Soft Subgrade Research Articles

Evaluation of back-calculated elastic moduli of unreinforced and geocell-reinforced unbound granular material from full-scale field tests

This paper presents a field-scale experimental track over a poor subgrade with an unreinforced section and a geocell-reinforced section subjected to in-situ performance tests. Plate load tests and Benkelman beam tests were carried out distributed in several unreinforced and reinforced layers. The objective was to: (1) examine the variability of the elastic modulus of unbound granular material (UGM) due the influence of its thickness and the presence of poor subgrade in its base, (2) evaluate the modulus improvement factor (MIF) generated by the geocell reinforcement in the UGM and (3) verify the most appropriate condition to apply the MIF to transport infrastructure design. The results showed that there is a significant influence of the thickness of the UGM layer on its elastic modulus when the layer is supported directly over a soft subgrade. The MIF values obtained in field suggest that its determination is mostly related to the UGM maximum elastic modulus rather than its decreased values (by virtue of poor subgrade or reduced thicknesses), and that the analytical formulation presented for MIF calculation has good predictive capability to be applied to pavement design.

Quantifying the Benefits of Geosynthetics Reinforcement in Flexible Pavement Design Using Cyclic Plate Load Testing

Development of pavement design over the past decades has focused on moving from empirical design equations to more powerful and adaptive design schemes. The AASHTO mechanistic–empirical pavement design guide (MEPDG) has been developed to model pavement structure and predict its service life more accurately. Although MEPDG has been widely implemented to design conventional pavement, it is not yet capable of predicting the service life of pavement reinforced with geosynthetics. Given the above concerns, seven full-scale test sections that were constructed at Louisiana Transportation Research Center-Pavement Research Facility (LTRC-PRF) were devoted to a structural experiment to investigate the performance of geosynthetic reinforced pavements. The benefits of using geosynthetics to enhance the performance of pavements constructed over soft subgrades was evaluated using cyclic plate load testing. Cyclic load at a frequency of 0.77 Hz was applied through a 305 mm diameter steel plate. The test results clearly show the benefits of geosynthetics in significantly reducing pavement rutting. The test section with double geosynthetic layers performed better than the other six sections studied. After eliminating the effect of variations in construction, the benefits of geosynthetic reinforcement are quantified within the context of the AASHTOWare Pavement ME Design Guide. The developed design procedure is capable of quantifying the contribution of geosynthetics in pavements to base reinforcement as well as subgrade stabilization. A design methodology is proposed that falls within the context of MEPDG.

Seismic Subsidence of Soft Subgrade with Considering Principal Stress Rotation

Seismic Subsidence of Soft Subgrade with Considering Principal Stress Rotation

Time Dependence of Neutral Plane Position in a PHC-Supported Soft Subgrade

This case study focuses on an International Circuit project in China where unmanned automobiles are to be tested in the future. The subgrade undergoes extensive settlement because of the widely distributed lacustrine deposits with high void ratio and compressibility. Prestressed high-strength concrete (PHC) pile-supported earth platforms are used for ground improvement in this area, but their effectiveness has not yet been confirmed. Associated with this adoption, the negative skin friction (NSF) of the piles and the related neutral plane (NP) position is of importance. Although NSF has been examined in detail, the NP position has not, especially with regard to its time dependence. This paper reports the settlement of the pile-supported soft soil subgrade as obtained from long-term observations of full-scale tests. The NP position is determined according to the layered settlement of the soil and the pile settlement, both of which are obtained from long-term field observations. The results reveal that the NP position has a clear time dependence that develops in stages. The NP is initially elevated with time, and then it gradually approaches a constant value. The existing models fail to predict this time dependence of the NP position in the studied sites. A exponential model is used, the fitting parameters of which have practical implications. The adopted model works well with the studied sites, especially in terms of reflecting the time dependence of the NP position and distinguishing the different stages of its development. This paper enhances the understanding of the NP and provides a reference data set for related engineering practices.

