Dynamic Properties of Compacted Cohesive Soil Based On Resonant Column Studies

Dynamic properties of compacted cohesive soils are required in most of the civil engineering constructions. These include construction of dams, dikes, embankments, liners and levees. Dynamic properties are highly important for determining the engineering behaviour under dynamic as well as cyclic loading. In the present study a series of resonant column tests were performed on compacted cohesive soil to determine its dynamic properties. The dynamic properties studied were shear modulus, damping ratio and Poisson’s ratio of soil. The effects of confining pressure and shear strain on the dynamic properties of the compacted cohesive soil are discussed. It is observed that there is increase in shear modulus and decrease in damping ratio as well as Poisson’s ratio of the soil with the increase in confining pressure. It is also observed that there is reduction in shear modulus and increase in damping ratio and Poisson’s ratio of the soil with increase in shear strain

Effect of Confining Pressure, Relative Density and Shear Strain on the Poisson’s Ratio of Clean Sand

In comparison to other dynamic properties of soil, Poisson’s ratio is mostly considered as an elasti c constant. The effect of different para meters on the Poisson’s ratio of soil is neglected in most of the available literature. Proper estimation of the Poisson’s ratio is required as it signifies the stress and deformation characteristics of the soil. In this study a series of resonant column tests were performed to determine the variation of Poisson’s ratio with confining pressure, relative density and shear strai n. Clean sand free from fines content was used in this study . The tests were performed on sa nd sample of size 50 100 mm compacted at relative densities of 30%, 50% and 75 %. The sample preparation was done by using a split mold. B y using a funnel , t he sand was gently poured into the split mold. Then each layer was compac ted by means of a tamping rod which weighs 150 g. The sample preparation was done in 5 equal thick layers. By performing few trial sample pre parations , the number of blows required per layer for a desired relative density was determined. A fixed free type of resonant column was used in this study . By performing the resonant column test in torsional mode as well as flexural mode of excitation , it is possible to estimate the Poisson’s ratio of the soil. The confining pressure was varied from 100 kPa to 800 kP a. The shear strain vari ed from 10 - 4 % to 10 - 1 %. Figure 1 gives the variation of Poisson’s ratio with confining pressure for various relative densiti es. From the figure i t is observed that there is a continuous decrease in Poisson’s ratio of the soil wit h an increase in the confining pressure as well as relative density. The percentage reduction in Poisson’s ratio obtained for the soil subjected to a confining pressure of 800 kPa as compared to soil subjected to 100 kPa confining pressure is 19 %, 21 % an d 22 % for 30 %, 50 %, 75 % relative densities of the soil. In addition, the percentage s reduction in Poisson’s ratio of sand prepared at 75 % relative density as compare to 30 % relative density are 14 %, 15.7 %, 16.7 % and 17.6 % for sand subjected to 100 kPa, 400 kPa, 60 0 kPa and 800 kPa respectively . It can be concluded that the percentage reduction in Poisson’s ratio increases with the increase in confining pressure as well as relative density

Evaluation of Flexural Fatigue Behavior of Two Layered Asphalt Beams with Geosynthetic Interlayers Using Digital Image Correlation

In the current study, the flexural fatigue performance of two layered asphalt beams with and without geosynthetic interlayers has been evaluated using digital image correlation (DIC) technique. The two layered asphalt specimens consist of an old deteriorated pavement slab as the bottom layer, a conventional tack coat along with geosynthetic interlayers and a bituminous concrete (BC) mix compacted as an overlay. The digital images are recorded at a specific interval of load cycles during flexural fatigue testing (four point bending) using a high definition digital camera. The displacement fields obtained from the digital images are analyzed to provide an information on the crack width, crack height and the tensile strains to study the crack initiation and crack propagation stages of beam specimens. The deformation data obtained from the DIC analysis is validated with the vertical deformations measured during the fatigue testing. The results correlate well with an acceptable level of accuracy. The DIC data depicted that the tensile strains are as high as 4.75% at the crack tip in the control specimen at 335 load cycles against 1.42% in a polyester grid interlayered specimen at 13116 cycle

Rutting Behavior of Geocell Reinforced Base Layer Overlying Weak Sand Subgrades

In this study, a series of large scale repeated model load tests are performed on geocell reinforced and unreinforced base layers overlying weak sand subgrades. The weak sand subgrades are prepared at 30% relative density (RD) through pluviation (sand raining) technique in a test tank of dimensions 1m × 1m × 1m (length × width × height). Two different base courses consisting of 75% RD sand and a granular base material have been tested. The 75% RD sand base course is also prepared by pluviation technique, while the granular base course is prepared in 5 layers, each of 50mm thick, by static compaction. A repetitive load of 0.97kN and 9.7kN was applied on the prepared base layer through a 150mm diameter plate to replicate the traffic load equivalent to a contact pressure of 550kPa. Loading was applied through a graphical user interfaced multi-purpose test software along with the help of a hydraulic power unit, hydraulic service manifold and sophisticated double acting linear dynamic 100kN capacity actuator which is connected to a 3.5m high, 200kN capacity reaction frame. Four different tests are conducted on both the base courses (75% RD sand and granular base) with and without reinforcement overlying the weak sand subgrade separately. There is a considerable amount of improvement observed for different number of cycles and plate settlements on quantification of traffic benefit ratios (TBR), cumulative plastic deformations (CPD) and rut depth reduction (RDR) for geocell reinforced base courses. However, geocell reinforced granular base course have shown a better improvement comparatively

