Soft Subsoil Research Articles

Deformation Analysis of Soft Subsoil on Sloping Bedrock Loaded by Highway Embankment with Stabilization Methods

A highway embankment built in Akita prefecture using ground stabilization methods was constructed on very soft peat and clay deposits on a sloping base. This paper presents a case study of a soil/water coupled elasto-viscoplastic finite element simulation of an actual embankment construction, that is, the deformation analysis of a soft clayey foundation deposited on an inclined base with embankment loading. For execution of the numerical simulation, determining the input parameters needed in the analysis and evaluating the shape of sloping bedrock under the ground were problems. The former is discussed for a practical procedure of determining input parameters using unconfined compression strengths and the latter is accomplished by drawing a contour map of the top face of the bedrock under the ground, with reference to the geographical features of the area around the site and to a topographic map. The performance of the numerical simulation is discussed in comparison with in-situ monitored records. The effectiveness of ground stabilization methods employed at the site is also a topic of discussion.

捨石防波堤全体に作用する波力ならびに堤体内部への波圧伝播の数値計算

A simplified theory to calculate the total wave pressure exerted on a slope of mound breakwater is proposed. Its applicability is examined by comparing the theory with the results of numerical computations. Discussionis also made on the properties of the pressure attenuation inside the core of breakwater. Throughout this study, it is clarified that the theory will assist the design of a rubble mound breakwater to be built on a soft subsoil by providing an estimate of wave force on the breakwater and the subsoil.

Development of Stabilizer Selection Tables for Low-Volume Roads: Arlington, Texas

Expansive soil movements cause damage to low-volume traffic roadways, which is attributed to the low rigidity of the materials used in the pavements. Several treatment methods have been used to stabilize expansive, soft subsoils, which have yielded mixed results due to the presence of sulfates. Because of the ambiguity of these results, the city of Arlington, Texas, established a task force of researchers and practitioners in the area of soil stabilization to develop matrix tables of the various treatment methods and their applicability to stabilize expansive, soft, and sulfate-rich soils. Extensive literature compiled on the stabilizers, several new and previous research studies on stabilizers, and the expertise of the task group members were considered in the evaluation process. The task force developed seven matrix-form selection tables that showed various treatment methods and if their applications were acceptable or unacceptable for the 12 types of soft and sulfate-rich subgrade soils. A summary of the tables was prepared, along with guidelines on how to use these tables to screen the potential stabilizers for pavement construction projects.

Traffic-Load-Induced Permanent Deformation of Road on Soft Subsoil

A method for predicting the traffic-load-induced settlement of road on soft subsoil with a low embankment is proposed. The traffic-load-induced dynamic stress in subsoil is calculated by the multilayer elastic theory (not covered in this paper). Then the plastic vertical strain in subsoil is calculated by an empirical equation, in which constants are related to the physical and mechanical properties of subsoil. The method was applied to analyze three different cases in Saga, Japan. Comparisons of the calculated values with field data indicate that the proposed method can provide a reasonable prediction of traffic-load-induced permanent settlement of the road on soft subsoil with a low embankment. The method is useful for designing the road with a low embankment on soft subsoil. For the cases studied with embankment thickness of 0.75 to 2.7 m, the depth significantly influenced by traffic load is about 6 m below the base of the embankments.

Performance of embankments with and without reinforcement on soft subsoil

A case history of both reinforced and unreinforced embankments on soft subsoil built-to-failure is described and analyzed. The effect of geotextile reinforcements on embankment behavior is discussed by comparing the field and numerical analysis results of cases with and without reinforcement. The results of a laboratory model test on the behavior of embankments on soft subsoil are discussed. Both field and laboratory tests, as well as analysis results, indicate that the reinforcement had a positive effect on embankment stability. However, at a working state (for a factor of safety of FS = 1.2~1.3) the reinforcement did not have an obvious effect on the subsoil response. The effect of reinforcement on subsoil deformation could be noticed only when the unreinforced embankment was close to failure. The laboratory model test results indicated that if the reinforcement is stiff and strong enough, the effect of reinforcement is considerable. It is suggested that although the geotextile has a beneficial effect on embankment over soft subsoil due to its relative lower stiffness, to achieve a substantial improvement on embankment behavior, the stiffer and stronger reinforcements should be used. This case history also demonstrated that the rate of lateral displacement and excess pore pressure development are sensitive indicators of the stability of embankment on soft subsoil.Key words: embankment, reinforcement, soft ground, field tests, laboratory tests, FEM analysis.

