Soft Subgrade Research Articles

Finite Element Analysis of Geogrid-Incorporated Flexible Pavement with Soft Subgrade

Improving the durability of flexible pavements and constructing new roads on weak soil foundations present significant challenges, prompting designers to explore alternative methods to prolong pavement lifespan. Geosynthetics have emerged as a promising solution for soil stabilization, with various materials developed for this purpose. The current study concentrates on using the finite element (FE) method to examine the effectiveness of geogrid-incorporated flexible pavements on soft soil substrates. A three-dimensional layered pavement is constructed with an FE model, incorporating subgrade layers of varying strengths based on their California bearing ratio (CBR) values, with a geogrid layer implemented to enhance subgrade stability. Additionally, attention is also given to investigating the effect of base course thickness. The findings reveal that the geogrid layer primarily influences the formation of plastic strains in the subgrade rather than resilient strains, effectively reducing vertical compressive strain by approximately 40%. With increasing CBR values, there is a reduction in vertical strain, although the influence zone extends up to a depth of 300 mm within the subgrade. At the surface of the subgrade, vertical strain decreases by around 17%, 39%, and 49% as the CBR values increase from 1% to 3%, 5%, and 8%, respectively.

Lightweight rPPU Eco-block Using Recycled Plastic (rP) and Polyurethane (PU) for Soft Subgrade Soil Stabilisation

The stability of the soil plays a crucial role in the safety and longevity of various civil engineering structures. However, problematic soil conditions, such as clay soil and peat soil, often pose challenges due to their high-water content, poor strength, low bearing capacity, and high compressibility. This research focuses on addressing these challenges by utilizing lightweight fill materials as an effective solution for backfilling applications. The study aims to investigate the characteristics and performance of a novel lightweight fill material using recycled plastic (rP) and polyurethane (PU) known as rPPU ecoblock. The eco-block is mainly fabricated using recycled plastic bottles, recycled plastic packaging, and polyurethane, offering a sustainable approach to waste management and construction materials. To achieve the research objectives, a series of experiments will be conducted. The strength and density of plastic bottles filled with recycled plastic packaging will be analyzed, providing insight into their mechanical properties. Furthermore, the water absorption and buoyancy of the lightweight rPPU eco-block are evaluated, considering its suitability for various construction scenarios. The research findings contribute to the understanding of the physical and mechanical characteristics of the rPPU eco-block, providing valuable information for its potential application as a backfill material. The compressive strength, water absorption, and buoyancy tests conducted per ASTM standards provide quantitative data on the eco-block's performance. The use of lightweight fill materials, particularly the rPPU eco-block, offers benefits such as reduced embankment self-weight, improved stability, and minimized environmental impact. Moreover, the recycling of plasticwaste contributes to the circular economy and aligns with sustainable development goals (SDG): SDG9 and SDG11. The results of this research provide valuable insights for engineers, construction professionals, and stakeholders involved in geotechnical and civil engineering projects.

Improving Soft Subgrade Stability Using a Novel Sustainable Activated Binder Derived from By-Products

Soft soil concerns, due to high compressibility and low bearing capacity, prompted an investigation into stabilizing clay soil. Traditionally, binder including cement or lime has been used as stabilizers though a current requirement of alternatives is stem from environmental concerns. The study focused on the viability of using a novel binary activated blended binder composed of environmentally friendly materials, namely ground granulated blast furnace slag (GGBS) activated by cement kiln dust (CKD). The experimental work included investigating the impact of the developed binders on the Atterberg limits, standard Proctor compaction, California Bearing Ratio (CBR), unconfined compressive strength (UCS), and field-emission scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy. CBR tests were conducted after 7 days of curing or soaking, while UCS and SEM analyses were conducted after 7 and 28 days of curing. A fixed binder ratio of 9% was maintained, with GGBS blended at 25%, 50%, and 75% with CKD. For comparison, samples of untreated and treated soils with unary binders from GGBS and CKD were also prepared. Results indicated that activated binders notably decreased soil plasticity and maximum dry density, while elevating optimum moisture content, CBR, and UCS, especially in later stages of treated soil and unary GGBS binder. Unary CKD binder exhibited a similar trend to activated binders. The activating of 25% GGBS with 75% CKD provided the optimum binder which increased the mechanical strengths by about 6 times than untreated soil. SEM revealed substantial formations of C-S-H and C-A-H gel, along with ettringite, intensifying with time. This research provides viable outcomes for stabilizing clay soil using environmentally friendly binders, demonstrating significant improvements in soil properties, particularly when using the binary activated blended binder consisting of GGBS and CKD.Graphical

