Base stability of multi-propped excavations in soft clay subjected to hydraulic uplift

2013 ◽  
Vol 50 (2) ◽  
pp. 153-164 ◽  
Author(s):  
Y. Hong ◽  
Charles W.W. Ng
Keyword(s):  
Soft Clay ◽  
Three Dimensional ◽  
Hydraulic Pressure ◽  
Ground Settlement ◽  
Overburden Pressure ◽  
Passive Resistance ◽  
Centrifuge Tests ◽  
Sand Aquifer ◽  
Base Stability ◽  
Three Dimensional Finite Element

Excavations in soft clay underlain with an aquifer may be destabilized by hydraulic uplift. Previous studies on this subject are based mainly on field observations. Dewatering from the aquifer is a common method to improve base stability where ground settlement is not a major concern. Alternatively, piles readily installed as part of the top-down construction method for multi-propped excavation may be considered to provide base stability and minimize ground settlement outside the excavation. This paper presents results from two centrifuge tests that were conducted to simulate multi-propped excavations in-flight (with and without piles) in soft clay destabilized by hydraulic pressure from an underlying sand aquifer. Moreover, coupled three-dimensional finite element analyses were carried out to back-analyse the centrifuge tests. Numerical parametric studies were also conducted to study the influence of pile length on the effectiveness of base stabilization. It is revealed that both for excavations with and without piles, the artesian pressure required to initiate uplift inside the excavation is about 1.2 times the overburden pressure of the clay. By using “anti-uplift” piles inside the excavation, the ultimate hydraulic uplift resistance increases by 16%, while the uplift movement can be reduced by 80%. The presence of piles also increases the passive resistance in front of the wall by 70%, but reduces the mobilized undrained shear strength, cu, of clay by 53%.

Three-dimensional centrifuge and numerical modeling of the interaction between perpendicularly crossing tunnels

2013 ◽  
Vol 50 (9) ◽  
pp. 935-946 ◽  
Author(s):  
Charles W.W. Ng ◽  
Thayanan Boonyarak ◽  
David Mašín
Keyword(s):  
Shear Stress ◽  
Tensile Strain ◽  
Control Volume ◽  
Three Dimensional ◽  
Vertical Direction ◽  
Stress Transfer ◽  
American Concrete Institute ◽  
Overburden Pressure ◽  
Novel Technique ◽  
Centrifuge Tests

Tunnel driving inevitably induces changes in stress and deformation in the ground, which could cause ultimate and serviceability problems to an adjacent tunnel. The effects of induced stress on an existing tunnel and crossing-tunnel interaction are still not fully understood. In this study, a series of three-dimensional centrifuge tests were carried out to investigate the responses of an existing tunnel in sand to the excavation of a new tunnel perpendicularly below it. Three-dimensional tunnel advancement was simulated using a novel technique that considers the effects of both volume and weight losses. This novel technique involves using a “donut” to control volume loss and mimic soil removal in-flight. To improve fundamental understanding of the stress transfer mechanism during the new tunnel advancement, measured results were back-analyzed three-dimensionally using the finite element method. The maximum measured settlement of the existing tunnel induced by the new tunnel constructed underneath was about 0.3% of tunnel diameter, which may be large enough to cause serviceability problems. The observed large settlement of the existing tunnel was caused not only by a sharp reduction in vertical stress at the invert, but also by substantial stress transfer of overburden pressure at the crown. The section of the existing tunnel directly above the new tunnel was compressed vertically because the incremental normal stress on the existing tunnel was larger in the vertical direction than in the horizontal direction. The tensile strain and shear stress induced in the existing tunnel exceeded the cracking tensile strain and allowable shear stress limit given by the American Concrete Institute.

Effect of Fins on Combined Loaded Suction Caisson in Deepwater Clay Soils. A Numerical Analysis

2021 ◽  
Author(s):  
Pablo Castillo Garcia ◽  
Stylianos Panayides
Keyword(s):  
Soft Clay ◽  
Three Dimensional ◽  
Cost Savings ◽  
Clay Soils ◽  
Suction Caisson ◽  
Dimensional Finite Element ◽  
Three Dimensional Finite Element ◽  
In The Beginning ◽  
The Impact ◽  
Fin Length

Abstract Suction piles are a form of foundation widely adopted in the offshore energy industry. Efforts to enhance the combined Vertical-Horizontal (V-H) performance of piles with the addition of fins, attracted interest from the engineering community in the beginning of the 21st century. Design of this enhancement was surfaced whilst examining foundation solutions for renewable energy projects. Studies to date have primarly considered relatively shallow waters comprising sandy soils, with the behaviour of fin-enhanced piles in very soft to soft clay soils, receiving less attention. The present study emphasis is on typical deep-water deposits of soft clay and attempts to evaluate the impact of varying fin length, shape, orientation and location, on the combined capacity of suction piles by means of three-dimensional finite element analyses. The paper investigates two types of load configuration; in the first instance loading at the pile head and secondly with the load attachment point located at approximately two thirds of the pile embedded length. These two configurations cover different foundation solutions, such as support for subsea infrastructure and anchoring for floating facilities, respectively. Optimum fin-enhanced suction pile configurations are presented for each application, with the results from this study indicating an increase of the load-carrying capacity in V-H space, whilst reducing the overall suction pile size. The efficiency of various configurations is presented with composite plots of increase in holding capacity, plotted against the increase in steel surface area. Preliminary recommendations on fin length, location, shape and orientation for typical suction pile applications are presented with intent to demonstrate the potential for cost savings and reduction in both operational and schedule risk.