Geotechnical Reconnaissance Study on Damage Caused by 2001 Gujarat Earthquake, India

The Gujarat earthquake in India occurred in January, 2001, and caused significant damage in the province of Gujarat. The Japanese Geotechnical Society sent a small reconnaissance team to the damaged region after the quake. The present text presents the report from this activity. The major attention of the team was focused on geotechnical aspects of the damage which were related to earthfill dams and harbor land fills. It was found that a significant portion of damage was concentrated in fills resting on soft natural soils. Swedish weight soundings which were conducted at two places demonstrated the existence of such a soft subsoil. From this, it was concluded that amplification of earthquake shaking as well as the permanent deformation in the soft foundation soil induced such kinds of damage as subsidence, cracking, and lateral spreading in overlying artificial fills. For the better seismic resistance of those affected fills during future earthquakes, the improvement of soft subsoil seems essential.

盛土底面に敷設したプラスチック水平ドレーンの排水機能に関する研究

The discharge capacity (qw) of prefabricated horizontal drain (PHD) placed at the base of embankment on soft subsoil is investigated by laboratory tests. For two PHDs tested, although their qw values reduced with confining pressure and elapsed time, for the conditions investigated, their long-term confined in clay qw values were more than 16, 000m3/yr/m. The field performance of PHD has been investigated by FEM analysis. Under the assumed conditions, the analysis results indicate that when qw>1, 000m3/yr/m, qw value will have almost no effect on embankment settlement. This suggested that the two PHDs tested are suitable as drainage material under embankment on soft subsoil

Seismic Design of Cut and Cover Tunnel Based on Damage Analyses and Experimental Studies.

Some cut and cover tunnels were damaged by the Hyogoken-Nambu Earthquake in 1995. In most cases, except for important facilities and those constructed in soft subsoil, a seismic resistant design had not been considered. And its design method was not for such large earthquakes. From this viewpoint, a new seismic design method of cut and cover tunnels for larger earthquakes is desired. To investigate the behavior of underground structures at the time of earthquake, damage of three underground structures is first investigated and analytical simulations are carried out. Several model shaking table tests of cut and cover tunnel are then performed and the results are numerically analyzed. Finally, a new seismic design method of cut and cover tunnel is presented based on these test and analysis results.

Generalized Stability Analysis of Embankments on Granular Piles

A generalized method has been proposed to analyze the stability of embankments constructed over soft subsoil reinforced with granular piles. The problem of determining the critical slip surface and the corresponding minimum factor of safety has been formulated as a constrained minimization problem. The shear strength of the improved subsoil has been estimated using both the conventional and the equivalent anisotropic concepts. The proposed method has been validated comparing the results obtained with solutions available in the literature. With reference to a typical problem, limited results are presented to highlight the influences of the various parameters on the stability of an embankment.

Analyses of Lateral Displacement of a Pile Supported Abutment Constructed in a Soft Subsoil Profile

A case study on the measurement of lateral movement of an abutment supported by pile groups constructed in a cohesive soil is reported in this paper. Based on prediction at the design stage and observational construction control, the abutment was successfully constructed within the range of allowable lateral movement. After the completion of construction, the 3D elasto-plastic coupled FE analysis based on the Cam-clay model was applied to the problem. Although the analysis overpredicted the lateral movement, it was indicated that such analysis is useful for evaluating the mechanism of lateral movement of such an abutment and the effect of the weak surface soil, replaced by more “competent” material.

Structural analysis of concrete pavements on soft subsoils with differential settlements : Tan; Yao Proc International Conference on Geotechnical Engineering for Coastal Development, GEO-COAST'91, Yokahama, 3–6 September 1991 V1, P845–848. Publ Japan: Coastal Development Institute of Technology, 1991

Structural analysis of concrete pavements on soft subsoils with differential settlements : Tan; Yao Proc International Conference on Geotechnical Engineering for Coastal Development, GEO-COAST'91, Yokahama, 3–6 September 1991 V1, P845–848. Publ Japan: Coastal Development Institute of Technology, 1991

MGEOT: A low-volume road designer

A calculation program is described to design geotextiles in low-volume roads of soft subsoil. The theory is based on membrane action. The program specifies the required quality of the geotextile as a function of the acceptable rut depth.

