Excavation In Soft Clay Research Articles

Investigation of Three‐Dimensional Deformation Behavior due to Long and Large Excavation in Soft Clays

By analyzing the extensive field data from a long and large excavation in soft clay, this study investigates the three‐dimensional deformation behavior induced by excavation. Significant inhibition effects of corner on both wall deflections and ground settlements were observed and quantified. Thus, a modified function for estimating the distribution of deformation parallel to the excavation is proposed and evaluated. Further analyzing shows that the pipeline settlement can be well estimated by the modified function combining with settlements profile proposed by Hsieh and Ou, and the reduction coefficient is about 0.8; the calculated maximum distortion of the pipeline can provide reliable reference. In addition, it is found that the cement‐soil partition walls can also considerably reduce the wall deflections and ground settlements even after the part that is above the excavation base were removed.

Investigation of the Axial Force Compensation and Deformation Control Effect of Servo Steel Struts in a Deep Foundation Pit Excavation in Soft Clay

This study presents a comparative analysis of the deformation control effect of the hydraulic servo steel struts and ordinary steel struts of a foundation pit based on the measured axial force of the steel struts, lateral wall deflection, and ground surface settlement due to pit excavation. The results indicate that ordinary steel struts installed via axial preloading exhibit a disadvantageous axial force loss with a maximum value equal to 86.7% of the axial preloading force. When compared with ordinary steel struts, the hydraulic servo steel strut exhibits a superior supporting effect. The hydraulic servo steel strut adjusts the axial force in real time based on the deformation of the retaining structure and the axial force of the struts. Thus, the ratio of maximum lateral deflection to the excavation depth of a deep foundation pit in soft soil is less than 0.3%. Concrete struts undergo unsupported exposure during the excavation process, leading to sharply increasing deformation of the retaining structure. Therefore, regarding a foundation pit with strict requirements for deformation control, the use of hydraulic servo steel struts rather than concrete struts is recommended.

Numerical investigation of pile responses caused by adjacent braced excavation in soft clays

ABSTRACT The pile responses to soil movements induced by adjacent braced excavation have attracted increasing attention from geotechnical design engineers. The lateral displacement of soil caused by deep braced excavation could produce significant effect on adjacent piles according to field measurement data. In this paper, the finite element (FE) analyses considering soil of the hardening small strain (HSS) constitutive model were carried out to establish the excavation-induced pile behaviours by varying the depth of excavation, pile diameter, pile length, pile distance away from the excavation, pile-head fixity, unsupported depth of excavation and axial loadings exerted on the pile head. A set of design charts is developed to estimate the behaviours of piles subjected to adjacent braced excavation in soft clay, targeting to provide design guidelines for the lateral wall deflection assessment of pile foundations and stability evaluation.

Predicting deformation of multipropped excavations in soft clay with a modified mobilizable strength design (MMSD) method

Although serviceability (i.e., deformation) governs the performance of deep excavations in urban areas with soft clay deposits, the common design methods are only able to check the potential of base stability, rather than the deformation. This limitation has led to the development of an analytical approach entitled the mobilizable strength design (MSD) method, which can predict both serviceability and safety in a single step of calculation. The modification of the MSD method (i.e., the MMSD method) was recently made by the authors, by implementing a more realistic plastic deformation mechanism into the calculation. The objectives of this study are (a) to validate the MMSD method by comparing its predicted deformation with eight well-documented case histories in Shanghai and Hangzhou soft clay and (b) to quantify the influence of some potentially important factors on the deformation of deep excavations in Shanghai and Hangzhou. The accuracy of the MMSD predictions for the maximum lateral wall displacement (δh-max) have been found to fall within 35% of the field data in most of the cases. This accuracy is more satisfactory than the accuracy (i.e., 120%) of the MSD predictions for the corresponding case histories, suggesting the effectiveness of the modified plastic deformation mechanism considered in the MMSD calculation. Based on the verified MMSD method, preliminary design charts that predict δh-max of narrow excavations (specifically for a metro station) considering different geometries and strength mobilization characteristics have been developed.

