Mohammad Sadik Khan

Influence of Soil Reinforcement on Horizontal Displacement of MSE Wall

Mechanically stabilized earth(MSE) walls offer simpleconstructiontechniques,pleasing aesthetics,and cost-effectivesolutions as an alternative to conventional gravity walls. However, design and construction should be carefully evaluated to achieve satisfactory perfor- mance of the wall. A case study is presented on a MSE wall located on State Highway 342 in Lancaster, Texas. The horizontal movement of the MSE wall was between 300 and 450 mm within 5 years of construction. A forensic investigation was performed to determine the causes of the excessive movement. It was identified that inadequate reinforcement length was one of the contributing factors that caused horizontal displacement of the MSE wall. The objective of this study was to determine the effects of soil reinforcement on excessive movement of the MSE wall. As a part of the forensic investigation, two inclinometers were installed at the site to monitor any additional movement of the MSE wall. The inclinometer results suggested that the wall continued to move at an average rate of 4.5 mm/month during the investigation period. A finite-element (FE) program was used to simulate horizontal displacement and stability of the MSE wall. It was observed that the numerical modeling results were in good agreement with inclinometer results. A parametric study was conducted to identify the effects of soil reinforcement on horizontal movement at varied wall heights and backfill conditions. Numerical analyses results indicated that the effect of reinforcement stiffness was not significant at a wall height of 4 m compared with 8 and 12 m. The wall movement varied from 74 to 29 mm for an increase in reinforcement stiffness from 250 to 42,000kN= ma t 1:0H reinforcement length. The variations in displacement with reinforcementlengthssuggestedthatsubstantialreductionindisplacementoccurredforanincreaseinlength-height(L=H)ratiofrom0.5to0.7. FEmodelingresultswereusedforsensitivityanalysisemployingastatisticalanalysisprogram.Basedontheanalyses,reinforcementlengthand stiffness wereidentified as influential factors for thehorizontal displacement of MSE walls at a specific height.DOI:10.1061/(ASCE)GM.1943- 5622.0000297.

Evaluation of Unknown Foundation Depth Using Different NDT Methods

Many of the older bridges in the United States have no original contract documents available, and about 26,000 bridges that are rated as scour critical have unknown foundation conditions. Thus, no information is available regarding the type, depth, geometry, or material of these scour critical bridge foundations. The unknown bridge foundations pose a significant problem to bridge owners because of safety concerns. This paper presents the determination of an unknown bridge foundation depth at Forth Worth, Texas. The bridge was supported by driven steel H-piles. Three nondestructive testing (NDT) techniques were utilized: (1) the parallel seismic (PS) method, (2) the sonic echo (SE) method, and (3) resistivity imaging (RI). The main objective of this present study was to compare the suitability of NDT techniques to determine the unknown bridge foundation depth. Based on the field test results, both PS and RI methods provided foundation depth close to actual foundation depth. However, the SE method was determined to be unsuitable for determining the unknown steel H-pile depth.

Effects of Backfill Soil on Excessive Movement of MSE Wall

AbstractThe use of mechanically stabilized earth (MSE) retaining walls has gained popularity as an alternative to conventional cast-in-place concrete walls. The construction of MSE walls is cost effective, requires less site preparation, and is technically more feasible compared with conventional concrete retaining walls. However, use of backfill with high fine content and poor drainage behavior can cause excessive wall movement or even failure. The current paper presents the case study of a MSE wall located at State Highway 342 in Lancaster, Texas. The top of the MSE wall has moved as much as 300–450 mm only 5 years after construction. An extensive site and laboratory investigation testing program was conducted to determine the possible causes of the MSE wall movement. The site investigation included soil test boring and resistivity imaging (RI). Perched water zones were identified at a few locations in the backfill area using RI. The bulging of the MSE wall facings was observed where the perched water z...

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 of a Slope Reinforced with Recycled Plastic Pin

The shallow slope failures are predominant in the North Texas area and pose significant maintenance problems for the Texas Department of Transportation (TxDOT). Typically, TxDOT maintenance team considers the traditional slope stabilization techniques, mainly concrete retaining wall. However, conventional remedial methods can be expensive in some instances and Recycled Plastic Pin (RPP) could be utilized to stabilize the shallow slope failure as a cost effective alternative. RPP are fabricated from recycled plastics and waste materials (i.e. polymars, sawdust, fly ash). It is a lightweight material and less susceptible to chemical and biological degradation compared to alternative reinforcing element. RPP are driven into the slope face that provides an additional resistance along the slip plane and increase the factor of safety. During the current study, a highway slope located over highway US 287 near the St. Paul overpass in Midlothian, Texas was stabilized using RPP. Surficial movement and cracks over the shoulder were observed near the bridge abutment due to rainfall. Two cracked section of US 287 slope were selected and reinforced using RPP in March 2011. The width of each reinforced section was 15.25 m (50 ft). Another 15.25 m (50 ft) section between two reinforced sections was used as a control section. To monitor the performance of the reinforced and control section, RPPs were instrumented with strain gauges and were installed in all sections. Based on the performance monitoring data of first year, the instrumented RPP at the unreinforced control section had larger strain due to rainfall compared to the reinforced section. In addition, no significant increment in strain was observed in the instrumented RPPs driven at the reinforced zone.

