Soft Soil Foundation Research Articles

Unloading Mechanical Effect Analysis of Retaining Structure of Deep Foundation Pit in Soft Soil

The large-scale foundation pit construction site is small in area and complicated in conditions, which is easy to affect the environment. At the same time, it is prone to cracking or even collapse of nearby buildings due to uneven surface. The low-ranking official network around the foundation pit is easily damaged, and the surrounding roads are cracked. Many problems. At the same time, soft soil has the characteristics of high compressibility, low strength, large water content and small bearing capacity. It shows remarkable structural and rheological characteristics, which makes it more and more challenging to ensure the safety and stability of soft soil foundation pit engineering. In this paper, the deep foundation pit deformation of super soft soil is predicted and controlled, and the deep foundation pit engineering of soft soil is taken as the research object. The mechanical characteristics of soil unloading process are discussed, and the strength and deformation law of unloading soil are studied. The actual constitutive model and soil parameters are numerically simulated by the finite element method, which objectively reflects the mechanical state of the foundation pit excavation unloading soil, which is of great significance to ensure the safety and long-term stability of large underground engineering construction.

Research on Deformation Law of Soft Soil Foundation under Load

This study takes a road as the main object of long-term settlement of the road foundation, and analyses the settlement deformation and long-term settlement deformation of the soft soil foundation in the construction process from the aspects of numerical analysis and field test respectively, so as to obtain the long-term settlement characteristics of the road soft soil foundation in Tianjin port area, and further analyse the generation mechanism and key influencing factors of the long-term settlement.

Experimental Evaluation of Consolidation Behavior of Double-Layer Soft Soil Ground

Abstract Double-layer grounds are characterized by one layer of soft soil on the top of another, which are frequently encountered in land reclamation projects in China. Consolidation behavior of the double-layer grounds is complicated compared with that of single-layer soil because of the complex drainage conditions that exist between two soil layers and the different consolidation properties of the two soils. The consolidation behavior of the double-layer ground becomes more complicated when it is subjected to a surcharge preloading. Therefore, it is necessary to understand the consolidation behavior of individual soils that make up the double-layer ground as well as the double-layer ground as a whole. In this article, double-layer specimens were prepared to simulate double-layer grounds, which include an upper layer clay with high plasticity and a lower layer clay with low plasticity. Consolidation tests were performed on both single-layer specimens and double-layer specimens using a modified consolidation test device. The influences of surcharge ratio and surcharge preloading duration on the consolidation behavior of double-layer ground, such as consolidation settlement and consolidation rates, were studied in addition to the influence of the thickness ratio (i.e., the ratio of the thickness of the upper layer of soil to the lower layer of soil). The experimental results show that a longer surcharge preloading duration could reduce the time needed to complete the rebound of the double-layer ground. The secondary consolidation settlement of double-layer ground can be reduced by increasing either the surcharge ratio or surcharge preloading duration. In addition, an increase in the thickness ratio could result in an increase in the primary consolidation settlement and a decrease in the consolidation rate. However, the effect of the thickness ratio on the consolidation settlement and the consolidation rate gradually decreased with the increase in the thickness ratio.

Reliability Design of Soft Soil Foundation Preloading Method for Airport Runway

Focusing on the rapid assessment of the reliability of airport runway soft soil foundation, taking the soft soil foundation preloading method as the research object, we established the corresponding settlement calculation model and studied the soil parameters and how they affect the settlement values, and the reliability of the soft soil foundation preloading method was obtained by the checking point method (JC). In order to realize the fast calculation of reliability, Visual Studio 2010 is used to design the visual reliability calculation program of the airport runway soft soil foundation preloading method. The analysis results showed that the soil index with the greatest influence on the final consolidation settlement was the compression coefficient, followed by the soil layer thickness, the compression modulus, the void ratio, and the consolidation coefficient. On the basis of the limit state equation, we got the final failure probability formula. Combined with an airport project example, the reliability calculation results showed that the longer the preloading time was, the more reliable the treatment plan was, and the less the probability of failure was; the larger the preloading pressure was, the more reliable the treatment plan was, and the less the probability of failure was. In the engineering example, it can be found that the treatment plan with the preloading of 100 kPa and the stacking time of 150 days is the best solution.

