Soil Nail Pullout Research Articles

Numerical investigation of gravity-grouted soil-nail pullout capacity in sand

Abstract Soil nailing reinforces soil slopes and retaining walls by installing reinforcing elements into the soil, which rely on bonding strength with surrounding soil to transfer axial forces. This study focuses on the pullout capacity evaluation of grouted soil-nailing, which is conventionally determined through costly and time-intensive site-specific field pullout tests or simplified design methods, Federal Highway Administration (FHWA), and Effective Stress Method (ESM). However, these methods neglect important soil nail configuration parameters governing the pullout capacity. To address this limitation, we present a parametric study on a validated Finite-Element model using PLAXIS 3D to comprehensively investigate the pullout characteristics of grouted soil nailing. The study considers various factors such as sand relative densities, lengths, diameters, and inclination angles of the soil nails. The results reveal that the pullout capacity is significantly influenced by the sand relative density, diameter, and the length of the nailing, while the angle of inclination has only a marginal effect. The comparison with FHWA and ESM methods shows significant scatter, attributed to their lack of consideration for these governing parameters in determining pullout capacity. These findings provide valuable insights into the design and optimization of grouted soil nailing pullout capacity for geotechnical applications.

Effect of Degree of Saturation on soil-nail Pullout Resistance

Soil nailing is a technique which is used to reinforce and strengthen the existing ground conditions. This is done by installing closely spaced, passive, structural inclusions known as nail into the soil and these nails helps to improve the overall shear strength of soil. The nail pullout resistance is the shear stress at the grout-soil interface. The soil-nail pullout resistance depends on many parameters. Among the factors influencing the soil-nail pullout resistance, degree of saturation of the soil is an important factor. As degree of saturation of soil mass change with the moisture content of the soil, the pullout resistance may decrease during the intense rainfall. However, verification of pullout tests on soil-nail are not conducted under the worst condition. Hence, measured pullout resistance may not be a safe parameter for design. As such, in this research study, the effect of degree of saturation on pullout resistance was studied by conducting a series of laboratory tests using a laboratory pullout box. Especially designed waterproof cap was used to apply back pressure to saturate the soil within the pullout box. Variation of earth pressures close to the grouted nail were observed during the tests. It was evident from the results that higher the degree of saturation, lower the pullout resistance. Maximum pullout resistance was observed when the degree of saturation is near the optimum moisture of the soil. When the soil is sufficiently dry, lower pullout resistance was observed due to low bond strength between grout surface and surrounding dry soil.

A shallow artificial neural network for mapping bond strength of soil nails

Soil nails are extensively used to stabilize existing slopes and new cuts in Hong Kong. The current method adopted for estimation of nail bond strength is the effective stress method (ESM), which has been shown to be excessively conservative and highly dispersive in prediction. A more robust and accurate model for estimation of nail bond strength is thus highly desired. The present study provides an artificial neural network (ANN) model for mapping of soil nail bond strength. The ANN model is trained, validated, and tested using 522 nail bond strength data contained in Hong Kong soil nail pullout database and is publically accessible in the literature. The accuracy of the developed ANN model is examined and compared with the default ESM. The results showed that ANN model is accurate on average and the predicted dispersion is low. The practical value of ANN model is highlighted by a reliability-based design example of soil nails against the pullout limit state. This work demonstrates the opportunity of applying machine learning approaches to design of soil nail walls.

Evaluation of the Pullout Resistance of Soil Nails in Tehran Alluvium by Considering the Overburden Pressure Effect

The present study evaluates the pullout and bond resistance of soil nails by considering the overburden pressure effect in Tehran alluvium consisted of four units depending on the deposition age. In order to investigate the pullout resistance, a series of 42 in situ pullout tests are performed in unit A, B and C contained coarse grained soils and clayey soils chosen from unit D of Tehran alluvium. Moreover, a one-way analysis of variance has been conducted to evaluate the proportion of soil and overburden pressure effect on the pullout resistance. The obtained results were compared with a conventional code (FHWA-NHI-14-007) criterion and it was concluded that the SPT base pullout resistance is more conservative method for evaluating soil nail pullout resistance. The results indicate that the bond resistance of soil nails increases as the overburden pressure increases in coarse grained soil, but the overburden pressure does not have a significant effect on the bond resistance in clayey soil. It seems that the overburden pressure effect is mobilized by inter granular stress transmission. Furthermore, it was found that the soil nail bond resistance is depended on the construction quality. As the quality of construction reduces, the bond strength decreases. The construction quality is highly affected the soil nails nearby the excavation ground surface where the overburden pressure does not have a significant influence on the pullout capacity.

