Geo-Congress 2019: Soil Improvement

This study evaluates the quality of ground surface pre-grouting (GSPG) in shallow-buried sections of mountain tunnels. A fuzzy analytic hierarchy process (FAHP), which enables objective evaluation based on multiple criteria, is presented. Three controlled factors (i.e., integrity, continuity, and robustness of rock mass after grouting) are selected in the criterion layer of the analytic hierarchy process (AHP) model in which five indices are established, including increased ultrasonic wave speed and unconfined compressive strength, reduction rate of permeability, and curve types of the water pressure test. The performance of the model is evaluated on the basis of the field test results of a GSPG project in a shallow-buried section of the Tongluoshan Tunnel in the southwestern mountainous region of China. The test data of the five indices are applied to evaluate the quality of grouting by using the FAHP. The results of the FAHP can be classified as “good” because they are consistent with the field test data, thus verifying the reliability of this method.

The two oldest methods, both developed in the late 1920s and early 1930s, for reducing settlements and increasing the shear strength of foundations on soft clays are sand drains and the Kjellman paper “wick” drain. These early developments are described, including some special equipment developed for the installation of paper drains. Full-scale field tests were conducted by the Swedish Geotechnical Institute in the 1940s using Kjellman paper drains and in the 1950s on sand drains by Hansbo. In the early 1970s, the author and colleagues at the SGI investigated sand drain performance 14 years, after installation. About the same time, the first modern PV drain was developed by Wager, and two early case histories are presented: one a full-scale loading test and the other in which PV drains were used on timber piles to reduce excess pore pressures during piling installation. Suggestions for successful PV drain design and installation, especially for preloading and surcharging, are also given.

At the I-295/I-76/Route 42 Direct Connection project in Camden/Gloucester Counties, New Jersey, column supported embankment systems (CSES) are used to limit settlement and accelerate construction of large embankments bearing on compressible soils. At present, the I-295/I-76/Route 42 interchange does not provide a direct connection for I-295 thru traffic. The reconfiguration of the interchange will allow access from I-295 directly to I-76 and Route 42 without the use of low-capacity ramps. Due to the project size, traffic flow, and safety concerns, expedited reconstruction of the existing roadway system is necessary and CSES are an ideal solution to treat the soft soils. Without ground improvement, unimproved settlements were estimated to be up to 30.5 cm; with ground improvement, the post-construction settlement was limited to 2.5 cm. This paper presents the project background, site challenges, ground improvement design, analysis of load test data, and quality control measures used on-site at the 295 Direct Connect Project Contracts 1 and 2. The project is divided into 4 contracts. The majority of the rigid inclusions were installed in a relatively homogenous soil profile consisting of 15.2 meters of low plasticity clay whose stiffness increases with depth. Thus, the rigid inclusions CSES design calculations relied on skin-friction capacity in cohesive soils. The design met the project performance requirements for settlement, bearing capacity, and external stability of the walls and embankments. The design was completed using finite element software including Plaxis and RocScience Slide as well as traditional deterministic analyses. Ten rigid inclusions static load tests were performed in Contract 1 and seven rigid inclusions static load tests were performed in Contract 2 to validate design assumptions. The rigid inclusion shaft resistance estimated in the clays during design was compared with actual shaft resistance values from the strain gauge data collected during the load tests. The walls were instrumented and monitored and the instrumentation indicated the system performance was well-aligned with design models. Finally, the numerous quality control measures employed on the project are discussed.

The Seminole Tribe of Florida initiated construction of a $1.8 billion extension to its Hard Rock Hotel and Casino in Hollywood, Florida. The overall project includes an iconic, guitar-shaped, 36-story hotel structure on the edge of a lake, a subsurface valet tunnel accessing an existing, multi-story parking structure, and various low-level structures housing resort amenities. Extensive ground improvement and geotechnical construction was required for both the building foundations and the tunnel construction, requiring several different techniques to address the subsurface conditions, site restraints, and project requirements. Technologies used included wet mass soil mixing, sheet piling, auger cast piles, micropiles, vibro-compaction, vibro-replacement/stone columns, and compaction grouting. The ground improvement and deep foundation program was accomplished by a single specialist contractor, significantly streamlining site operations and allowing each area to be completed ahead of schedule. This paper discusses each work area of this high-profile project, including design and installation challenges.

