Sahadat Hossain

The effects of daily cover soils on shear strength of municipal solid waste in bioreactor landfills.

Bioreactor landfills are operated to enhance refuse decomposition, gas production, and waste stabilization. The major aspect of bioreactor landfill operation is the recirculation of collected leachate back through the refuse mass. Due to the accelerated decomposition and settlement of solid waste, bioreactor landfills are gaining popularity as an alternative to the conventional landfill. The addition or recirculation of leachate to accelerate the waste decomposition changes the geotechnical characteristics of waste mass. The daily cover soils, usually up to 20-30% of total MSW volumes in the landfill, may also influence the decomposition and shear strength behavior of MSW. The objective of this paper is to study the effects of daily covers soils on the shear strength properties of municipal solid waste (MSW) in bioreactor landfills with time and decomposition. Two sets of laboratory-scale bioreactor landfills were simulated in a laboratory, and samples were prepared to represent different phases of decomposition. The state of decomposition was quantified by methane yield, pH, and volatile organic content (VOC). Due to decomposition, the matrix structure of the degradable solid waste component was broken down and contributed to a significant decrease in the reinforcing effect of MSW. However, the daily cover soil, a non-degradable constituent of MSW, remains constant. Therefore, the interaction between daily cover soil particles and MSW particles will affect shear strength behavior. A number of triaxial tests were performed to evaluate the shear strength of MSW. The test results indicated that the shear strength of MSW was affected by the presence of cover soils. The friction angle of MSW with the presence of cover soil is higher than the friction angle of MSW without any cover soils. The friction angle of MSW increased from 27 degrees to 30 degrees due to the presence of cover soils for Phase 1 samples. The increased strength was attributed to the friction nature of sandy soil that was used as daily covers soils. Therefore, the effects of cover soils on the shear strength properties of MSW should be evaluated and taken into consideration during stability analyses and design.

Monitoring extent of moisture variations due to leachate recirculation in an ELR/bioreactor landfill using resistivity imaging

Bioreactor or enhanced leachate recirculation (ELR) landfills are designed and operated for accelerated waste stabilization, accelerated decomposition, and an increased rate of gas generation. The major aspects of a bioreactor landfill are the addition of liquid and the recirculation of collected leachate back into the waste mass through the subsurface leachate recirculation system (LRS). The performance of the ELR landfill largely depends on the existing moisture content within the waste mass; therefore, it is of utmost importance to determine the moisture variations within the landfill. Traditionally, the moisture variation of the ELR landfill is determined by collecting samples through a bucket auger boring from the landfill, followed by laboratory investigation. Collecting the samples through a bucket auger boring is time consuming, labor intensive, and cost prohibitive. Moreover, it provides the information for a single point within the waste mass, but not for the moisture distribution within the landfill. Fortunately, 2D resistivity imaging (RI) can be performed to assess the moisture variations within the landfill and provide a continuous image of the subsurface, which can be utilized to evaluate the performance of the ELR landfill. During this study, the 2D resistivity imaging technique was utilized to determine the moisture distribution and moisture movement during the recirculation process of an ELR landfill in Denton, Texas, USA. A horizontal recirculation pipe was selected and monitored periodically for 2.5years, using the RI technique, to investigate the performance of the leachate recirculation. The RI profile indicated that the resistivity of the solid waste decreased as much as 80% with the addition of water/leachate through the recirculation pipe. In addition, the recirculated leachate traveled laterally between 11m and 16m. Based on the resistivity results, it was also observed that the leachate flow throughout the pipe was non-uniform. The non-uniformity of the leachate flow confirms that the flow of leachate through waste is primarily through preferential flow paths due the heterogeneous nature of the waste.

