Soft Soil Foundation Research Articles

Research and Engineering Application of Soft Soil Unloading and Deformation Model

Soft soil is a multiphase medium composed of solid soil particles, pore water, pore gas and combined water film adsorbed on the surface of particle group and colloid between particles. It is widely distributed in coastal, middle and lower reaches of rivers and delta of lakes. Its physical and mechanical properties and engineering characteristics are complex and regional differences are obvious. With the rapid development of social economy and urbanization, it will inevitably lead to serious problems such as urban land tension, traffic congestion and environmental pollution. These coastal cities are the leaders of China’s economic development. After more than 30 years of rapid economic development, the construction scale of these cities in China is also expanding, and the supply and demand of construction land is becoming increasingly tight. The contradiction between supply and demand is more prominent. With the scarcity of land resources and the rapid development of economy, a large number of construction projects in Qianhai area have been started, among which the construction of subway is also on a large scale. The shortage of land resources and the increase of land development make the high density and high strength building development along the subway line. The development of these building properties is bound to appear a large number of complex deep foundation pit projects, and the excavation of deep and complex foundation pit near the side of subway tunnel is a common form of excavation. In practical construction projects, due to the influence of external factors such as complex construction site conditions and urgent time requirements, people often pursue the progress of foundation pit excavation too much in actual construction, ignoring the research and application of mechanical properties of soft soil foundation, which leads to the safety accidents of subway tunnel structure from time to time. This is closely related to the mechanical properties of soft soil with high moisture content, high compressibility and low bearing capacity. Therefore, deep and large foundation pit engineering in soft soil area is prone to heavy engineering safety problems such as underground continuous wall and other retaining structures. Therefore, the further study of the unloading deformation model of soft soil, the physical and mechanical properties and engineering characteristics of soft soil has a positive and important economic value and social role in engineering in the future.

In-situ test on impact loads of a five-module 100% low-floor tram and the prediction of damage characteristics of a pile-plank-supported tram track

To mitigate the differential settlement of the soft soil foundation and to reduce the maintenance work of the embedded tram track, an innovative pile-plank-supported tram track configuration is proposed and utilized in Shanghai Tram Line T1/T2. Since this is the first application of this new structure in tram track projects, the dynamic properties and service performance of this structure are still not clear. In this work, an in-situ test on the dynamic performance of the pile-plank-supported ballastless tram track is carried out. The contact forces between the independently rotating wheelset of a five-module 100% low-floor tram and the channel rail of tram track are measured and investigated as well as the vertical acceleration of the plank. The measured wheel-rail contact forces are employed as a loading condition for a track-subgrade coupling model. The influence of multi-factors (i.e. dynamic loading, thermal conditions and subgrade settlement) on the tram track damage is investigated. The results indicate that the long-term service performance of the pile-plank-supported tram track is stable under the dynamic tram loads.

Mechanism of pore water seepage in soil reinforced by step vacuum preloading

A laboratory model experiment of step vacuum preloading (SVP) for dredger fills was performed. The soil moisture content and water discharge rate were measured. The seepage mechanism of pore water was analyzed based on the theory of drainage consolidation. The results indicated that the seepage fluid mostly consisted of free water during self-weight sedimentation and under 10 kPa vacuum pressure in the SVP test. The stage corresponding to the application of 20 kPa vacuum pressure was the transitional stage in which both the free water and the loosely bound water participated in the seepage. When higher vacuum pressure was applied, the loosely bound water mostly participated in the seepage. In the SVP test, the complete potential difference channels were established in the test bucket during self-weight sedimentation. The vacuum channels at the center began to form when 10 kPa vacuum pressure was applied, and the free water near the sidewall was discharged through the potential difference channel. With 20 kPa vacuum pressure, the vacuum channel developed continuously from the drainage board to the sidewall of the test bucket, completing the process and ultimately stabilizing as the vacuum pressure increasing. The study of the seepage mechanism of pore water during SVP test is significant for the analysis of pore water seepage under vacuum pressure during the consolidation of soft soil foundation. This study contributes to the existing theory related to vacuum preloading consolidation methods, providing theoretical guidance for the analysis of soil consolidation under drainage conditions.