Ground improvement is often an effective option for mitigating liquefaction-induced hazards for new or existing structures, embankments, tanks, or other facilities. In these cases, ground improvement typically includes one or more of the following modifications to the soil: 1) densification, 2) shear reinforcement, 3) cementation, and 4) enhanced drainage conditions. The ground improvement ‘toolbox’ contains techniques capable of seismic hazard mitigation for a wide range of soil conditions. Following a brief discussion on liquefaction triggering and the engineering mechanics involved for using ground improvement for liquefaction-hazard mitigation, three case histories are presented herein. The case histories will describe the design and execution of projects using the following ground improvement methods: 1) vibro-replacement for soil densification, 2) deep soil mixing for soil shear reinforcement, and 3) earthquake drains for improved drainage conditions.

This paper is a sequel to a paper presented in an April, 1995 conference on karst geohazards. It extends our experiences over the decades in Reading and surrounding areas of south-east Pennsylvania. Project experiences included review of site geology and hydrogeology, historical maps/photos, fracture and lineament study, past usage of the area, past sinkhole occurrences maps, site reconnaissance visit, geophysical investigations, soil borings, probing, and testpit excavations. Drainage of the site, proximity of water bodies such as streams, drainage channels, ponds, wells, and other such hydrogeological information are important to consider when studying the sinkhole potential of the area. The analyses consider the observed and explored site subsurface conditions, structural loading, and design settlement limits. The most important aspect is to make sure that water is drained properly not eroding the ground as it flows along its passage and away from the structure foot-print, and daylighted in a way that does not endanger the foundation bearing medium. Sinkhole experiences are described and illustrated with designed and constructed projects along with remediations implemented.

Soil surcharging has been a traditional and economical method of ground improvement for minimization of post-construction settlements of structures located in soft soil areas. An embankment was constructed and monitored between 2014 and 2018 at the Harold J. “Babe” Landry Landfill in Berwick, Louisiana, as part of a drop-off ramp for citizens of St. Mary Parish to dispose their household wastes in a safe manner when coming to the landfill. The drop-off ramp consists of a sand backfill embankment with a maximum height of approximately 8 feet overlaid by asphaltic concrete paving and supported by reinforced concrete retaining walls. The long-term settlement for the ramp was estimated to be on the order of 6 to 8 inches, which was above the allowable settlement for the structure, which was established equal to 2 inches. The present case study demonstrates that surcharging is an economical and feasible method for ground improvement even in relatively small projects such as the construction of the Citizens’ Drop-off Ramp of the Harold J. “Babe” Landry Landfill.

The Christina River Bridge is a three-span bridge about 143.3 m long and 13.7 m wide. The project site was contaminated with heavy metals. The west approach for the bridge is located over an old buried stream/canal and the elevation is near EL +3.048 m, the elevation at the east approach is near EL+1.52 m. The subsurface soils consisted of a thin crust of fill underlain by as much as 12.19 m of very soft, highly-plastic silt and clay. Geotechnical design challenges included global stability, long-term settlement, and downdrag on the bridge foundation. The proposed design treatments included expanded poly-styrene retaining walls along the west approach, ground modification using deep mixing methods (DMM) along the east approach adjacent to the bridge and surcharge and quarantine of the easternmost portion of the embankment. The contractor proposed using controlled modulus columns (CMCs) in lieu of DMM to save time.

This paper describes the use of rigid inclusion ground improvement techniques at an energy facility located in a coastal New England city. Difficult geotechnical conditions and proposed loading at the site drove the recommendation of typical deep foundations for structural support, e.g. piles or drilled shafts. However, in some areas, rigid inclusions ground improvements were employed. Rigid inclusions, when designed and constructed properly (that is, grid spacing, element depth, diameter, etc.), allow traditional, cost-effective spread footing, and slab-on-grade foundation support. This approach often reduces overall project schedule and budget when compared to deep foundations with pile caps and structural slab construction. This paper presents a ground improvement case history with discussion of geotechnical investigations, design considerations, methods of analysis, construction quality control, installation, and a large, pre-production single-element, and full-scale load testing program, with focus on the test program results and evaluation, particularly the full-scale load testing results.

Double fluid jet grouting is a technique for ground improvement which employs high-pressure cement slurry and high-pressure air. Because of the high energy with which the cement slurry is injected into the ground, the soil structure breaks up and mixes with the cement slurry in situ, creating a high-strength soil-cement material. In this study, sound monitoring was conducted to confirm the condition of the ground cutting, and the rotation cycle was changed according to the ground conditions in order to carry out the jet grouting. In order to confirm the strength of the improved body and the adhesion of the joints, core samples were gathered by vertical and inclined core boring. Consequently, it was confirmed that by controlling the rotation cycle, it is possible to construct improved columns with less variation of diameter. It was also confirmed that a homogeneous structure can be constructed with the overlapping sections of adjacent columns having the same strength as the improved body itself.

