Weak Subgrade Publications

High Water Table

Staged Road Construction in Very Weak Subgrade Using Polymeric Geocell

Chatterjee, A., Poharel, S.K.,
Breault, M.
Innovations in Pavement Management, Engineering and Technologies TAC Conference & Exhibition
2022

Abstract
The ongoing development of the energy industry in Canada, especially for pipeline-related structures, demands innovative ways of access construction. Most of these are at remote locations and under difficult geotechnical conditions making access a challenge. A 4.1km long unpaved access road leading to a proposed compressor station in Blainville, Quebec was planned for the construction of the station pad and future access. The road was expected to serve as a material haul road for the 200m x 250m gravel pad, regular compressor station construction loads, and future traffic. Construction of the pad was scheduled to start before the winter of 2020. The proposed road alignment had very soft to soft subgrade conditions with saturated peat. There were problems with limited right of way, a landslide-prone zone, a creek crossing, and overhead power lines. The road geometry was to be designed such that there was enough clearance from the overhead power lines and the slide-prone area and maintain the minimum turning radius required for the long and heavy trucks carrying compressor units. Additionally, there were constraints imposed in permits and other aspects of the project that left a short 6-week window for the construction of the road.

Soft Soil

High-Strength Geocell and Geogrid Hybrid Reinforcement- Compressor Station Gravel Pad on Soft Subgrade

Poharel, S.K., Yii, T., Breault, M., Norouzi, M.
IFAI [ATA] Geosynthetics Magazine
2020

Abstract
Planar geosynthetics have been used as soil reinforcement for many years; three-dimensional geocells are comparatively new. Geocell reinforcement usually utilizes geotextile for separation. Geogrid improves the stiffness of the reinforced soil by interlocking, lateral restraint and tension membrane; it reduces the applied stress on the soft soil and increases the bearing capacity while decreasing settlement (Qian et al. 2013). The performance of geogrid depends on aperture size and shape, material stiffness at junctions, and shape and stiffness of ribs (Giroud and Han 2016). Ever since the U.S. Army Corps of Engineers used geocell for reinforcing beach sand in the 1970s (Webster 1979), numerous research programs, experiments and monitored applications have been carried out to further understand the geocell-reinforcement mechanisms (Han et al. 2013). Vertical and lateral confinement, wider stress distribution and beam/slab effect are identified as the main reinforcement mechanism of geocell. Higher tensile stiffness, strength and creep resistance of geocell material provide the reinforced base with improved bearing capacity, higher modulus and extended design life (Pokharel et al. 2010, Thakur et al. 2013 and Kief et al. 2015). High-strength NPA geocell reinforcement also improves the creep resistance of the reinforced structure, which is a very important factor in the repetitive loading conditions, as it significantly reduces the initial deformation and rate of creep of the reinforced material (Thakur et al. 2013).

Experimental Study on Triaxial Geogrid-Reinforced Bases Over Weak Subgrade Under Cyclic Loading

Qian, Y., Han, J., Pokharel, S.K.,
Parons, R.L.
GeoFlorida 2010 [Annual Geocongree of the Geo-Institute of ASCE]

Abstract
Geogrids have been successfully used to improve soft subgrade and reinforce weak base course by providing lateral confinement. However, uniaxial and biaxial geogrids cannot provide uniform resistance in all directions. A new product termed “Triaxial geogrid” was developed to overcome this limitation. The triaxial geogrid is expected to have a more stable grid structure to provide uniform resistance in all directions as compared with uniaxial and biaxial geogrids. However, the effects of the triaxial geogrids on the performance of reinforced bases have not been well evaluated. In this study, unreinforced and triaxial geogrid-reinforced bases over a weak subgrade were constructed in a large geotechnical testing box (2m x 2.2m x 2m high) at the University of Kansas and tested under cyclic loading. During the tests, the surface deformations and the vertical stresses at the interface between the base and the subgrade were monitored. The test results showed that triaxial geogrids increased percentage of resilient displacement and reduced permanent displacement and vertical stresses at the interface as compared with the unreinforced base.

Validation of Geocell Design for Unpaved Roads

Norouzi, M., Pokharel, S.K.,
Breault, M., Martin, I.
IFAI [ATA] Geosynthetics Conference
2015

Abstract
Based on cyclic plate load and moving wheel tests Pokharel (2010) proposed a design method for unpaved roads using Neoloy based Geocell with a theoretical base from the Giroud and Han (2004) design method for planar reinforcement. To validate it under real time traffic, unpaved haul roads, well pads, and logging yards were designed. To the possible extent locally available cheaper poorly graded but environmentally friendly materials were used as infill. In case of extremely soft subgrades, construction layer of poorly graded sand reinforced with Geocell provided a driving surface for the construction equipment. The performance of the structures was evaluated on the rutting criteria, maintenance requirement, and visual serviceability after one year of service. This paper discusses the design method used in a number of projects in Western Canada including the Oil Sands region that face extreme cold weather conditions. The limitations of the design method are also discussed.

