Environmental and material controls on desiccation cracking in engineered clay embankments

PhD ThesisDesiccation cracking is a natural phenomenon commonly associated with drying of expansive soils. The role of cracks in surface permeability increase and overall deterioration of infrastructure slopes makes it a key factor in climate-related slope instability processes. Despite this significance, the controls on soil cracking in engineered slopes still represent a poorly understood area. In this study, soil cracking behaviour in clay embankments exposed to cyclic wetting and drying was investigated to improve understanding of this phenomenon for application in geotechnical practice. A complimentary field and laboratory study was undertaken, approaches commonly conducted in isolation in the literature. The field program involved direct investigation of natural crack development in a heavily instrumented, clay embankment (BIONICS, Newcastle University). Crack morphology parameters were quantified under engineering, meteorological and near surface soil hydrological conditions to understand how temporal change influences these. Laboratory experimentation was carried out on materials representative of typical embankment fills and construction methods in the UK in a bespoke climate control system. Time series photographs of the crack networks were analysed using image processing technique to compare their intensities across the experimental conditions. Syntheses of field and laboratory results show the influence of factors related to the embankment geometry (i.e. slope aspect, layer thickness), material properties (i.e. soil density and plasticity) and environmental condition (i.e. wetting and drying cycles) on the cracking behaviour in engineered clay slopes. The sensitivity of cracking intensity under given climate conditions critically relates to the rate of moisture loss and the material strength. Overall, this research presents how newly gained understanding of cracking can potentially impact upon improved construction techniques of engineered clay embankments and the susceptibility of historic embankments constructed to lower densities to climatic changes, including how drying/wetting cycles can exacerbate crack development.Akwa Ibom State University, TETFUN

Evaluating the Effectiveness of a Hybrid Geosynthetic Reinforcement System to Mitigate Differential Heave on Flexible Pavement due to Expansive Subgrades

Transportation industries encounter substantial challenges with respect to ride quality and serviceability when they deal with expansive soils underneath roadway structures. These soils exhibit swell-shrink behavior with moisture variations, which cause surficial heaving on the pavement structure and cost billions of dollars for the maintenance of pavements. For the past four decades, a particular stretch of US-95 (Oregon line to Elephant Butte) exhibited recurrent swelling distresses due to the underlying expansive soils. Despite remedial measures that exhibited satisfactory results for most of the sections, recurrent damage still continued in few sections. Further research indicated that the problematic soils were located at a depth below 1.82 m. Conventional chemical remediation methods typically performed at a depth no greater than 0.9 to 1.2 m. To be able to address the adverse effects of this swell-shrink behavior of soil at a deeper depth, hybrid geosynthetic systems were proposed. Hybrid geosynthetic systems were successfully used to mitigate expansive soil swelling in railroad applications. Hence, this research study explored this idea of using hybrid geosynthetic reinforcement systems (geocell-geogrid combination) to mitigate differential pavement heaving resulting from underlying expansive soils. To evaluate the use of hybrid geosynthetic systems in reducing differential heaving from expansive subgrades, a large-scale box test was developed to simulate a pavement section with a base course and expansive subgrade (asphalt overlay was ignored). The surficial heaving on the base course reinforced with geocell, geogrid and hybrid geosynthetic reinforced system (HGRS) were measured over time and compared with the unreinforced case. The large-scale box test results showed that the geosynthetic systems significantly reduced the maximum surficial heave along with the differential swelling on the pavement section. HGRS exhibited better performance than geocells and geogrids. Numerical analysis using the finite element approach was conducted to study the response of other soil types not tested in the box. The numerical model was first calibrated using using the box test results and the calibrated model was used to change soil properties for two other soil types with different swelling charecteristics. In the numerical model, swelling behavior of expansive soils was simulated using material models that incorporate volumetric swelling and suction as a function of moisture content. The modulus of the unreinforced base was determined using laboratory tests while the modulus that for the reinforced sections was calibrated using large scale test data. The calibration of control model was performed by controlling the moisture percolation through subgrade. The improvement of reinforced models were quantified by higher modulus of reinforced base. These calibrated models were used to conduct a parametric study by varying the subgrade soil properties and their performance with respect to the modulus of reinforced base. The parametric study revealed that the expansive soils with high PI exhibited higher swelling than the expansive soils with low PI. It was observed that the reinforcing effect was higher for soils with lower swelling characteristics