This paper presents the preliminary results for the use of filler-stabilized biopolymers to enhance the mechanical properties of sands. Biopolymers include natural polymers produced by micro-organisms or artificially manufactured fully degradable polymers. Several studies in the literature have reported the successful use of biopolymers (e.g. xanthan gum, agar gum, guar gum) as additives for soil improvements. However, concerns were also raised about the strength dependency of these improved soils on moisture content as well as the expected increase in the swelling potential of these soils. In this study, xanthan gum was first stabilized using bentonite clay as a filler to strength the resulting gel, which was then mixed with a clean sand. The results of triaxial consolidated-undrained tests on the clean sand, sand treated with un-stabilized xanthan gum, and sand treated with xanthan gum stabilized with two percentages of bentonite clay are presented. Also, the results of free swelling tests on sand samples treated with un-stabilized xanthan gum, and xanthan gum stabilized with different fillers are reported. While more experiments are needed, the main conclusion of this study is that stabilizing biopolymers with a small percentage of a filler—such as clays—will provide a viable ecofriendly bioinspired soil improvement technique. This could also help providing the nutrition needed for microbially induced calcite precipitation (MICP) without compromising the soil strength.

Previous studies on the use of fly ash as a binder agent to improve the geotechnical properties of mine tailings are limited, despite the potential application. When fly ash is mixed with mine tailings and water, the calcium oxide in fly ash reacts with water and forms calcium hydroxide. The calcium hydroxide then reacts with the aluminate-silicate phase in the tailings to produce calcium-silicate-aluminate hydrates (hydration gel). The hydration gel can strengthen the tailings structure by bonding the solid particles and reducing the void ratio. In this study, gold mine tailings were mixed with two types of fly ash with different chemical compositions. Initial solid contents of 70% and 78% were used to evaluate the effect of the initial solid content (SC) on the unconfined compressive strength (UCS) of the tailings. Specimens were prepared by mixing tailings with 3% and 7% fly ash contents by dry mass and cured for 3 and 28 days. The amended specimens of mine tailings with an initial 78% of solid content and 7% of fly ash show measurable UCS while the other specimens yielded no measurable UCS. The test results show the potential application of fly ash as the major binder content to improve the tailings mechanical properties at the paste consistency (SC=78%).

Biochar has been identified as an environmentally friendly soil amendment material that inhibits greenhouse gas emission from solid waste disposal sites, and that enhances nutrient removal capacity of site soils. A literature survey indicates that there is a lack of research focusing on the mechanical properties of soils amended with biochar. This work examines the behavior of compacted soil-biochar specimens. Standard Proctor compaction curves obtained for a silty sand soil, a 6 percent biochar-soil mixture, and a 10 percent biochar-soil mixture are investigated. It was hypothesized and proven correct that the dynamic compactive effort of the hammer breaks the biochar particles, and therefore creates inconsistent compaction curves when the biochar-soil mixture was reused for the preparation of subsequent specimens at increased water content. The compaction curves were reproducible when the compacted mixture was discarded from use in preparing subsequent specimens. Comparing the compaction curves obtained with and without remixing, a reduced maximum density was obtained for the later. It was also observed that, with increasing biochar percentage (by volume), the optimum water content of the mixture increased and the maximum dry density decreased.

In order to fully understand the thermo-hydro-mechanical behavior of the geotechnical infrastructures, the effects of temperature variations on soil properties and soil behavior have to be studied. Hydraulic conductivity, strength, volume change, moisture content, and pore pressure generation and dissipation rates depend on temperature variations. Thermal loading might induce excess pore water pressure and volumetric changes. Temperature changes in the fine-grained soils will cause expansion in water and soil particles. Since the coefficient of expansion for soil particles is much smaller than that for water, a generation of pore water pressure is expected. This thermally induced pore water pressure and then its dissipation during the relaxation period results in a time dependent consolidation. Thermal consolidation in fine grained soil is more dominant and can be irreversible in normally consolidated clay. However, the volumetric changes of highly over consolidated soil caused by temperature increment is reversible by temperature reduction. In this research, a modified consolidation testing device is used to study the effect of temperature increments (e.g., increasing step by step temperature increments to 80°C) on the consolidation of fine grained soils. In another words the effect of temperature increments during the test on the consolidation process is studied. Time of applying the heating, target temperature, and initial void ratio are parameters affecting the rate and the amount of consolidation in the samples.