Experimental Investigation of the Deformation Characteristics of Tianjin Clays under Coupled Dynamic Stress and Seepage Fields

The causes, prevention, and control of clay disasters are of great concern in practical engineering applications. Recently, due to unprecedented economic development, increasing numbers of tunnels and subgrades of tidal flat areas have been built in Tianjin, China. Soft soil ground not only bears the vibration load during operation and construction but also receives seepage effects caused by the bursting of pipelines and variations in groundwater levels. Under the coupling action of dynamic and seepage loads, large settlement can occur in soft soil since the deformation is related to both cyclic stress and seepage. Therefore, it is significant to understand the dynamic deformation characteristics of soil to ensure the safety of engineering applications. In this study, a series of laboratory cyclic triaxial tests were conducted to study the deformation behaviours of Tianjin clays under coupled cyclic stress and seepage fields. To simulate the seepage field, water pressure is applied at the bottom of the specimen, and the water can be drained from the top of the specimen to form a head difference in the specimen during the tests. The effects of the seepage pressure, cyclic stress ratio (CSR), and a number of cycles (N) on the hydraulic conductivity, axial deformation, hysteresis curve and dynamic modulus of Tianjin clays were systematically investigated. The laboratory results show that the hydraulic conductivity (k) decreases with the increasing seepage pressure, CSR, and N, and its range is from 4.6 × 10−8 cm/s to 7.4 × 10−8 cm/s. The axial deformation of the soil increases with increasing CSR and seepage pressure. With an applied water pressure of 150 kPa at the bottom of the specimen and a CSR of 0.3, the maximum cumulative plastic strain reaches 4.78%, increasing by 53.2% compared with a CSR of 0.1. The hysteresis curve gradually becomes denser or thinner with increasing CSR and N; the hysteretic curve tends to a straight line for N of 5,000. The dynamic modulus increases with increasing N and depends on the CSR and seepage pressure. Empirical equations are suggested for calculating the dynamic modulus with different CSR values and seepage pressures. The results of this study can provide reference parameters for the construction and design of tunnels and subgrades of tidal flat areas in clay strata under the influence of water environments.

Application of geotextiles in strengthening soft soil foundation

Application of geotextiles in strengthening soft soil foundation

Analysis of Deformation Characteristics of Long‐Short Pile Composite Foundation in Salt Lake Area, Iran

To study the settlement and deformation characteristics of a long‐short pile composite foundation and its influencing factors, the indoor centrifuge model test and numerical simulations were performed under the actual working conditions to treat the saline soft soil foundation in the Soltan salt lake area of Iran. Firstly, the settlement of a long‐short pile composite foundation and the law of pile‐soil stress ratio development with time were studied using a centrifuge model test. In addition, FLAC3D was used to simulate the test conditions, and it has been observed that the numerical and experimental results were in good agreement. Based on the numerical model, the effects of the replacement rate of long and short piles, cushion modulus, and pile length on the settlement of the composite foundation and the stress ratio of long and short piles were analyzed. The results show that the long‐short pile composite foundation can effectively control settlement. Compared to the untreated foundation, the total settlement of a long‐short pile composite foundation was reduced by 67%, and the postconstruction settlement was reduced by 47.7%. Compared to the final settlement of the composite foundation obtained using the centrifuge model test, the error range of the numerical simulation results was between 2.4% and 8%. The influence of pile spacing on the settlement and stress characteristics of a long‐short pile composite foundation is greater than that attributable to the arrangement of the piles. Under this geological condition, it is reasonable to choose a pile spacing of 4d∼5d. When the long pile reaches the bearing stratum, the length of the short pile should not exceed 0.6 times the length of the long pile. The setting cushion not only reduces uneven settlement of the composite foundation but also improves the joint bearing capacity of the pile and the soil between the piles.

