is 10% compared with Hansbo's solution. For the case of one layer or multilayers and considering both vertical and radial drainages with the parameters adopted here, the maximum error of the proposed method is 5%. The multilayer case was analyzed by FEM method, and the proposed simple method is compared with that of using 1D drainage elements. Then, 2D finite-element analyses were conducted for three case histories of embankments on PVD improved subsoils. One case is discussed in detail. The analyses using both the simple method and 1D drainage elements, were conducted. It is shown that for all three cases, the simple method yielded results as good as those using 1D drainage elements.     

Rapid field evaluation of the strength of cement-stabilized clayey soil

Bulletin of Engineering Geology and the Environment - This paper presents an investigation into the hardness and unconfined compressive strength (UCS) of cement-stabilized Shanghai clay. Various...