Soft Subsoil Research Articles

Field Evaluation on the Strength Increase of Marine Clay under Staged Construction of Embankment

This article presents a case history of an embankment constructed on the soft Ariake clay on the bank of Rokkaku River, Saga, Japan. The staged method was used in the construction of embankment. To shorten the time of construction, the soft subsoil was treated by a prefabricated vertical drain (PVD). The physical and mechanical properties of soft subsoil, such as water content, unit weight, and unconfined compressive strength, were investigated before and during embankment construction. Based on the field data, the strength increase of Ariake clay during construction was revealed. A nonlinear relationship between strength increment and the degree of consolidation was found. Finite element method (FEM) was employed to investigate the strength increase behavior. The results showed that it is difficult to predict the filed behavior of strength increase using Cam-clay model. Based on the concept of structure loss of structured clay, the strength variation of Ariake clay during consolidation is discussed.

Arched Soil-Cement Walls for Soft Ground Reinforcement of Dike Slope

New structure of long straight walls with arched retaining wall was raised to improve the anti-sliding capacity of embankment soft ground. The arched walls embedded in the soft subsoil will transfer lateral forces in reasonable ways. Detailed research shows that the structure of arched walls can greatly increase slope stability and reliability.

Influence of Uneven Soft Subsoil on a Brick Building

According to an evaluation on a six story, brick residential building with severe cracks and inclination, the influence of uneven soft subsoil on building was analyzed. Such damaged wall could be avoid, if the treatment to the basement and subsoil is correct in design and the brickwork is regulated strictly in construction. In order to minimizing the lost and keep the building safety, influence of uneven soft subsoil on buildings should be taken into account much more seriously, not only in design, but also in the construction process.

Improvement of Underwater Soft Subsoil in Tidal Zones by Vacuum Preloading Technique

In view of the characteristics of the natural conditions in tidal zones, systematic innovation and improvement have been made in the techniques to execute conventional vacuum preloading technology in combination with sand cushions, technology for installation of prefabricated drains, sealing membrane and technology for laying sealing membrane, and technology for vacuum pumping, to overcome the difficulties in improving underground soft soils in tidal zones. Key techniques in improving underwater soft soils in tidal zones have been solved and the vacuum preloading technique was successfully used in improving soft soils in tidal zones, producing very good results.

Testing and Simulation Analysis on the Settlement of Geosynthetic-Reinforced Pile-Supported Embankments on Soft Ground for the High Speed Railway

The settlement of railway subgrade has an important connection with the quality, safety and natural life of the railway, the excessive settlement of subgrade will directly cause too many problems in the process of railway using. Especially in high speed railway, these problems will be more outstanding, thus affecting the development and progress of the railway. For the foundation of traffic infrastructure in areas with soft subsoil often embankments supported by piles are necessary due to the settlement requirements. For sufficient load transfer into the piles different construction methods are possible. For the construction of a high speed railway, a cement stabilization of the embankment material and geogrid reinforcement on top of the piles were used. Before the temporary load the settlement is monitored continuously for 10 months. In this paper, the method of construction is described and important field monitoring results are presented.

Estimation of Flow Characteristics in Peat

This paper presents the behavior of the soft subsoil under structures such as embankment dams, particularly tailing dams which are nowadays usually placed in swampy areas often dominated by organic soils, such as peat. Structures construction on soft organic soils makes it necessary to calculate subsoil deformations with special consideration of the water flow. Constructions of embankments on soft organic soils cause many problems because of their specific properties. The organic soils, such as peat, are characterized by high porosity and low shear strength as well as high initial permeability which decreases during consolidation. In the deformation process of soil skeleton under loading the porosity decreases and causes changes of flow characteristics. Therefore, the water flow characteristics are very important to be considered in predicting and calculating the subsoil deformation under load. The specific behavior of soft subsoil demands estimation reliability of the flow characteristics. Therefore in the statistical analysis suitable statistical methods and proper measures in laboratory tests should be used. Several statistical functions have been applied to analyze the flow-pump test data in order to provide a correct estimation of flow characteristics in soft soils. In this paper, the results of water flow characteristics obtained in laboratory tests and the estimation of flow characteristics in soft organic soils are presented.

Plane strain FE analysis of arching in a piled embankment

The results of a series of linearly elastic-perfectly plastic plane strain finite-element (FE) analyses are reported, investigating the arching of a granular embankment supported by pile caps over a soft subsoil. The objective of the research was to contribute further to generic understanding of arching in a piled embankment, and the contribution of tensile reinforcement near the base of the embankment. The stress state in the embankment and load on the subsoil were examined. The analyses demonstrate that the ratio of the embankment height to the centre-to-centre pile spacing is a key parameter in determining the behaviour. The potential contribution of tensile reinforcement near the base of the embankment was also considered. It was found that arching in the embankment occurred at relatively small settlement of the underlying subsoil, whereas much more settlement was required for sag of the reinforcement to have significant effect in further reducing the stress on the subsoil.