IMPROVEMENT OF DYNAMIC PROPERTIES AND SEISMIC RESPONSE OF CLAY USING FIBER REINFORCEMENT

ABSTRACT Title of Document: IMPROVEMENT OF DYNAMIC PROPERTIES AND SEISMIC RESPONSE OF CLAY USING FIBER REINFORCEMENT Behzad Amir Faryar, Doctor of Philosophy, 2012 Directed By: Professor M. Sherif Aggour, Department of Civil and Environmental Engineering In recent years, earthquakes have caused heavy damage to buildings and infrastructure. One of the causes of heavy damage due to earthquake motions is the role of soft clay in amplifying bedrock ground motions. Improving the soil conditions at a site in order to mitigate earthquake damage can be one of the methods of modifying site conditions and thus reduce its effects on the seismic site response. The inclusion of randomly distributed short virgin polypropylene fibers (C3H6) in clay has proven to significantly improve the static geotechnical properties of clay such as shear, compression, tensile strengths, and so on. These improvements have triggered great interest in the possibility of mixing fibers with clay to improve the clay's dynamic properties. Because the percentage of fibers is currently arbitrarily chosen by users, a procedure was set up in this study to determine the optimum fiber content for a fiber-clay composite. Experimental testing was performed using the Resonant Column Method to obtain both the shear modulus and the material damping for a clay and the fiber-clay composite to determine the effect of fiber inclusion on the dynamic properties of clayey soil. The research showed that the inclusion of fiber at optimum fiber content as a ground improvement technique can improve the dynamic properties of soft clayey soils at low shear strain. Test results indicated that both the shear modulus and damping increased. Hence, the inclusion of fiber in clay can provide a double benefit for the dynamic response of a site by increasing the stiffness of the site and reducing its amplitude of vibration. General formulas for shear modulus and damping were developed as functions of the shear strain amplitude for the clay and for fiber reinforced clay. The effect of fiber inclusion on the seismic site response using two different earthquake motions was also studied. One-dimensional wave propagation analysis was performed to investigate the effect of the modification of the clay dynamic properties using fibrillated fiber reinforcements on the site response. The results indicated that by modifying the clayey soil using fiber, the seismic site response can be improved

Resistance to Desiccation Cracking in Clay-Polymer Mixtures

This thesis investigates how polymer treatment can mitigate desiccation cracking in Na-bentonite. It aims to address key unanswered questions, regarding conditions under which polymers are effective, underlying mechanisms, and durability of improvements through a three-part experimental investigation. Sodium carboxymethyl cellulose (Na-CMC) with varying degrees of substitution (DS) is added to Na-bentonite following by two different mixing methods, namely wet mixing (WM) and dry mixing (DM). Part 1 investigates petri dish-dehydration experiments to compare cracking intensity and evaporation characteristics of amended and unamended mixtures under different conditions. Part 2 examines the mechanisms contributing to the reduction in desiccation cracking and investigates water retention ability, swelling, shrinkage, tensile strength, and microstructural changes using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). Part 3 studies the probability of polymer elution from DM andWMsamples, during consolidation and permeation, and its effect on hydraulic conductivity (k). The research findings indicate that adding 2% Na-CMC to bentonite had minimal impact on water retention, but reduced swelling/shrinkage potential, and improved tensile strength. The two latter parameters likely contributed to cracking resistance of amended mixtures in dehydration experiments. DM reduced bentonite k by 60%, but its polymer leaching was more pronounced than WM, leading to increased k over time. Despite polymer loss, both WM and DM samples retained lower k than untreated bentonite. Adding a slit-film geotextile did not significantly enhance polymer retention in amended samples, but reduced the polymer migration into the effluent of k test, suggesting polymer retention by geotextile, not bentonite. These results show Na-CMC as a promising agent for reducing desiccation cracking, but its practical viability and long-term sustainability require further investigations