Influences of Urban Viaduct Pier Caps on Uneven Settlement of Roadbed

Uneven settlement widely occurs on the roadbed under urban viaducts in operation. Previous studies focused on the effect of soft soil foundation on the uneven settlement of roadbed, without considering the phenomenon and mechanism of uneven settlement at the junction of pier cap and the roadbed under viaduct. The interaction is now considered between the buried depth of cap, the relative stiffness between cap and road subgrade, and the uneven settlement of roadbed. Using the Midas GTS NX software, a numerical analysis model of the deformation of roadbed under viaduct is constructed to study the influence of viaduct cap foundation on the uneven settlement of roadbed under viaduct. The simulated results of uneven settlement of roadbed caused by urban viaduct pier cap show that (1) when the buried depth of cap exceeds 2.35 m, with the increase of buried depth, the fall of roadbed settlement tends to decrease, but the decreased amount is not significant. Considering the factors such as construction cost and complications of construction technology, it is suggested that the best buried depth of cap is about 2.5 m. (2) The fall of roadbed settlement decreases with the decrease of relative stiffness between cap and roadbed. When the relative stiffness exceeds 6.3, the decreasing trend of settlement fall is not significant. The concrete roadbed should have more stiffness in order to reduce the settlement.

The Settlement Models of Deep Vacuum Dewatering Method

The deep vacuum dewatering method is effective for groundwater control in projects. By now, although the vacuum consolidation in soft soil foundation treatment has been analyzed much, the settlement of deep vacuum dewatering has not been researched sufficiently. Because of the extra vacuum pressure, the settlement should be analyzed further. In this paper, the settlement models are derived and analyzed based on the 2 vacuum pressure distribution models (plane seepage model and Johnson’s model), Imai and Chai’s vacuum consolidation models, and elastic model of traditional soil mechanics. And then a project case is provided to verify the theoretical models. The results show that the proposed model is suitable to predict the settlement and provide new references for vacuum dewatering engineering.

Field instrumentation and settlement prediction of ground treated with straight-line vacuum preloading

The vacuum preloading method has been used in many countries for ground improvement and land reclamation works. A sand cushion is required as a horizontal drainage channel for conventional vacuum preloading. In terms of the dredged-fill foundation soil, the treatment effect of the conventional vacuum preloading method is poor, particularly in Tianjin, China, where a shortage of sand exists. To solve this problem, straight-line vacuum preloading without sand is widely adopted in engineering practice to improve the foundation soil. Based on the engineering properties of dredged fill in Lingang City, Tianjin, this paper presents field instrumentation in five sections and analyzes the effect of a prefabricated vertical drain (PVD) layout and a vacuum pumping method on the soft soil ground treatment. Through the arrangement of pore water pressure gauges, settlement marks and vane shear tests, the settlement, pore water pressure and subsoil bearing capacity are analyzed to evaluate the effect of the ground treatment. This study demonstrates that straight-line vacuum preloading without sand can be suitable for areas with a high water content. Furthermore, the consolidation settlement and consolidation degree system is developed based on the grey model to predict the consolidation settlement and consolidation degree under vacuum preloading; the validity of the system is also verified.

INFLUENCE OF PVD-ENHANCING SOFT SOIL GROUND ON THE ADJACENT BRIDGE PILES – A 3D FEM ANALYSIS


 
 
 The influence of prefabricated vertical drain (PVD)-enhancing soft soil on the adjacent bridge pile foundation is not sufficiently clear. The related studies are scarce and far behind the requirement of engineering practice. Three-dimensional finite element model (3D FEM) is established to investigate this important issue. Plastic drainage boards in the PVD are modelled by a user-defined element, which considers the axial force and permeability of the plastic drainage board, whereas its weight, shear, and torsion are not considered. The elastoplastic-consolidation coupled model is used for the soils. Field measured results of pore water pressure, ground settlement, and pile cap displacement are used to validate the 3D FEM with good accuracy. A number of useful conclusions can provide guidelines for similar projects and inform designers of the adverse effects of PVD-enhancing soft soil ground on the adjacent bridge pile foundation.
 