Long-term settlement of soft subsoil clay under rectangular or semi-sinusoidal repeated loading of low amplitude

By applying Yin and Graham’s one-dimensional (1-D) equivalent-time rheological model, which can predict both primary and secondary settlement, the expressions for calculating the long-term subsoil settlement under rectangular or semi-sinusoidal repeated loading of low amplitude are obtained without considering the fluid–solid coupling. A property has been proven that ultimate settlement of clay is independent of stress increasing history under 1-D conditions. According to this property, the expressions are derived considering the solid–fluid coupling to approximately calculate the long-term total settlement and the long-term settlement of soft subsoil clay after construction of the road embankment. Because the expressions of the long-term settlement are analytical solutions in the present paper, they can not only avoid cumulative calculation errors in the numerical analysis, but also have more solid theoretical foundations than empirical equations.

낮은 심도의 연약지반에 대한 비선형 지진응답해석

본 연구에서는 얕은 연약지반에서 구조물-지반 상호작용의 영향을 받는 구조물의 비탄성거동을 정확히 나타낼 수 있는 유한요소해석 방법을 연구하였다. 이를 위하여, 국내의 지반특성을 반영한 얕은 연약지반과 단자유도 구조물로 2차원 유한요소모델을 구성하고, 다양한 지진파와 지반에 대해 OpenSees 해석프로그램을 이용한 비선형 시간이력해석을 수행하였다. 연약지반의 비선형거동을 반영하기 위하여 일반적으로 흔히 사용되는 등가선형 주파수영역 해석 결과와 비선형 시간이력 유한요소해석 결과의 차이를 검토하였다. 그 비교결과는 등가선형강성을 사용하고 지반-구조물 상호작용을 고려하지 않는 주파수영역해석은 단주기영역의 구조물의 응답스펙트럼을 과대평가할 수 있음을 보여주었다. 응답스펙트럼에 대한 지반-구조물 상호작용의 영향은 기초크기와 구조물의 질량의 변화와 큰 관계 없이 일정하게 나타났다. This study presents a finite element analysis method that can accurately evaluate the nonlinear behaviour of structures affected by shallow soft subsoils and the soil-structure interaction. A two-dimensional finite element model that consists of a structure and shallow soft subsoil was used. The finite element model was used for a nonlinear time domain analysis of the OpenSees program. A parametric study was performed to investigate the effects of soil shear velocities, earthquake input motions, soft soil depth, and soil-structure interaction. The result of the proposed nonlinear finite element analysis method was compared with the result of an existing frequency domain analysis method, which is frequently used for addressing nonlinear soil behavior. The result showed that the frequency domain analysis, which uses equivalent secant soil stiffness and does not address the soil-structure interaction, significantly overestimated the response of the structures with short dynamic periods. The effect of the soil-structure interaction on the response spectrum did not significantly vary with the foundation dimensions and structure mass.

Settlement prediction for soft ground improved by columns

A simple method of calculating the consolidation settlement of soft subsoil improved by a floating cement-stabilised column–slab system subjected to embankment loading has been proposed, and the method has been applied to three case histories in Fukuoka, Japan. The proposed method adopts the methodology of treating a part of the soil–cement column improved layer with a thickness of Hc as an unimproved layer and calculating its compression using the properties of the soft soil only. Based on coupled finite-element analysis results using a unit cell model, two bilinear functions with variables of the improvement area ratio (α) and the improvement depth ratio (β) by the columns have been proposed to determine the value of Hc. Both the proposed method and the method currently used in Japan (proposed by the Japanese Institute of Construction Engineering (JICE)) were used to calculate the settlement (compression) of the soft layers for three case histories in Fukuoka, Japan. Comparing the calculated results with the field values shows that the proposed method yielded much better predictions than that of the JICE method. It is suggested that the proposed method can be used to design the improvement of soft clayey ground using the floating cement-stabilised column–slab system.