Influence of geosynthetic-interlayers on the performance of asphalt overlays on pre-cracked pavements

The functions of geosynthetic-interlayers in retarding reflection cracking and improving fatigue performance of hot mix asphalt (HMA) overlays in flexible pavements are evaluated in this study. The delamination or debonding mechanisms of the overlays are studied when geosynthetic-interlayers are adopted. A polyester grid coated with polymer modified binder (G1), a woven geo-jute mat (G2), and a bi-axial polypropylene grid (G3) interlayer are examined based on their adhesion properties. A two stage experimental program, in which, during the first stage, the performance of the geosynthetic-interlayers sandwiched between the pre-cracked old pavement and new asphalt layers are evaluated using flexural fatigue testing. A digital image correlation (DIC) technic was employed to record the failure modes and the corresponding tensile strains in the overlay system. During the second stage, the effect of interlayers on the interface bond strength was evaluated with the help of shear and tensile bond strength tests. The results show that the inclusion of interlayers retards the propagation of reflection cracking, however, results in the delamination of overlays. The debonding effect is prominent in G3 interlayers due to their high initial stiffness. Overall, interlayers with high interfacial shear and pull-off tensile bond properties proved effective in controlling the reflection cracking and increasing fatigue life of the overlays

Study on a Combined Ground Improvement Technic to Facilitate Rapid Embankment Construction on Soft Soil Deposits

Soft soils are characterized by their high compressibility, and low shear strength. Highway embankments proposed to construct on such soils will undergo excessive consolidation settlements. The failure of embankments on soft soils is attributed also to the undrained shear when the embankments are constructed in a short time. Hence, the construction of high embankments is taken up in stages by maintaining enough waiting period for consolidation between stages due to stage loading, which delays the whole construction period. Among the several ground improvement technics available to reduce the construction period, pre-consolidation of soft soils through the application of surcharge, use of vertical drains are common methods to achieve required degreed of consolidation (usually 90-95%). In any of these methods, time and cost of the project takes a major role in achieving the required degree of improvement. The stability of an embankment is an issue if the embankment is proposed to build in a very short time

PERFORMANCE ASSESSMENT OF GEOCELL REINFORCED FLEXIBLE PAVEMENTS

The influence of introducing a three dimensional geo-synthetic material, called geocells in the base layer of flexible pavements is investigated in the current study. This study compares the reinforced and unreinforced pavement sections constructed on a subgrade having a moderate California bearing ratio (CBR) value of 5%. A series of model tests were carried out to understand the influence of geocell reinforcement on the load carrying mechanism of the pavement section under static and repeated loading conditions. The parameters studied were as follows: load-settlement response of the pavement sections, actual rut at the subgrade level, pressure transmitted to the subgrade soil underlying the geocell reinforced base layer and surface deformation profile of the test sections. Different flexible pavement design methodologies proposed by Indian Roads Congress (IRC) and American Association of State Highways and Transportation Officials (AASHTO) were used for the design of pavement sections and were compared through experimental program conducted in the laboratory. Large scale repeated load tests were performed to replicate the actual field conditions. The test results indicate that the geocell reinforcement reduces the rutting in the pavement. The performance improvement was presented in terms of traffic benefit ratios (TBR), layer coefficient ratios (LCR), rut depth reductions (RDR) and rut benefit ratios (RBR). Besides, under the same load repetitions, the thickness of the reinforced pavement is considerably less than that of the unreinforced pavement section. In addition to the cost savings, this would conserve natural materials like aggregates used in the pavement construction. Overall, the inclusion of geocell in the base layer helps in improving the life, uniform distribution of load, reduction in rut depth and it also provides an economical and sustainable solution to the present practices

The function of basal geogrids in minimizing rutting of geocell reinforced subgrades

Rutting is a common phenomenon encountered in flexible pavements supported by weak subgrades. Reinforcing the weak subgrades is one of the promising alternatives to alleviate the pavement surface rutting. This paper presents the results of laboratory model tests on a circular plate supported by geocell reinforced sand subgrades. A series of tests were carried out by varying the height of the geocell mattress with an additional layer of basal geogrid placed underneath the geocell mattress. The surface settlements (rutting) were measured through displacement gauges. Strain gauges were placed along the width of the basal geogrid to verify their performance as a base layer. A substantial reduction in surface rutting is observed in the case of geocell reinforced beds with basal geogrids. A seven fold improvement in bearing capacity was obtained with the provision of an additional geogrid layer over unreinforced subgrades. Overall, a basal geogrid layer provides higher structural support mobilized through membrane effect to the geocell reinforced pavement layer

Repeated Load Tests on Geocell Reinforced Sand Subgrades

In this research, results from a series of large scale dynamic model tests on geocell reinforced and unreinforced homogeneous sand beds are presented. The placement density of sand in all the tests was maintained at 70%. The loading was applied through a circular steel plate which replicates the load application from a passenger car. A single axle wheel load of 40 kN was assumed on the pavement surface of which 7 kN was calculated to be applied on the subgrade layer. The influence of the width and height of the geocell reinforcement on the cyclic behavior of the loading system was studied and the performance improvement in terms of traffic benefit ratios and cumulative plastic deformations/rutting was determined. A traffic benefit ratio was observed to be as high as 45 for the case of geocell size h/D=1, b/D=4 at 10% plate settlement. The cumulative permanent deformations were reduced by 8 fold for the same case against the unreinforced case at 5% plate settlement