Effectiveness of deep cement mixing walls with top-down construction for deep excavations in soft clay: case study and 3D simulation

This paper presents the observed and simulated effectiveness of deep cement mixing walls created using top-down (DCM-TD) construction techniques for a deep excavation in soft Bangkok clay. The wall system consisted of four rows of 0.7-m-diameter DCM columns, and the bracing system consisted of two 0.25-m-thick basement slabs and seven temporary struts. The effectiveness of the wall system compared to that of other wall systems was evaluated using the measured results of previous case studies. A 3D numerical analysis was performed to calculate forces in the basement slabs and bending moments in the DCM wall. Finally, series of parametric analyses of both DCM-TD and deep cement mixing walls created using bottom-up (DCM-BU) construction techniques were carried out, and their results were compared to highlight the effectiveness of DCM-TD and its applicability to excavations at greater depths. The field and numerical results show that DCM-TD is more effective than DCM-BU in terms of the limitations of lateral wall movement, the bending moment in a DCM wall and the thickness of a DCM wall for various depths because of a larger system stiffness. Therefore, DCM-TD is very effective and suitable for use in potential future deep excavations in urban areas.

Settlement and load transfer mechanism of a pile group adjacent to a deep excavation in soft clay

Three-dimensional coupled consolidation analysis is conducted to gain insight into the response of a 2×2 floating pile group adjacent to deep excavation in soft clay. By using a validated finite element model, the influence of the excavation depth, pile length, pile group location from excavation, the supporting system stiffness, soil state and permeability, and working load are systematically studied. The analysis revealed that the maximum settlement occurs when the pile group is founded at the excavation level and at a distance of 0.75-times the excavation depth, although the induced bending moment is minimum. In contrast to pile group settlement, tilting is maximum when it gets closer to the wall and minimum at a distance of 0.75-times the excavation depth. It is also observed that the influence of system stiffness is more pronounced for the flexible wall and when the pile group is located at the excavation level. Applied working load influences pile group settlement but has relatively minor effects on pile group tilting. Excess negative pore water pressures are generated in the soil elements due to excavation. Pile group experiences progressive long term settlement with the dissipation of excess negative pore water pressure.

Failure Investigation at a Collapsed Deep Open Cut Slope Excavation in Soft Clay

Collapse of a 7.08-m-deep open cut excavation for the bank of a navigation lock in sensitive soft clay in Zhejiang, China, is presented in this paper. Cone penetration tests, field vane shear tests (VSTs), and traditional laboratory tests have been performed to investigate the soil properties after the collapse. According to the test results, the soft soil layers of mucky clay and muck have been found to be disturbed because of the effects of the boring holes. For bored concrete piles, the stresses release due to the cut slope, thus its shear strength is much lower than that measured from laboratory tests. The shear strength reduction technique based on a finite-element method program has been used to study the excavation behavior and overall factor of safety (FOS) of the excavated slope considering geometries of original design, later adopted schemes and soil properties associated with various test types. The computed FOS values obtained from VST results are lower than the corresponding recommended values and just a little higher than the limited value. Thus the excavated slope may easily collapse triggered by additional vehicles and surface surcharge loads. The reasons of excavation collapse can be explained as the misuse of soil properties for designers, and inadequate construction process management. The investigation of the failure provides experiences and lessons for the design and construction of similar projects in sensitive soft clay.

Responses of a Newly Built Metro Line Connected to Deep Excavations in Soft Clay

AbstractExcavation-induced ground settlement causes adverse impact or damage on adjacent facilities. If a foundation pit is connected to the facilities, the responses of the facilities can be influ...

Deterministic and reliability assessment of basal heave stability for braced excavations with jet grout base slab

For braced excavations in deep deposits of soft clays, it is common to construct a jet grout slab (JGP) beneath the excavation in order to restrain the wall deformation, reduce the forces acting on the struts and to increase the basal heave factor of safety. In this paper, finite element analyses were carried out to assess the basal heave factor of safety for excavations in soft clays supported by JGP. The finite element analyses indicate that the interface friction between the jet grout slab and the wall is a key component contributing to the resistance of the excavation system to basal heave failure. Comparison of the factor of safety from the finite element analyses with limit equilibrium predictions based on the slip circle method and the modified Terzaghi method was then performed. Since the conventional factor of safety approach does not explicitly reflect the uncertainties of the soil and JGP properties, and the excavation geometry on the excavation system performance, a series of reliability analyses were also carried out to assess the basal heave factor of safety for excavations supported by JGP. The provided spreadsheet template can be used to estimate the probability of basal heave failure for deep excavations supported by jet grout slabs.

