PERFORMANCE OF GEOSYNTHETIC-REINFORCED UNPAVED ROADS

Abstract

Geosynthetics have been used to improve the performance of pavement. The performance of an unpaved road can be measured by determining the California bearing ratio (CBR) and this value represents the strength of subgrade soil. Unpaved roads constructed on weak soil subgrade are frequently subjected to severe damage and hence, they require regular maintenance and repair. One of the main stabilization methods of improvement of the serviceability of these roads is to reinforce them with geosynthetics. In the present research, firstly an experimental investigation was carried out to evaluate the performance of the subgrade soil by placing a single layer and double layers of geosynthetic reinforcements horizontally at varying depths from the top surface of subgrade soil. Through a series of CBR tests in the laboratory, an attempt was made to determine the optimum depth of

the reinforcement layer. The single layer of reinforcement has been placed at the middle, one- third and one-fourth of the height of the CBR specimen from the top surface of the soil in the

CBR mould. The double layers of reinforcement were placed at one-fourth of the specimen height from the top surface and the bottom surface. The results show the significant contribution in terms of increased CBR value of the soil, resulting in reduced design thickness of the pavement layers above the subgrade soil. The results indicate that for the maximum benefit, the Tenax 3D grid reinforcement should be placed in between 0.3H and 0.36H where H is the height of the soil specimen. For Glasgrid and Tenax multimat reinforcements, the maximum effect of reinforcement is obtained when they are placed between 0.41H and 0.62H. An attempt has been made to conduct laboratory CBR tests on unreinforced and reinforced soil-aggregate composite systems. The improvement in the strength of subgrade-aggregate composite system was determined through the tests conducted in the standard CBR mould in terms of CBR value. Unreinforced soil–aggregate composite system is prepared by compacting

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the soil layer in the mould, and placing the aggregates layer above the soil, where the soil represents the existing subgrade and aggregate layers, represents the base course material of an unpaved road. In reinforced soil–aggregate composite system, the reinforcing layer was installed at the soil-aggregate interface. The geosynthetics used in the study as reinforcing layers are geotextile, geogrid and geomat. Some more tests were conducted on reinforced soil– aggregate composite system with double layers of reinforcement such that the first reinforcing layer was laid at the soil-aggregate interface and another reinforcing layer was laid at middle half of the compacted aggregate layer. Unreinforced and reinforced soil–aggregate composite systems were subjected to standard penetrating load while performing the tests, and the performance of reinforced soil–aggregate composite system was compared with that of the unreinforced systems. The effect of type of reinforcement on the load–penetration curve and the relative performance of various types of geosynthetics have also been investigated. In this thesis, an effort has been made to analyses the geosynthetic reinforced unpaved roads using digital static and dynamic cone penetrometer tests. DCP test has been used widely as a pavement evaluation technique. Field experiments were conducted on unpaved test sections reinforced with geotextile and geogrid, with the potential use of dynamic cone penetrometer (DCP) and digital static cone penetrometer (SCP) to assess benefits of geotextile and geogrid reinforcement. Digital SCP was used to measure the load–displacement behavior of geosynthetic-reinforced test section in the field. The field test results of DCP were expressed in terms of dynamic cone penetration index (DCPI, mm/blow). DCPI value represents the penetration depth of the cone per hammer blow and recorded along with the depth profile. Reinforced test section observed reducing DCPI value as compared to the unreinforced test section. DCP results were able to detect transition zone and significant change in the strength of unpaved test section along with penetration depth. The field results indicate the greater resistance to penetration in the geosynthetic-reinforced test section and the penetrometer

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resistance increases with the depth. Higher penetration resistance offered by the geotextile has more contribution to the performance improvement of the test section. In addition, this thesis also presents a fuzzy logic-based modeling approach which is employed for geosynthetic-reinforced subgrade soil of unpaved roads. A review of the

literature reveals that fuzzy logic has not been used for predicting the behavior of geogrid- reinforced subgrade. FL-based two models were developed with fuzzy Triangular and Gaussian

membership functions for input and output variables. It consists of eight input parameters/factors, namely, reinforced/unreinforced section, depth of reinforcement, plasticity index, plastic limit, liquid limit, optimum moisture content, maximum dry unit weight, and soaked/unsoaked condition and California bearing ratio (CBR) as an output parameter. The fuzzy rules are deduced from the experimental data. The laboratory CBR tests were performed on the subgrade soil reinforced with geogrid. The precision of models was examined by comparing the predicted CBR values with the experimental CBR values for Triangular and Gaussian membership functions. The sensitivity analysis reflects a set of dominant parameters. The results indicated a significant improvement in the CBR value of geogrid-reinforced subgrade soil due to the inclusion of geogrid. The range for optimal depth of geogrid reinforcement is found to be 36% to 60% of the thickness of the soil layer. The potentialities of FL were found to be satisfactory. Furthermore, to show the influence of geosynthetic reinforcement on rutting in unpaved roads, field test results of unpaved test section have been presented. The common cause of pavement failure during the unpaved road construction is rutting. Geosynthetic is a solution to this pavement distress and have been widely used for reinforcing unpaved roads. Moving wheel load tests were carried out on unpaved road test sections to investigate the influence of geosynthetic reinforcement in the improvement of pavement surface deformation. One unreinforced and two geosynthetic-reinforced unpaved test sections were constructed in a test

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pit of dimension 2700mm×9000mm×800mm at the Delhi Technological University. Geogrid and geotextile were used for reinforcing the unpaved test sections and this geosynthetic reinforcement layer was embedded at the base-subgrade interface. Unreinforced and geosynthetic-reinforced test sections were examined after moving wheel load tests. Rut depth was measured at three grid locations of each test sections in the transverse direction of the wheel path after certain numbers of vehicle passes. A total number of 350 vehicle passes were applied to the unpaved test section. The contributions of geosynthetic reinforcement were evaluated by calculating traffic benefit ratio (TBR) based on the rut depth measurements in the fields. Test results indicate that inclusion of geosynthetic reinforcement significantly improves the rutting resistance and stability of reinforced test sections comparing to the unreinforced test section.