 

Propagation and attenuation characteristics of the vibration in soft soil foundations induced by high-speed trains

The vibration caused by high-speed trains (HSTs) can result in reductions of strength and bearing capacity of the railway foundation, unexpected settlement, and other geotechnical engineering problems. Thus it is essential to estimate the vibration level at different depths of the foundation. This study investigates the propagation and attenuation characteristics of the vibration in the soft soil foundation induced by high-speed trains. A series of field experiments were conducted to measure the HST-induced vibration in the soft soil foundation in Shanghai, China. The measurement provides a unique dataset for numerical model validations. Based on the Bornitz equation, a new attenuation model is proposed for layered homogeneous media. The material damping coefficient and the geometric damping coefficient, two input parameters for the model, are thoroughly investigated using measurements in this study and from the existing literature. The proposed model was applied to estimate the vertical vibration levels at different depths in typical subgrade and foundation of HSTs in the soft area. Model performance is supported by the good agreement between predictions and observations. Our model provides an alternative tool to further investigate the effect of HST-induced vibration on foundation soils and adjacent underground structures.

Cumulative ground deformation induced by train operations

With the rapid development of railway construction, many railway projects have crossed the soft soil deposit. In order to obtain ground settlement data after train operation to ensure the safety of railway, the train-track dynamic model and track-ground finite element model were established to predict accumulated deformation of soft soil foundation induced by dynamic train load. Compared to the measured vibration acceleration on the sleeper and ground, the predicted vibration acceleration can match well with the measured vibration acceleration across the full range of time history shown. The results showed that after many times of train operations, the cumulative deformations of the ground at different depth are quite different, it may affect the safety of the railway operation, the foundation reinforcement measures can be considered during construction.

The Influence of Vacuum Preloading Method Process on the Nearby Foundation in a Project

The vacuum preloading method is a useful and common method in treating soft soil foundation. Lots of pore water is discharged and the foundation occur large deformation in vacuum preloading procedure. The large deformation may affect the stability of the nearby foundation, leading to cracking and differential settlement. This paper analysed the influence of vacuum preloading process on the nearby foundation with numerical simulation method based on an engineering case. The vertical and lateral displacement was calculated and the effect of implement vacuum preloading on nearby foundation was evaluated.

Analysis of temperature field of embankment-CFG piles composite soft soil foundation system in cold regions

For the major projects built on the soft soil area in cold regions, the effective treatment of deep soft soil layer and the stable bearing capacity of foundation are of great importance. In this paper, the finite element model of embankment-CFG piles composite soft soil foundation for D3K629-D3K629+950 of Harbin-Dalian railway is established based on ABAQUS. The depth and regional distribution of freezing and thawing and distribution law of temperature field of CFG piles are studied. The results show that the horizontal position of 0-degree line is higher and the protective effect of embankment on the foundation is obvious when the embankment is higher. The embankment almost has no effect on the CFG piles near embankment foot. For the middle pile at the center of the embankment, the protection effect of the embankment is remarkable.

Earthquake responses of a base-isolated structure on a multi-layered soft soil foundation by using shaking table tests

Soil-structure interaction (SSI) has a significant effect on the earthquake response of a base-isolated structure, particularly on the rotation response of the SSI system and the isolation performance of the isolation layer, as demonstrated by previous shaking table tests (Zhuang et al., 2014). On a softer soil foundation, the SSI should have a greater influence on the seismic response of an isolated structure. To this end, a new shaking table test is conducted to estimate the effect of SSI on the dynamic characteristics of a base-isolated structure on a multi-layered soil foundation including a soft clay layer. As expected, the isolation efficiency of the isolation layer is reduced by the SSI effects, especially with increasing peak ground acceleration (PGA) of the input motion. Compared with the test results for an isolated structure on a harder soil foundation, the rotation responses of the pile cap and the isolation layer in this study are stronger. Additionally, the rotation responses of the pile cap are significantly amplified by the isolation layer. This type of amplification effect can become stronger with increasing PGA of the input motion, which differs from the results for previous tests with a base-isolated structure on a harder sand foundation. Meanwhile, when the natural isolation property of the softening soil layer is considered, the seismic responses of a base-isolated structure are reduced by the SSI effects because the natural isolation of the soft soil layer can compensate for the lost isolation ability of the isolation layer.