Effective CBR and Elastic Modulus of Geogrid-Stabilized Prepared Subgrades Overlying Existing Soft Subgrades

Effective CBR and Elastic Modulus of Geogrid-Stabilized Prepared Subgrades Overlying Existing Soft Subgrades

Numerical assessment of geocell-reinforced pavement performance subjected to cyclic loading conditions

A numerical analysis was conducted using PLAXIS 3D software on a geocell-reinforced pavement over a soft subgrade under cyclic loading conditions. This study illustrates surface deformation and vertical stress within the subgrade layer of a reinforced pavement. The accuracy of the FE model was confirmed by comparing the results ofthe present study with those reported in the literature. This study investigated the influence of various factors, such as different infill and subgrade materials inreinforced pavements, on the overall pavement performance. These findings were compared with those observed for conventional planar-reinforced pavements. For improved infill and subgrade materials, the total surface deformation of the reinforced section decreased by approximately 15%–40%. Compared with the improvement in subgrade properties, a significant reduction of approximately 10% was observed inthe pressure response at the subbase-subgrade interface when using improved infill material. Moreover, compared to the planar-reinforced pavement, the geocell-reinforcedpavement reduced vertical stress at the interface between the base and subgrade by approximately 26%.

Subgrade soil response to rail loading: Instability mechanisms, causative factors, and preventive measures

The demand for more efficient heavy-haul rail networks over soft subgrades poses significant geotechnical challenges and requires a comprehensive understanding of stress conditions as well as the failure potential of subgrade soil under moving wheel loads and increasing rail speeds. Unfavourable stress conditions in the subgrade can result in various types of failures, three of which are identified in this article: (i) excessive plastic settlement, (ii) progressive shear failure, and (iii) subgrade fluidisation (mud pumping). Through a series of advanced testing schemes using cyclic triaxial, hollow cylinder, and an in-house dynamic filtration apparatus, critical stress conditions and soil characteristics prone to subgrade instability are discussed. The results demonstrate that under adverse combinations of loading frequency (f) and cyclic stress ratio (CSR) the continuous application of cyclic loads can lead to an unstable state of soil where excess pore pressure and axial strain increase rapidly. This study also reveals that low to medium plasticity soils (PI < 22) are more vulnerable to subgrade fluidisation, where the rapid internal migration of pore water transforms the upper soil to a fluid-like state with substantial loss in soil stiffness. The layered response of soil through dynamic filtration tests showed larger hydrodynamic forces induced by differential hydraulic gradients in the top layer during cyclic loading causes moisture to move upwards. Various factors that can influence soil instability such as the degree of compaction degree, clay content, soil fabric and stress rotation are also addressed in this paper. Finally, novel solutions for stabilising subgrade such as a vertical drain-composite system and the use of eco-friendly biopolymers are presented.

Pre-Design of Pavement Behavior of Nailed-Slab System to Soft Soils Due to Eccentric and Lateral Axial Loads

Under natural conditions, low to medium-density soils tend to experience considerable deformation when crossed by repeated loads from vehicles. Therefore, a pavement structure is needed to protect the soil from excessive loads caused by vehicles. Rigid pavement is one of the common types of road pavement used on soft subgrade soils. A proposed method to address road pavement issues on soft soils is the nailed-slab system. This research is a quantitative study through an experimental method conducted in the laboratory using a 1:1 scale. A container houses a prototype of rigid pavement featuring a nailed-slab system. Strain gauges are installed at specific points on the pile and pavement slab, and they are connected to a computer to record any changes in deflection that occur on the pile and the pavement slab when subjected to eccentric and axial lateral loads. Numerical analysis is also performed using Plaxis for comparison. It is found that lateral loads can affect the interaction between the soil and the pile. Repeated lateral loads can create gaps that reduce the soil friction against the pile. These gaps tend to become larger as the lateral loads are increased and applied repeatedly, thus causing a greater settlement of the slab above them as well.

Foam Mortar Layer in Soft Subgrade Embankment: A Review

Soils are the basis of the construction of civil engineering structures that are designed to support and withstand the weight of the structures above them. The main problems of soft soil include low bearing capacity and large settlement. In road pavements, the sub grade as the bottom layer of the road must have a good bearing capacity. Subgrade is a part that must be considered because of the weight of the pavement structure and the embankment above it. Soft soil conditions cause the need for reinforcement of the sub grade layer. Solving the problem of road construction is done by engineering so that the sub grade as the bottom layer of the road is safe and stable and has a good bearing capacity. The effort made to increase the bearing capacity of the subgrade is by reinforcing the soil (soil improvement). One of them is with lightweight material mortar-foam is an alternative construction material in soil reinforcement for road construction. Mortar-foam is formed from a mixture of foam, cement, sand and water. Research on foam mortars has been conducted in various places and countries in various ways and methods. This article will discuss the latest developments in the use of foam mortars as backfill in soft soils in the subgrade layer presented based on information provided by the author.