Evaluation of soil-nail pullout resistance using mesh-free method

Soil nailing has been widely used in the world during the last two decades; because of its technical and economical advantages. The shear strength versus displacement behavior of soil-nail interface has significant effects on economic and safety design of a soil-nail system. In this paper, by application of mesh-free method, MFM, a new approach is presented to investigate the behavior of interface in soil-nail pullout process and also the interaction between nail and surrounding soil. Simulation of soil-nail interface in mesh dependent methods such as finite element method, FEM, has been associated with problems of heterogeneous deformations.In the presented method, the problem domain and its boundary are represented by using sets of nodes scattered in the problem domain and on its boundary. Interaction between nail and surrounding soil is simulated using interface layers with small thickness which is able to model slippage between nail and surrounding soil. The model is based on a Mohr-Coulomb failure criterion, including deviatoric hardening/softening during pre and post peak pullout point and ultimate state. A computer code has been developed to simulate elasto-plastic soil-nail pullout behavior. Simulation results show good performance of MFM in numerical simulation of soil-nail interaction and also soil-nail pullout resistance.

Analysis of resistance factors for LRFD of soil nail pullout limit state using default FHWA load and resistance models

Resistance factors for load and resistance factor design (LRFD) of pullout limit state of both permanent and temporary soil nails are calibrated against a wide design space using the current Federal Highway Administration (FHWA) nail load and resistance models. The calculated resistance factors were shown to scatter broadly among design scenarios that differ in wall face batter, soil friction angle, nail ultimate bond strength, and surcharge live load. An important lesson learned from the analysis results is that the current practice of using a single resistance factor for LRFD of nail pullout limit state could not result in uniform reliabilities across different design scenarios. Simple artificial neural network (ANN) models were developed for computation of resistance factors. Design examples demonstrated the ability of the ANN models in providing resistance factors that yield satisfactory and consistent reliabilities in different nail pullout designs.

Element Nail Pullout Tests for Prediction of Soil Nail Pullout Resistance in Expansive Clays

Abstract The performance of soil nail systems relies primarily on soil-nail interaction, which is especially complex for nails embedded in clay materials. Soil-nail interaction is difficult to predict in these soils because of the effect of moisture-induced stress and swelling along the interface between soil and nail. As part of this investigation, tests were conducted to characterize the soil-nail interface shearing mechanisms using a novel element nail pullout device that involves comparatively short nails embedded in soils under varying degrees of saturation. Additionally, a large-scale pullout test, involving inundation and loading phases, was conducted using soil nails embedded in expansive clays. The soil-nail interface shearing mechanisms identified with the element nail pullout device and the vertical rise measurements of the clay from the inundation test were used within a load transfer theoretical framework aimed at predicting the response of soil nails in expansive clays. This load transfer framework provided a very good prediction of the large-scale pullout test results using the data collected from the element nail pullout tests.

Laboratory pullout resistance of a new screw soil nail in residual soil

The ultimate shear strength at the interface between the soil nail and surrounding soil is of practical importance in the design and performance of a soil nail system. The most commonly adopted method of measuring this interface shear strength is by soil nail pullout testing. This study introduces a novel soil nail system in the form of a screw nail and compares its performance with a conventional grouted soil nail. Both types of soil nails are tested in a controlled laboratory setting using residual soil in a large purpose-made pullout box. The development of the screw nail and the laboratory testing procedures are briefly discussed first, followed by presentation and discussion of the results of the interface shear behaviour measured from pullout tests. It is shown that the screw nail offers many advantages in terms of pullout load–displacement behaviour and the interface shear mechanism than that of the conventional grouted soil nail.

Influence of Critical Parameters on the Peak Pullout Resistance of Soil Nails Under Different Testing Conditions

This paper presents a comprehensive analysis of the correlation between influencing parameters and peak pullout resistance (PPR) of soil nails installed in typical completely decomposed granite (CDG) or sand. A total of eight soil nail pullout tests were conducted in a field to examine the influence of overburden pressure (OP) and grouting pressure (GP) on the PPR of soil nails. Comparative analysis indicates PPR values of soil nails pulled out in soils with low degrees of saturation (30%) are almost twice of that in almost fully saturated soils. Apparent coefficient of friction (ACF) increases linearly as the increase of GP, but decreases linearly as the increase of OP when GP is constant in both laboratory and field tests. Extensive studies also show that the magnitude of normalized ACF is more sensitive to OP compared with GP and degree of saturation (Sr). Diameter increase in percentage of soil nails pulled out from field are almost twice of the soil nail diameters in laboratory tests, indicating that due to complex geological condition in field (such as possible high void ratio of field soils), failure surface of soil nails presents substantial shift into surrounding soils.