The analysis of consolidation in organic soils

The analysis of consolidation in organic soils. This paper is devoted to the specifi c diffi culties connected to construction on problematic soils. Different type of structures like: road embankments, fl ood control levees, dykes and dams are often located in soft subsoil areas, which consists mainly of peat, calcareous soil with a very high content of calcium carbonate and other high plasticity organic and no organic soils. These soils can be characterized as highly deformable with low initial shear strength and an insuffi cient bearing capacity. Soft soils show a large deformation, both vertically and horizontally, under load. The settlements often appear very quickly and can also continue for a long time. The consolidation process consists here of two main stages: primary settlement and secondary (and tertiary) settlement (creep). It is essential to have a good quality description of physical and mechanical properties of soil before the calculation and construction stage. For calculations of each stage of settlement the different physical and mechanical parameters of problematic soil are applied. In this paper the analysis of organic soils deformation process is presented. The deformation characteristics were defi ned on the basis of laboratory tests results. Soil investigations were performed on peat samples taken from test site located in Olsztyn region. Laboratory test of physical properties and consolidation tests in oedometer were carried out. Based on laboratory test results the empirical relationships between stress and deformation as well as stress and time were elaborated in order to describe the primary consolidation in organic soils.

The uncertainty of the water flow velocity data obtained in the laboratory test

The uncertainty of the water flow velocity data obtained in the laboratory test The uncertainty of the water flow velocity data obtained in the laboratory test. Some of the engineering constructions are significant for the environment and flood protection. There are dams for water retention as well as waste tailings dams and road embankments founded on organic soils. Constructions on soft organic soils make it necessary to calculate subsoil deformations with a special regard of the water flow. In this thesis, the approach of the water flow test in soft organic soils is presented. In the permeability test the modern flow-pump technique was applied. This technique takes into consideration organic fiber structure, large soil strain, short time of research and "real" hydraulic gradients appearing in situ. The specific behavior of soft subsoil demands to calculate the test reliability results. Therefore in the statistical analysis of the test results the suitable statistical method and the proper laboratory technique should be used. Several statistical models have been applied to the flow pump test results in order to find the correct description of water flow velocity in soft subsoil.

Inflection point method for predicting settlement of PVD improved soft clay under embankments

Prefabricated vertical drains (PVDs) are being used to accelerate the consolidation of subsoil for construction of high embankments on soft ground. The construction is carried out in stages and the height of each stage construction depends on gain in strength of soft subsoil and target factor of safety. The inflection point method for estimating the degree of consolidation for vertical drainage has previously been published. The degree of consolidation was estimated to be 70% at the inflection point. After 70% consolidation, the rate of consolidation reduces to a minimum value and it is economical and technically feasible to allow the second-stage loading. Additional load at this stage would ensure increase in shear strength of the subsoil. In this paper, the authors have extended the procedure of inflection point method for vertical drainage to a three-dimensional drainage when PVDs are adopted for subsoil improvement. Theoretical graphs have been developed which can estimate the percent consolidation at inflection point (% U i) for different subsoil thickness, drain spacing ratios and C r/ C v ratios. Different factors viz. smear, drain spacing, depth of clay thickness and well resistance were also considered in the analysis. The degree of consolidation at inflection point is observed in the range of 61–78%. The inflection point method has been applied to a settlement data from a case history of two sites and the estimated percent consolidation at inflection point has been compared with the values predicted from developed theoretical graphs. The total primary settlement estimated from inflection point has been compared with that of Asaoka method, which is widely used in the field. It is concluded that inflection point method has the potential for field application and provides an alternate method for estimating of total settlement in field applications using PVDs and surcharge, and to determine the appropriate required waiting period for stage loading.

Geotechnical foundation conditions of Mesologion swimming pool center

Although geotechnical tests are standard and commonly used in determining the foundation type to be applied, in some cases the use ofin-situ tests are essential in order to obtain the correct values needed in the geotechnical calculations. In the case of the foundation of Mesologi Swimming-pool Center, an attempt was made to propose a foundation type, suitable from geotechnical point of view, for sediments that lie very close to the sea! The selection of the most suitable foundation type encloses the examination of several parameters. The subsoil there, consists of soft, fine grained, low plasticity materials, CL to SL with alternations of high plasticity clay layers CH. Additionally, the groundwater level is 0,8 meters under the soil surface, demonstrating constant saturation conditions. The combination of soft subsoil layers quality and the high level of groundwater leads to difficult foundation conditions in relation to immediate and secondary consolidation settlement. Geotechnical calculations and calculations of settlement were performed and the most suitable foundation type was semi-compensated mat foundation.

Vacuum consolidation and its combination with embankment loading

A method is proposed for determining the optimum penetration depth of prefabricated vertical drains (PVDs) in cases where vacuum consolidation is combined with the use of PVDs in a clayey deposit with two-way drainage. The advantages of combining vacuum pressure with embankment loading are discussed in terms of reducing preloading-induced lateral displacement of the subsoil, increasing the effective surcharge loading, and reducing construction time in the case of road construction. A vacuum consolidation project conducted in Saga, Japan, is described, and the results from a fully instrumented test section are presented and analyzed using a two-dimensional finite element approach. The numerical simulations compare well with the field measurements. The validated numerical approach is then used to examine the response of soft subsoil subjected to vacuum consolidation. The results confirm the usefulness of the proposed method for determining the optimum penetration depth of PVDs and the advantages of combining vacuum pressure with embankment loading.Key words: vacuum consolidation, preloading, prefabricated vertical drain, FEM analysis, embankment.

