Case Studies of Problematic Expansive Soils: Characterization Challenges, Innovative Stabilization Designs, and Novel Monitoring Methods

This presentation describes key research works on expansive soils, the methods employed to characterize them, and fallacies in the current characterization of expansive soils. Novel swell characterization models that account for hydro, chemical, and mechanical behaviors of soils are introduced and used in various case studies to improve expansive soil stabilization practices. An innovative design method for successful stabilization of expansive soil is introduced in one case study which incorporated both basic clay mineralogy and unsaturated soil behaviors, as well as performance-based durability studies. Sulfate soil stabilization works on medium-to-high sulfate soils are presented in another case study. The last case study involving steep earthen embankment built with expansive clayey soils and experiencing recurring surficial slope failures and maintenance issues is presented along with forensic studies explaining the causes of slope failures. All case studies reveal the need for understanding of soil chemistry, including clay mineralogy and sulfate screening studies, to improve the current field stabilization and infrastructure design on expansive soils. The last section of the talk focuses on recent innovations for better health monitoring and management of civil infrastructure built on expansive soils using unmanned aerial vehicle (UAV) platforms and visualization tools, which will be valuable in validating the application of new materials in infrastructure design and construction processes as well as for health monitoring and asset management practices

Effect of Cementation on Cone Resistance in Sands: A Calibration Chamber Study.

An understanding of the effect of cementation on geotechnical properties of soil deposits is gaining increasing attention in the profession. When low levels of cementation in sands are neglected, pile capacity and slope stability are underestimated and liquefaction is overestimated. It is essential to devise schemes to identify cementation in soil investigations and develop methods in evaluating engineering characteristics of cemented deposits. The objective of this study is to develop a method to identify cementation in sands and assess the engineering characteristics of cemented sand deposits using the cone penetration testing scheme. The scope of the study includes evaluation of the effect of cementation on cone penetration testing (experimental model) and comparison of these experimental results with theoretical models of penetration mechanism in cemented sands. Existing models based upon the bearing capacity theories and cavity expansion models are utilized in theoretical modeling. A constitutive model is developed for strength-deformation behavior of cemented sands and is used in theoretical modeling. Artificially cemented Monterey No. 0/30 is used in the calibration chamber study. A total of 30 tests are conducted at three ranges of relative density (45-55, 65-75 and above 85%), three confining pressures (100, 200 and 300 kPa) and three different cement content (0, 1 and 2%). Pluviation method is used for specimen preparation. Specimens are cured for 7 days, transferred into the flexible wall calibration chamber and then consolidated under K\sb0 conditions. Penetration testing was conducted with a 1.27 cm diameter miniature cone. Separate drained triaxial tests provided the necessary parameters for strength-deformation modeling of cemented sands. The experimental model results coupled with the theoretical model predictions provide a semi-empirical and empirical schemes for evaluating engineering characteristics of cemented sand deposits. An assessment of the applicability of these models in prediction of cementation in such deposits is also provided. The results indicate that tip resistance and sleeve friction in cone penetration testing provide a reasonable assessment of cementation. The charts and the analysis method provided can be used to estimate the engineering characteristics of such deposits

Strength and Stiffness Characterization of Controlled Low-Strength Material Using Native High-Plasticity Clay

A research attempt was made to design a controlled low-strength material (CLSM) mix that can be used as bedding and haunch material for a pipeline by using the native soil as fine aggregate. Several CLSM mix designs were attempted using native high-plasticity clay as fine aggregate material. Comprehensive material characterization studies including flowability to strength tests were performed. These results were analyzed to address the applicability of each mix to serve as pipe bedding/backfilling zones in a pipeline construction. Both flowability and density test results are first evaluated, and as a result, several mixes are formulated. These mixes were further subjected to engineering characterization-related studies, and this paper presents these test results. Setting time, strength, and stiffness results as well as excavatability evaluations of these mixtures are covered as a part of these studies. These results indicate that the CLSMs can be produced using native high-plasticity soils with strength properties always matching specified requirements. Certain relaxation on setting time periods could further help in developing economical mix designs. CLSMs that meet project specifications are recommended for field implementation