Long-term cyclic behavior of soft clay under different variable confining pressures and partially drained conditions

Traffic loading is one of the essential causes of the excess settlement of soft subgrade. The stress path induced by traffic loading is more complex than a single dimensional path and coupling of both vertical and horizontal stresses. Also, under long-term cyclic loading, the pore water pressure (PWP) generated in soft clay eventually dissipates. Therefore, to study the cyclic behavior of soft clay under a long-term traffic loading, a series of long-term (50,000 cycles) cyclic triaxial tests involving variable confining pressure (VCP) on saturated remolded clay were conducted under partially drained conditions in this study. The result shows both the cyclic vertical stress and VCP have detrimental effects on the soft clay. Thus, a new index (CSRη) is proposed to synthesize these two factors. The peak PWP and final volumetric strain are all linearly related to CSRη. An empirical model involving CSRη is proposed to predict the final permanent axial strain. Compare to the empirical models in previous studies in which both the constant confining pressure (CCP) and VCP tests are necessary, only three CCP tests are needed to obtain the parameters in this model.

Investigating the Hydraulic Properties of Unsaturated Crushed Stone Aggregate to Enhance Horizontal Drainage Systems on Soft Ground

Crushed stone aggregate materials are commonly used for basal embankment support on soft subgrades or for horizontal drainage in soft soils. When the drainage material is constrained by earth pressure and water pressure, it changes to an unsaturated water‐permeable state. If the pore water pressure decreases as the compaction of the soft ground increases, the water does not drain through the drainage material. Therefore, it is necessary to measure the critical pressure value (air entry value) going from saturation to desaturation. That is, even if the pore water pressure is small, the water must be discharged through the drainage material. Therefore, this study presents experimental results for gravity drainage and soil‐water retention curves (SWRCs) using mixed crushed stone aggregate samples at different particle size distributions and different compaction conditions. SWRC experiments indicate that porosity, hydraulic conductivity under saturated conditions, and air entrapment decrease with increasing sand content. Also, the samples retained more water at higher suction values, although the porosity decreased with increasing sand content. This phenomenon is due to the tendency of the uniformity coefficient to increase as more fine particles are added to the crushed stone aggregate. In addition, it is considered that the function as a drainage material is possible only when it shows a higher air entry value than that of the loose crushed stone aggregate drainage material. Therefore, for drainage materials using crushed stone aggregate, analysis of the soil‐water retention curve for unsaturated soil should be preceded.

Performance of the bamboo geotextile system ‘GEOBAMTILE’ for road construction on deep soft ground

Infrastructural development faces the dilemma of attempting to construct over deep soft ground. Practical methods to support heavy engineering construction over soft ground has evolved tremendously. Proper ways to treat a depth of softness that extends to a great depth technique are pretty expensive and non-functional. Furthermore, those available methods have their practical limitation during implementation. Bamboo Geotextile System “GEOBAMTILE” is designed to solve a critical contemporary civil engineering problem of supporting hefty construction safely over deep soft subgrade without attracting excessive settlements in due course. This study aims to evaluate the performance of Geobamtile on settlement reduction. Methodology to achieve the objective of the study is by data collection from the site. The next phase continues with construction of road work and analysing the result collected. Based on this study, it is found out that the implementation of Geobamtile reducing the settlement rate of foundation effectively. Compared to the predicted settlement design, the field data result is 88mm to 100mm compare the predicted settlement for the first 100 days is 301mm to 341mm. Soil settlement reaches a relatively constant value despite an increase in backfill height. From this study it shows that Bamboo Geotextile System “Geobamtile” is effective for heavy construction over deep soft ground compare to the predicted settlement design.

