Soil Nail Walls Research Articles

Statistical evaluation and calibration of two methods for predicting nail loads of soil nail walls in China

The design of soil nail walls in China is dictated by two national technical specifications, namely, Technical Specification for Retaining and Protection of Building Foundation Excavations by China Academy of Building Research (CABR) and Specifications for Soil Nailing in Foundation Excavations by China Association for Engineering Construction Standardization (CECS). Each specification proposes a method for computation of maximum nail loads while the prediction accuracy of the method is still not yet fully assessed. To fill the gap, this study evaluates the accuracies of the two methods using 144 measured nail loads collected from fully instrumented soil nail walls in China. The influences of three wall working conditions on the prediction accuracy of the methods are also examined. The results show that the two default methods overestimate the maximum nail loads by about 40% on-average and the spread in prediction generally ranges from 70% to 100%. However, the prediction accuracy is not significantly influenced by wall working conditions. Simple calibrations with two or three empirical constants are introduced to the two methods to achieve great improvements in prediction accuracy. The calibrated CABR and CECS methods are demonstrated to be unbiased on average and the prediction dispersions are reduced to about 50%.

Application of UAV Photogrammetry in Displacement Measurement of the Soil Nail Walls Using Local Features and CPDA Method

The high cost of land across urban areas has made the excavation a typical practice to construct multiple underground stories. Various methods have been used to restrain the excavated walls and keep them from a possible collapse, including nailing and anchorage. The excavated wall monitoring, especially during the drilling and restraining operations, is necessary for preventing the risk of such incidents as an excavated wall collapse. In the present research, an unmanned aerial vehicle (UAV) photogrammetry-based algorithm was proposed for accurate, fast and low-cost monitoring of excavated walls. Different stages of the proposed methodology included design of the UAV photogrammetry network for optimal imaging, local feature extraction from the acquired images, a special optimal matching method and finally, displacement estimation through a combined adjustment method. Results of implementations showed that, using the proposed methodology, one can achieve a precision of ±7 mm in positioning local features on the excavated walls. Moreover, the wall displacement could be measured at an accuracy of ±1 cm. Having high flexibility, easy implementation, low cost and fast pace; the proposed methodology provides an appropriate alternative to micro-geodesic procedures and the use of instrumentations for excavated wall displacement monitoring.

Calibration of Resistance Factors for Load and Resistance Factor Design of Internal Limit States of Soil Nail Walls

AbstractA general approach for calibration of resistance factors for load and resistance factor design (LRFD) of pullout and tensile failure internal limit states of soil nail walls is presented. T...

Effect of surcharge magnitude on soil-nailed wall behaviour in a geotechnical centrifuge

The current research was undertaken to study the effect of surcharge magnitude on soil-nailed wall behaviour by modelling an excavation using a geotechnical centrifuge and analytical software. An acceleration of 50g was applied to the models in the geotechnical centrifuge and the excavation was modelled in an in-flight centrifuge at 50g by lowering the saltwater level inside a transparent Latex container in front of the soil-nailed wall. Changes in the safety factor (SF) and stability of the soil-nailed wall were investigated. Next, the centrifugal acceleration was increased to 80g in 5g increments and the effect on the parameters was determined. An increase in centrifugal acceleration (N) was found to decrease the SF of excavation gradually until it reached the stability limit (SF = 1), which is the failure initiation criterion. The occurrence of failure in the physical model agreed with the pattern developed in the numerical models and illustrates and verifies the accuracy of models. Nail density (Dn) is introduced as a functional parameter having a major role on the performance of soil-nailed walls. An empirical relation is developed between the external stability SF and the deformation components which can be normalised using the value for nail density.

Accuracy assessment of default and modified Federal Highway Administration (FHWA) simplified models for estimation of facing tensile forces of soil nail walls

The accuracy of the default Federal Highway Administration (FHWA) simplified model for estimation of facing tensile forces for soil nail walls under in-service conditions was evaluated using a large quantity of measured long-term and short-term facing tensile force data collected from the literature. The estimation accuracy was quantified by the mean and coefficient of variation (COV) of the bias where bias is defined as the ratio of measured to calculated facing tensile force. Based on the available data, the default FHWA simplified model equation was found to overestimate long- and short-term facing tensile forces by about 15% and 23% on average, respectively. The corresponding spreads in estimation accuracy expressed as the bias COV were about 43% and 67%. Undesirable correlations between bias values and calculated facing tensile forces using the default FHWA simplified model equation were detected. A modified FHWA simplified model was proposed to improve the on average accuracy, reduce the spread in estimation accuracy, and remove the hidden correlations noted above. In addition, the modified equation has fewer empirical coefficients than the current formulation (i.e., four versus five). The facing tensile force equations developed in this study are a contribution to the design of facing of soil nail walls within the current FHWA soil nail wall design framework.