scholarly journals Effect of embedment on consolidated undrained capacity of skirted circular foundations in soft clay under planar loading

2017 ◽  
Vol 54 (2) ◽  
pp. 158-172 ◽  
Author(s):  
Cristina Vulpe ◽  
Susan M. Gourvenec ◽  
Alexander F. Cornelius
Keyword(s):  
Soft Clay ◽  
Three Dimensional ◽  
Soil Mass ◽  
Relative Magnitude ◽  
Interface Roughness ◽  
Engineering Practice ◽  
Straightforward Method ◽  
Failure Envelopes ◽  
Dimensional Finite Element ◽  
Three Dimensional Finite Element

The effect of foundation embedment ratio and soil–skirt interface roughness on the consolidated undrained capacity of skirted circular foundations under planar loading in normally consolidated clay has been investigated through coupled three-dimensional finite element analyses. Results are presented as failure envelopes, and changes in shape and size of the normalized vertical–horizontal–moment (VHM) failure envelopes are described as a function of relative magnitude and duration of applied preload. Results show that embedment ratio and interface roughness affect the load distribution within the soil mass, but that consolidated undrained capacity under planar loading scales proportionately with the (unconsolidated) undrained capacity of the foundation. This latter feature enables the results to be neatly synthesized into a relatively straightforward method for use in engineering practice for prediction of gain in undrained VHM capacity due to preload and consolidation.

A Three-Dimensional Finite Element Model of Hydraulic Progressive Damage

2006 ◽  
Vol 324-325 ◽  
pp. 375-378 ◽  
Author(s):  
Bing Xue ◽  
Heng An Wu ◽  
Xiu Xi Wang ◽  
Zhi Long Lian ◽  
Jin Zhang ◽  
...  
Keyword(s):  
Finite Element ◽  
Flow Behavior ◽  
Three Dimensional ◽  
Element Model ◽  
Fluid Loss ◽  
Hydraulic Pressure ◽  
Progressive Damage ◽  
Cohesive Elements ◽  
Dimensional Finite Element ◽  
Three Dimensional Finite Element

Three-dimensional finite element simulations were carried out to investigate the hydraulic progressive damage and associated flow behavior in rock. In this study cohesive elements were used to simulate the damage of rock. A three-dimensional coupled pore fluid flow and stress model was proposed. The commercial engineering software ABAQUS is employed to simulate the damage process in rock along several predefined paths. A user-subroutine named FLOW was developed to enhance the capability of ABAQUS to deal the moving loadings. With the proposed coupling model, we studied the stress distribution, the pore pressure, the fluid loss, the geometry of the progressive damage. The results show that the length and the width of the path of the progressive damage are strongly influenced by both the hydraulic pressure and the injection time. The results provide good interpretation and understanding of the mechanism of hydraulic progressive damage in rock. This study is very useful and important to the oil engineering and some other rock engineering fields.

scholarly journals Effect of Geometric Parameters and Construction Sequence on Ground Settlement of Offset Arrangement Twin Tunnels

Geosciences ◽  
2022 ◽  
Vol 12 (1) ◽  
pp. 41
Author(s):  
Md Shariful Islam ◽  
Magued Iskander
Keyword(s):  
Three Dimensional ◽  
Ground Surface ◽  
Geometric Parameters ◽  
Horizontal Distance ◽  
Ground Settlement ◽  
Construction Sequence ◽  
Settlement Behavior ◽  
Total Settlement ◽  
Three Dimensional Finite Element ◽  
Twin Tunnels

A parametric study that examines the ground surface settlement due to the excavation of shallow offset arrangement twin tunnels is presented. Offset arrangement tunnels are those that run parallel to each other, but at different elevations. The study focuses on the influence of both the construction sequence and various geometric parameters on the induced soil settlement. A series of three-dimensional finite element analyses was carried out to investigate the settlement behavior and interactions between offset arrangement twin tunnels excavated in clay using a simplified mechanized excavation method. Analyses were carried out for three cover-to-diameter (C/D) ratios, three possible construction sequences, five angular relative positions, and five angular spacings. In addition, settlement data were also investigated by varying horizontal and vertical spacings while keeping the angular spacing constant. The total settlement of the excavated twin tunnels and the settlement induced solely by the new second tunnel are both presented, and special attention was paid to identifying the dominant geometric parameters. The observed data trends from this study are generally consistent with the limited data available in the literature. This study confirmed a few perceived behaviors. First, angular relative position better describes the settlement behavior in comparison to angular spacing. Second, the effect of the vertical distance is noticeably more significant than that of the horizontal distance between the two tunnels. Third, excavation of the lower tunnel at first induces higher total ground settlement than when the upper tunnel is excavated first or when both tunnels are excavated concurrently. Fourth, settlement due to the construction of the newer tunnel decreases with the increase in the cover depth. In addition, two design charts have been proposed to calculate the settlement induced from a new second tunnel excavation and the eccentricity of the maximum total settlement relative to the center of the new tunnel.