Mechanical Characteristics of Variable Cross‐Section Immersed Tunnels under Spatial Differential Settlement

This work establishes a fine model reflecting the characteristics of immersed tube structures to study the spatial mechanical characteristics of variable cross section immersed tunnels. Then, the mechanical behavior of the pipe segments under differential deformation is explored, based on which we propose a formula for estimating the shear key force and analyze the corresponding failure mechanism and failure mode. Thus, the critical points of the pipe segment design are specified. The results show that, first, the variable cross section segment bears more sectional internal force (over 47%) than the uniform cross section segment in actual cases, which should be fully considered in practical reinforcement design. Bearing deformation alternatively occurs at different shear keys, implying an apparent spatial stiffness matching problem. Second, the pipe segments show overall settlement under longitudinal bending, failing to satisfy the assumption of deformed plane sections. The force on the shear keys varies with the quadratic function of the longitudinal bending degree, and the dislocation magnitude has a positive near‐linear relationship with the bending degree. Third, when subjected to torsion, the shear keys of the weak sidewalls are sheared to resist torsion. Then, this force is laterally transferred to the top and bottom plates and jointly shared by the adjacent shear keys, explaining why shear is adjusted with structural deformation. Fourth, under the action of spatial differential deformation, the shear force is mainly undertaken by the vertical shear keys. When the structure is subjected to bending, compression and shear failure occur at the end angle of the middle wall shear key, and its root junction experiences tension and shear failure. When subjected to longitudinal torsion, the local tension and shear failure at the root of the shear key on the soft subgrade side cause a joint failure. Finally, based on the bearing characteristics, failure mechanism, and failure mode of variable cross section immersed tunnels, the idea of strengthening immersed tube structures is put forward, which can guide similar engineers.

Influence of Geogrid Properties on Rutting and Stress Distribution in Reinforced Flexible Pavements under Repetitive Wheel Loading

AbstractGeogrid reinforcement effectively controls the rutting of pavements under traffic loads, especially when constructed over soft subgrades. In this study, extensive large-scale model pavement...

Effects of Principal Stress Rotations and Excess Pore-Water Pressure Changes on Soft Subgrade Deformation of Airfield Mat Structures under Pseudodynamic Loading

AbstractThe Airfield Mat 2 (AM2) matting system typically is employed as an expedient surface for airfields. It usually is emplaced over soft existing subgrades due to logistical, material, and equ...

EVALUATION OF BUND STABILITY USING GEOSTUDIO SOFTWARE WITH GROUND IMPROVEMENT METHOD USING SAND-KEY AT RECLAMATION WORK ON SOFT SOIL

The use of sand-key reclamation works in soft and very soft soils to increase the stability of the bund construction. Reclamation using the hydraulic fill method requires a stable embankment structure to withstand the potential for landslides from landfills, and in sea areas with very soft subgrade soils, with small undrained cohesion values causes the bund to be unstable and do not exceed the critical limit. The purpose of this study is to evaluate the improvement of soft soil in bund construction so that it is stable using GeoStudio. The Bund stability analysis results show that the value of the sand-key depth depends on the variation in the depth of the seabed. At a seabed depth of -6 m, there is a sand-key depth of 7 m, -5 m the sand-key depth of 10 m, -4 m thesand-key depth of 10 m, -3 m the sand-key depth is 5 m. The final safety number of 1.404,1.438,1,675, and 1.354 respectively.

ALTERNATIVE IMPROVEMENT OF SUBGRADE AND REINFORCEMENT OF EMBANKMENT ON PELABUHAN KUALA TANJUNG TOLL ROAD, INDERAPURA - KUALA TANJUNG SECTION IN NORTH SUMATRA (STA 0+000 S/D STA 3+500)