High-Strengh Geocell and Geogrid Hybrid Reinforcement for Compressor Station Gravel Pad on Very Soft Subgrade

Poharel, S.K., Norouzi, M., Breault, M., Yii, T.
IFAI [ATA] Geosynthetics Conference
2019

Abstract
Planar geosynthetics have been used as soil reinforcement for many years; three-dimensional geocells are comparatively new. Geocell reinforcement usually utilizes geotextile for separation. Geogrid improves the stiffness of the reinforced soil by interlocking, lateral restraint and tension membrane; it reduces the applied stress on the soft soil and increases the bearing capacity while decreasing settlement (Qian et al. 2013). The performance of geogrid depends on aperture size and shape, material stiffness at junctions, and shape and stiffness of ribs (Giroud and Han 2016). Ever since the U.S. Army Corps of Engineers used geocell for reinforcing beach sand in the 1970s (Webster 1979), numerous research programs, experiments and monitored applications have been carried out to further understand the geocell-reinforcement mechanisms (Han et al. 2013). Vertical and lateral confinement, wider stress distribution and beam/slab effect are identified as the main reinforcement mechanism of geocell. Higher tensile stiffness, strength and creep resistance of geocell material provide the reinforced base with improved bearing capacity, higher modulus and extended design life (Pokharel et al. 2010, Thakur et al. 2013 and Kief et al. 2015). High-strength NPA geocell reinforcement also improves the creep resistance of the reinforced structure, which is a very important factor in the repetitive loading conditions, as it significantly reduces the initial deformation and rate of creep of the reinforced material (Thakur et al. 2013).

Performance of Triangular Aperture Geogrid-Reinforced Base Courses Over Weak Subgrades Under Cyclic Loading

Qian, Y., Pokharel, S.K., Parson, R.L.

ASCE Journal of Materials in Civil Engineering
2013

Abstract
Geogrid (uniaxial or biaxial) is one type of geosynthetics that has been successfully used in slopes, walls, roads, and other applications. The main application of biaxial geogrid is to stabilize soft subgrade and reinforce weak base courses by providing lateral confinement. The confinement due to the interaction between aggregates and the ribs of biaxial geogrid depends on the geometry properties of the geogrid, such as rib shape and apertures size, the stiffness of the ribs, and the properties of aggregates. Research has shown that biaxial geogrid cannot provide uniform tensile resistance in all directions. To overcome this problem, a geogrid product with triangular apertures was developed and introduced into the market. Recent studies showed that the triangular aperture geogrid can provide nearly uniform tensile resistance in all directions and is more efficient in improving the performance of reinforced bases as compared with biaxial geogrid. However, the performance of triangular aperture geogrid-reinforced bases under dynamic loading and the influence of base course thickness on the confinement effect of triangular aperture geogrids have not been well evaluated. In this study, unreinforced and triangular aperture geogrid-reinforced bases at different thicknesses over a weak subgrade were constructed in a large geotechnical testing box at the University of Kansas and tested under cyclic loading. During the tests, surface deformations and vertical stresses at the interface between the base and the subgrade were monitored. The test results indicated that triangular aperture geogrids reduced permanent deformation and maximum vertical stress at the interface as compared with the unreinforced bases. The geogrids improved the performance of the aggregate bases at different thicknesses. The benefit became more pronounced when a heavier-duty geogrid was used. The back-calculations from the test data showed that the stress distribution angle and the modulus ratio of base course to subgrade decreased with an increase of the load cycles. The stress distribution angle increased with the increase of the base thickness. The vertical stress distributions were compared with the computed distribution by the layered linear elastic theory. These test data provide the basis for the development of a design method for triangular aperture geogrid-reinforced bases over weak subgrade in the future.

A Large Test Box Study on Geocell-Reinforced Asphalt Pavement [RAP] Bases over Weak Subgrade under Cyclic Loading

Thakur, J.K., Han, J., Pokharel, S.K., Parson, R.L.