A new concept of precast concrete retaining wall: from laboratory model to the in-situ tests

A new concept for the soil nail walls is here proposed and validated through experimental and numerical approaches. This process, based on the use of precast elements that are easier to install, is cheaper and more aesthetic than the classical methods, but the main advantage is reducing the cement consumption which conducts to divided carbon footprint by three. In order to characterize the structural capacity of this new process, this article present an investigation on two in-situ representative walls, one in shotcrete which is the old way of construction, and the other, consisting the precast reinforced concrete slabs, which is the new process. We thus have a demonstrator on a real scale, and perfectly representative, since the constructive modes, as well as the mechanical, thermal, and hydric loadings are the real ones associated with the environment in situ. Substantial instrumentation has been realized over a long period (nearly 2 years), enabling to follow the evolution of the displacements of each wall and the efforts in the anchor nails. To determine the bearing capacity of the constituent element of the precast nail wall, an experimental study coupled with a numerical simulation has been conducted in the laboratory on a single precast slab. This study allows the evaluation of the load associated to crack initiation and the bearing capacity associated to the ultimate state, at the scale of the constituent elements. Finally, in order to evaluate the behaviour of the two concepts of nail walls in the case of extreme solicitation, a dynamic loading induced by an explosion has been conducted on the site.

Reliability-Based Internal Limit State Analysis and Design of Soil Nails Using Different Load and Resistance Models

AbstractA general approach for reliability-based analysis and design for pullout and tensile failure internal limit states of soil nail walls is presented. Reliability index values are computed usi...

Estimation of ultimate bond strength for soil nails in clayey soils using maximum likelihood method

ABSTRACTThis paper presents a maximum likelihood estimation of the ultimate bond strength for soil nails in clays. Both uncensored and censored ultimate bond strength data for soil nails are collected from the literature. Based on the concept of maximum likelihood, a log-likelihood function is constructed for estimating the mean and coefficient of variation (COV) of the ultimate bond strength jointly using the two types of data. The mean and COV are determined as the pair that maximises the log-likelihood function. Two distribution models (normal and lognormal) are used for the estimation. A comparison of the relative competence between the two candidate distribution models that are adopted for describing the collected uncensored and censored data is performed using the Bayesian Information Criterion. Example designs of soil nail walls against internal pullout limit state of nails and overall stability limit state are provided to demonstrate the benefit of taking censored data into account for estimation of the ultimate bond strength of soil nails.

Probabilistic Prediction of Maximum Tensile Loads in Soil Nails

This paper presents the development of a simplified model for estimation of maximum nail loads during or at completion of construction of soil nail walls. The developed simplified nail load model consists of two multiplicative components: the theoretical nail load and the correction factor. The theoretical nail load is computed as the product of lateral active Earth pressure at nail depth and the nail tributary area. The correction factor is introduced to account for the difference between the theoretical and the measured nail loads. A total of 85 measured nail load data were collected from the literature; out of which, 74 were used to develop a simple formulation for the correction factor, whereas the remaining 11 were used for validation. After the validation, the model was updated using all 85 data. The updated simplified nail load model was demonstrated to be accurate on average (mean of model factor equal to 1), and the spread in prediction quantified as the coefficient of variation of the model factor was about 40%. Here, model factor is the ratio of measured to estimated nail load. The randomness of the model factor was also verified. Finally, the model factor was demonstrated to be a lognormal random variable. The proposed simplified nail load model is beneficial due to its simplicity and quantified model uncertainty; thus it is practically valuable to both direct reliability‐based design and load and resistance factor design of soil nail wall internal limit states.

Seismic performance of soil-nailed walls using a 1g shaking table

To assess the seismic performance of soil-nailed walls, a series of 1g shaking table tests was conducted on 0.8 m high wall models with different nail lengths. It was found that the seismic deformation mode of walls highly depends on the nail length and input motion parameters. The combination of a base sliding and rotating deformation mode was observed as the predominant mode of deformation and the length to height (L/H) ratio of 0.7 was presented as the critical ratio in seismic conditions. The pattern of the observed failure mechanisms was similar to a third-degree polynomial failure surface with a specific inflection point. Irrespective of different nail lengths, the threshold acceleration corresponding to the onset of plastic displacements was similar and equal to 0.5g for all models. Based on the formation of initial cracks, a range of Δx/H = 0.33%–0.56% (where Δx is the lateral displacement of the facing) was found as a transitional level from quasi-elastic to plastic state. Also, based on the initial development of the active wedge failure, a range of Δx/H = 3.3%–4.3% was found as a transitional level from plastic to failure state.