Enhancing soft subgrade performance with geocomposite-embedded capping layer: utilising Australian granular pavement design chart for assessing geocomposite benefits

Improving soft subgrades in road construction is essential for efficient machinery use, reducing gravel layer thickness, and enhancing pavement performance. Traditional methods like gravel replacement and chemical stabilisation are costly and harmful to the environment. Geosynthetics offer a cost-effective, eco-friendly alternative, but their full potential in Australian pavement design remains untapped due to limited research on estimating their modulus. This study addresses this gap by modeling geosynthetic-stabilised subgrades in a controlled environment and conducting plate load tests. Results show that incorporating geocomposites reduces capping layer thickness by 25% and overall granular cover by 20%. By developing a method to estimate geosynthetic-stabilised subgrade modulus and integrating it into design charts, this research aims to promote more efficient and sustainable road construction practices in Australia.

Behavior of Bamboo and Jute Geocell Overlaying Soft Subgrade under Repeated Wheel Loading

Behavior of Bamboo and Jute Geocell Overlaying Soft Subgrade under Repeated Wheel Loading

On the effect of subgrade strength on the performance of geogrid-reinforced railway ballast

This paper presents the results of a series of ballast box tests aimed at investigating the effectiveness of geogrid reinforcement in reducing track settlement in a 300mm-thick layer of railroad ballast supported by three different artificial subgrades. In each experiment, the ballast layer supports a model tie subjected to cyclic compressive loading applied at a frequency of 0.8Hz with stress extrema at the tie-ballast interface of 57kPa and 400kPa for a total of 40,000 cycles. The three artificial subgrades considered in this study have CBR readings of 25, 13, and 5. For each subgrade, four tests are performed whereby one corresponds to an unreinforced condition (i.e., no geogrid) and three are reinforced with a single geogrid placed at either 150mm, 200mm, and 250mm below the bottom of the tie. The results indicate that geogrids exhibit a superior ability to minimize the tie’s settlement when the ballast layer is supported by a weak subgrade. The experiments further allude to the fact that the influence of the geogrid’s placement depth is exacerbated by the subgrade’s strength. In ballast layers supported by competent subgrades, the geogrid placement depth wields a marginal influence on the resulting tie settlement. However, the geogrid’s location becomes a key factor in ballast beds underlain by soft subgrades, with geogrids placed closer to the bottom of the tie being the most effective at minimizing the tie settlement.

Use of high-performance reinforcement geosynthetics to stabilize access roads

With the rapid infrastructure development, the availability of competent soils is decreasing. The access roads and tracks practicability should be ensured when crossing these weak soils. The practicability of access roads and tracks is therefore important. Depending on the mechanical properties of the subgrade, the construction of access roads requires the use of granular foundation materials of significant thickness, which can result in substantial costs and construction delays. This is especially true when the runway is to be constructed on soft subgrade and possibly in the presence of water. This article presents the use of a high-performance reinforcement geosynthetic to stabilize unpaved access roads, control settlement and prevent contamination between the subgrade and the foundation. A laboratory case well documented is used. Two calculation methodologies based on the subgrade soil characteristics are provided and compared.

Effect of gravel size and geogrid opening variation on the reinforced of the unpaved roads

In unpaved roads, the bare soil of the soft subgrade or the covered grading layer of subgrade soil has traditionally been the focus of embankment and pavement loads. Using geogrid as isolation and reinforcement between road layers is a widely accepted method to solve this problem. This study used the California Bearing Ratio (CBR) test to evaluate the geogrid reinforcing effects among geogrids with various aggregate particle sizes and the size of the openings of the geogrid. The research results show that when the particle size of the subbase layer was controlled and the ratio of the geogrid opening was variated, the bearing capacity was improved relatively more significantly. The contribution of geogrid to CBR value increased from 12% to 50%. In the reinforced system with a fixed geogrid opening aperture, increasing the particle size of the aggregate will increase the overall bearing capacity of the reinforced system; however, when the size of the aggregate increases to a certain level, the contribution rate of the geogrid CBR value changes relatively indignantly. Additionally, in the reinforced system, by fixing the particle size of the aggregate and adjusting the ratio between the aggregate particle size and the geogrid opening aperture, the overall bearing capacity of the reinforced system is still improved, and the CBR contribution rate provided by the geogrid changes relatively significantly. Finally, the research results also show that the 3Dprinting geogrid had a similar effect to the commercially available geogrid. Therefore, different types of geogrids could be made in follow-up research, and further analysis had the best effect. The particle size of the gravel with the best-reinforced effect and the ratio of the geogrid opening can provide a reference for engineering design.