Planning the Kuala Tanjung Ruas Port of Inderapura - Kuala Tanjung Toll Road is one of the efforts to support economic growth in North Sumatra. The toll road will be built on a pile with a height of 2 ms / 9 m at STA 0 + 000 to STA 3 + 500. Due to limited land, the toll road section must be built on the poor subgrade. Based on the N-SPT test results, it is known that the subsoil consists of a clay layer with a thickness reaching 36 meters and a layer of sand at the top and bottom with a low N-SPT value. If the soft subgrade is burdened with a high heap, it is estimated that the subgrade will experience landslides, so it is necessary to plan the improvement of the subgrade and reinforcement of the pile.The soil improvement method chosen is the pre-loading method with a combination of the prefabricated vertical drain (PVD). Pre-loading method aims to spend the compression that occurs on the subgrade so that there is no compression during the service life of the road. The combination with PVD is intended to speed up the compression time in thick clay layers. The stability of the embankment will be calculated using a computer-assisted program, and geotextile reinforcement planning will be carried out if the value of the safety factor does not meet the requirements.Based on the analysis of compression computation will be divided into five zones. The amount of compression obtained under various piles of 2 ms / d 9 m was 2.25 m, 1.49 m, 1.75 m, 2.57 m, 3.65 m, 4.3 m, 4.69 m, and 5.06 m with an initial heap height of 5.02 m, 5.26 m, 6.51 m, 8.34 m, 10.41 m, 12.07 m, 13.46 m, and 14.83 m. Compression time required for subgrade is relatively long with a period of more than five years so that the planned acceleration with PVD. To accelerate the compression time to 5-6 weeks, a triangular PVD installation pattern is used with a spacing between PVD of 1.5 m and a depth of PVD installation as thick as a soft soil layer. From the results of the pile stability control, it is predicted that the pile will experience slides so that the reinforcement is installed with geotextile.Rp1.598,213,522,582.08.

Predicting method on settlement of soft subgrade soil caused by traffic loading involving principal stress rotation and loading frequency

Since the alarming settlement of transport infrastructures on soft subgrade caused by long-term traffic loading, a method for predicting settlement is of considerable significance in the engineering design. In this study, we proposed a practical and simple predicting method on the settlement of soft subgrade considering traffic loading effects. The model used in this method involves the principal stress rotation and loading frequency, which needs only four parameters with clear meaning obtained by several cyclic triaxial tests. The applicability of the model has been verified by previous results from other researchers. Moreover, the engineering practicality of this method was validated by the field data of a highway. The settlements under low-frequency and high-frequency are the upper limit and lower limit of the traffic load-induced settlements, respectively. With the increasing embankment thickness, the settlements decrease because the cyclic stress ratio decreases and the detrimental effect of the principal stress rotation on the settlement reduces. The vehicle load is a more influential factor in the settlement than the traffic volume.

Discussion and Analysis on Construction Technology Treatment of Soft Soil Subgrade in Highway Construction

Discussion and Analysis on Construction Technology Treatment of Soft Soil Subgrade in Highway Construction

Experimental study on bamboo grid reinforced copper slag overlying soft subgrade

Design and construction of pavements and structural foundations on soft ground are risky and costly projects for civil engineers because of the low bearing capacity of soil and the massive amount of granular fill required for replacing it. Improvement of the soft ground using natural materials is a sustainable, efficient, and cost-effective technique for providing a stable and supportive base for various civil engineering structures. This paper focuses on the use of bamboo grids to effectively improve the performance of soft subgrade underlying copper slag. Two types of bamboo grids in different aperture shapes were used to compare their effectiveness. The performance of the bamboo grids was evaluated on the basis of placement depths and their sizes. Model tests were carried out to analyse the effect of thickness of copper slag layer on footing pressure and settlement response of the base-subgrade system. A substantial improvement in footing pressure and footing settlement was observed at different levels of settlement. The tridirectional bamboo grid (hexagonal aperture) reinforced foundation base exhibited better performance than the bidirectional bamboo grid (square aperture). The footing pressure improvement ratio in the foundation bed using the tridirectional and bidirectional bamboo grid was found to be 2.28 and 1.89, respectively, while footing settlement reductions were obtained as 87% and 78%, respectively, for the same size of reinforcement. From the present study, it has been suggested that the bamboo grid and copper slag can be used for improving the soft subgrade, especially considering their availability and cost.