Electrokinetic stabilisation as a subsidence remediation technique

This study comprises a three part investigation into electrokinetic stabilisation (EKS): numerical modelling, laboratory trials and site trials. The numerical modelling was introduced to identify issues in the physical trials and assess proposed site trial solutions and associated configurations. The laboratory trials concentrated on electrode development and chemical stabiliser combinations in conjunction with known practical issues such as electrode polarisation and current intermittence effects. English China Clay batches combined with predetermined chemical stabiliser mixes were cured for up to 540 days and tested for standard engineering indexes and electrodes were tested for durability in liquids and under electrolysis. Optimum electrodes, determined through current transfer and durability results, along with optimum chemical combinations, determined through increase in undrained shear strength and reduction is linear shrinkage, were then combined to determine the effects on English China Clay under a mock strip footing for 30 days. It was shown that EKS has a negligible effect on the footing itself through either damage or level changes. The methodology was developed from the laboratory trials where the optimum electrode type, chemical combination and current intermittence times were developed, and applied to a site trial near Watford, UK. Four trials were conducted consisting of: a control; a vertical electrode set‐up, a vertical electrode set‐up using current intermittence and a raked electrode set‐up. All trials consisted of a 0.3x0.3x1.0m strip footing with treatment targeted at beneath the concrete. EKS was conducted for 56 days on a predominantly London Clay based made ground material with apparent resistivity measurements taken pre‐, peri‐ and posttreatment where apparent resistivity measurements showed the movement of the stabilising fluids over time. Site monitoring included voltage, current, air temperature and footing underside temperatures. Post treatment testing included standard engineering index testing, chemical analysis and SEM photography to determine the effects of the treatment regime. It was determined that the vertical electrode system showed most beneficial clay improvements through the laboratory testing and apparent resistivity results and as in the laboratory trials, negligible effect on the strip footings

Development of novel apparatus for establishing swelling and water retention characteristics of bentonite

Despite the breakthroughs in suction control and measurement for laboratory testing of unsaturated soils at the range of high suctions, existing equipment faces limitations in either the precision, the range, or the practicality. The aim of the present thesis is to develop novel apparatus for establishing the swelling and water retention characteristics of compacted bentonite, which may exhibit very high suctions at its as-compacted state. The development of the equipment is based upon the best elements of existing equipment, combined in a simple and efficient manner and extends the range of existing suction-controlled facilities in the Geotechnics Laboratory of Imperial College London. A new humidity generator is presented, based on the principles of the divided-flow method. The divided-flow humidity generator (dfHG) controls the relative humidity (RH) in a sealed chamber, allowing suction control of soil samples through the Vapour Equilibrium Technique (VET). The control of RH is automated and is obtained by the proportional mixing of dry and wet air based on the feedback given on RH measurements in the chamber with affordable, individually calibrated capacitance hygrometers. Direct comparison with other suction-control methods, and derivation of water retention curves for reference soil materials validate the method. The automated control and the continuous operational range of suction provides the versatility of pressure generators, while the simplicity of the system is comparable to the practicality of the air-regulation method. The humidity generator is subsequently adapted to a newly-developed oedometer, to assist in the investigation of highly expansive clays, such as bentonites. The device allows the application of continuous, automated control of suction in the range between 30 MPa and 300 MPa, while applying complex stress and hydraulic paths for swelling tests. Equilibrium on each suction step is assessed through the combined evaluation of swelling stress, RH of the outflow air and water exchanged with the soil sample. The method is validated through successful investigation of the swelling behaviour of a natural sodium bentonite.Open Acces