ASCE Geocongress
2012

Abstract
Currently, great emphasis is placed on sustainable construction and infrastructure with green technologies. As a result, the demand for sustainable and environmental friendly roads is increasing day by day. More technologies for sustainable roadway construction are needed. One way to construct environmentally sound roads is through the use of recycled asphalt pavement (RAP) materials as a base course. However, limited research has been done to quantify its structural capacity with engineering properties. It is expected that RAP base courses may have excessive deformation under traffic loading due to the existence of asphalt in RAP. It is believed that geocell confinement can reduce the permanent deformation and improve the performance of RAP bases. In this study, three-dimensional novel polymeric alloy (NPA) geocell was used to confine RAP bases. To investigate the effect of geocell confinement on the performance of road bases, unreinforced and geocell-reinforced RAP bases were constructed over weak subgrade (target CBR of 2%) in a large geotechnical test box (2 m x 2.2 m x 2m high) and tested under cyclic loading. The test results showed that geocell confinement reduced the permanent deformation and vertical stress distribution at the interface between base and subgrade, and increased percentage of elastic deformation as compared with the unreinforced base.

Experimental Evaluation of Recycled Asphalt Pavement Bases Reinforced by Geocells under Cyclic Plate Loading

Thakur, J.K., Han, J., Pokharel, S.K., Parson, R.L.

TRB 91st Annual Meeting 
2011

Abstract
Geocells, one type of geosynthetics manufactured in a form of three-dimensional interconnected cells, have been reported to effectively provide lateral confinement to infill material to increase the modulus and bearing capacity of base courses. Most studies so far have been focused on the behavior of geocell-reinforced bases under static loading. Geocells used for pavement applications are subjected to repeated loading. Limited studies have been conducted so far to investigate the performance of geocell-reinforced bases under repeated loading. In this study, single and multiple geocell-reinforced granular bases with three types of infill materials (Kansas River sand, quarry waste, and AB-3 aggregate) were tested and compared with the unreinforced bases under repeated loading. This study experimentally investigated the effect of the geocell reinforcement on the permanent deformation and percentage elastic deformation of the granular bases. The test results showed that the geocell reinforcement reduced the permanent deformation and increased the percentage elastic deformation of the granular bases. Multiple geocell-reinforced sections demonstrated even better performance as compared with single geocell-reinforced sections.

Accelerated Pavement Testing of Geocell-Reinforced Bases Over Weak Subgrade

Han, J., Pokharel, S.K., Manandhar, C., Yang, X.M., Leshchinsky, D., Halahmi, I., Parsons, R.L.

Journal of the Transportation Research Board
2011

Abstract
To evaluate the effect of geocell reinforcement on base courses for low-volume unpaved roads over weak subgrade, full-scale trafficking tests were conduced using the accelerated pavement testing facility at Kansas State University. Three different types of infill materials including AB3 aggregate, quarry waste (QW), and Recycled Asphalt Pavement (RAP) were used for the base courses and A-7-6 clay was used as the subgrade. Four unpaved sections that included one unreinforced control section of 30 cm thick AB3 aggregate and other three 15 cm polymeric alloy geocell-reinforced sections with 2cm cover were tested under the single-axle dual tire wheel loading. The benefits of alloy geocell reinforcement are evaluated in terms of rut depths at a number of passes of the wheel load and the angle of stress distribution from the surface to the base course-subgrade interface. The test results demonstrated that the alloy geocell reinforcement improved the performance of unpaved AB3 and RAP sections in terms of rut depth and angle of stress distribution compared to the unreinforced section. The QW section also showed better performance in terms of stress distribution angle. The road sections were exhumed and evaluated after the moving wheel test. 

Performance of Geocell-Reinforced RAP Bases over Weak Subgrade under Full-scale Moving Wheel Loads

Han, J., Pokharel, S.K., Manandhar, C., Yang, X.M., Leshchinsky, D., Halahmi, I., Parsons, R.L.

ASCE Journal of Materials In Civil Engineering
2011

Abstract
Recycled asphalt pavement (RAP) has been increasingly used as an energy efficient and environmentally friendly paving material and is currently the most reused and recycled material in the United States. RAP has been used in new hot mix asphalt (HMA) mixtures and in base courses for pavement construction. When RAP is used as a base course material, the presence of asphalt in RAP may cause excessive deformation under traffic loading. Geocell, three-dimensional (3D) polymeric geosynthetic cells, was proposed in this study to minimize the deformation by confining the RAP material. Full-scale accelerated pavement tests were conducted to evaluate the effect of geocell reinforcement on RAP base courses over weak subgrade. Two types of RAP were used and a total of seven geocell-reinforced and unreinforced RAP sections were tested under full-scale traffic loads. The road sections were excavated and examined after each moving wheel test. The benefits of geocell reinforcement were evaluated in rut depths for a specific number of passes of the wheel load and the angle of stress distribution from the surface to the base course-subgrade interface. The test results demonstrated that the novel polymeric alloy geocell reinforcement improved the performance of unpaved RAP sections by widening the stress distribution angle and reducing the rut depth if the base courses were equally compacted in unreinforced and reinforced sections.