Improvement of soil characteristics has always been of interest to civil engineers. This research aims to propose an environmentally-friendly material to improve dune sand strength using sodium alginate biopolymer. For this purpose, unconfined compressive tests were performed. The effect of temperature on the variation of compressive strength was also studied. It was shown that curing at temperatures of higher than 45°C led to a gradually decrease of compressive strength up to 80°C. Curing time was also an effective parameter on the compressive strength which considerably increased until 7 days and samples reached to 90% of their total strength. CBR tests were also conducted to evaluate the ability of using sodium alginate for base and subbase of road stabilization. The results indicated that sodium alginate as a biodegradable material greatly increased the CBR strength in comparison to pure sand. A microstructural analysis was also carried out through SEM images which showed that sodium alginate effectively linked soil articles together through different types of chemical bonds. Consequently, it was pointed out that sodium alginate has a good potential to improve the mechanical properties of poorly graded sand, and also it has no detrimental effects on the environment.

This paper presents the results of a study conducted on expansive soil using alumino-silicate binders known as geopolymers. As part of the study, an experimental program was developed to characterize the native soil obtained from North Texas. A metakaolin-based geopolymer was synthesized using potassium hydroxide as the alkali-activator. The soil was treated with a pre-determined dosage of geopolymer and tested for swell-shrink behavior, using one-dimensional swell, and linear shrinkage bar tests. Results show significant reduction in swelling, shrinkage, and plasticity characteristics. These outcomes indicate that geopolymer stabilized materials are beneficial in mitigating volume-change behavior of expansive soils, and should be further evaluated as an effective soil stabilizer for the efficient design of sustainable and distress-free infrastructure for problematic soil conditions.

Recently, soil strengthening using gel-type biopolymers have been attempted by many researchers. However, most previous studies have been conducted by focusing on feasibility of biopolymers as a new soil binder with lack of in situ considerations (e.g., confinement and strain-stress related variation). This study aims to investigate the shear behavior of xanthan gum-treated sand under different in situ confinement conditions. Laboratory tri-axial test is performed under three different confinement conditions (σ₃=50 kPa, 100 kPa, 200 kPa) with different xanthan gum biopolymer contents (m_bp/m_s=0.5%, 1.0%, 2.0%). It is revealed that high shear strength is developed when the biopolymer film matrix is established within the pore space when the biopolymer is thoroughly dried in the laboratory (oven). The strengthening effect by biopolymer film is substantial although its enhancement is varied according to xanthan gum contents and confinement conditions.

Growing biofilm in saturated sand has been shown to reduce the permeability of soil by one or more orders of magnitude and may be a viable approach to reducing seepage in the field; however, obtaining laboratory permeability results in soil samples where biofilm is being developed is difficult. Adding nutrients results in the formation of biofilm in the soil but also the formation of biofilm in the piping and other areas of the permeability testing apparatus. In addition, some bacteria produce gas as a product of metabolism and this gas can collect in the apparatus and interfere with fluid flow. This paper presents an approach to permeability testing that effectively minimized the growth of biofilm and the collection of gas in the testing apparatus for multiple sand samples treated with a nutrient solution over a period of more than 60 days.

In the present study, the utilization of cement kiln dust (CKD) and unground cement clinker (UgCC) containing high amount of alkali is used in the production of geopolymerized black cotton (BC) soil is investigated. CKD and UgCC are cement industrial waste by products. Due to high alkali content of CKD and UgCC, its disposal as well as recycling poses problem. The objective of the present study is to utilize CKD and UgCC to be geopolymerized with BC soil using mixture of sodium silicate and sodium hydroxide at different molarity to induce cementation of BC soil to increase its strength and durability. The effect of molarity of sodium hydroxide on unconfined compressive strength (UCS) of the BC soil treated with CKD and UgCC is investigated. The effect of geopolymer on strength characteristic of BC soil have been studied for a curing period of 7 days, 14 days, and 28 days. The compressive strength of BC soil increases with geopolymer dosage and curing. The density of BC soil also increases with the increase in geopolymer. The test result indicates that geopolymer is very effective in stabilizing the soft soil.

Microbial induced calcium carbonate precipitation (MICP) has drawn significant attention as a more sustainable and eco-friendly ground improvement technique compared to conventional methods in recent decades. It has been shown that bio-treatment improves the shear strength of sands remarkably, and this improvement should be predictable. To predict the shear strength after bio-treatment, different types of failure envelopes with various forms can be employed. In the majority of studies on MICP-treated sands, the shear strength parameters have been calculated based on linear failure envelopes, in spite of the fact that linear failure envelopes can overestimate the shear strength at low confinements. The intercept cohesion, indeed, may be overestimated using a linear model. Therefore, this paper presents a nonlinear failure envelope which was developed based on thirteen drained triaxial tests at two levels of cementations on Ottawa 20–30 (a clean coarse sand). The obtained results indicate that nonlinear failure envelope has a higher accuracy in predicting the shear strength of MICP-treated sands at low confinements.

