Soft Subsoil Research Articles

Influences of number of geosynthetic reinforcement layers on soil arching under cyclic loading

Geosynthetic-reinforced pile supported (GRPS) embankments have been widely used in highways and railways on soft subsoil. During the operation period, these embankments are often subjected to cyclic loading, such as traffic loading. Considering that soil arching is a key load transfer mechanism in these embankments, the development of soil arching under cyclic loading is vital important to operational safety of these embankments. Conventional trapdoor system cannot represent actual conditions of soil arching in GRPS embankments because it have a forcibly-moved trapdoor and cannot simulate load-induced subsoil settlement. Hence, this paper used a self-moving trapdoor supported by compression springs that moved up and down dependent on its overburden pressure. To investigate influences of number of geosynthetic reinforcement layers on soil arching under cyclic loading, two self-moving trapdoor tests were conducted, including a single layer and two layers, where the total reinforcement stiffness were approximately same. The trapdoor test results show that the inclusion of two reinforcement layers with a fill layer of a certain height in between reduced surface settlement of the fill and tensile forces of reinforcement, as compared with a single reinforcement layer. It also enhanced soil arching effect and improved the stability of soil arching under cyclic loading.

Analysis of deformations of road embankments founded on displacement columns improving soft subsoil

Drilled displacement columns, constructed in the form of unreinforced or reinforced concrete elements, are currently a very commonly used method of improving soft subsoil, creating an alternative to more expensive pile foundations. A frequently used solution for improving soft soils of road or railway embankments is to design a regular pattern of columns of relatively small diameter. Columns along the perimeter of the improved area are reinforced with rigid steel profiles, while the internal ones are made as concrete elements. Column heads are usually covered with a load transfer platform (layer of compacted granular fill) which is additionally reinforced with geosynthetics. The application of soil improvement with displacement columns is not always successful. It is due to the errors and shortcomings occurring at the design stage, including simplifications in modelling, to construction faults, which may include insufficient experience of contractors and/or improper supervision. Referring to the real object that failed, the article provides the results of numerical parametric analyses taking into account the influence of the key design parameters such as: the stiffness of the load transfer layers, the amount and stiffness of the geosynthetic reinforcement as well as the column spacing. The article presents comparisons of numerical results obtained with the finite element analyses for various approaches to geometry modelling (axisymmetric, 2D and 3D). The simulations indicate that the use of the axisymmetric model of a single column in routine design may lead to the deformations exceeding the serviceability limit states.

Vertical bearing capacity of spun precast concrete pile in liquefiable soil: a case study

Spun precast prestressed concrete (SPC) pile has been used in many parts of the world as a viable foundation alternative. This study assesses the load-carrying capacity of SPC piles passing through a deep soft subsoil layer and resting on a dense sand layer. The vertical bearing capacity of the SPC pile has been estimated using various analytical methods and static pile load tests for the Jolshiri area of Dhaka. The liquefaction potential of the site has been assessed using local seismic site conditions, field and laboratory test data. The liquefaction analysis suggests that the topsoil layer is liquefiable to a depth of 4.5 m. The capacity obtained from the load test is compared with those obtained from the different methods, ultimate push-in load, and local design guidelines. Finite Element Analysis (FEA) was performed considering the hardening soil (HS) model using PLAXIS 3D to simulate field conditions. The load settlement response obtained from FEA shows a good agreement with the test results. The capacity examinations primarily suggest that the SPC pile can be a viable foundation solution for the subsoil conditions of Jolshiri Abasion area of Dhaka.