A Prediction Method for Postconstruction Settlement of Pile-Soil Composite Subgrade Based on Fuzzy Comprehensive Evaluation

This paper presents a postconstruction settlement prediction method for pile-soil composite subgrade based on the multilevel fuzzy comprehensive evaluation principle. In this method, the variation range of postconstruction settlement can be obtained from a simple calculation based on the basic data of actual engineering. Firstly, according to the characteristics of influencing factors in the construction of soft soil subgrade, the evaluation index set and two-level factor index sets were selected. The grading standards of the evaluation index and factor index were determined according to the allowable value of the standard and the numerical simulation results. Secondly, each factor index was standardized, and the normal distribution function in the form of exponential was used to construct the standard membership function for the first and second factor indexes. Finally, the comprehensive evaluation matrix of postconstruction settlement of composite subgrade was constructed based on the entropy weight method. The variation range of postconstruction settlement was predicted by the principle of maximum membership. The example analysis shows that the predicted results of the prediction method and the field measurement method are in good agreement, indicating that the proposed method can realize the postconstruction settlement prediction of composite subgrade, and the results are more accurate and more instructive.

Research on Construction Technology Treatment of Soft Soil Subgrade in Highway Construction

Research on Construction Technology Treatment of Soft Soil Subgrade in Highway Construction

Behaviour of jute and bamboo geocell with additional basal mat filled with different infill materials overlaying soft subgrade

Behaviour of jute and bamboo geocell with additional basal mat filled with different infill materials overlaying soft subgrade

Using Geotextile to Reduce the Required Thickness of Sub Base Layer of the Road and Improvement in CBR Value

Soil fabrics (geotextiles) are a permeable textile structural composition, and are mainly use in civil engineering applications associated with soil, rocks, or water. The American specifications (ASTM-D1316) indicated in the definition of this type of fabrics that they are use in some installations for civil and structural engineering, as the traffic increases day by day, many road related problems happen. Due to the heavy volume and heavy traffic the roads were damage very early, to avoid such a situation, we can use the geotextile.Geotextiles can be roughly in to two-type woven and nonwoven, in this research, non-woven (geotextile-50 pressed) was use in the work. The effect of geotextile sheets on improving the load-settlement characters of five-layered soil (three-layer clay, two-layer recycle concrete aggregate); moreover, the use of geotextiles has been study to reduce the required thickness of the sub-base layer of the road. Given the widespread use of bearing testing rate in the projects of road construction, it has used this test in this research, it was a test California bearing ratio (CBR) on the soil of five layers including the rubble of recycled concrete layer recycled (RCA) in the top (sub base)) and the clay soil place bottom(subgrade)These tests were conduct different the situations, including the use of test geotextile and test not use geotextile. In this research, the performance of nonwoven geotextiles, interconnect between the soft subgrade and the sub-base of the recycle of the recycle concrete aggregate (RCA) in the flexible paving system was good as the Geotextiles improve and increase the CBR value about (20%) of the road layers as well as the geotextile worked on reduce permeability about (50%).Geotextile also worked to separate or (isolate) two materials that are not alike, such as two soil layers with different properties, such as separating the subgrade layer from the subbase layer.

The role of geosynthetics in reducing the fluidisation potential of soft subgrade under cyclic loading

The instability of railway tracks including mud pumping, ballast degradation, and differential settlement on weak subgrade soils occurs due to cyclic stress from heavy haul trains. Although geotextiles are currently being used as a separator in railway and highway embankments, their ability to prevent the migration of fine particles and reduce cyclic pore pressure has to be investigated under adverse hydraulic conditions to prevent substructure failures. This study primarily focuses on using geosynthetics to mitigate the migration of fine particles and the accumulation of excess pore pressure (EPP) due to mud pumping (subgrade fluidisation) using dynamic filtration apparatus. The role that geosynthetics play in controlling and preventing mud pumping is analysed by assessing the development of EPP, the change in particle size distribution and the water content of subgrade soil. Using 3 types of geotextiles, the potential for fluidisation is assessed by analysing the time-dependent excess pore pressure gradient (EPPG) inside the subgrade. The experimental results are then used to evaluate the performance of selected geotextiles under heavy haul loading.