Hurricanes Irma and Matthew caused significant damage to the coastal dune systems in northeast Florida. Dunes are the first line of protection in storm events for the prevention of beach erosion and inland floods. It would be beneficial if a sustainable, rapidly-deployable strengthening mechanism could be used to fortify the dunes prior to hurricanes to help mitigate damage. One potential technology for dune protection is microbial induced calcite precipitation (MICP). Dune sand was collected from Neptune Beach, FL, and the feasibility of treating the sand using a surface-spray technique was investigated. Specifically, microbes, urea solution, and calcium chloride were applied to a bench-scale synthetic dune surfaces using a garden hose attachment. The treated dunes were allowed to cure for 48 hours. Results showed this application technique may provide erosion protection via three mechanisms. An erosion resistant crust formed along the outer surface of the dune; large blocks of cemented material are formed on the interior of the dune; and treatment appears to increase median grain size diameter. However, the increase in grain-size diameter may have a relatively small effect on erosion mitigation.

This research focuses on investigating the effectiveness of microbial-induced carbonate precipitation (MICP) to reduce the potential for wind erosion. This paper summarizes the results of preliminary wind tunnel experiments. Wind tunnel experiments were performed on untreated and MICP-treated samples. Soil samples prepared at the same relative density were treated using different treatment procedures. Soil samples were sprayed with bacteria (Sporosarcina pasteurii) suspended in a urea solution followed by the application of a calcium chloride cementation solution. The solutions were applied on the soil surface and allowed to percolate through the sample. To determine the wind velocity profile in the wind tunnel, pitot tubes were used to measure the differential pressure at various heights and verified analytically using the law of the wall. Digital image correlation (DIC) combined with wind velocity measurements were utilized to identify the conditions at which the surface fails (i.e., erosion initiation and sustained erosion). The wind velocity was gradually increased until particle movement was initiated. This paper summarizes the experimental setup and results of preliminary experiments.

Microbially induced calcite precipitation (MICP) has been used for a number of years as a technique for the improvement of various geological materials. MICP has been used in a limited capacity in organic matter-rich soils with varying degrees of success. Investigators hypothesized that microbially-induced cementation of organic soil could be improved using a surfactant. Varying amounts of Sodium Dodecyl Sulfate (SDS) were added to soils of varying organic content and a mixing procedure was used to treat these soils via MICP. Treated specimens were tested for unconfined compressive strength (UCS). Results appeared to show direct relationships between SDS content and treated specimen strength although significant variability was present in the data. The addition of SDS during MICP treatment improves the organic soil strength, which is likely due to the formation of a calcium dodecyl sulfate complex and little to do with the microbially-induced calcite.

Sandy slopes supporting coastal roadways are susceptible to failure during large storm events due to erosion and slope stability. Investigating the application of a natural and sustainable soil improvement method is required to increase the shear strength and erosion resistance of sands while having less impact on the coastal ecology. Microbial induced calcium carbonate precipitation (MICP) is a novel and promising soil improvement technique that utilizes indigenous soil bacteria to hydrolyze urea and induce calcium carbonate precipitation, which improves the mechanical properties of the soil. The chemical concentrations and recipe ratios used for this process can affect the level of improvement. This study aims to draw a relationship between the chemical ratios, shear strength, and erodibility resistance of MICP treated sands. In an attempt to tailor the laboratory experiments closer to field requirements, industrial grade chemicals were used to treat the soil specimens. A submerged impinging jet system was used to evaluate the erodibility parameters of soil specimens treated with different ratios of urea to calcium chloride. Unconfined compression strength tests (UCS), an indication of shear strength, were performed on specimens with similar properties treated with different ratios of urea to calcium chloride. Shear wave velocity measurements were used to detect the level of improvement in all soil specimens during the treatment. The results indicate that the ratio of urea to calcium chloride can affect the number of treatments required to reach a target level of improvement. In addition, the ratio of chemical concentrations can change the erosion strength differently compared to the UCS. These results can also provide a basis for choosing an optimum treatment recipe for field application, by keeping a balance between the mechanical properties of MICP treated soils, mass of calcium carbonate, and cost of chemicals used for treatment.