Strength and load-bearing behaviors of cement-industrial by-products solidified crust layer over soft subsoil

Industrial by-products have a broad application prospect in sustainable soft soil treatment. The applicability and characteristics of a cement-slag-phosphogypsum based ternary cement (TC) are investigated for solidifying soft clay to create a crust layer. The strength and load-bearing behaviors of the solidified crust are characterized using unconfined compressive strength (UCS), triaxial compression, and small-scale loading tests. The ascending trends in UCS with increasing binder content (Cb) differ between TC solidified clay and cement solidified clay, and the UCS ratio between them increases with Cb. Besides, the failure strain and secant modulus of TC solidified clay have functional relationships with UCS, and the same functions work for cement solidified clay as well. In triaxial compression tests, the relationship curves of the deviatoric stress and excess pore water pressure with axial strain change to present more significant shear dilation characteristics as Cb rises from 6% to 8%, along with a sharp rise in the friction angle, suggesting the solidification performance improved to a higher level. The results of small-scale loading tests show that the ultimate bearing capacity (pu) increases with Cb and crust thickness (h), along with the potential crust failure mode transiting from punching to bending failure; the pu gets a sharp rise as Cb and h are just raised high enough to ensure the crust integrity. In more details, it is suggested to prioritize the effective solidification critical for stress spreading before enlarging h in order to achieve higher pu. Finally, TC has been applied and proven effective in engineering practice.

Extended concentric arches model for piled beam-supported embankments

Geosynthetic-reinforced embankments founded on piled beams have emerged as a new ground modification technique for supporting earth embankments over deep soft subsoil. This paper presents an analytical model for this technique that combines parts of two existing analytical design models. The soil arching behaviour that divides the vertical embankment load into a part that is transferred to the piled beams directly, and the residual load part that rests on the geosynthetic, is described by the 3D part of the Concentric Arches (CA) soil arching model of van Eekelen et al. (2013 and 2015), which was adopted in CUR226:2016. The paper also compares the difference between adopting the 2D or the 3D part of this CA model. The tensioned geosynthetic effect was described by the membrane action equation of Pham (2020b). This equation calculates the geosynthetic strain and includes load–deflection behaviour, adhesion, and friction along the geogrid/soil interface. Two field studies and fully coupled hydromechanical finite element analyses were used to verify the proposed analytical approach and its assumptions. The paper compares the effect of founding geosynthetic-reinforced embankments on piles or piled beams. Finally, a parametric study showed the impact of beam width, center-to-center pile spacing, and geogrid stiffness on load transfer efficiency.

3D numerical modeling of a rigid inclusion reinforced railway embankment under cyclic loading

Rigid inclusion reinforcements of embankments are increasingly used in traffic route engineering to ensure the long-term system sustainability and to enhance the soft subsoils load-bearing capacity under static and cyclic loading conditions in an economical and practical way. This paper presents a three-dimensional numerical finite difference model to assess the performance of a rigid inclusion reinforced railway embankment subjected to cyclic loading. A slice model of the railway embankment is considered based on full-scale physical tests. The advanced elasto-plastic SANISAND constitutive model is considered for the granular layer in the rigid inclusion reinforced railway embankment. The responses of the system under cyclic loading are outlined concerning settlements, stress distribution and axial–flexural response of the rigid inclusions.

Response Analysis of a Ballasted Rail Track Constructed on Soft Soil by 3D Modeling

The displacement, stress, and strain distributions of railway embankments on the soft deltaic deposit of the Ganges–Brahmaputra floodplain are investigated. A numerical model developed in general-purpose finite element software is used to simulate the design train load on a deltaic deposit for a 100 km/hr rail speed. The numerical analysis analogy is grounded in the spring model, where a beam under the unit load is modeled based on the Winkler foundation model concept. In the moving load simulation on soil, the static point load relating to the axle load is assigned in the form of a dynamic multiplier, determined using auxiliary software. The calculated shear force in terms of the influence line is applied as a dynamic multiplier. The numerical results demonstrate that under a dynamic train load, the loose ballast undergoes larger and more erratic displacement than the subballast. Comparative analysis between varying subballast stiffnesses shows that stiffer subballast yields smaller displacements. Moreover, a high subballast stiffness can counterbalance the potential of forming permanent deformation by generating lower strains. However, a stiffer subballast does not play a prominent role in reducing the displacement of ballast or vertical stresses. The subgrade is found to carry the maximum load, withstanding the maximum vertical stress; thus, the importance of using an improved subgrade with higher stiffness is also observed. A greater subgrade stiffness improves its load-carrying capacity but fails to reduce the tension responsible for the lateral spreading of the soft subsoil. To reduce the high radial strain, the effects of improving the stiffness properties of two immediately adjacent soft soil layers are numerically investigated. The improvement of subsoil alone is effective in reducing the radial strain, whereas the improvement of both subgrade and subsoil produces further reductions. The critical train speed generating the maximum displacement is identified as 120 km/hr, and the dynamic velocity amplitude decreases with depth. Finally, an allowable limit of rail embankment settlement on a soft deltaic deposit is observed.