Shaking Table Test and Numerical Verification for Free Ground Seismic Response of Saturated Soft Soil

This paper investigates shaking table test (1g) and numerical simulation (fully coupled) of vertically propagating shear waves for saturated soft free field. A large-scale shaking table model test was performed to study seismic response characteristics of saturated soft soil free field. According to test results of seismic response features of free field system in saturated soft soil, the free field nonlinearity fully coupled numerical model of dynamical effective stress of saturated soft soil was established using OpenSEES, based on the u-p formulations of dynamic consolidation equation as well as effective stress solution method for saturated two-phase media. The numerical simulation of the free field seismic response of saturated soft soil under various test conditions was performed and the calculated results were compared with the shaking table test results. The results show the following. (1) With the increase of input ground motion intensity, the characteristic frequency of the saturated soft free ground decreases and the damping ratio increases gradually. (2) The saturated soft soil ground has short period filtering and long period amplification effect on the horizontal input seismic loads. The failure foundation takes on the isolation and shock absorption under strong ground motions. (3) The peak pore pressure ratio of the saturated soft soil ground is located in the shallow buried soil layer, and with the increase of the input ground motion intensity, the advantage of dynamic pore pressure ratio in this area is gradually weakened. (4) The numerical simulation results are consistent with the results of the shaking table test. This fully coupled effective stress numerical method can reasonably simulate the seismic response characteristics of free field in saturated soft soil, which lay the foundation for other more complex parameter extrapolation models of saturated soft soil sites. This research can provide the necessary technical experience for experimental study on non-free field.

Theoretical Analysis on Soft Soil Foundation Long-term Settlement for Road Engineering

Long-term settlement is a big issue which should be consider for dredger fill soft soil foundation. This paper analyzed the long-term settlement of a road engineering located at Tianjin Nanjiang Port. The long-term settlement was calculated based on “Code for Foundation of Port Engineering”, and the predicted result is compared with the in-site monitoring data. The comparison results show that the long-term settlement calculation results based on code should be modified before using for settlement prediction. The modify factor is 1.1 to 1.3 for Tianjin area, and the factor should rises to 1.5 when the value of preloading is small and the foundation stiffness is small.

Influence of vacuum preloading on vertical bearing capacities of piles installed on coastal soft soil

Land reclamation has increased significantly in the eastern coastal areas of China. The increased exploitation of offshore resources has made cast-in-situ piles more preferable in these regions. However, precise prediction of axial forces and shaft resistances of piles is particularly difficult because geological conditions are complex after the foundation is treated by vacuum preloading. In this study, two groups of cast-in-situ piles, each of which consisted of two piles installed in soft soil in Oufei Project, Wenzhou, China, were compared by conducting tests using the slow static loading method to evaluate the influence of applying vacuum preloading to deal with soft soil foundation on the vertical bearing capacities of the piles. Two piles were located in an untreated area, while the other two were located in a vacuum preloading treating area. All the piles had the same length and diameter. In addition, the axial forces and shaft resistances of piles were calculated based on the measured strains. The field tests revealed that the ultimate bearing capacities and shaft resistances of test piles were significantly improved compared to those of the piles in untreated area. The experimental results presented in this study are expected to be highly beneficial for practical engineering.

Field experiments and numerical analysis on the ground vibration isolation of wave impeding block under horizontal and rocking coupled excitations

In this paper, the effectiveness of a semi-analytical BEM in studying the vibration mitigation of wave impeding block (WIB) in layered soil under horizontal loads is verified by field tests, subsequently the performance of WIB in mitigating the vibrations of ground subjected to horizontal-rocking excitations were investigated by numerical analysis using the verified BEM. Two types of layered half-space model under the horizontal-rocking excitations (site A: the stiff soil ground with relatively soft overlying soil layer, site B: the soft soil ground with relatively stiff overlying soil layer) are considered. Test and numerical results indicate that WIB can achieve desired vibration isolation effectiveness in layered soils under both the horizontal and horizontal-rocking coupled excitations. With the decrease of the embedded depth, the performance of WIB for vibration mitigation improves in layered ground under horizontal excitation. For horizontal-rocking coupled excitation, WIB reduces ground vibration by 50–70% at zones over one diameter of WIB (10 m) away from the vibration source; at zones within one diameter of WIB, peak AR value occurs, and the peak values are beyond 1.0 (2.0 in site A and 12.5 in site B at normalized distance D = 0.75) in radial direction. WIB behaves generally better in site A than in site B.