A series of tests with models of the dam (reinforced and unreinforced) to compare the stability of structures with horizontal and vertical deformations. The results of the experiment are shown in the form of photos and graphics. The objects of study in this article are the deformation model of the dam, located on the subgrade. Purpose of this research a comparison of admissible deformations models earth dams, reinforced and non-reinforced, with the partial collapse of the base and at the same time the horizontal and vertical strains of dam.

View full abstract

The basal heave stability of braced excavations in clay soils is one of the key considerations in both design and during construction. The finite element method with shear strength reduction (SSR) is increasingly being used for estimating the safety factor for geotechnical engineering problems. In this paper, results of basal heave stability for deep narrow braced excavations in terms of factor of safety (FS) and stability number (N_s) using two-dimensional finite element method with SSR are presented. Total stress finite element simulations are carried out to parametrically analyse the effects of system stiffness, undrained shear strength and excavation geometry on the base stability of braced excavations in different clay soils. For the cases considered, the study shows that the system stiffness has minimal influence on the factor of safety against basal heave. The stability number N_s calculated from finite element analysis is independent of the undrained shear strength of clays. Also, the N_s value decreases almost linearly as the ratio of the depth to the width of the excavation increases. The failure zone from finite element analysis is more extensive than that proposed by Terzaghi. Comparison of the factor of safety between the calculations from conventional methods and from finite element analysis indicate that the modified Terzaghi’s method that considers the wall embedment depth are in close agreement with the results based on the plane strain finite element analyses.

View full abstract

Two failure deep excavations in soft clay were investigated in this study using the finite element method (FEM) with reduced shear strength. A comprehensive model of support system was built as regard the existence of wall, struts, and center posts. In order to determine the causes of collapse of the excavations, both elastic and elastoplastic behaviors of support system were analyzed. Result shows that failure of the excavations starts from large soil heave at the final excavation grade as considering an elastic behavior of support system and from yielding of support system if its behavior is elastoplastic. A failure mechanism is proposed for deep excavations in soft clay. Due to an upward movement of center posts, secondary bending moment on struts will reduce their capacity, originally designed for bearing axial load. Yielding of struts is followed by yielding of wall and then failure of excavation. Factors of safety estimated by FEM with modeling elastoplastic support system are also in good agreement with stability of the excavations observed in the field.

View full abstract

With the fast increasing of urban underground space developments in China, a new kind of underground construction, deep excavation group, has been developed, which means a group of adjacent / related excavations constructed at same time or the supper large excavation divided into small pits. In this paper, the concept and characters of the deep excavation groups are introduced with cases in Shanghai. The excavation groups are divided into three kinds according to the special relation between different parts, which are divided excavation group, jointed excavation group, and adjacent excavation group. Some key scientific issues for further studies about excavation groups are also presented and discussed, i.e., soil behavior and constitutive model, earth pressure under limited soil zone, performance of dividing walls, coupling and interactive behavior of excavation group, environment response and control of excavation group, and huge scale computing method.

View full abstract

Deep mixing and jet grouting with cement are widely used to improve soft clayey soils as part of stability intervention in deep excavation and tunnelling. The quality of the improvement is often evaluated by measuring the strength of cored samples after the installation of deep mixing or jet grout columns. Although the spatial variability in the strength of cement-treated soil in such operations has been reported, this variability, as well as its influence on design and construction control, has still not been well-characterized. Using data from two field cases, this paper examines two issues associated with strength measurement and its usage in design. The two field cases are the ground treatment works for Marine Bay Financial Centre and the Marina One projects, which lie in the same locality, overlying thick layers of soft marine clay. The first issue examined is the time interval between ground treatment and core testing. The effect of this parameter on the measured strength and its implications on ground treatment construction control are discussed. The second issue relates to the specification of design strength, which takes into account the strength variability of the ground. The robustness of several criteria with respect to sample sizes is examined using the data from these sites, leading to recommendations for design strength and control measures.

View full abstract

The inevitable influence of large excavation in soft soils on adjacent tunnel is greatly concerned in practice. A case in Ningbo Metro Line 1 in China is presented in this paper. The constructed metro line of which axes were at a depth of 12 m to 18 m was almost parallel to the excavation. The minimum horizontal distance between the metro line and the retaining structures was 7.2 m. Field montoring indicated that the left tunnel suffered considerable deformation during the 3rd excavation step of the adjacent excavation, and the maximum increment of horizontal displacement of left tunnel reached 33.5 mm. From the completion of left tunnel to the completion of the excavation, the maximum increments of settlement and convergence of left tunnel reached 33.6 mm and 28 mm, respectively. As a result, these deformations caused the appearance of visible cracks and leakages of linings and threatened left tunnel severely. Numerical simulation was conducted with Plaxis 3D. The performance of surrounding soils and structures due to the nearby excavation, especially of left tunnel, are investigated. It is found that displacement growth of left tunnel and surrounding soils caused by adjacent excavation has intense correlation. In addition, the effects of divided excavation on alleviating the displacements and internal forces of left tunnel during the 3rd excavation step are also discussed. The above problems and findings should be paid more attention to to ensure the safety of the nearby twin tunnels.

View full abstract

3D FEM has been commonly used for analyzing complicated soil-structure interaction problems thanks to the fast development of computer technology and geotechnical softwares. The 2D analysis is favorable to be performed in the conceptual design. However in many cases the 2D analysis can hardly simulate a real 3D situation. In this paper such a case is presented, a vertical shaft constructed in a slope beside water. The main purpose of the analysis is to clarify if the soil movements due to the shaft construction may cause damages to the adjacent existing structures. The soil displacements obtained from the 2D analysis are extremely large and tieback anchors must be needed to keep the shaft stable. Due to 3D effects the displacements become much smaller and more realistic.

View full abstract