Laboratory and Numerical Studies on the Performance of Geocell Reinforced Base Layer Overlying Soft Subgrade

The use of geocells for support of foundations on weak subgrades has been gaining popularity in recent years. Detailed studies were performed in this work through laboratory tests and numerical simulations to investigate the influence of different parameters on the geocell supported foundations on soft subgrades. A series of large-scale laboratory plate load tests were performed on the geocell-reinforced base layer overlying soft subgrade. The soft foundation soil was simulated using blocks of Expanded Polystyrene (EPS) in the laboratory tests for obtaining consistent subgrade conditions between the tests. The numerical analyses were performed in three-dimensional finite difference numerical program Fast Lagrangian Analysis of Continua in Three Dimensions (FLAC3D). The numerical models were calibrated against the data obtained from laboratory tests. This paper deals with investigating the influence of different parameters such as pocket opening size, height and size of the footing on the performance of geocell-reinforced base layer overlying soft subgrade. The provision of three-dimensional cellular reinforcement (geocells) in the base layer has substantially increased the bearing capacity of the pavement section and the optimum aspect ratio of geocell pocket was found to be close to unity. From the parametric study, it is observed that the potential benefits of geocell reinforcement increases with reduction in weld spacing and height of geocell while the influence decreases with increasing loading area for constant height of geocell layer.

Field Monitoring of Geogrid-Reinforced and Pile-Supported Embankment at Bridge Approach

Owing to the growing traffic demands, many embankments have to be constructed on very soft soil. To improve the undesirable geotechnical properties of the soft soil, the geogrid-reinforced and pile-supported (GRPS) embankments are gradually gaining a great reputation as a reliable and cost-effective treatment for time-limited construction projects or complicated geological conditions. This paper describes an in-situ monitoring of the GRPS embankments with three different pile spacings at the bridge approach over the soft soil foundation. The earth pressures and the settlements both on top of the pile caps and the soil surface around the piles, the pore water pressures, and the lateral displacements of the foundation soil were recorded during and after the filling stage. Based on the important measurement data within 280 days, the behaviors of the GRPS system were monitored and discussed in detail. The monitoring data revealed that the measured earth pressures acting on top of the pile caps were 9.56–16.03 times greater than those acting on the soil surface around the piles, reducing simultaneously the excess pore water pressures in the foundation soil. The measured settlements and the lateral displacements of the soil were relatively stable and gradually converged after about 140 days. The stability of the embankments was significantly strengthened by the GRPS technology. Moreover, the change of the pile spacings made the pavement smoother. As a result, an acceptable treatment effect was achieved for vehicle bump at the bridge approach.

Study on Seismic Spectrum and Amplification Effect of Soft Soil in Tianjin New Coastal Area

Tianjin new coastal area has a large area of soft soil with high salinity and high humidity. This kind of foundation has unique spectral characteristics and amplification effect on ground motion, which is a deep factor to increase earthquake damage. Based on the geotechnical data of Tianjin new coastal, we established a simplified calculation model and adopted a nonlinear method to study the seismic spectrum and amplification effect of three typical soft soil foundation with different thickness in Tianjin new coastal area, which real earthquake motion EL-Centro seismic wave passing through, that is in order to provide a basis for the seismic design of engineering structures in this area. The results show that: The soft soil foundation in Tianjin new coastal area has an obvious amplification effect on EL-Centro wave within 8HZ, while the seismic wave greater than 8HZ rapidly attenuates after filtering through the soft soil, and the amplitude greatly reduced. After the EL-Centro wave passes through the three types of soft soil, the acceleration, velocity and displacement coefficient are always in the state of amplification. The amplification coefficient is the largest in the muddy soil, followed by the muddy clay and the smallest in the silty clay. The softer the soil, the more obvious the amplification effect will be. Except the displacement amplification coefficient does not change much with the increase of soft soil thickness, the acceleration and velocity coefficients increase gradually with the increase of soft soil thickness, and the amplification effect is obvious after reaching the surface.