Basal-heave analysis of pit-in-pit braced excavations in soft clays

Recently, the pit-in-pit braced excavations are frequently adopted in the geotechnical engineering projects in China due to the large demand of urban underground space utilization. In this study, a simplified approach for basal-heave stability analysis of pit-in-pit braced excavations in soft clays is developed based on the limit equilibrium method. The configuration of failure surfaces is obtained by using finite element method with the shear strength reduction technique. The factors of safety computed with the simplified approach are comparable to those from the finite element simulation. This simplified approach is demonstrated through a case study of pit-in-pit excavations in Shanghai.

Construction Technology of Floor Slab for Deep and Large Top–Down Excavation in Soft Clay

The construction quality and elevation of floor slab obtained using the top–down construction method in soft clay may be affected by soil uplift due to excavation unloading and recompression settlement due to the weight of concrete. Based on the Shanghai EXPO axis and underground complex project (Shanghai EXPO axis excavation), soil uplift is studied by conducting centrifugal model test and numerical simulation. The soil uplift process for soft clay in Shanghai EXPO Axis excavation is mainly concentrated during the 14 days after excavation, attaining approximate 84 % of the final values. The configuration of soil uplift is serrated. The maximum soil uplifts for excavation depths of −1.08, −9.10, −12.80, and −17.30 m (WSEY) are 85, 127, 143, and 153 mm respectively. On-site monitoring reveals that the floor slabs and beams are settling during concrete casting. The settlement values are related to concrete weight, bearing capacity of soils under the excavation surface, and stiffness of scaffold. Based on these findings, exposure time duration between soil excavation and concrete casting and reasonable pre-camber values of scaffold are proposed. The results of this paper demonstrate that the application of these values is effective in controlling construction quality and elevation of floor slab and beam.

Time-dependent ground movements induced by shield tunneling in soft clay: a parametric study

In this work, the effects of coupled hydromechanical (consolidation) processes associated with shield tunneling excavation in soft clays are investigated with particular attention to the prediction of ground movements at the ground surface. A series of 2d FE analyses have been carried out in parametric form in order to investigate the effects of tunnel excavation velocity relative to the soil consolidation rate and the hydraulic boundary conditions at the tunnel boundary. The shield advancement process has been simulated with a simplified procedure incorporating both volume loss and ovalization of the tunnel section. In order to investigate the relative importance of soil consolidation during the excavation process, different characteristic times for the tunnel face advancement and for the consolidation process around the tunnel have been considered, for the two limiting conditions of fully permeable liner and impervious liner. The potential damage induced by the tunnel excavation on existing structures, based on computed ground surface distortions and horizontal deformations, has been found to vary significantly with time during the consolidation process. The results of the simulations allowed to obtain useful information on the minimum tunnel face advancement speed for which the assumption of fully undrained conditions for the soil during the excavations is acceptable, as well as on the speed range for which solving the fully coupled hydromechanical problem is necessary.

Finite element analysis of failure of deep excavations in soft clay

Finite element analysis of failure of deep excavations in soft clay

Determination of failure mechanism of very soft clay behind L-pile wall with physical model tests

This paper presents physical model tests to verify the failure mechanism of soil behind the gap of contiguous L-pile retaining wall used to support the excavation in soft clay. The ratio of pile gap to pile width (Sg /B) of 0.1, 0.3, 0.5, 0.7, 1.2, 1.6, 2.0, and 3.0 was used. The undisturbed soft clay samples were obtained from an open cut in Samutprakarn province. The physical model tests were conducted in the vanity of the soil sampling area to avoid the effect of sample disturbance. The pressures were applied to the physical model using screw jack until the failure of soil between the L-piles was observed. The digital photos of soil movement were recorded during the test to monitor the failure mechanism. The test results show that the normalized failure pressures reduce as the gap width ratio (Sg /B) increases. The failure pressure of L-pile is lower than that of circular pile proposed by Winit [9] due to soil arching size. The failure arches are clearly observed in the model with Sg /B of 0.1–1.2, but not with Sg /B of 1.6–3.0.

Use of 3D finite element method for back study of a failed basement excavation in soft clay

Use of 3D finite element method for back study of a failed basement excavation in soft clay

A study of failure mechanisms of deep excavations in soft clay using the finite element method

In this study, four failure mechanisms of excavations were investigated using the finite element method (FEM) with reduced shear strength. Center posts were considered in this finite element model. Both the elastic and elastoplastic support systems were used for comparison. The results showed that when the elastoplastic support system was used, reasonable stability of the excavations was estimated by the FEM. The yielding of the support system caused failures of the excavations. When the elastic support system was used, the FEM overestimated the stability of the excavations. The failures of these excavations were due to a large soil heave present at the excavation bottoms. For the two excavations in thick clay, the horizontal struts were bent until yield due to an upward movement of the center posts and a downward sinking of the wall. The yielding of the struts was followed by the yielding of the wall and failures of the excavations. For the remaining two excavations, which had either a shallow hard stratum or an insufficient support system, the failure was caused by the yielding of the wall due to an inward movement of the soil and the yielding of the struts because of the transfer of a large axial load from the wall.