Evaluation of intermixing application of geotextiles at subgrade-base interface using accelerated pavement testing

This study focuses on identifying and quantifying the potential benefits of a geotextile separator in flexible pavements placed at the interface between a granular base and an underlying soft subgrade with high water content and a granular base. While benefits of using separators in roadway applications have been generally acknowledged, quantification of such benefits has been limited. The evaluation in this study is based on the results from two accelerated pavement tests using a one-third scale Model Mobile Load Simulator (MMLS3) device. Both models have identical configuration except that a non-woven geotextile was placed at the interface between the subgrade and base layer to serve as a separator in one of the pavement models. The subgrade material used in this study involved a soil mixture containing fifty percent kaolin and fifty percent Monterey sand #30. The base material was an angular AASHTO #8 aggregate. Additionally, an instrumentation program was implemented involving sensors to track the earth pressure distribution, the subgrade excess pore pressures generation, the change in moisture content of the subgrade and the horizontal particle displacement of the base layer. During exhumation of the pavement models, base and subgrade samples were collected to quantify the levels of intermixing. The results of the APT were quantified in terms of the traffic benefit ratio (TBR), which was as high as 6.4 for 25.4 mm rutting showing the significant benefit of using a non-woven geotextile as a separator to extend the design life of flexible pavements. The differences in performance observed between the control and the separation pavement models were found to correlate well with the amount of intermixing observed at the interface between subgrade and base layers. A series of soil columns were tested using cyclic loading to extend the intermixing analysis to different subgrade types with different fine content and geotextiles. The study concluded that higher levels of fine contain in the subgrade would lead to higher levels of intermixing. A correlation between the pore size of the geotextiles and the pumping of subgrade into the base was identified suggesting that smaller pore size would mitigate the pumping intermixing mechanism more efficiently.

Cost Comparison of Flexible Pavement on Weak Sub-Grade Soil Modified with Lime and SD

The effectiveness of flexible pavement is affected by the subgrade quality. The subgrade refers to a compacted layer of soil that provides sideways support to the pavement. When constructed on a weak subgrade, it negatively impacts the pavement's performance, leading to a shorter lifespan. Traditionally, the common method to stabilize a soft subgrade involves removing the weak soil and replacing it with stronger soil. However, due to the high expenses associated with soil replacement, highway agencies are exploring alternative approaches to constructing highways on soft subgrades. Soil stabilization is a commonly employed alternative in pavement construction, serving as an effective method to enhance the engineering characteristics of soil, including its strength and stability. This paper focuses on the utilization of lime and stone dust (SD) as admixtures for an efficient ground improvement technique over weak subgrade soil deposits. The California Bearing Ratio (CBR) test is conducted by making the specimens of weak subgrade by adding the variable percentages of a mixture of lime and SD. First, the soil was mixed up with lime to 12% by weight with an increment of 3% again the soil was mixed with SD with increments of 10% up to 50% by weight of soil. The study determined an optimal lime content of 3% based on the geotechnical properties of the mixture and the cost considerations of lime and the weak subgrade. Following this, SD was added to the optimized lime-weak subgrade mixture in varying increments of 10% by weight, up to a maximum of 40%. The modified mixes were then evaluated for their CBR and maximum dry density values. The CBR is increased to 15% and the total pavement thickness decreased to 725 mm for 50% SD addition with 4.89 % in cost reduction.

Numerical analysis of the influence of granular trench reinforcement of soft clay subgrade on the undrained dynamic response of a railway track

Soft subgrade poses significant challenges to the designer of railway tracks due to its high compressibility and low shear wave velocity. The low shear wave velocity might affect the maximum design speed of the train. The stabilization of this soft soil improves its performance and reduces the expected settlement. However, little attention has been paid to understanding the feasibility of different stabilization techniques, as there are very few studies on this topic. Thus, this research study aims to determine the impact of granular trench reinforcement of soft subgrade on the dynamic response of a railway track subjected to a moving train load. Three-dimensional finite element analysis has been used in this research. The effect of the train speed and the depth of the granular trench has been carefully evaluated. It has been found that the critical velocity (which is the velocity that corresponds to the highest settlement) is not affected by the granular trench reinforcement. In addition, it has been noted that the granular trench reinforcement reduces the maximum settlement of the railway track under the moving loads, with a percentage reduction ranging between 1% and 24%. Furthermore, the granular trench reinforcement did not affect the acceleration induced in nearby areas due to the train’s vibration. The results of this research provide useful insight into the feasibility of the granular trench and help design engineers compare different solutions for the problems of soft subgrade soil.