System reliability assessment of soil nail walls

An analytical model defining soil nail walls' probability of failure with respect to external and internal stability is developed in the present study. Unit weight, shear strength properties and ultimate bond strength along the soil-nail interface are modelled as random variables and computations of reliability are performed using Monte Carlo simulations. The proposed methodology is illustrated through a case example, in order to identify the critical failure modes and the dependency among them. Furthermore, the influence of the random variables’ probability distribution function on the probability of failure is examined, as well as the importance of the cross-correlation between them.

Observations from a parametric study of the seismic design of soil nailing

A parametric study of the seismic design of soil-nailed walls is performed. Values of a set of geometrical and mechanical parameters for the nail and soil of a slope have been varied to assess their effects on the global factor of safety FSG. Equations of limit equilibrium of forces based on the Coulomb criterion for a two-block-failure mechanism are adopted. Pseudo-static force analyses with a horizontal seismic coefficient of 0·15 are carried out, which is normally used in engineering practice for temporary soil-nailing projects. It was found that FSG clearly increases with soil cohesion and friction as well as nail length, diameter and inclination, although nail inclinations higher than 15° can become detrimental. It was found that there is a combination of nail spacing and wall inclination for which the trend of FSG changes from decreasing to increasing. This situation is caused by a software mathematical optimisation routine, which does not take into account whether it is possible for the failure surface shape to occur or not. Additionally, perforation diameters larger than 100 mm are necessary for FSG ≥ 1·1 and soil–nail shear stress rs values of more than 200 kPa are not significantly beneficial.

Numerical investigation of the static behavior of stepped soil nail walls

This paper presents a numerical investigation on the static behavior of stepped soil nail walls constructed in dry sandy soils. The finite element method was used to study influence of wall geometry and soil parameters on behavior of the walls. We analyzed walls with heights of 10, 15, and 20 m .This study shows that the wall deformation and the nail tensile forces of the stepped soil nail wall are smaller than those of the typical soil nail wall. If properties of soils such as friction angle, cohesion, and elastic modulus decrease, these will result in more decrease in the wall lateral displacement and nail tensile forces of a stepped soil nail wall. Therefore, stepped soil nail walls are more effective in soft soils than hard soils. When a step is located at the middle of the wall height, the wall lateral displacement and nail tensile forces are minimized; hence, the ratio of the optimal step depth to the wall height is 0.5. As the step width increases, the wall deformation and nail forces decrease. Numerical analysis demonstrates that the minimum step width is approximately 0.1 times of the wall height.

Numerical Simulation of Soil Nailing Wall in Deep Foundation Pit

Based on finite difference method and Mohr-coulomb criteria, the numerical model of representative profile of a deep foundation pit was built. The excavation process and soli nailing retaining structure are simulated. The stability of retaining structures which meets the requirement of technical specification is verified by analyzing the displacement and stress. And the force mechanism of soil nailing and concrete surface is studied.

Experimental study on seismic response of soil-nailed walls with permanent facing

A series of 1-g shaking table tests were performed on five reduced-scale soil-nailed wall models to investigate the influence of peak acceleration, loading duration, and nail length on seismic response of the soil-nailed walls in terms of the distribution of shear modulus (G) and damping ratio (D) in soil-nailed mass, the axial force distribution along the nails and the distribution of dynamic lateral earth pressure behind the surface. It was found that the seismic response of walls highly depends on the length of nails and input motion parameters. By introducing a new non-dimensional parameter (Gglobal) for soil-nailed walls, it was observed that the values of G, G/G0, and D are strongly dependent of confining pressure, L/H ratio, and shear strain levels, so that the variation trend of these parameters with γ is well expressed as an exponential equation with a high correlation coefficient. Additionally, a proper convergence was found between Tmax/H.γs.SV.SH and L/H ratio at different levels of acceleration and duration, so that Tmax/H.γs.SV.SH can be defined as a function of L/H ratio and seismic parameters for different rows of nail. Also, It was discovered that the values of predicated earth pressure by conventional methods in static and seismic conditions are too conservative and these methods predict the location of the resultant lateral earth pressure higher than the actual point.

Limit analysis of ground anchor forces

An anchored sheet wall is commonly used in retaining deep excavations. Anchor force is one of the critical design parameters in practice. Based on the kinematical admissible failure mechanism, a limit analysis approach to determine the anchor force is presented. The explicit formula for the anchor force is given, which makes it easy to calculate using a simple calculation program such as MS Office Excel. Anchor force is mainly influenced by seven parameters: the internal friction angle; cohesion of the soil; wall friction angle; surcharge on the ground surface; dip angle of the anchor; penetration depth of the wall; and depth from the anchor force action point on the wall to the ground surface. The relevant quantitative calculation can be performed by the proposed method. In addition, the design anchor force under a specified design safety factor and the anchor forces of multiple rows of anchors are also illustrated in this paper. To verify that the method is reasonable, the predicted and measured anchor forces are compared in two classical soil-nailed wall experiments. The result shows that the presented method is applicable.