Expansive soils, also known as swell-shrink soils, undergo substantial volumetric changes due to moisture fluctuations from seasonal variations. These volumetric changes cause millions of dollars in damages annually. Microbial induced calcite precipitation (MICP) is a promising soil improvement technique, which uses urease producing bacteria to precipitate calcium carbonate. In this study, a stabilization alternative for expansive soils was studied using MICP. Specifically, indigenous bacteria were stimulated by mixing enrichment and cementation solutions with expansive natural soils to precipitate calcium carbonate and make soil stronger and less expansive. This study examined three expansive soils with varying plasticity and mineralogical characteristics. Two protocols for shallow mixing were studied. In Protocol-1, soil samples were mixed with enrichment solutions at optimum moisture content and allowed to mellow for 1, 2, 3, and 4 days. In Protocol-2, soil samples were mixed with enrichment solutions at moisture content corresponding to 95% of maximum dry unit weight on the wet-side of a standard Proctor curve. Moisture was allowed to escape from the mix during the mellowing period under both protocols. Following the mellowing periods, the lost moisture is replaced with cementation solution to reach optimum moisture content, and the soil sample was compacted to its maximum dry unit weight. Unconfined compression strength test was used to evaluate the strength improvements due to treatments. The treatment effectiveness was also evaluated with measurements of calcium carbonate precipitation. The results show promise for this method as an alternative to current shallow stabilization methods. An increase in mellowing period for low and medium plastic soils was determined to be beneficial. The current results also showed that the presence of higher amounts of enrichment solution and addition of less cementation solution is not advantageous for this procedure based on the performance of Protocol-2.

Microbially induced calcite precipitation (MICP), or bio-cementation, has advanced rapidly in the past decade as a promising environmentally-conscious ground improvement alternative. In the ureolytic bio-cementation process, microorganisms containing urease enzymes are used to mediate the degradation of urea, thereby producing total ammonium, dissolved inorganic carbon, and hydroxide ions. Despite recent advances, concerns regarding the fate of produced ammonium by-products have persisted. Methods to manage and remove ammonium following bio-cementation will likely be needed as the technology transitions toward industrial field-scale application. In this study, a preliminary investigation of the removal of ammonium from a bio-cemented poorly-graded sand was performed using soil column experiments to improve our understanding of the effect of rinse solution chemistry on ammonium removal efficiency, bio-cementation integrity, and potential for long-term attenuation. Four soil columns received identical treatments for 14 days targeting a final calcite content near 4 to 5% and daily rinse injections were subsequently applied to investigate post-treatment ammonium removal. Results suggest that the injection of 5 pore volumes of a high pH, high ionic strength solution may offer the ability to remove near 99.1% of post-treatment pore fluid ammonium concentrations with minimal bio-cementation degradation.

Microbial induced calcium carbonate precipitation (MICP) is a promising technology to improve engineering properties of soil. The goal of this paper is to explore the possibility of using the MICP to treat coal combustion products to increase the mechanical strength and the limit the metal leaching from these products. Class C fly ash was used in the MICP treatment. Experimental results show that unconfined compressive strength (UCS) of MICP-treated fly ash samples increased from 1.3 MPa to 2.6 MPa when the fly ash content increased from 3% to 6% (by weight). However, the UCS of MICP-treated samples dropped to 0.07 MPa when the fly ash percent was up to 20%. Toxicity characteristic leaching procedure (TCLP) results showed that concentrations of boron, chromium, and lead decreased with fly ash in the MICP-treated sample. Compared to the fly ash sample without MICP, the MICP-treatment can limit the metal leaching with stronger bonding of precipitated calcium precipitation.

In this study, injection configuration system to stabilize soil mass around a pile foundation with the microbially induced carbonate precipitation was developed and a simplified scheme for field implementation was proposed using seepage and chemical transport analysis. Experimental testing included a tracer experiment in a large-scale soil box with the results used to calibrate transport parameters for the numerical model. Results indicated that the use of a central injection through the pile lead to effective distribution of the tracer within the flow domain. The radius of the zone of influence was estimated as 8.5D based on the induced flow hydraulic gradient of 0.7. Target concentration was used as a design criterion to determine the injection duration. Analyses indicated 7.5 days of injection duration was needed to affect a zone having 4D extent with a tracer concentration of 100%. Recommendations are presented regarding volume treatment scenarios with the use of single point injections.

Shrinkage cracking in pavement subbase layer is a major cause of distress in pavements, especially for pavements on expansive soils. Polymeric soil stabilizing agents can potentially mitigate such problems, because of its ability to improve soil mechanical properties, such as reducing shrinkage cracking and improving the soil strength. In this research, experiments have been conducted to investigate two types of polymeric stabilizing agents, i.e. one hydrophilic polyurethane foam and one hydrophobic polyurethane foam, for mitigating soil shrinkage. Bentonite clay was used in the experiment due to its high shrinkage potential. Two types of polymeric foam were mixed with bentonite at different weight ratios, and the moisture behavior reflected by water content change were studied over time. The behaviors of each chemical foam agent were also quantified by the video camera. The results show when PU foams were added at a relatively low weight ratio (< 5%), the shrinkage property has been improved, because 3%~5% more moisture can be retained inside samples. Besides, comparing with hydrophobic PU foam, hydrophilic PU foam has a greater effect on alleviating shrinkage distress at the early stage. In the meanwhile, samples with PU foam show earlier cracking than the pure bentonite sample, possibly because the application of PU foam makes sample be stiffer and of higher stress. Nonetheless, it is notable that the de-polymerization of hydrophobic PU foam occur in the experiment. Therefore, it is worth being studied further before applying polymeric stabilizing agents in engineering.