Determining Unknown Bridge Foundation Depth by Resistivity Imaging Method

Determining unknown foundation length and type of bridge foundation is becoming an important concern for state departments of transportation (DOTs) to ensure the safety of the bridge. Based on specific site conditions and type of foundation to be tested, there are several methods available to determine unknown foundation depth. Resistivity Imaging (RI) is a nondestructive method to investigate the subsurface condition. The objective of the current paper is to investigate the applicability of RI for determining unknown foundation depths. RI tests were conducted on two bridges located in North Texas. The first investigated bridge was supported by driven steel H-pile and the other bridge had drilled shaft as foundation. The investigated bridge foundation depths were compared with the actual foundation depths. The determined foundation depth utilizing RI was found very close to actual foundation depth for the first bridge supported by steel H-pile. However, for the second site, the determined depth of foundation did not match with actual depth of foundation.

Potential Applicability of Impact Echo Method on Pavement Base Materials as a Nondestructive Testing Technique

The use of both recycled asphalt pavement and recycled concrete aggregates is increasing considerably in pavement construction. These materials are relatively weak and have to be stabilized with cement or other stabilizers. However, because of product variability and lack of strength and stiffness, their applicability has to be evaluated extensively. Traditionally practiced methods of evaluation might be unreasonable in terms of time, cost, reliability, and applicability. Rapid nondestructive methods, such as the spectral analysis of surface wave, impact echo, pulse velocity, and so forth, have the potential to be inexpensive and less time-consuming, as well as offering low variability of the test results. The objective of the study was to assess the potential applicability of the impact echo method in evaluating recycled pavement base materials. Six combinations (0%–100%, 10%–90%, 30%–70%, 50%–50%, 70%–30%, and 100%–0%) of recycled asphalt pavement and recycled concrete aggregates, respectively, treated by four amounts of portland cement (0%, 2%, 4%, and 6%), were evaluated by impact echo, unconfined compression, and repeated-load triaxial test. From the test results, the range of P-wave velocity was between 5,500 in./s and 18,000 in./s, the compressive strength varied from 10 pounds per square inch (psi) and 415 psi, and the tangent modulus range was from 2.8 kips per square inch (ksi) to 41 ksi. Statistical models based on P-wave velocity data were derived for predicting elastic modulus, compressive strength, and resilient modulus. It was found that impact echo has significant potential in characterizing the strength and stiffness properties of cement-treated recycled base materials, which confirms the effectiveness of recycled materials in pavement applications.

Parametric Study on Slope Stability Using Recycled Plastic Pin

Expansive clay soil in the North Texas region contributes to frequent shallow slope failures on highway embankments, which require a substantial maintenance budget to repair. Through several field demonstration projects, the recycled plastic pin (RPP) has been proven to be a sustainable and cost effective alternative for the Texas Department of Transportation (TxDOT) in stabilizing shallow slope failure. In the current study, an extensive numerical analysis using the finite element method (FEM) was conducted to evaluate the most influential parameters in designing a slope stabilization scheme with RPP. A parametric study was conducted using five different soil strengths, six slope heights (1.52 m increments from 4.57 m to 12.19 m), three slope ratios (4H:1V, 3H:1V and 2H:1V), three RPP lengths (2.44 m, 3.05 m, and 3.66 m) and five RPP spacing configurations (0.91 m, 1.22 m, 1.52 m, 1.83 m, and 2.13 m). By employing the strength reduction technique in PLAXIS 2D, the factor of safety (FS) was determined for all combinations of the above parameters. The obtained FS values ranged from 1.51 to 3.76. Regression analysis showed that soil strength and RPP spacing had the greatest influence on the FS of the reinforced slope. INTRODUCTION Soil in North Texas is expansive and clayey, making it particularly susceptible to shallow slope failure (Khan et al. 2013). The depths and plan dimensions for such slides vary with soil type and slope geometry and are generally characterized by sliding depths of less than 3.05 m (10 ft.); however, depths of 0.9 m (3 ft.) to 1.8 m (6 ft.) have become more common (Loehr and Bowders 2007). Slope stability issues are typically solved with retaining walls, soldier piles and lagging, and grating cribs (Khan 2013). North Texas roadways are plagued with slope stability problems, and repairs often require a significant maintenance budget. Across the United States, costs associated with the repair of shallow slope failures (resulting in sliding depths between 0.9 (3 ft.) and 3.05 m (10 ft.)) are estimated to be greater than or equal to costs associated with major landslides (Turner and Schuster 1996). In addition, shallow failures often cause significant damages to guard rails, shoulders, and portions of roadways. If not properly maintained, these structures require more extensive and expensive repairs (Loehr et al. 2000). Slope stabilization Geo-Risk 2017 GSP 283 226

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.

Investigation of geohazard potential of highway embankment slopes on expansive clay by using geophysical methods

Natural disasters due to failure of geo-structures present an important threat all over the world. Each year, rain induced slope failures cause significant damages in infrastructures and environments, as well as tragic losses of human lives around the world. Therefore, it is important and necessary to improve our knowledge of slope failure mechanism and ways to mitigate slope failures. At present, soil drilling and sampling method are used for the determination of geo-hazard potential of a site. However, this method offers information at certain points, not a general view of the site. Geophysical methods have the possibility to give an image of the subsurface. The objective of the current paper is to present the results of a site investigation using Resistivity Imaging (RI) and Multi-channel Analysis of Surface Wave (MASW) for a highway slope failure in Texas, USA. The site investigation was also conducted using soil test borings. The investigation results were utilized to identify possible failure plan and to determine the associated geo-hazard potential of the site. The preliminary results indicate that geophysical methods can be successfully utilized to determine the geohazard potential of a site.