http://manuscript.elsevier.com/S0266352X21005942/pdf/S0266352X21005942.pdf

Further development of distinct lattice spring model for stability and collapse analysis of deep foundation pit excavation

The dewatering of foundation pits with a suspended waterproof curtain causes different groundwater drawdowns inside and outside the pit, resulting in the drawdown difference between the inside and outside the pit. Maintaining a groundwater drawdown difference between the inside and outside of a foundation pit can eliminate the adverse effects of dewatering on the surrounding environment. According to previous studies on unsteady flow, an analytical solution of the groundwater drawdown with a suspended waterproof curtain under unsteady flow has been proposed. The analytical solution of unsteady flow and the formula of groundwater drawdown difference with a suspended waterproof curtain were validated by comparing pumping tests and finite-element method (FEM), in which a good agreement was observed. The magnitude of the drawdown difference generally represents the extent of surrounding groundwater affected by groundwater drawdown inside the pit. This paper also investigated the effects of sensitivity parameters on the drawdown difference for minimizing the effect of surrounding environment. During the process of dewatering with a suspended waterproof curtain, the groundwater drawdown (Sh) should not exceed the length of the waterproof curtain (L), and the optimal radius of foundation pit (Rw) and length of waterproof curtain (L) were found, i.e., Rw/H0 = 0.781 and L/H0 = 0.813 (H0 is 32 m). Beyond these values, the drawdown difference tends to be stable. The drawdown difference is also significantly affected by the dewatering time. When t < 48 h, the groundwater drawdown difference decreases rapidly; when t > 48 h, the groundwater drawdown difference stabilizes.

/pdf/analytical-analysis-of-the-groundwater-drawdown-difference-14zm281d.pdf

Analytical Analysis of the Groundwater Drawdown Difference Induced by Foundation Pit Dewatering with a Suspended Waterproof Curtain

Hydro-mechanical analysis of a braced foundation pit affected by rainfall and excavation in unsaturated soils

Seepage in rock fractures is influenced by the geometry of the contact area. A parallel model with two circular cylinder contact areas is studied by numerical simulation. The variation of the seepage field in the fracture model is analyzed by changing the size (radius r) and position (Mutual distance s and Upstream angle θ) of the two contact areas. Changing r, s, and θ is essentially changing the width and angle of the flow channel on both sides of the contact area. The contact area compresses the flow channel so that there is an obvious pressure drop around the contact area, and the reduced pressure potential energy is mainly converted into the kinetic energy of flow around the contact area and the water flow obtains a larger velocity. The resulting inertial effect controls the generation and development of the low-velocity region and eddy behind the contact area. θ is about 30{degree sign}, which is the critical point of eddy current generation behind the contact area, the changing trend of -∇ P and s, -∇ P and sin θ. Two conceptual models are proposed that there is a quadratic function relationship between -∇ P and s, and an exponential function relationship between -∇ P and sin θ. The parameter N is proposed to represent the joint action of two kinds of the flow channel (contact area and contact area, contact area and lateral), and the relationship between -∇ P and N is a quadratic function.

Study on the influence of geometric characteristics of contact area on fracture seepage

This paper investigates the performance of a top-down deep excavation in soil-rock composite stratum. The behavior of the excavation bracing system, consisting of ground anchors and end-suspended piles, has not been well understood due to the lack of relevant research. Based on the observed data of a typical deep excavation case history for the May Fourth Square Station in Tsingtao, China, the characteristics of the horizontal and vertical pile displacements, ground surface settlements, building settlements, axial forces in ground anchors, earth pressure, and pore water pressure during excavation were analysed. Two-dimensional finite element simulations were carried out to further explore the deformation and internal force responses of end-suspended piles and to capture the effects of pile diameter, embedded depth, and rock-socketed depth on the horizontal displacement and bending moment distributions along the pile shaft. It was found that the pattern of the vertical pile displacements could be categorized into three types: rapid settlement, slow settlement, and rapid heave. The magnitudes of the ground and building responses can be well controlled within allowable limits by combining the top-down method with the adopted bracing system. Among the investigated parameters, pile diameter is dominant in affecting the horizontal pile displacement. The primary influence zone for pile bending moment varies, depending on the parameters. It is recommended that a combination of top-down method, ground anchors, and end-suspended piles be adopted for restraining excavation deformation and lowering construction costs of similar deep excavations in soil-rock composite stratum.

/pdf/observed-performance-and-fem-based-parametric-analysis-of-a-3o9vv6dsux.pdf

Observed Performance and FEM-Based Parametric Analysis of a Top-Down Deep Excavation in Soil-Rock Composite Stratum

Water is one of the major risk sources in the excavation of deep-large foundation pits in a water-rich area. The presence of intrusive broken diorite porphyrite in the stratum aggravates the risk level of deep foundation pits. Based on a geological survey report and design documents of parameter information, MIDAS/GTS software was used to perform the numerical simulation of an engineering example of a deep foundation pit project of ultradeep and water-rich intrusion into the broken rock station of subway line 4 in a city. The simulation results show the characteristics of seepage path evolution, seepage aggregation areas and points, and the effect of seepage on the deformation of a deep foundation pit during the whole construction of this deep foundation pit. The results show that with the precipitation-excavation of the deep foundation pit, the pore water pressure at the bottom of the foundation pit follows a distribution of three “concave” shapes. High-permeability pressure zones are found around the foundation pit, intruding broken diorite porphyrite zones, and middle coarse sand zones. With further excavation of the foundation pit, the seepage pressure in the middle part of the foundation pit gradually decreases, and the two “concave” distributions in the middle gradually merge together. After excavation to the bottom of the pit, the pore water pressure at the bottom is distributed in two asymmetrical “concave” shapes, and the maximum peak of pore water pressure is found at the intrusion of fractured porphyrites prone to water inrush. The four corners of the foundation pit are prone to form seepage accumulation zones; therefore, suffosion and piping zones are formed. The surface settlement caused by excavation is found to be the largest along the longitudinal axis of the deep foundation pit, whereas the largest deformation is found near the foundation pit side in the horizontal axis direction of the foundation pit. With the excavation of the deep foundation pit, the diaphragm wall converges to the foundation pit with the maximum deformation reaching about 25 mm. After the first precipitation-excavation of the deep foundation pit to the silty clay and the bottom of the pit with the largest uplift, with further precipitation-excavation of the deep foundation pit, the uplift at the bottom of the deep foundation pit changes only slightly.

/pdf/numerical-simulation-of-seepage-and-deformation-in-1l0ng5wnio.pdf

Numerical Simulation of Seepage and Deformation in Excavation of a Deep Foundation Pit under Water-Rich Fractured Intrusive Rock

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Numerical Simulation of Seepage and Deformation in Excavation of a Deep Foundation Pit under Water-Rich Fractured Intrusive Rock