Stability and Bearing Capacity Analysis of Breakwater with Cylindrical Jacket & Pile Foundations

In view of deep water and soft soil foundation, a new type of breakwater with cylindrical jacket and pile foundations is proposed. According to the hydrological and geological conditions of Tianjin Port, the stability and bearing capacity of breakwater are numerically simulated by ABAQUS software. The results show that when the pile is an elastic model, the failure of soil is the main cause of structural instability. In the limit state, the structure is unstable in rotation in addition to arc sliding, and its turning point is below the bottom of the cylindrical jacket and deviates from the structural axis near the front-pile side. When the pile is a damage plastic model, the loss of bearing capacity is the cause of structural instability. The safety coefficient is greater than 1, but it is obviously smaller than the one of elastic model. The research results provide a reference for structural design and stability study of the breakwater with cylindrical jacket and pile foundations.

Influence Analysis of Large-area PHC Pipe Pile Construction on the Surrounding Environment in Deep Silt Site

There are a large number of deep and thick soft soil foundations along the river and coastal areas. PHC pipe piles have become one of the foundation treatment methods widely used in soft soil foundations. Taking the construction of a pile foundation project in Wenzhou as an example, the thickness of the dredger fill and silt at the site is 27.20∼33.60m, and the impact of large-area pile group construction on the surrounding environment is discussed. The results show that: (1) The load imposed on the pile top and the pile sinking depth during the construction of the foundation pile increase according to an exponential function. (2) The direction of horizontal soil squeezing deformation around the field is related to the position of the measuring point. The corner range is small and the middle range is larger, but the direction of maximum displacement is basically along the shortest line between the pile position and the measuring point. (3) The vertical direction of the measuring point varies greatly with the construction time and pile position, and the uplift and settlement appear repeatedly. The monitoring results can provide a reference for the construction of similar projects.

Experimental study on flocculation-Vacuum-Electroosmosis method for strengthening soft soil foundation in coastal area

In view of the traditional vacuum preloading method to reinforce soft soil foundation, puts forward a new method of foundation reinforcement: flocculation - vacuum - electroosmosis method, and indoor test, the test shows that flocculation - the vacuum - electroosmosis method can greatly reduce the moisture content of reinforced soil, enhance the vane shear strength, and the reinforcement effect was homogeneous.

Application of machine learning for predicting ground surface settlement beneath road embankments

Predicting the maximum ground surface settlement (MGS) beneath road embankments is crucial for safe operation, particularly on soft foundation soils. Despite having been explored to some extent, this problem still has not been solved due to its inherent complexity and many effective factors. This study applied support vector machines (SVM) and artificial neural networks (ANN) to predict MGS. A total of four kernel functions are used to develop the SVM model, which is linear, polynomial, sigmoid, and Radial Basis Function (RBF). MGS was analysed using the finite element method (FEM) with three dimensionless variables: embankment height, applied surcharge, and side slope. In comparison to the other kernel functions, the Gaussian produced the most accurate results (MARE = 0.048, RMSE = 0.007). The SVM-RBF testing results are compared to those of the ANN presented in this study. As a result, SVM-RBF proved to be better than ANN when predicting MGS.

Test and research on mechanical characteristics of geotechnical filling engineering based on inclined soft ground

In order to improve the analysis accuracy of the mechanical properties of soil filling in inclined soft soil foundation, the experimental research method of engineering mechanical properties of rock and soil filler based on inclined soft soil foundation was put forward. CEO-SLOPE software is used to simulate the deformation and stress of foundation and embankment in the process of rock-soil filling. The mechanical properties of bedrock and filling engineering in inclined soft soil are obtained. From the test results, it can be seen that the filling load will compacted the soil of the inclined soft foundation, leading to deformation phenomena such as expansion and sloshing at the foot of the slope.

地下综合管廊软土地基沉降曲线机理及预测

地下综合管廊软土地基沉降曲线机理及预测

Laboratory Model Tests on Consolidation Performance of Soil Column with Drained‐Timber Rod