Long-Term Cross-Slope Variation in Highways Built on Soft Soil under Coupling Action of Traffic Load and Consolidation

The variation in road cross slope with service life affects the pavement drainage and has an adverse effect on vehicle operation safety. This paper describes a cross-slope variation prediction method influenced by the coupling effect of traffic load and soil consolidation, considering characteristics of embankment to cover the shortage for insufficient consideration of compacted embankment. First, the traffic load-induced settlement equation of a highway on soft soil foundation was introduced, which considers the effects of traffic load stress, confining pressure, soil structure, strength weakening and cyclic loading times on different positions along the cross-section. Then, the finite element model of a highway on soft soil foundations under soil consolidation is established, considering the influence of soil seepage. Finally, predictions of long-term settlement and cross-slope variation generated by coupling action of traffic load and soil consolidation were carried out with contrastive analysis with measured data. We find the following: (1) the long-term settlement was mostly from the consolidation of natural soft subsoil, while the cross-slope variation was mainly affected by traffic load; (2) variation in the cross slope of highway in soft soil areas mainly occurs within 1 year of operation. The effect of passenger cars and trucks on cross-slope variation shows diametrically opposite results, as the passenger car lane decreases while the truck lane increases; (3) the relative error of the cross-slope variation prediction results with the measured data are 2.86% and 2.5% for the left and right lane cross slopes, respectively.

Impacts of open dumpsite leachates on soil and groundwater quality

Groundwater remains a vital natural resource and its security should be the top priority for everyone. Continuous indiscriminate waste dumping and unruly human behaviour towards environmental management and strategies require a holistic approach to guarantee environmental security. This study assesses the impacts of dumpsite leachate on soil and groundwater quality using geophysical and physicochemical water analysis approaches. Data were collected at dumpsites and a sample station using geo-electrical resistivity. The impact of the dumpsite's leachate plumes on the soil and groundwater quality was investigated. The resistivity values distributed within the dumpsites and the sample station indicate that the dumpsite leachate plumes have impacted the ground both vertically and horizontally. The physicochemical water analysis was carried out on four hand-dug wells within the radius of 61 m of the open dumpsites. The results indicate the presence of heavy metals, physical drilling depicts soft subsoil, while the geophysical result is characterized by low resistivity values, which show a good match. The soft soil, heavy metal presence, and low resistivity values suggest that the soil and the aquifer systems within and around dumpsites have been compromised by the leachate plums, which had seeped the ground down to the water table. High ion concentrations associated with leachate plumes have critically influenced the conductivity of subsoil and the groundwater within and around dumpsites. However, unpolluted fresh water can be achieved below 23 m depth. The combined geophysical investigation and physiochemical water analysis can give an account of dumpsite leachates activities on ground resources.

DEM Simulation of the Load Transfer Mechanism of a GRPS Embankment with a Fixed Geogrid Technique