Flowability and Density Characteristics of Controlled Low Strength Material (CLSM) Using Native High Plasticity Clay

In pipeline construction projects when high plastic clayey soils are encountered in the excavated trench material, they are typically landfilled and better quality materials are imported from outside quarry sources for use as bedding and haunch zone materials. This practice has detrimental environmental and cost impacts; therefore, an efficient reutilization of this high plastic excavated material to produce controlled low strength materials (CLSMs) to use as bedding and haunch zone materials will have major sustainability benefits. As a part of an on-going research study, novel CLSM mix designs were developed by utilizing native high plastic clayey soils from the excavated trench material. Due to the high plasticity nature of the soils, it is essential to address both flowability and density property requirements prior to validating them against other engineering properties. Hence, several CLSM mixtures with the native clayey soils as ingredients were initially designed as per flowability criterion to establish the optimum quantities of chemical binders and water quantities. Later, these mixes were verified for satisfying density property criterion. This technical note presents the step by step procedure followed in preparing these mixes along with test results obtained from various mixes designed as a part of the testing program. Based on these results it was evident that CLSM mixes with high plastic clays can be developed that meet both flowability and density criteria. The success of this research has enhanced the sustainability efforts in pipeline construction projects as this study showed excavated clayey soils can be successfully reused in CLSM applications than landfilling them

Addressing Clay Mineralogy Effects on Performance of Chemically Stabilized Expansive Soils Subjected to Seasonal Wetting and Drying

Premature failures in chemically stabilized expansive soils cause millions of dollars in maintenance and repair costs. One of the reasons for these failures is the inability of existing stabilization design guidelines to consider the complex interactions between clay minerals and the stabilizers. It is vital to understand these complex interactions, as they are responsible for the strength improvement and swell/shrink reduction in these soils, in turn affecting the overall health of the infrastructure. Hence, this research study examined the longevity of chemically stabilized expansive soils subjected to wetting/drying conditions with a major focus on clay mineralogy. Eight different natural soils with varying clay mineralogy were subjected to wetting/drying durability studies after stabilizing with chemical additives including quicklime and cement. Performance indicators such as volumetric strain and Unconfined compressive strength trends were monitored at regular intervals during the wetting/drying process. It was observed that clayey soils dominant in the mineral Montmorillonite were susceptible to premature failures. It was also noted that soils dominant in other clay minerals exhibited early failures at lower additive contents. Also, an attempt was made for the first time to address the field implications of the laboratory studies by developing a correlation that predicts service life in the field based on clay mineralogy and stabilizer dosage

Swell and Shrinkage Strain Prediction Models for Expansive Clays

A comprehensive laboratory investigation was conducted to study volume change behaviors of five different types of expansive clayey soils sampled from various regions in Texas, USA. The laboratory test results, which were presented in an earlier paper, are analyzed here to evaluate existing correlations that can be used to predict swell and shrink-related displacements in these soils. The test database is also used to develop newer and practical models for predicting volume change-related soil properties. Models developed here used soil plasticity and compaction properties as independent variables. Newer models, that rely on seasonal compaction moisture content variations in the subsoils, were introduced to estimate both volumetric and vertical swell and shrinkage-induced soil deformations expected under civil infrastructure. The developed correlations, along with the existing models, were then used to predict vertical soil swell movements of four case studies where swell-induced soil movements were monitored. This comparison analysis showed that the model dependency on the volume change test procedural information and moisture content variation due to seasonal changes will lead to better prediction of swell movements in subsoils. Future research directions and recommendations are provided on implementation of the developed models in a realistic estimation of swell movements of infrastructure construction projects