Evaluations of load-deformation behavior of soil nail using hyperbolic pullout model

Soil nailing, as an effective stabilizing method for slopes and excavations, has been widely used worldwide. However, the interaction mechanism of a soil nail and the surrounding soil and its influential factors are not well understood. A pullout model using a hyperbolic shear stress-shear strain relationship is proposed to describe the load-deformation behavior of a cement grouted soil nail. Numerical analysis has been conducted to solve the governing equation and the distribution of tensile force along the nail length is investigated through a parametric study. The simulation results are highly consistent with laboratory soil nail pullout test results in the literature, indicating that the proposed model is efficient and accurate. Furthermore, the effects of key parameters, including normal stress, degree of saturation of soil, and surface roughness of soil nail, on the model parameters are studied in detail.

Estimation of Maximum Pullout Shear Stress of Grouted Soil Nails Using Bayesian Probabilistic Approach

AbstractThe pullout resistance of soil nail is a key parameter in soil nailing design. Pullout resistance is affected by many factors, such as overburden pressure, grouting pressure, soil dilation, and degree of saturation of soil. Because of the complexity of the pullout mechanism, some factors have not been well incorporated in the current soil nail design methods. In this study, Bayesian analysis is performed to investigate the relative importance of several key factors and to build a new design formula to estimate maximum pullout shear stress of grouted soil nails. By using a series of laboratory soil nail pullout test data, Bayesian analysis is performed to select a predictive formula with suitable complexity and to identify its parameters. It is found that the most important factors are the degree of saturation and the product of grouting pressure and overburden pressure. It is shown that the proposed optimal model exhibits significantly stronger correlation with measurements than the existing effec...

Influence of Grouting Pressure on the Behavior of an Unsaturated Soil-Cement Interface

The strength of soil-nails depends on the behavior of the soil-cement grout interface at saturated and unsaturated conditions. Nowadays, pressure grouted soil-nails are considered to provide better interface strength than gravity grouted soil-nails. Soil-nail pullout tests have limitations to control some boundary conditions. To overcome these limitations, direct shear tests can be used to determine the actual soil-cement grout interface behavior. In the present study, a series of interface direct shear tests are performed between compacted completely decomposed granite (CDG) soil and cement grout at both saturated and unsaturated conditions under different grouting pressures. The interface shear stress increases with matric suction for different grouting pressures, and a strain softening behavior is observed for different suctions except at saturated conditions. A dilative behavior is obvious for interface as the suction value is increased from a saturated condition. However, the dilation values of soil-cement interface for different grouting pressures are less than that of CDG soil under different suctions. The interface shear strength increases with grouting pressure at saturated conditions, whereas, a downward trend is obvious as the suction value is increased from saturated conditions. The rate of increase of shear strength with matric suction is greater for CDG soil compared to soil-cement interface. Interface shear strengths for different grouting pressures are greater than CDG soil at a lower suction range but become less than CDG soil at a higher suction range.

Finite element modelling of pullout testing on a soil nail in a pullout box under different overburden and grouting pressures

In this paper, a three-dimensional (3D) finite element (FE) model is developed to simulate the pullout behaviour of a soil nail in a soil-nail pullout box under different overburden and grouting pressures. The FE model simulates all the procedures of a pullout test on a grouted soil nail in a compacted and saturated completely decomposed granite (CDG) soil. The stress–strain behaviour of the CDG soil is described by a modified Drucker–Prager/Cap model, while that of the soil–nail interface is represented by the Coulomb friction model. Triaxial experiment data are used to calibrate the soil parameters in the soil constitutive model. The interface parameters are determined from back-analysis with the laboratory soil-nail pullout data. The soil stress variations surrounding the soil nail during drilling, grouting, saturation, and pullout are all well simulated by the FE modelling and compared with available test data. The comparisons between the modelling and experimental data have shown that the established FE can well simulate the pullout behaviour of a soil nail in a soil mass. Based on this, the verified FE model has the potential to simulate the performance of a soil nail in a field soil slope.

Influences of overburden pressure and soil dilation on soil nail pull-out resistance

Abstract Soil nailing is the most popular technique for stabilizing newly formed and existing sub-standard slopes in Hong Kong because of its economic and technical advantages. The nail–soil interface shear resistance is an important parameter in design of soil nailed structures. A three-dimensional finite element model was established and used for simulating soil nail pull-out tests. The finite element model was verified by comparing simulated results with measured data. The agreement between the experimental and simulated results in terms of both average pull-out shear stress and stress variation was very good. Using this finite element model, a parametric study was carried out to study the influences of the overburden pressure and soil dilation angle on the soil nail pull-out resistance. The simulated peak pull-out resistance was not directly related to the overburden pressure, which was coincident with the observations in laboratory pull-out tests. The simulated pull-out resistance increased significantly with the increase in dilation angle of the shearing zone. This analysis indicated that the constrained dilatancy of the nail–soil interface and the soil surrounding the nail contributed a lot to the development of peak pull-out resistance.