Numerical evaluation of a deeply buried pipe testing facility

2018 ◽  
Vol 21 (16) ◽  
pp. 2571-2588 ◽  
Author(s):  
Van Thien Mai ◽  
Neil Hoult ◽  
Ian Moore
Keyword(s):  
Boundary Conditions ◽  
Three Dimensional ◽  
Friction Angle ◽  
Buried Pipe ◽  
Overburden Pressure ◽  
Testing Procedures ◽  
Novel Approach ◽  
Dimensional Finite Element ◽  
Three Dimensional Finite Element ◽  
The Impact

A new facility for testing pipes under deep burial has been developed. However, before the facility was commissioned, the influence of the loading scheme and boundary conditions on the pipe behaviour was investigated so that the most appropriate experimental setup could be developed. Two- and three-dimensional finite element analyses were used to assess the impact of the top and side boundary conditions on both flexible and rigid pipes with varying diameters. The vertical overburden pressures expected in the field are simulated using actuators applying vertical forces to two steel grillages. The numerical results show that the use of two independent grillages on the surface produced a more uniformly distributed ‘overburden’ pressure, a novel approach that performs significantly better than previous loading systems. Proximity of the test facility’s walls to the pipes was also investigated and found to have less than a 0.2% impact on pipe response when compared to simulations of field geometries. Results examining five different approaches to reducing the effect of sidewall friction were compared to the case of zero friction (i.e. the field case), and it was found that while lubricating the wall to create a friction angle of 5° over the full height produced the most accurate results, lubrication of only the top 2.5 m of the wall also produces thrust forces and bending moments within 10% of values from the zero-friction case. Finally, the effect of the position of the pipe within the test cell was investigated, where pipe testing with 0.3 m of bedding is expected to produce results like those for pipes close to rock foundations in the field. These results are already being used to inform testing procedures using this unique facility.

Influences of Vehicle Loads on Braced Excavation in Soft Clay

2013 ◽  
Vol 353-356 ◽  
pp. 146-151 ◽  
Author(s):  
Chang Jie Xu ◽  
Yuan Lei Xu ◽  
Hai Hu Lin ◽  
Feng Ming Sun
Keyword(s):  
Finite Element Method ◽  
Soft Clay ◽  
Three Dimensional ◽  
Design Work ◽  
Reinforcement Scheme ◽  
Braced Excavation ◽  
Plane Method ◽  
Dimensional Finite Element ◽  
Vehicle Loads ◽  
Three Dimensional Finite Element

Economic development and urbanization give rise to the excavation complexion, and higher requirements on design work are put forward. The risk of excavation in soft clay area is high and the design shall be comprehensive. While in practice, the dynamic characteristic of vehicle loads is seldom calculated. In this paper, conventional one-side plane method (beam on elastic foundation method) and three-dimensional finite element method were adopted to analyze the effects of vehicle loads on the retaining strutting system of braced excavation. Moreover, the reinforcement scheme of excavation after collapse was given as references for similar situations.

Effect of Skirts on the Lateral Capacity of Mudmats in Soft Clay

2013 ◽  
Author(s):  
Jianchun Cao ◽  
Luo Yang
Keyword(s):  
Finite Element ◽  
Lateral Displacement ◽  
Soft Clay ◽  
Three Dimensional ◽  
Soil Mass ◽  
Element Analysis ◽  
Lateral Capacity ◽  
Fea Model ◽  
Rate Of Increase ◽  
Three Dimensional Finite Element

This paper presents the development of a three-Dimensional Finite Element Analysis (3D FEA) model using Finite Element Code PLAXIS 3D to investigate the skirt’s effect on the lateral capacity of a mudmat in soft clay. It is found that, for a skirted mudmat, 1) the stiffness of skirts had an influence on the mobilization of its ultimate lateral capacity; 2) the required lateral displacement to mobilize the ultimate lateral capacity was about 1% of its width; 3) the lateral capacity is proportional to skirt depth under a soft clay condition with a linearly increasing strength; 4) the ultimate lateral capacity can also be increased by adding internal skirts, but the rate of increase becomes less prominent with the increase of internal skirt number; 5) an empirical formula in ISO19901-4 [5] was confirmed to conservatively estimate the lateral capacity of a skirted mudmat. Moreover, the influence of skirt on the involved soil mass was discussed.

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