Numerical Study of Slope Stabilization Using Recycled Plastic Pin

Moderate (3H : 1V) to steep (2.5H : 1V) slopes and embankments underlain by expansive clayey soils are susceptible to shallow land sliding during intense and prolonged rainfall events. These failures are predominant in the North Texas area and cause significant maintenance problems for the Texas Department of Transportation (TxDOT). The recycled plastic pin (RPP) has potential to be utilized in slope stabilization as a remedial measure of shallow slope failure. A parametric study was conducted to evaluate the effect of different spacings of RPP on the factor of safety and deformation of the slope. The objective of the current study was to investigate the effect of spacing and length of RPP over the factor of safety using the finite element method (FEM). The analysis was conducted using 2D FEM software package PLAXIS 2D, and factor of safety of the slope was determined using the strength reduction technique. A highway slope constructed using high plastic clayey soil, located over US 287 near St. Paul overpass in Midlothian, Texas, was selected as the reference slope. The strength of the slope was back analyzed using PLAXIS 2D at factor of safety equals to unity. As a remediation technique, RPP was selected for slope stabilization, and the slope was analyzed with different lengths (2.44 m to 3.65 m, 8 ft to 12 ft) and spacings (0.61 m c/c to 2.44 m c/c, 3 ft c/c to 8 ft c/c) of RPP. The FEM results indicated that RPP provided resistance against the shallow failure and resulted increment in factor of safety by shifting the failure plane to a deeper depth toward deep-seated failure. In addition, the factor of safety increased with the increment in RPP length as higher resistance was observed from the foundation soil. The factor of safety of the reinforced slope remained almost constant up to RPP spacing of 1.5 m (5 ft) c/c and then decreased with further increments in spacing. The horizontal deformation of the slope was greater with the increments in spacing of RPP.

Performance Evaluation and Numerical Modeling of Embankment over Soft Clayey Soil Improved with Chemico-Pile

The stabilization of soft clay is one of the most important construction techniques in geotechnical engineering. The injection method using chemical grouting in the ground has been widely used to improve soft ground. Chemical admixture stabilization (with lime and cement as the chemical admixture) has been extensively used in both shallow and deep stabilization to improve the inherent properties of soil, such as strength and deformation behavior. These methods involve mixing soil with chemicals. The other technique—the so-called chemico-lime pile or chemico-pile method—involves forming columns of chemically treated lime in the soil without mixing. This method is widely used in Japan and Singapore and is a fast, effective, and well-recognized way of improving soil with soft clay deposits. The chemico-pile method has been used in a full-scale test embankment project at the Nong Ngo Hao site near Bangkok, Thailand. The site is part of the second Bangkok International Airport project, located in the Samutprak...

Real-Time Modeling of Moisture Distribution in Subgrade Soils

Swelling and shrinkage of expansive subgrade soils in North Texas causes deep cracks and surficial distresses in pavements. Because this behavior is affected by changes in water content of the soil, evaluating moisture variation in the subgrade soil could provide an estimation of pavements deformation. The objective of the current research is to investigate the moisture variation of a pavement site due to real-time environmental factors. For this study, a two-lane rural road in Kaufman County, Texas, was selected for data acquisition and monitoring purposes. Soil test borings were conducted to determine subgrade condition, and moisture sensors were installed at 1 m and 4.5m depth to monitor in situ moisture content every hour. In addition, a rain gauge was used to record the precipitation at the site. The main wetting branch of the soil water characteristics curve (SWCC) was estimated from soils physical properties and incorporated into finite element (FE) software PLAXFlow for transient analysis of moisture variation. The FE results were compared with direct field measurements. Model outputs and sensor readings indicated significant variation of moisture content due to rainfall. Lower moisture content accompanied with considerable variations was recorded at shallow depths, while moisture variation at greater depths was mainly controlled by groundwater. The FE results were in line with direct field measurements, confirming the validity of the estimated parameters. The FE analysis could be utilized for determination of the hydraulic balance of the subgrade and has the potential to be considered as part of future design procedures.

Prediction of Strength and Stiffness Properties of Recycled Pavement Base Materials Using Non-destructive Impact Echo Test

Reclaimed Asphalt Pavement (RAP) and Recycled Crushed Concrete Aggregates (RCCA) are being used increasingly as an alternative to the conventional base materials in pavement construction. However, product variability and lack of strength-stiffness characteristics are limiting the use of recycled materials in pavement application. A non-destructive evaluation technique could provide a better quality control tool for the highway officials during the construction. Therefore, the objective of the current study is to develop a correlation between the non-destructive impact echo and Unconfined Compressive Strength (UCS) test for RAP and RCCA materials. Both impact echo and UCS tests were performed on specimens prepared with 100% RAP and RCCA. The RAP and RCCA were separately treated using four (0, 2, 4 and 6%) different dosages of Portland cement. Based on the experimental results, the range of P-wave velocity was found between 175 m/s to 475 m/s, the compressive strength varied between 67 kPa to 2860 kPa and tangent modulus range was 19 MPa to 280 MPa. Dynamic modulus of elasticity was also calculated from the P-wave velocity, density, and Poisson’s ratio. At 4% and 6% cement contents, dynamic modulus of elasticity was within 10% of the tangent modulus found from UCS test. At 0% and 2% cement content, higher variation was observed. Inadequate fines to fill the voids might be the reason of lower P-wave velocity at 0% and 2% cement contents. This could eventually have predicted lower values of dynamic modulus.

