Full-Scale Field Test On Retaining Structure Enhanced With Soil Nails And Prestressed Anchors

Retaining structure enhanced with soil nails and prestressed anchors is found good at constraining the horizontal displacement and therefore ensuring the stability of the foundation pit during excavation. Based on these advantages, such retaining structure is widely used in foundation excavation practice. This paper presents results of a series of in-situ tests conducted to investigate the mechanical behaviors of retaining structure enhanced with soil nails and prestressed anchors. Behaviors of three different retaining structures enhanced with i) soil-nails; ii) soil-nails and prestressed anchors without unbonded part; iii) soil-nails and prestressed anchors with a 2.5m unbonded length, were monitored during staged excavation to investigate the influences of i) the prestressing force and ii) unbonded length of the prestressed anchors on the performance of the entire retaining system. It was found that the affecting the stress and deformation of composite retaining system, which is in agreement with the other published results in the literature. The variation of the magnitude and distribution of soil nail force responding to the anchor prestressing force however showed no systematic trend. The unbonded length of anchors, which is suggested to be the main factor affecting the structural stability in dense materials in the literature, is found to have little influence in loose fill materials used in this study. Studies presented in this paper are useful for the rational design and serviceability analysis of the composite soil-nailed retaining structure enhanced with prestressed anchors.

Comparisons of Two Approaches for Geotechnical Model Calibration with Scarce Data

Geotechnical models are usually built upon assumptions and simplifications, inevitably resulting in discrepancies between model predictions and measurements. To enhance prediction accuracy, geotechnical models are typically calibrated against measurements by bringing in additional empirical or semiempirical correction terms. Different approaches have been used in the literature to determine the optimal values of empirical parameters in the correction terms. When measured data are abundant, calibration outcomes using different approaches can be expected to be practically the same. However, if measurements are scarce or limited, calibration outcomes could differ significantly, depending largely on the adopted calibration approach. In this study, we examine two most commonly used approaches for geotechnical model calibration in the literature, namely, (1) purely data-catering (PDC) approach, and (2) root mean squared error (RMSE) method. Here, the purely data-catering approach refers to selection of empirical parameter values that minimize coefficient of variation of model factor while maintains its mean value of one, based solely on measured data. A real case of calibrating the Federal Highway Administration (FHWA) simplified facing load model for design of soil nail walls is illustrated to thoroughly elaborate the differences in practical calibration and design outcomes using the two approaches under scarce data conditions.

Numerical Modelling of Soil-Nail and Secant Pile in Plaxis 2D: A Case Study of Tomb of Jam Nizam-al-Din Samoo, Makkli Thatta

The majority of historical heritage structures of Makli, Thatta require rehabilitation. As the Tomb of Jam Nizam-al-Din Samoo is near the slope’s edge, the settlement increased due to slope degradation. This study aims to investigate the effect of structural elements such as secant pile and structural nails on the settlement of Tomb. From this study, it was observed that with the installation of secant piles on the slope, Tomb’s settlement can be reduced considerably; moreover, the anchors did not have an appreciable role in the settlement. The secant pile of 12 m sufficient to reduce settlement. This will be economical and easy approach as compare to the retaining wall. The Factor of safety (FOS) reduced from 1.36 to 1.10 with increase of slope from 45 to 35. The FOS of slope also increased with the installation of nails from east side of slope. The will reduce the chances of slope failure and consequently the tilting/collapse of Tomb.

Experimental Study on Chloride-Induced Corrosion of Soil Nail with Engineered Cementitious Composites (ECC) Grout

Conventionally, a soil nail is a piece of steel reinforcement installed inside a hole drilled into the slope and filled with cement paste (CP) grout. Chloride penetration is a major deterioration mechanism of conventional soil nails as the CP grout is easy to crack with an uncontrollable crack opening when the soil nail is subject to loading or ground movements. Engineered Cementitious Composites (ECC) are a class of fiber-reinforced material exhibiting excellent crack control even when loaded to several percent of strain, and therefore, ECCs have great potential to replace traditional CP grout in soil nails for achieving a long service life. In this study, the chloride ion transport characteristics and electrically accelerated corrosion process of steel rebar in ECC and CP grouts are systematically studied. The rapid chloride ion penetration test results showed a reduction of 76% and 58% passing charges in ECC with 0.15% and 0.3% pre-loading strain, respectively, as compared to that in un-cracked CP. Furthermore, the accelerated corrosion experimental data showed that ECC under pre-loading strain still exhibited a coefficient of chloride ion diffusion that is 20–50% lower than CP grout due to the ability to control crack width. Service life calculations based on experimentally measured parameters showed that the predicted corrosion rate and corrosion depth of soil nails in ECC grout were much lower than those in CP grout. The findings can facilitate the design of soil nails with excellent durability and long service life.

Optimization of geometric parameters of soil nailing using response surface methodology

Optimization of parameters of soil nailing is an important task in reinforcement soil problems. This paper focuses on the effect of nail geometric parameters on soil nailed wall analysis and identifies which factors that most affect their stability and cost using response surface methodology (RSM). RSM has been chosen to achieve an optimum combination of the soil nailing wall design. The influence of three factors has been considered; it included nail length, its inclination, and vertical spacing between nails. After a finite element analysis to model and perform the soil nailing simulations, a Box–Behnken design was applied, based on a set of experiments using various combinations. For this purpose, 15 runs were conducted to analyze tested parameters and to determine their interactions. The analysis of variance (ANOVA) and contour lines plots were investigated, and therefore, the most important parameters affecting the safety factor and the cost were identified. From the goodness-of-fit analyses of the model and the illustrative example, the proposed regression model provides a reasonably good estimate of the overall safety factor for soil nail walls and their cost. The results obtained from this study showed that RSM is an efficient and effective tool to identify the optimal combination, and it emerges that the safety factor and cost are most influenced by nail length and vertical spacing.