As a new technique, a fixed geogrid in a geogrid-reinforced and pile-supported (GRPS) embankments has been used to reduce the total and differential settlement. To investigate the load transfer mechanism of the fixed geogrid technique of a GRPS embankment, three discrete element method (DEM) models of pile-supported embankments were established, including an unreinforced embankment, a geogrid reinforced embankment, and a fixed geogrid reinforced embankment. The efficacy of the pile, the evolution law of the contact force chain and the axial force of the reinforcement, and the microscopic load-bearing structure of the soil were investigated. Numerical simulation results showed that the embankment self-weight and surcharge load were transferred to the pile through the soil arching and tensile membrane effect. The settlement could be effectively reduced via the addition of the reinforcement, and the fixed geogrid technique was more conducive to improving the load-bearing ratio of the pile than the traditional reinforcement technique. Compared with the traditional technique of a GRPS embankment, the fixed geogrid technique had a better effect on reducing the total and differential settlement. With the increase in the surcharge load and the settlement of the soft subsoil, the reinforcement transferred a greater load to the pile. The results also showed that the stress of the embankment fill was concentrated at the pile top in all three models. The GRPS embankment with a fixed geogrid technique had a lower soil stress concentration than the other two cases. The contact force chain and stress in the embankment also showed that the reformation of the microscopic load-bearing system of the embankment fill was the internal mechanism that caused the development of the soil arching and the redistribution of stress. Furthermore, the evolution of the fabric parameters in the arching area could reflect the evolution of the soil arching structure. In the fixed geogrid case, the proportion of the load transferred to the pile from the soil arching effect was reduced, and the vertical load transferred to the pile top by the tensile membrane effect accounted for 22–28% in this study. Under the combined effect of the tensile membrane and the soil arching, the efficacy of the pile could increase by 10%.

Soil Arching of Piled Embankment in Equal Settlement Pattern: A Discrete Element Analysis

Soil arching, which occurs in the piled embankments, plays an important role in stress redistribution between the relatively soft subsoil and the stiffer piles. The formation of the soil arching depends on the differential settlement of the embankment fill above the pile and the subsoil. The soil arching effect is barely investigated in the literature from the perspective of differential settlement of piles and soils. Based on the discrete element method (DEM), this paper develops a classic trapdoor test model to investigate the differential settlement in piled embankment during the downward movement of the trapdoor, and to explore the formation mechanism of soil arching in equal settlement pattern by changing the width of the pile cap and the height of the embankment. Due to symmetry, only one section of the laboratory test model is simulated herein. It was found that the soil arching formed under the equal settlement pattern remained unchanged after a certain degree of development, and the height of the equal settlement did not change at 0.7(s-a), where s is the pile spacing, and a is the width of the pile cap. The height of the embankment (H) and the width of the pile cap (a) have a significant influence on the formation of the equal settlement pattern when the width of the trapdoor is kept constant. Both the decrease in “H” and the increase in “a” facilitate the differential settlement of the soil between the piles and the pile-soil, enabling the slip surface to develop upward gradually, thereby hindering the formation of the equal settlement pattern.

Spring-Based Trapdoor Tests Investigating Soil Arching Stability in Embankment Fill under Localized Surface Loading

Pile-supported (PS) embankments have been used increasingly to support highways and railways on soft subsoils. In addition to the self-weight of the embankment, this embankment system is often subjected to surface localized loading, such as traffic loading. In this embankment system, soil arching is a key load transfer mechanism. Stability of soil arching under localized surface loading is important because traffic loading applied on the embankment surface can transfer onto and between pile heads and affect the degree of mobilization and degradation of soil arching. Conventional trapdoor systems have a rigid trapdoor, control its displacement manually/automatically, and cannot represent load-induced subsoil settlement below the embankment. This study utilized a trapdoor supported on low-stiffness or high-stiffness compression springs that moved under the load above the trapdoor (called a spring-based trapdoor) to evaluate the effects of continuous trapdoor displacement on the soil arching stability under static footing loading. To investigate the trapdoor rigidity effect, a trapdoor consisting of three segments (called a flexible trapdoor) was utilized in this study as well. The trapdoor test results showed that soil arching was mobilized during fill placement as the fill height and the trapdoor displacement increased. Subsequently, under static footing loading, the degree of soil arching increased at a low applied pressure; however, it degraded under higher footing loading that caused a larger trapdoor displacement. The high-stiffness trapdoor increased the degradation pressure required to eliminate soil arching even though it reduced the degree of soil arching under a low applied pressure compared with the low-stiffness trapdoor. The flexible trapdoor resulted in a uniform stress distribution on the trapdoor but reduced the total load transferred to the supports. The conventional trapdoor resulted in a lower soil arching ratio compared with the spring-based trapdoor during the soil arching mobilization, but had a higher soil arching degradation rate than the spring-based trapdoor due to continuous soil movement.