Centrifuge modeling of ground improvement under embankments

Stone columns are used in practice for ground improvement of soft subsoil. Current design procedures of these columns are based on simple empirical calculations. This research project investigates the behavior of a base reinforced embankment constructed on a soft clay layer, which is improved with stone columns. Centrifuge tests were performed in order to gain a deeper understanding of the interaction problem within the structure. Stone columns were constructed in-flight using a newly developed installation tool. First analysis of test data shows a significant change in stress state due to column construction and the influence on the clay behavior.

Busan–Geoje Link: Immersed tunnel opening new horizons

The Busan - Geoje Fixed Link will provide a road connection between the metropolis of Busan and Geoje Island. The Link comprises amongst others two cable stayed bridges and an under water tunnel constructed as a concrete immersed tube tunnel. The immersed tunnel has a number of special features: its length of 3,2 km, the water depth of over 35 m, the severe marine conditions, the soft subsoil and alignment constraints. Combined with the scale of the project these features make the design and the construction of the tunnel a major challenge. It is expected that the project will open up new horizons for the use of immersed tunnel technology. This paper highlights these special features and concentrates on the civil and structural aspects only. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. Reprinted with permission from Elsevier.

Analysis of field performance of embankments on soft clay deposit with and without PVD-improvement

This paper presents a case history of the performance of two full-scale test embankments constructed on soft clay deposit in the eastern coastal region of China. One embankment was constructed on natural subsoil and the other was constructed on prefabricated vertical drain (PVD) improved subsoil. The thickness of the soft clay deposit without PVD-improvement was 19 m and with PVD-improved case was 23 m. The PVDs were installed to a depth of 19 m with spacing of 1.5 m in a triangular pattern. Field performance of the two embankments was analyzed using the finite element method. The following influential factors: (i) hydraulic conductivity of subsoil in field and (ii) discharge capacity of PVDs were investigated numerically. The back-analyzed results for the embankment on natural subsoil showed that the hydraulic conductivity ratio ( C f) of field to laboratory values is about 6. To analyse the PVD-improved subsoil, a simple approach using the equivalent vertical hydraulic conductivity of PVD-improved subsoil was employed. The analytical results show that PVDs increased the bulk vertical hydraulic conductivity of soft subsoil by about 30 times compared to the original non-treated subsoil. The discharge capacity of PVDs in this field case is 79–100 m 3/a, which is consistent with the findings from laboratory tests and other reported values in literature.

Lateral Displacement of Ground Caused by Soil–Cement Column Installation

Closed form equations for calculating the lateral displacement caused by the installation of soil–cement columns are derived based on cylindrical cavity expansion theory. The radius of the cavity needs to be predetermined empirically, mainly with reference to the amount of admixture injected and injection pressure as well as the stiffness of the surrounding soil. Also, an empirical equation is proposed for considering the partial “plane strain” effect of installing a row of columns. The proposed method has been applied to four reported field tests conducted in Saga, Japan, using the slurry double mixing (SDM) method, the dry jet mixing (DJM) method, and the wet jet mixing (WJM) method. The radius of influence, i.e., where the lateral displacement in the soil is less than 5 mm during the installation of soil–cement columns, is found from the field data to be approximately 30, 40, and 50 m for the SDM, DJM, and WJM, respectively. It is shown that the proposed method yielded a reasonable prediction of these field measurements. The field data also indicate that the installation sequence has a considerable influence on the observed lateral displacement; but the proposed method can only consider two extreme conditions of this influence. It is suggested that the method is a useful tool for the design of soft subsoil improvement resulting from the installation of soil-cement columns.

Estimation of required seating length of bridge girders under non-uniform ground excitation and different ground conditions

This study addresses the simultaneous influence of soil-structure interaction, non-uniform ground excitations and pounding on the required seating length of bridge girders. The recently published Japanese design specification for bridge girder relative displacement and the recommendation of many current design codes for mitigating pounding and unseating potential are also discussed. The spatially varying ground excitation is simulated stochastically using an empirical spectrum and coherency loss function. The ground is a half-space or a soft layer of different thickness on hard soil. The results show that non-uniform ground motions together with soft subsoil can strongly amplify required seating length and pounding forces, and consequently increase unseating and pounding damage potential of bridge girders.