A Typical Design of Soil Nailing System for Stabilizing a Soil Slope: Case Study

Slope failures, is a common issue in construction industry, such that engineers have to avoid its risk on human lives and properties by an appropriate technical design of stabilizing methods. Soil nail is one of such stabilization methods. In this paper, soil-nailing system was studied in terms of inclination, spacing and length to determine the most appropriate values for effective stabilization of soil slope. To find the optimum soil nail system, different soil nail inclination, length and spacing were applied to a hypothetical homogenous soil slope (with inclination of 30o, 40o, 45o, 60o, 70o and 90o) and the Factor of Safety (FOS) was evaluated in each case. To validate the results, the optimum soil nail angle corresponding to a slope of 50o was applied to a case study slope, and the FOS of the case study slope was evaluated as well. The case study slope is located at Cadangan Menjalankan Kerja-Kerja Forensik Di Persiaran Endah, Seputeh Wilayah Persekutuan Kuala Lumpur. Results showed that the soil nails inclination; spacing and length have significant effect on the stability of the soil slope. For soil slope with steepness of 30o, 45o, 60o, 70o, and 90o; the best FOS was found with soil nail inclination (to the horizontal) of 50o, 40o, 20o, 15o, and 10o respectively. The effect of soil nails inclination on the stability of the soil-nailed walls is slight when the degree of inclination of the soil nails varies within 5o–20o to the horizontal. The stability of the slope decreases with the increase of spacing between soil nails. Soil nail length has significant effect on the stability of soil slope with deep-seated slip surface and less effect with shallow slip surface. The bond length behind the slip surface should be enough to allow the nail to use its allowable load.

Maximum Likelihood Estimation of Model Uncertainty in Predicting Soil Nail Loads Using Default and Modified FHWA Simplified Methods

Accuracy evaluation of the default Federal Highway Administration (FHWA) simplified equation for prediction of maximum soil nail loads under working conditions is presented in this study using the maximum likelihood method and a large amount of measured lower and upper bound nail load data reported in the literature. Accuracy was quantitatively expressed as model bias where model bias is defined as the ratio of measured to predicted nail load. The maximum likelihood estimation was carried out assuming normal and lognormal distributions of bias. Analysis outcomes showed that, based on the collected data, the default FHWA simplified nail load equation is satisfactorily accurate on average and the spread in prediction accuracy expressed as the coefficient of variation of bias is about 30%, regardless of the distribution type. Empirical calibrations were proposed to the default FHWA simplified nail load equation for accuracy improvement. The Bayesian Information Criterion was adopted to perform a comparison of suitability between the competing normal and lognormal statistical models that were intended for description of model bias. Example of reliability-based design of soil nail walls against internal pullout limit state of nails is provided in the end to demonstrate the benefit of performing model calibration and using calibrated model for design of soil nails.

Stability Analysis of Foundation Pit Under Bottom Over-Excavation and Top Overloading Conditions

The soil nailing wall is a common support method used in foundation pit supporting system. The overloading and over-excavation maybe be met during the processing of excavating and those all effect the stability of foundation pit. The effect of over-excavation and overloading on foundation pit stability are analyzed in this paper using FEM combined with the project in Luoyang city. The results show that local over-excavation may form a continuous slip surface in slope and lead to instability of foundation pit. The external load on the slope top may lead to greater lateral displacement and settlement. The distribution area of settlement under external load shows as triangular and the maximum depth is almost equal to load width. The overloading on the top of foundation pit may accelerate collapse processing when the toe of slope is over excavated, but over excavation still may lead to instability of foundation pit although no overloading on the top. This should be paid more attention in project.

Seismic analysis of soil nail performance in deep excavation

Deep excavation is a common part of development to utilize underground space in densely populated areas. Protection of contiguous building and properties is a primary design concern space. Soil nailing is one such technique to exchange conventional retaining system for deep excavation. It will also donate to significant saving in cost and time of construction compared to conventional retaining systems. In this study an attempt has been made to have a deep vertical excavation on ground of 10 m height using soil nail wall. Also studied the enactment of soil nail wall under different nail inclination to horizontal i.e., Ѳ = 0° and Ѳ = 15° with water table. The finite element analysis of soil nail wall was carried to study the behavior of maximum horizontal wall displacement, maximum horizontal nail displacement, base heave, maximum axial force in nail, maximum shear force in nail, maximum bending moment in nail under both static and seismic conditions using PLAXIS 2D. The process of construction is carried out in stages and a value of Global factor of safety (FSG) is maintained above 1.5 to make sure its stability. The length of nail has a major impact on the behavior of soil nail wall system; increase in nail length will increase the FSG. Results of the numerical analysis direct that the use of soil nail wall is desirable to impart stability to retaining systems.