Esplanade metro station in Kolkata, India: from concept to optimisation

Kolkata’s Line 2 metro is a major urban infrastructure project in India. Due for completion in 2022, it will connect the cities of Howrah and Kolkata by tunnelling through soft clayey subsoil under the Ganges and one of the world’s oldest and most congested urban areas. This paper describes the concept design of the new part of Esplanade station, one of the key interchanges on the project. It then explains how the configuration of the proposed four-level underground structure was optimised from a structural aspect, especially the depth and thickness of diaphragm walls.

Numerical simulation of the progressive development of soil arching in column-supported embankments

The soil arching mechanism is partly responsible for the transfer of stresses away from the soft subsoil and towards the relatively stiff column heads in column-supported embankments. Experimental studies have shown that the load transfer resulting from soil arching evolves progressively with increasing subsoil settlement. Past numerical studies exploring soil arching in column-supported embankments have typically not been able to capture this progressive development of load transfer. A series of improvements on past numerical studies are outlined that allow for improved simulation of the soil arching mechanism in column-supported embankments. These improvements include implementation of a strain-softening constitutive model, non-local integral type regularization and the application of the arbitrary Lagrangian–Eulerian finite element method. The benefits of these improvements are observed through comparison of simulation results to recent experimental studies of column-supported embankments. The comparison indicates that these techniques allow key aspects of soil arching kinematics and mechanics to be captured.

Analysis of unreinforced and reinforced shallow piled embankments under cyclic loading

Reinforced piled embankments are being increasingly used for construction over soft ground due to their efficiency in reducing potential settlement, speed of construction, and associated cost. Most of the previous studies focused on developing understanding of the behaviour of thick embankments that are loaded with a static surcharge load. Data on the behaviour of shallow piled embankments under cyclic loadings are scarce. In this study, an experimental programme was undertaken using a fully instrumented testing rig to generate data and improve our understanding of the behaviour of unreinforced and reinforced shallow piled embankments subject to monotonic and cyclic loadings that were applied over a predetermined area of the embankment. The experimental results showed that collapse of soil arching is instantaneous and occurs during the first few cycles of load. However, strength was regained, and recovery of the arching effect was observable during further stages of cyclic loadings due to densification of the embankment material and deformation of the soft subsoil. Inclusion of reinforcement layers was found to enhance the performance of load transfer mechanisms by concentrating stresses on pile caps. The results clearly showed a significant reduction in surface settlement, soft subsoil settlement and heaving with increasing numbers of reinforcement layers.

Settlement of a railway embankment on PVD-improved Karakore soft alluvial soil

This paper presents a case history of the settlement performance of a railway embankment built on a Prefabricated Vertical Drain (PVD) improved soft alluvial soil of Karakore area, Wollo province, north-east of Ethiopia. The embankment was constructed on Awash - Kombolcha - Haragebaya (AKH) railway project for the purpose of preloading so as to facilitate the consolidation settlement in a short period of time; and it was monitored for more than 750 days. Numerical predictions of Class A and Class C (based on back-calculation procedure) were performed using Plaxis 2D. The soft soil creep (SSC) model was employed for the soft soil layers, while the Mohr-Coulomb (MC) model was used for the drainage and fill layers. The predictions were compared with the field monitoring settlement data. Quite a reasonable agreement was achieved from the finite element modelling of Class C prediction when the prediction was compared with the actual field settlement values. Parametric sensitivity studies were carried out to examine the influence of parameters on the rate and magnitude of settlements. The results of the numerical predictions and parametric studies have been discussed in detail. The soil on the flood plain area of Karakore is found to be very soft to soft alluvial deposit and formed over Tertiary Eocene stiff clay. The variable conditions of the soil formation and deposition highly dictated the deformation and excess pore water pressure behaviour of the soft subsoil sediment. The parametric sensitivity study showed that, following the compression index, the initial void ratio, the hydraulic conductivity, and the OCR highly influenced the settlement responses of PVD-improved soft alluvium soil respectively.