Swell and Shrinkage Characterizations of Unsaturated Expansive Clays from Texas

Expansive soils have long been recognized as problematic because they cause failure to civil structures constructed above them. The main problem of these soils can be attributed to poor understanding of the volume changes caused by moisture fluctuations. Current swell and shrinkage characterization models are limited by both the lack of standardized tests and tests that employ volume changes in uniaxial direction. In the present research, a comprehensive laboratory investigation was undertaken to study the volume change related swell–shrinkage behaviors of five different types of expansive clayey soils sampled from various regions in Texas, USA. Extensive experimental programs consisting of basic, chemical and mineralogical soil properties were first determined. Three-dimensional free swell and shrinkage tests were performed on all soils at various compaction moisture content conditions. Soil–water characteristic curves (SWCCs) of all test soils were determined by studying the suction potentials of these soils over a wide range of moisture contents. Volume change measurements of soils showed a good correlation with soil properties, including plasticity and soil compaction properties. SWCC results also showed a clear variation in SWCC profiles of soils with respect to soil plasticity. Overall, a large database of soil properties was developed and is presented here. It includes physical and mineralogical properties, as well as engineering swell, shrinkage and SWCC test results

Analysis of bearing capacity of bored piles from bi-directional load test: A case study in Quang Ngai province

The paper presents the vertical bearing capacity of bored piles from the bi-directional load test (O-Cell method) at the Tra Khuc dam-bridge project in Quang Ngai province. The dam structure was supported by approximately 400 bored piles with the diameter of D1200mm and the length of 27 m to 50 m. The ground includes the sand, clay and weathered rock layers with the SPT index (N30) from 8 to 80. The pile's tips were socketed in the granite layer with the average compressive strength of 18.6 MPa. Two test piles with the length of 29.1 m (T1N) and 42.75 m (T8N) were conducted O-Cell test. The side friction of soil layers and pile tip resistance were analyzed. The axial strain obtained from strain gages were used to analyze the axial load distribution along the depth of the pile. The test results show that the side resistance of the piles in the weathered rock mixed is 77.14 kPa for the pile T1N and 72.34 kPa for the pile T8N (approximately 50% of the total side resistance) which are not the ultimate shaft resistance of the piles in this layer. As its’ advantages, the bi-directional load test could be applied widely in the narrow site or on river condition in Vietnam

Analysis of bearing capacity of bored piles from bi-directional load test: A case study in Quang Ngai province

The paper presents the vertical bearing capacity of bored piles from the bi-directional load test (O-Cell method) at the Tra Khuc dam-bridge project in Quang Ngai province. The dam structure was supported by approximately 400 bored piles with the diameter of D1200mm and the length of 27 m to 50 m. The ground includes the sand, clay and weathered rock layers with the SPT index (N30) from 8 to 80. The pile's tips were socketed in the granite layer with the average compressive strength of 18.6 MPa. Two test piles with the length of 29.1 m (T1N) and 42.75 m (T8N) were conducted O-Cell test. The side friction of soil layers and pile tip resistance were analyzed. The axial strain obtained from strain gages were used to analyze the axial load distribution along the depth of the pile. The test results show that the side resistance of the piles in the weathered rock mixed is 77.14 kPa for the pile T1N and 72.34 kPa for the pile T8N (approximately 50% of the total side resistance) which are not the ultimate shaft resistance of the piles in this layer. As its’ advantages, the bi-directional load test could be applied widely in the narrow site or on river condition in Vietnam

Evaluation of Sustainable and Environmentally Friendly Stabilization of Cohesionless Sandy Soil for Transportation Infrastructure

Ordinary Portland cement (OPC) is generally used to stabilize cohesionless sandy soils that are often found in coastal areas. Due to its high carbon footprint, many studies are being conducted to identify a suitable green alternative for stabilizing cohesionless soils. Previous studies have shown that partially replacing OPC with waste materials such as nano-silica and coal waste reduces the overall carbon footprint without significantly impacting the performance. Geopolymer (GP) received a lot of attention in the past few decades owing to its similar properties to that of OPC yet with a lower carbon footprint. This study investigated the feasibility of stabilizing cohesionless sandy soils with metakaolin-based GP. Engineering and characterization tests such as shrinkage, strength, pH, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) were performed to evaluate various characteristics of the stabilized mixes with different dosages of geopolymer and relate them to microstructural changes. Notably, GP-treated soils did not deteriorate during the durability tests, whereas the OPC-treated soil only retained about 75% of its strength. This is an indication that GP could be a better choice than OPC in coastal areas where cohesionless soils often experience heavy rainfall and flooding. Overall, an optimum dosage of GP improved both the mechanical properties and durability of cohesionless soils