A characterization of landslide occurrence in the Kigezi Highlands of South Western Uganda

The frequency and magnitude of landslide occurrence in the Kigezi highlands of South Western Uganda has increased, but the key underpinnings of the occurrences are yet to be understood. The overall aim of this study was to characterize the parameters underpinning landslide occurrence in the Kigezi highlands. This information is important for predicting or identifying actual and potential landslide sites. This should inform policy, particularly in terms of developing early warning systems to landslide hazards in these highlands. The present study analysed the area’s topography, soil properties as well as land use and cover changes underpinning the spatialtemporal distribution of landslide occurrence in the region. The present study focussed on selected topographic parameters including slope gradient, profile curvature, Topographic Wetness Index (TWI), Stream Power Index (SPI), and Topographic Position Index (TPI). These factors were parameterized in the field and GIS environment using a 10 m Digital Elevation Model. Sixty five landslide features were surveyed and mapped. Soil properties were characterised in relation to slope position. Onsite soil property analysis was conducted within the landslide scars, auger holes and full profile representative sites. Furthermore, soil infiltration and strength tests, as well as clay mineralogy analyses were also conducted. An analysis of the spatial-temporal land use and cover changes was undertaken using satellite imagery spanning the period between 1985 and 2015. Landslides were noted to concentrate along topographic hollows in the landscape. The occurrence is dominant where slope gradient is between 25˚ and 35˚, profile curvature between 0.1 and 5, TWI between 8 and 18, SPI >10 and TPI between -1 and 1. Landslides are less pronounced on slope zones where slope gradient is 45˚, profile curvature 18, SPI 1. Deep soil profiles ranging between 2.5 and 7 meters are a major characteristic of the study area. Soils are characterized by clay pans at a depth ranging between 0.75 and 3 meters within the profiles. The study area is dominated by clay texture, except for the uppermost surface horizons, which are loamy sand. All surface horizons analysed had the percentage of sand, silt and clay ranging from 33 to 55%, 22 to 40% and 10 to 30% respectively. In the deeper horizons, sand was observed to reduce drastically to less than 23%, while clay increased to greater than 50%. The clay content is very high in the deeper horizons exceeding 35%. By implication, such soils with a very high clay content and plasticity index are considered as Vertisols, with a profound influence in the occurrence of landslides. The top soil predominantly contains more quartz, while subsurface horizons have considerable amounts of illite/muscovite as the dominant clay minerals, ranging from 43% to 47 %. The liquid limit, plasticity index, computed weighted plasticity index (PIw), expansiveness (ɛex) and dispersion ranging from 50, 22, 17, 10 and 23 to 66, 44,34,54 and 64, respectively also have strong implications for landslide occurrence. Landslides are not normally experienced during or immediately after extreme rainfall events but occur later in the rainfall season. By implication, this time lag in landslide occurrence and rainfall distribution, is due to the initial infiltration through quartz dominated upper soil layers, before illite/muscovite clays in the lower soil horizons get saturated. Whereas forest cover reduced from 40 % in 1985 to 8% in 2015, cultivated land and settlements increased from 16% and 11% to 52% and 25% respectively during the same period. The distribution of cultivated land decreased in lower slope sections within gradient group < 15˚ by 59%. It however increased in upper sections within gradient cluster 25˚ to 35˚ by over 85% during the study period. There is a shift of cultivated land to the steeper sensitive upper slope elements associated with landslides in the study area. More than 50% of the landslides are occurring on cultivated land, 20% on settlements while less than 15 % and 10% are occurring on grassland and forests with degraded areas respectively. Landslides in Kigezi highlands are triggered by a complex interaction of multiple- factors, including dynamic triggers and ground condition variables. Topographic hollows are convergence zones within the landscape where all the parameters interact to cause landslides. Topographic hollows are therefore potential and actual landslide sites in the study area. Characterized by deep soil horizons with high clay content dominated by illite/muscovite minerals in the sub soils and profile concave forms with moderately steep slopes, topographic hollows are the most vulnerable slope elements to landslide occurrence. The spatial temporal patterns of landslide occurrence in the study area has changed due to increased cultivation of steep middle and upper slopes. Characterized by deep soil horizons with high clay content dominated by illite/muscovite minerals in the sub soils and profile concave forms with moderately steep slopes, topographic hollows are the most vulnerable slope elements to landslide occurrence. The spatial-temporal patterns of landslide occurrence in the study area has changed due to increased cultivation of steep middle and upper slopes. A close spatial and temporal correlation between land use/cover changes and landslide occurrence is discernible. The understanding of these topographical, pedological and land use/cover parameters and their influence on landslide occurrence is important in land management. It is now possible to identify and predict actual and potential landslide zones, and also demarcate safer zones for community activities. The information generated about the area’s topographic, pedological and land cover characteristics should help in vulnerability mitigation and enhance community resilience to landslide hazards in this fragile highland ecosystem. This can be done through designating zones for community activities while avoiding potential landslide zones. It is also recommended that, tree cover restoration be done in the highlands and the farmers encouraged to re-establish terrace farming while avoiding cultivation of sensitive steep middle and upper slope sections