Analysis of geosynthetic-reinforced pile-supported embankment with soil-structure interaction models

Abstract A geosynthetic-reinforced pile-supported (GRPS) embankment is a complex soil-structure system. The response of a GRPS system is affected by the interactions among its five linked elements, namely the geosynthetic (a synthetic product used in civil engineering to stabilise the terrain), foundation, granular platform, fill soil, and geological media underlying and surrounding the foundation. The key load transfer mechanism in a GPRS embankment is a combination of various phenomena that include arching in the fill layers, tensioned membrane effect of the geosynthetic, frictional interaction, and support of the soft subsoil. Referring to the current numerical and experimental studies, a new theory has been developed in this study by combining the arching theory for the soil layer and the tensioned membrane theory for the geosynthetic. The subsoil effect with both linear and non-linear models and the frictional interaction are also included in the proposed method, thereby providing a more comprehensive design approach than the earlier methods that considered only either the tensioned membrane theory or the arching theory. It is demonstrated in this work that the proposed method produces results that are in good agreement with the experimental observations.

Stability Control of Staged Filling Construction on Soft Subsoil Using Hyperbolic Settlement Prediction Method: A Case Study of a Tidal Flat in China

An improved method, which combines in situ measured settlement data, hyperbolic method, and deep lateral displacement rate, is presented in this study to predict the consolidation and stability of the ground, which can be used in conducting staged filling construction on soft subsoil. A case history of a highway embankment construction in a tidal flat with thick mucky clay is studied in Xia Pu, China. Preloading with the prefabricated vertical drain method is adopted to accelerate the consolidation of a subgrade. The field behavior of soft ground under filling load is observed through in situ monitoring sensors in four typical sections. The final ground settlement in each stage is determined using the field monitoring data based on the hyperbolic settlement prediction method. For each stage of graded filling load, the ground settlement with a strain consolidation degree of 95% is defined as the standard settlement, and the corresponding settlement time is set as the standard settlement time. The preloading period is estimated according to the standard settlement time. The deep lateral displacement rate of the ground is monitored to control the stability of the foundation and recommended to guide the embankment construction. Results indicate that the presented method can predict the preloading time of graded filling, reduce the frequency of observation, and ensure the consolidation and stability of the ground.

Influence of sand content on the flow characteristics of soft soil under cyclic and high-frequency vibration

The flow characteristics of foundation soils subjected to train loads can present engineering hazards in high-speed railways. In order to verify the feasibility of blending coarse sand in modifying soft subsoil, undrained pulling sphere tests were carried out and the train loads were simulated through localized and cyclic vibration at various frequencies. Laboratory testing results indicate that the flow characteristics of soft soil can be significantly enhanced by high-frequency vibration; meanwhile the continuous increase in flow characteristics caused by cyclic vibration may be an important reason for the long-term settlement of soft subsoil. The influence of sand content on flow characteristics is also studied in detail, and it is shown that the addition of coarse sand can weaken the flow characteristics of soft soil induced by sudden vibration at lower than 50 Hz. Under the condition of cyclic vibration, the growth of the flow characteristics of sand-clay mixtures is mainly caused by the first-time vibration in the cycle, and the increase in sand content can make the flow characteristics present a faster convergent tendency.

Probabilistic seismic hazard analysis of Nepal

Probabilistic seismic hazard analysis for Nepal has been carried out considering uniform density model. A detailed earthquake catalogue since 1255 A.D, within the rectangular area has been developed and historical earthquakes are plotted in the map of Nepal. Five hundred twenty eight numbers of areal sources are used within the study area to characterize the seismic sources. The completeness of the data has been checked by using Stepp's procedure. Seismicity in four regions of study area has been evaluated by defining 'a' and 'b' parameters of Gutenberg Richter recurrence relationship. Seismic hazard curve of Nepal for soft subsoil condition for 10% probability of exceedence in 50 years period i.e. for return period of 475 years has been plotted.