Experimental Evaluation of Relative Soil–Geotextile Displacements Induced by Strip Footing Pressure on a Reinforced Foundation Bed over Soft Subgrade

Experimental Evaluation of Relative Soil–Geotextile Displacements Induced by Strip Footing Pressure on a Reinforced Foundation Bed over Soft Subgrade

Numerical Modelling and Predicting Performance of Geogrid-Reinforced Low-Volume Unpaved Roads over Soft Subgrades

AbstractPavement performance is usually predicted by large-scale laboratory experiments, expensive field tests and/or comparatively cheaper numerical modelling alternative. In this study, a finite element limiting strain model has been developed to investigate the strain response of both unreinforced and geogrid-reinforced pavements on unpaved roads where the geogrid is placed at the bottom of the unbound granular base layer. A two-dimensional (2D) axisymmetric finite element model (FEM) is used to analyse the behaviour of both unreinforced and geogrid-reinforced granular base. The critical pavement responses (vertical surface deformation, compressive strain and compressive stress at the top of subgrade, etc.) are simulated numerically using ABAQUS. These critical responses are used to develop the pavement performance models (strain limiting models) and to predict long-term service life of pavement or the reduction of thickness of granular base for equivalent service life or a combination of both. The parametric results are then used to develop design charts to aid practitioners to use directly in designs. The numerical model is verified using published literature-based information and found to be reasonable. The results show that the traffic benefit ratio (TBR) of 3.1 at a 20 mm rut depth can be achieved for a thin granular base built over soft subgrade using a medium stiff geogrid.

The Shear Strength of Clay Stabilized with Palm Bunch Ash and Cement

Roads with soft subgrade have low bearing capacity. The result is non-uniform soil settlement. One way to overcome this is with soil stabilization. Stabilization of clay soil is done chemically with palm bunch ash and cement. The purpose of this study was to determine the bearing capacity of soil for the shear strength value of clay soil with variations in mixing 5%, 10%, 15%, 20% oil palm bunch ash, and 10% cement with a 7-day curing period, and then the samples were tested using a direct shear test tool SNI 3420: 2016. The results showed that in the addition of a mixture of oil palm bunch ash and cement with levels of 5%, 10%, 15%, 20%, and 10% cement, the cohesion value increased at a percentage of 20% oil palm bunch ash and 10% cement, namely 0.5448 kg/cm2 compared to the cohesion value in the original soil of 0.0081 kg/cm2, an increase of 98.51%, and a shear angle value of 53.12° while the value of the original soil shear angle was 16.93°. This research concludes that the shear strength value of clay soil stabilized with oil palm bunch ash and cement increases.

Performance evaluation of rail trackbed stiffness: pre and post stabilisation

Excessive deflection of a rail in response to axle loading can lead to discomfort for passengers and increased wear of both railway structures and trains. These oscillations are often caused by poor trackbed stiffness which may be due to either soft subgrade and/or contaminated ballast. A variety of trackbed stabilisation (TBS) techniques are available to remediate soft subgrades and increase the safety of tracks within the railway network. Traditional TBS methods require track removal, which is expensive, disruptive and often inefficient maintenance works. Micro-piling, using screw piles installed between sleepers, is an innovative low disruption TBS technique. This paper investigates the performance of a soft subgrade and contaminated ballast section of rail line in the UK, before and after screw pile TBS. Pre and post remediation, a computer vision-based system was used to measure rail vertical deflections during train passages and then analysed to quantify the trackbed stiffness. Additionally, 3D finite element models are created and validated by the site measurements. The finite element models are used to simulate a range of different scenarios exploring how changes to the TBS piling layout and/or further works, such as ballast improvement could add further improvements or design efficiencies. Site measurements show TBS reduced rail deflection by 20–30%, indicating that micro-piling is an effective technique for soft subgrades. The finite element analysis revealed the efficiency of micro-piling is highly dependent on the conditions of ballast, strength of the ground at the pile toe, and the pile arrangement. When the aforementioned are optimised the rail deflection could be reduced to approximately 50% of the pre TBS condition.