Stability and deformation of peaty soft subsoil for embankments : Hu, Z P Proc International Conference on Engineering Problems of Regional Soils, Beijing, 11–15 August 1988P411–414. Publ Beijing: International Academic Publishers, 1989

Stability and deformation of peaty soft subsoil for embankments : Hu, Z P Proc International Conference on Engineering Problems of Regional Soils, Beijing, 11–15 August 1988P411–414. Publ Beijing: International Academic Publishers, 1989

Damage caused by the November 25, 1988, Saguenay earthquake

The November 25, 1988, Saguenay earthquake prompted a site visit by a team representing the Canadian National Committee on Earthquake Engineering. This paper contains selected ground motion records in the form of acceleration-time histories, obtained from the Geological Survey of Canada, and corresponding response spectra. The horizontal acceleration spectrum obtained for Chicoutimi is compared with the design base shear coefficients from the 1980, 1985, and 1990 National Building Codes of Canada. Failures of natural slopes and embankments as well as both architectural and structural damage are discussed. The significant role played by the presence of soft subsoil in amplifying the ground motion and resulting damage is illustrated. Although no major structural failures occurred, there were many examples of poor performance, and in some cases failures, of unreinforced masonry walls. Concerns are expressed over the abundance of unreinforced masonry, particularly in schools and buildings such as hospitals, for which postdisaster performance must be ensured. Key words: earthquake, Saguenay, soils, structures, codes, masonry.

Les dommages dus au tremblement de terre du Saguenay du 25 novembre 1988

The November 25, 1988, Saguenay earthquake prompted a site visit by a team representing the Canadian National Committee on Earthquake Engineering. This paper contains selected ground motion records in the form of acceleration-time histories, obtained from the Geological Survey of Canada, and corresponding response spectra. The horizontal acceleration spectrum obtained for Chicoutimi is compared with the design base shear coefficients from the 1980, 1985, and 1990 National Building Codes of Canada. Failures of natural slopes and embankments as well as both architectural and structural damage are discussed. The significant role played by the presence of soft subsoil in amplifying the ground motion and resulting damage is illustrated. Although no major structural failures occurred, there were many examples of poor performance, and in some cases failures, of unreinforced masonry walls. Concerns are expressed over the abundance of unreinforced masonry, particularly in schools and buildings such as hospitals, for which postdisaster performance must be ensured. Key words: earthquake, Saguenay, soils, structures, codes, masonry.

Assessment and control of stability for road embarkments constructed on soft subsoils : Symons, I F Transport and Road Research Lab, lab report 711, 1976, 36P

Assessment and control of stability for road embarkments constructed on soft subsoils : Symons, I F Transport and Road Research Lab, lab report 711, 1976, 36P

Analysis of Load-Bearing Fills over Soft Subsoils

The protection afforded soft subsoils by the stress-distributing characteristics of load-bearing fills is examined. Using recently developed finite element techniques of analysis, including nonlinear soil properties, a complete stress-deformation solution of a fill-subsoil system is obtained. The proposed method is first examined by applying it to three problems, one hypothetical and two actual load test case histories. The method is then used to study the stress distributing characteristics of granular fills overlying clay subsoils. As would be anticipated, the stiffer the fill relative to the foundation soil, the greater the reduction in stress transmitted to the underlying soil. Although the complexity of the load-bearing fill problem is such that simple charts cannot be prepared to indicate fill-subsoil interface stresses for different soil conditions and geometries, it is shown that approximate values of maximum vertical and shear stress can be obtained by use of an easily applied formula for modulus characterization and a simple geometrical function.

Damage to Railway Embankments Due to the Tokachioki Earthquake

SYNOPSIS The 1968 Tokachioki Earthquake caused much damage to railway embankments built on soft subsoils. After having taken emergency action for restoration of traffic, the author started a thorough examination into the cause of embankment damage. The present paper outlines the results of the studies conducted in the first year of scheduled investigation period.