Soil stabilization has become a major trend in construction engineering and the researches regarding the effectiveness of using natural fibers are rapidly increasing. The aim of present study is to stabilize soil with bottom ash (BA) and reinforcing it with sisal fiber so as to achieve maximum load bearing capacity for the soil to be used as foundation material. Several studies on use of synthetic fibers for soil stabilization are available in the literature but these fibers are much expensive and not easily available. Stabilizing agent used in this study is BA and reinforcing material used are sisal fibers. BA is formed in coal furnaces and it is a waste product. The present study investigates the strength of the soil which is modified with BA and fibers which are added at a different percentage to the soil. A series of laboratory tests were conducted on soil sample stabilized with BA and different percentages of sisal fibers. The test method includes Compaction tests and unconfined compression strength (UCS) tests. The percentage of BA is varied from 0 to 40% and for fiber it is varied from 0 to 2%. Further model footing tests were conducted in a steel tank to evaluate the settlement-bearing pressure behavior of the stabilized soil under increment loading. The performance of sisal fiber was compared with that of poly vinyl alcohol (PVA) fiber. The test results indicate the competency of sisal fiber to be used as reinforcement in soils with better performance than PVA fibers.

Soil reinforcement by natural fibers is viewed as a viable, sustainable, and efficient ground improvement technique. An investigation of the published literature reflects a significant effort to study the undrained response of fiber-reinforced clays using triaxial tests. Such tests allow for an assessment of the short-term stability of geotechnical systems that are built with fiber-reinforced clays. For some geotechnical applications for which long term stability considerations govern the design, there is a need for studying the response of fiber-reinforced clays under fully drained conditions. The objectives of the work presented in this paper are to (1) investigate the drained shear strength of clays that are reinforced with natural hemp fibers and which are compacted to different initial water contents and (2) quantify the level of improvement in the drained shear strength due to the addition of fibers. These objectives will be achieved by implementing an experimental drained triaxial testing program on fiber-reinforced clay samples prepared using “static” compaction while changing the confining pressure (20, 100, and 200 kPa) and the moisture content of the clay (18% and 20%).

Expansive soils often exhibit volumetric changes with variations in the seasonal moisture. Pavements constructed on expansive soil subgrades may subject to cracking, waviness, and distress. This paper reports the laboratory CBR and UCS test results conducted on expansive soils treated with phosphogypsum (PG) and crumb waste rubber (CRW). The influence of several mix proportions of PG-CRW on the strength properties of expansive soil was studied through experimentation. A significant improvement was observed in the CBR and UCS values, at optimum percentages of 6% and 2% respectively for PG and CRW. Further, the durability of the proposed mix proportions was interpreted from the soil samples subjected to curing for 28 days. An attempt was made to correlate the CBR and UCS values of the treated soil matrix through linear regression analysis. The results of the study can be gainfully utilized in the design pavement subgrades on soft soils.

Coir fiber being eco-friendly, involves minimum amount of hard work for soil stabilization, and gaining popularity during past few years. This has added a new dimension to the development of coir fibers in the field of geo-textiles as coir possesses high tensile strength and biodegradability for its advantages over other natural and synthetic fibers. Coir fibers with random distribution (RDF) when used as soil reinforcement is found to increase in shear strength, post peak compressive strength, and ductility of the soil. The result from series S1 and S2 of unconfined compressive strength is compared with TCF and UCF of those obtained for BC soil alone. The effect of curing with 0.5% fiber content for varying dry densities and moisture content showed that TCF reinforcement with dry density of 15.5 kN/m3 and 23% moisture content compacted for unconfined compressive strength at 7 days curing was maximum in comparison with all the combinations tested at different curing periods. TCF reinforced BC soil indicated marginal reduction in UCS with curing period where compared with UCF reinforced BC soil, indicating TCF is suitable to be introduced as an admixture.