A Simplified Approach for Estimation of Braced Wall Deflection During Excavation in Soft Clay

The construction of braced excavations in urban areas requires control of surrounding ground movements since excessive ground movements damage adjacent properties. Reasonably good amount of work on numerical analysis and field studies on the estimation of ground surface settlement adjacent to excavation have been reported in the literature. However, theoretical studies or mathematical modeling have not been well addressed in the literature. This paper presents a simplified design approach for estimation of wall deflection where diaphragm wall is assumed as continuous beam. Earth pressure in the backfill side is taken as load, exerted on the beam and strut load along with earth pressure in cut side represents support of the beam. Peck’s earth pressure diagram for braced wall is used for load calculation while Terzaghi’s equation is considered to derive coefficient of modulus of subgrade reaction. Applicability of the proposed approach has been verified by comparing the estimated values of wall deflection with the measured values of three different case studies.

A simplified analysis for deformation behavior of buried pipelines considering disturbance effects of underground excavation in soft clays

Underground excavation, such as tunneling and deep foundation pit, will no doubt induce the soil disturbance and have result in uneven settlements of adjacent buried pipelines which adversely affect and even damage the structures. In order to explicitly point out construction interaction mechanism and rapidly predict the structure mechanical behavior, a simplified displacement-controlled two-stage method and stress-controlled two-stage method are presented for determining the deformation behavior of pipeline structures caused by underground excavation in soil clays. According to tunneling project, the free soil deformation calculated by the displacement-controlled boundary element solution is used to estimate the soil disturbance effects of underground excavation. The oval-shaped ground deformation pattern is imposed to the tunnel opening to consider the nonuniform convergence characters. According to foundation pit project, the free soil stress based on the Mindlin solution is used to predict the soil disturbance effects of underground excavation. The situations that the excavation unloading center is not acting on the pipeline axis and that the excavation boundary and pipeline axis are formed with an arbitrary angle can fully be considered. Then, the free soil deformation and free soil stress are imposed onto existing pipelines to analyze the interaction mechanics between the disturbance soil and buried structures. The accuracy of proposed method is demonstrated with existing calculation results, centrifuge model tests, and site investigation data. In addition, the parametric analyses for the deformation influence factors of existing tunnel induced by foundation pit excavation, including the horizontal distance between the excavation boundary and tunnel axis, the tunnel buried depth, the tunnel bending stiffness, and the crossing angle between the excavation boundary and tunnel axis, are presented to demonstrate the performance of the proposed method. The results indicate that the proposed method can be used to estimate the mechanical behavior of buried pipelines considering disturbance effects of underground excavation with higher precision.

Simplified Reliability-Based Design of Wall Displacements for Excavations in Soft Clay Considering Cross Walls

AbstractThis study proposes a simplified reliability-based design (RBD) method for the serviceability limit state (SLS) of excavations considering cross walls. Although the effects of cross walls are highlighted, the proposed method is applicable to cases without cross walls. In the proposed RBD method, partial factors are calibrated using rigorous reliability theory to meet the target reliability index. A design example is used to illustrate the use of calibrated partial factors and to demonstrate the effectiveness of cross walls.

Base stability analysis of braced deep excavation in undrained anisotropic clay with upper bound theory

It is imperative to evaluate factor of safety against basal heave failure in the design of braced deep excavation in soft clay. Based on previously published field monitoring data and finite element analyses of ground settlements of deep excavation in soft clay, an assumed plastic deformation mechanism proposed here gives upper bound solutions for base stability of braced deep excavations. The proposed kinematic mechanism is optimized by the mobile depth (profile wavelength). The method takes into account the influence of strength anisotropy under plane strain conditions, the embedment of the retaining wall, and the locations of the struts. The current method is validated by comparison with published numerical study of braced excavations in Boston blue clay and two other cases of excavation failure in Taipei. The results show that the upper bound solutions obtained from this presented method is more accurate as compared with the conventional methods for basal heave failure analyses.