Timber pile is an ancient technology applied in soft ground improvement for more than 1000 years. With the rise of many high‐rise buildings, many types of modern mechanized‐construction piles are widely developed and applied; for example, steel pile, precast concrete pile, sand pile, and gravel pile are widely used instead of timber piles. Yet, in some special conditions, timber piles have certain advantages due to their environment‐friendly characteristics, which result in obvious economic benefits and suitability. To overcome the weakness of the traditional timber pile technology and expand its application in engineering practice, a drained‐timber pile technique was put forward. This technology is to wrap the permeable filter‐type drainage geotextile around the timber pile, so that the timber pile not only has the replacement function to strengthen the foundation but also has the drainage function, can accelerate the pore water discharge, and speeds up the soft soil foundation consolidation. The reduced scale soil‐column consolidation model was designed to perform the consolidation tests for the soil column with a drained‐timber rod. In total, eight types of soil‐column consolidation tests were carried out to verify the feasibility and effectiveness of the drained‐timber pile technique. The results revealed that, under the same loading and consolidation time, the drained‐timber rod can obviously increase the degree of consolidation when compared with traditional timber rod. It can be expected that the drained‐timber pile technique has a good application prospect for the construction of medium‐small hydraulic structures and for the treatment of super soft clay.

Application of new pumping technology in dewatering in deep soft soil pit

The soft soil stratum has low carrying capacity, poor permeability and is hard to be drained, which affects the excavation speed of foundation pit. This paper introduces the application of a new type of dewatering technology in deep soft soil foundation pit dewatering. This technology adopts air compressor and vacuum pump to improve the water collection efficiency and pumping efficiency effectively and gains good economic and environmental benefits.

Consolidation analysis and effect analysis in the treatment of soft foundation by vacuum preloading

Vacuum preloading is a common method in the treatment of soft soil foundation. Combined with field test engineering cases, the plastic drainage board is equivalently processed into sand well units. The analytical solution of the sand well consolidation equation under the linear attenuation of vacuum preloading is derived to evaluate the consolidation degree of the site. The results showed that the vacuum negative pressure reached -85 kPa at 20 d, and the pore water pressure in the silt soil continued to dissipate to produce negative pore pressure, and finally reached a stable value at 60 d. Considering that the vacuum negative pressure affects the change of pore pressure, it is concluded that the vacuum negative pressure attenuates 3.4 kPa/m along the depth of the soil. Based on the data obtained on site, the solution in this paper is compared with the Hansbo solution, which is closer to the actual project.

Numerical simulation of the effect of moving loads on saturated subgrade soil

The substantial increase in excess pore pressure and plastic strain in subgrade soil due to faster and heavier trains is a major cause of track instability. Stresses that occur in track substructure during the passage of trains have not often been captured accurately due to insufficient rigour in dynamic analysis. This paper presents a numerical investigation into the plastic/dynamic characteristics of a saturated porous medium (capping and subgrade) subjected to moving axle loads. In this study, a detailed numerical analysis is described, whereby a coupled fluid-dynamic framework is developed for the saturated porous medium in conjunction with a generalized plasticity model, in order to examine the cyclic loading response of a soft subgrade soil. The constitutive model is calibrated by undrained cyclic triaxial testing carried out on a soft foundation soil retrieved from a coastal railroad site having a historical record of mud pumping. Kelvin elements are attached to the transmitting boundary to absorb wave energy and to prevent any back-propagation of waves into the saturated subgrade domain. This approach is then validated against a semi-analytical solution and wayside measurements. The results show that the stress path corresponding to the applied moving load exhibits a series of heart-shaped envelopes along which the deviator stress and mean effective stress gradually decrease with the successive loading cycles. The build-up of pore water pressure and the associated soil deformation (e.g. accumulated settlement) are exacerbated when the train speed approaches a critical velocity of the saturated subgrade. A relationship between the train speed, track settlement and drainage capacity of a sub-ballast (capping) layer is also established. As a practical guide, the influence of permeability of the capping layer is highlighted in relation to controlling track settlement for a given operational train speed.

Suitability of Seismic Isolation for Buildings Founded on Soft Soil. Case Study of a RC Building in Shanghai