Expansive soils for residential projects are often treated with liquid ionic soil stabilizer using deep pressurized injection method. Liquid ionic soil stabilizers (LISSs) have a long history of application in practice. However, its treatment effect has not been well documented both in the laboratory and in the field due to the proprietary nature of the liquid chemicals. In this paper, a home construction site in Burleson, TX, was selected for the study of a liquid soil stabilizer. Both Shelby tube and bulk soil samples were collected from the construction site before chemical injection treatment and after the treatment. The Atterberg limits test, linear shrinkage test, 1-D swell, swell pressure, and swell fatigue tests were carried out on the untreated and treated samples in the laboratory before and after treatment. Furthermore, the results from the hand-mixed sample were compared with those of Shelby tube samples to evaluate the feasibility of the lab mixing method. The test results show LISS is effective in the reduction of PI, shrinkage, and swell pressure. Similar swell reductions were observed between Shelby samples and bulk samples treated by the same LISS concentration. However, Shelby tube specimens swell much faster and have higher swell fatigue resistance as compared with the remolded specimens.

The paper presents results of a comprehensive investigation on utilization of lime sludge (a waste material from water treatment plant) and polypropylene fiber on unconfined compressive strength (UCS) and shrinkage behavior of commercially available kaolinite clay. Effects of adding various quantities of fiber i.e., 0%, 0.5%, 1.0%, and 1.5% and lime sludge i.e., 2%, 4%, 6%, and 8% were investigated and evaluated. Both fiber and lime sludge samples were added to kaolinite (EPK clay) by dry weight of the EPK clay. The UCS samples were prepared at optimum moisture content (OMC) and corresponding maximum dry unit weight from standard Proctor compaction test results. The addition of fibers and lime sludge resulted in significant influence on engineering properties with curing period. Scanning electron microscopy (SEM) analyses were conducted on samples after shearing. Results show that with the increase in the lime sludge content, UCS value initially increased, followed by a slight decrease. An increase in curing period resulted in an increase in the unconfined compressive strength. Also, an increase in the lime sludge content led to a reduction in shrinkage potential. Increase in the fiber content increased the UCS value. Also an increased shrinkage potential was observed with the increase in fiber content. Based on the SEM analyses, addition of fiber created a physical interaction between the fiber and the EPK clay. Whereas, the use of lime sludge induced a chemical reaction between the EPK clay and the lime sludge which changed the soil fabric significantly.

The primary aim of study is to evaluate the influence of hydrated lime and bottom ash on strength properties of expansive soil by two different kinds of dry-weight-based ratios. In this research, bottom ash is used as a stabilizing agent for expansive soil stabilization due to the potential benefits of its additional pozzolanic components in combination of hydrated lime. Bottom ash is a by-product of coal-fired process which was collected from Eraring Power Station in New South Wales, Australia. Meanwhile, expansive soils were artificially prepared using a proper combination of kaolinite, bentonite, and Sydney fine sand to constitute soil samples typically representing expansive soil in the region. To determine the optimum combination ratio of bottom ash to lime to stabilize expansive soil, different contents of randomly distributed bottom ash from 5% to 30% were mixed with soil and 5% hydrated lime to investigate the engineering behaviour of stabilized expansive soils. It is noted that the additive contents of lime and bottom ash, adopted in this study, were calculated based on both the dry weight of soil alone and the total weight of bottom ash-lime-soil admixture for the comparison purpose. The results of indirect tensile (IDT) strength and California bearing ratio (CBR) tests after various curing times are presented and discussed. The experimental findings show that a ratio of 5% lime to 20% bottom ash mixed with expansive soil is considered as their optimum combination ratio for achieving the maximum bearing capacity and tensile strength for soil-dry-weight-based additives. However, the optimum combination ratio of 5% lime to 25% bottom ash is determined in the case of the entire mixture-dry-weight-based additives. It is concluded that applying the latter optimum ratio to a combination of lime and bottom ash can improve the expansive soil strength better than the former.

Many soil improvement/stabilization methods are available for modifying the soil mass in order to increase the overall engineering performance of soil. In the conventional method of reinforced soil construction, the inclusions are normally oriented in a preferred direction and sequentially in alternating layers. Another soil strengthening technique is mixing of randomly distributed discrete fibers into the soil, similar to admixture stabilization in its preparation. The discrete fibers are simply added and mixed randomly with the soil. In the present study, the laboratory triaxial compression tests were carried out on soil specimens uniformly mixed with steel fibers at different confining pressures. Steel fiber contents having two different sizes (1 mm×2 mm and 1 mm×5 mm) were mixed with the cohesionless soil, and the effects of reinforcement content and size on failure strain, peak deviator stress, ultimate strength ratio, and initial tangent modulus/secant modulus were studied. The failure occurred between 5–9% of vertical strain and the optimum reinforcement content was around 4%.