Expansive Subgrade Behavior on a State Highway in North Texas

Expansive soil in pavement subgrade experiences volume change due to the seasonal moisture and temperature. For the clear understanding of the subgrade behaviour, it is necessary to study the pavement behaviour in real time which connects the gap between moisture, temperature and suction variation with induced pressure on the pavement and pavement deformation. The current study presents an extensive instrumentation program on a subgrade soil and performance of an asphalt pavement over expansive clay subjected to seasonal variation of moisture temperature and suction. A section over 2- lane State Highway (SH) 342 in Lancaster, Texas, maintained by Texas Department of Transportation (TxDOT) was instrumented with moisture and temperature sensor, water potential probe, pressure sensor and piezometer to monitor the seasonal variation on continuous basis using automated data collection system. In addition, horizontal inclinometer was installed across the pavement and routine topographic survey was conducted to monitor the pavement deformation. Based on the preliminary monitoring results of almost a year, a 5-7% variation of moisture was observed in the subgrade soil whereas; 2 inch of movement was noticed over the pavement. Additionally, a temperature prediction model was developed based on 15 months monitoring data.

Deformation of MSW bioreactor landfills: Properties and analysis approach

Bioreactor landfills are operated for rapid stabilization of waste, increased landfill gas generation for cost-effective energy recovery, increase in landfill space, enhanced leachate treatment, and reduced post closure maintenance period. Due to rapid stabilization and settlement of solid waste, bioreactor landfills are gaining popularity as an alternative to conventional Subtitle D landfills. However, the addition of leachate to accelerate waste decomposition changes the physical and engineering characteristics of Municipal Solid Waste (MSW), which affects the compressibility and shear strength behavior of MSW. Settlement during the active landfilling period is beneficial as it increases the landfill capacity, however, large differential settlement may cause serious damage to the existing leachate recirculation pipe system and interim covers. Also, due to accelerated decomposition and changes in shear strength properties, the stability of landfill slopes is expected to be affected. The objective of this paper is to analyze the compressibility of MSW in a bioreactor landfill as a function of construction sequence, time and waste placement using the finite element program PLAXIS. In this analysis, the layer properties are adjusted to account for extent of decomposition. The results from PLAXIS are compared with waste settlement data collected during the filling of a landfill cell at Calgary Biocell in Canada.

Data-Based Real-Time Moisture Modeling in Unsaturated Expansive Subgrade in Texas

Moisture variations significantly influence the strength and stiffness of expansive subgrade soils, shortening the service lives of pavements and increasing the associated maintenance costs. Accurate measurements of soil moisture can be obtained through soil sampling and testing, but the process can be extensive and costly. Empirical models can accurately predict the moisture variations in an expansive subgrade in a shorter period of time, with lower accompanying costs. The objective of the current study was to develop moisture models, using real-time field monitoring data from two hot mix asphalt roads in North Texas. The collected data were analyzed in a statistical environment to solve two first degree Fourier series. The solution produced a moisture variation model that captured variations associated with seasonal effects and temporary variations associated with rainfall. The outputs of this model were within 90% of the values measured on site. Application of the developed models will facilitate noninvasive estimations of the response of soil strength and stiffness properties to variations in moisture.

Monitoring of Moisture Variation in Highway Slope through Resistivity Imaging

Moisture flow dynamics in roadway systems affect the performance and serviceability of pavements. Accurate measurement of moisture requires direct collection of soil samples, followed by testing in the laboratory. Nondestructive measurements from neutron probes, time or frequency domain reflectometry, and radiometry in remote sensing are also used by researchers to record changes in moisture content. However, these methods either yield discrete point information or have depth limitations. Geophysical testing provides a continuous portrayal of the subsurface moisture flow. During this study, a geophysical testing method known as electrical resistivity imaging (ERI) was adopted. Monthly testing was performed on a selected two-lane highway slope. The objective of the study was to determine seasonal moisture fluctuations in a side slope of highway pavement. Over a monitoring period of 12 months, two distinct seasons were observed. April to October was determined to be the dry season, whereas November to March was found to be the wet season. Resistivity values ranged from as high as almost 25 Ohmm in the dry period to as low as 10 Ohm-m in the wet period. In addition, the zone of moisture variation was found to be nearly 3 m. Moreover, the possible zone of moisture intrusion could be detected from resistivity imaging after rainfall. The ERI data was further statistically analyzed and correlated with temperature and precipitation data. The findings will further advance understanding of the dynamics of moisture flow in roads.