Base (seismic) isolation is a promising technology for seismic protection of buildings and other constructions. Nowadays, it is accepted that such a technique is efficient and reliable; however, it has two major limitations: soft foundation soil, and tall buildings. The first issue restrains the seismic isolation spreading, given that soft soil is frequent in densely populated areas, and usually such a soil type concentrates the highest seismicity levels. This paper aims to contribute to demonstrating that base isolation, if properly implemented, can be suitable for soft soil. A representative case study is analyzed: a 6-story reinforced concrete (RC) building with base isolation that has recently been built in Shanghai. Since the building is founded on soft soil, concern regarding base isolation suitability arose; even the Chinese design code does not recommend this solution for soft soil. To clarify this issue, non-linear time-history analyses are carried out for a number of natural and artificial seismic inputs that represent the site seismicity; the superstructure behavior is linear, while nonlinearities are concentrated in the isolation layer. The adequacy of base isolation is assessed in the superstructure (in terms of reduction of interstory drift, absolute acceleration and shear force) and in the isolation layer (in terms of axial force, torsion angle and shear strain). The relevance of soil–structure interaction is discussed. The behavior when the mechanical parameters of the isolation units have experienced important changes is also analyzed. The major conclusion is that base isolation of ordinary mid-height RC buildings founded on soft soil can perform satisfactorily in medium seismicity regions.

Assessment of building damage due to excavation-induced displacements: The GIBV method

This paper presents an original methodology for the rapid assessment of “potential” building damage caused by an excavation. The combination of the Ground-work Impact (GI), in terms of induced greenfield displacements, and the Building Vulnerability (BV) form the basis for the GIBV damage assessment method. Both short- and long-term displacements are considered in the impact evaluation. In addition to short-term displacement due to undrained shear deformations, groundwater drawdown can result in significant consolidation settlements at considerable distance from the excavation site. In the proposed method for soft soil ground conditions, the short-term excavation-induced settlement is derived using empirical expressions and long-term displacement is estimated by coupling empirically-derived relationships for excavation-induced groundwater drawdown with a soil stratification model and consolidation theory. The building vulnerability is assessed by considering different physical characteristics and conditions of the buildings.The GIBV method has been implemented in a GIS tool to predict damage classes for buildings exposed to excavation-induced settlements. Two different case studies in Norway were used to test the reliability of the predictions of the tool and validate the methodology. A comparison between the predicted damage classes with the back-calculated ones shows a relatively high accuracy of the methodology. Moreover, the coupled model, which considers both impact and vulnerability for the damage assessment of buildings, has a higher accuracy and lower overestimation rate compared to the results obtained by just considering the impact.

Failure mode of monopile foundation for offshore wind turbine in soft clay under complex loads

Offshore wind turbines (OWT) subjected to long-term complex marine loads are prone to foundation damages, affecting their regular operation. However, the failure mode of the large-diameter monopile foundation under complex marine loads is not well understood. Hence, failure modes of monopile foundation for OWT in soft-soil seabed under unidirectional and multidirectional loads are investigated. Firstly, the mechanical characteristics of soft soil are tested indoors. Subsequently, the monopile foundation failure modes under unidirectional loads are examined, which indicates capacity failures of monopile foundation are progressive, and two distinct soil failure mechanisms are identified. Then, the failure modes and failure envelopes under multidirectional loads (horizontal load, vertical load, and bending moment）are explored. Results show that the failure envelopes in H–V and M–V load space are oval-shaped, and failure modes in H–V and M–V load space are characterized by wedge failure. More critically, the service performance of the monopile foundation in soft soil in a full lifecycle can be estimated by the location of the actual loads in the failure envelope. Finally, the effect of pile diameter on the bearing capacity of the monopile foundation for OWT under unidirectional and multidirectional loads is discussed.

Planar geosynthetic-reinforced soil foundations: a review

Geosynthetics-reinforced soil (GRS) foundation can improve the bearing capacity of the foundation and reduce the settlement of the footing, which has been widely applied to the treatment and improvement of soft soil foundation. In recent years, scholars have carried out a large number of studies to reveal the influence of different factors on the load and settlement behaviors of GRS foundations. In this study, the reinforcing mechanisms of reinforced materials are first summarized, and then the literature review of planar geosynthetic-reinforced foundations from the aspects of experimental studies, numerical simulations and theoretical analyses are presented. Finally, the current research trend of reinforced foundation and the prospects in the future study are discussed. Although the researches on the performance of GRS foundations are conducted extensively, there is no unified understanding on the failure mode and reinforcing mechanism of reinforced foundation. It is necessary to propose the accurate and simple calculation method to evaluate the ultimate bearing capacity of reinforced foundations based on the reinforcing mechanism and failure mode. In addition, the dynamic response of GRS foundations under cyclic loading and earthquake needs to be studied intensively. The paper summarizes the past and present developments of the GRS foundations and provide the views for the further researches.