The engineering properties and mechanical behaviour of fibre reinforced clay

Embankment slope failure due to insufficient strength, weak bearing capacity, excessive deformation and desiccation cracking of problematic soils is commonly observed on the UK road network, and this leads to huge expenditure in the maintenance and repair of highway projects every year. It is necessary to reduce these engineering problems and economic losses through environmentally and economically friendly methods. Previous studies have shown that randomly distributed fibres can significantly improve various soil properties. However, there is a lack of comprehensive study on the engineering properties of fibre reinforced high plasticity clay. Also, limited mechanical models have been proposed for predicting the shear strength behaviour of fibre reinforced clay. In order to investigate these problems, a series of laboratory investigations including compaction, bearing capacity, one-dimensional consolidation, linear shrinkage, desiccation cracking, direct tensile strength, triaxial compression tests were conducted on unreinforced and polypropylene fibre reinforced London Clay. A mechanical model was proposed for predicting the shear strength of fibre reinforced clay. The experimental results showed that fibres can significantly improve the engineering properties of London Clay. As fibre inclusion ratio increased, the bearing capacity, coefficient of consolidation, tensile strength and shear strength of the soil increased; the compression index, swelling index, linear shrinkage and desiccation cracking area of the soil decreased. As fibre length increased, the bearing capacity and desiccation cracking area of the soil decreased; the tensile strength and shear strength of the soil increased. The mechanical model was proposed based on the conception of equivalent confining stress and the predicted stress-strain-pore water pressure response and stress path behaviour of fibre reinforced soil were compared with the experimental results. The comparisons showed that the model was capable of predicting the shear strength behaviour of fibre reinforced clay

A Novel Clustering Model Based on Set Pair Analysis for the Energy Consumption Forecast in China

The energy consumption forecast is important for the decision-making of national economic and energy policies. But it is a complex and uncertainty system problem affected by the outer environment and various uncertainty factors. Herein, a novel clustering model based on set pair analysis (SPA) was introduced to analyze and predict energy consumption. The annual dynamic relative indicator (DRI) of historical energy consumption was adopted to conduct a cluster analysis with Fisher’s optimal partition method. Combined with indicator weights, group centroids of DRIs for influence factors were transferred into aggregating connection numbers in order to interpret uncertainty by identity-discrepancy-contrary (IDC) analysis. Moreover, a forecasting model based on similarity to group centroid was discussed to forecast energy consumption of a certain year on the basis of measured values of influence factors. Finally, a case study predicting China’s future energy consumption as well as comparison with the grey method was conducted to confirm the reliability and validity of the model. The results indicate that the method presented here is more feasible and easier to use and can interpret certainty and uncertainty of development speed of energy consumption and influence factors as a whole