Influence of Grouting Pressure and Overburden Stress on the Interface Resistance of a Soil Nail

Grouted soil nails are widely used in slope stabilization. The influence of both grouting pressure and overburden stress on the soil-nail pullout interface shear resistance is still not well understood due to the complex of soil-grout interactions. A series of laboratory soil-nail pullout tests have been carried out on a completely decomposed granite soil in nearly saturated condition under a combination of different grouting pressures and overburden stresses. The pullout tests simulate the real construction process of a soil nail, including establishment of initial soil stresses in a soil slope, drilling a hole with stress release, grouting, and soil-nail pullout when the slope is sliding. The pullout box is well instrumented. Test data are collected automatically by a data logger. Typical test results are presented, explained, and discussed in this paper. The soil-nail interface shear resistance data from all tests are analyzed and interpreted. The study shows that the grouting pressure and overburden stress have interactional influence on the soil-nail pullout resistance. Based on the test results, a new empirical liner equation with two grouting pressure dependent parameters is proposed for calculation of soil-nail pullout resistance considering both grouting pressure and overburden stress. New understandings and findings from the study are presented.

The influence of grouting pressure on the pullout resistance of soil nails in compacted completely decomposed granite fill

The interface shear strength between a soil nail and the surrounding soil is required in the design analysis and safety assessment of a soil nailed structure, such as a slope, retaining wall or excavation. It has been found that a number of factors may influence the interface shear strength, such as the normal stress, soil properties, soil nail surface roughness, degree of saturation and grouting pressure. Some initial results from field pullout tests on soil nails grouted under pressure have indicated that the grouting pressure contributes to the pullout resistance. Pressure grouting is a cost-effective method for increasing the soil nail pullout resistance, thus improving the performance of the nailed structure. Studies on the soil nail failure mechanisms and quantitative data from tests investigating how grouting pressure influences the pullout resistance are, however, very limited. First, this paper introduces a laboratory soil nail pullout box with full instrumentation and a special pressure grouting device and its set-up. Second, the paper describes the test procedures and a programme for studying the influence of pressure grouting. The paper presents representative test results. The influence of the grouting pressure together with overburden soil pressure on the pullout resistance is examined. From the test results, relationships between pullout resistance and grouting pressure are obtained and discussed.

Influence of Overburden Pressure on Soil–Nail Pullout Resistance in a Compacted Fill

Soil nailing has been widely used in many places in the world in the last two decades because of its technical and economical advantages. The nail–soil interface shear strength is an important parameter in soil nail design. This parameter is governed by a number of factors, among which the influence of the overburden pressure (or soil depth) is the most controversial. There are differing views concerning the effect of overburden on the nail–soil interface shear strength. In order to examine the influence of the overburden pressure, a series of laboratory pullout tests on soil nails installed in compacted completely decomposed granite fill have been conducted using two pullout boxes. Numerical simulations have also been carried out and the results are compared with the pullout test data. The procedures of the pullout tests and new features of the pullout boxes used are briefly described. Changes of the vertical stress in soil close to the nail throughout the course of soil nail installation and pullout are...

Numerical Simulation of Pressure Grouting in Soil Nail Pull-Out Tests

Soil nailing is a widely used technique for stabilizing slopes and excavations. In all current design methods, the nail-soil interface shear strength, that is, the pull-out resistance of a soil nail is an important parameter which controls the design and safety assessment of the soil nailing system. The pressure grouting is a cost effective method for increasing the soil nail pull-out resistance and in turn improving the performance of the nailed structure. In this paper, a three dimensional (3-D) finite element (FE) model for pull-out tests is established and verified by comparing simulated results with measured data. This model is then used to simulate the effect of grouting pressure on the soil nail pull-out resistance.

Influence of Soil Dilatancy on Soil Nail Pull-Out Resistance in Completely Decomposed Granite Fill

Soil nailing, developed from the New Austrian Tunnelling Method, has been widely used in many countries and regions in the world since the 1970’s. This technique is used to stabilize in situ soil mass by installing a large number of closely spaced unstressed inclusions into the soil mass to increase its strength and stability. Since the mid 1970’s, several design methods have been proposed based on laboratory and field tests. Among the design criteria in these methods, the pull-out resistance of a soil nail is a key parameter that controls the stability assessment of soil nail structure. In the previous investigations, the soil dilatancy was found to be an important factor that influences the soil nail pull-out resistance especially for drill-and-grout soil nails. In order to study the influence of soil dilatancy on soil nail pull-out resistance, laboratory pull-out tests and numerical parametric studies have been carried out for soil nails in Completely Decomposed Granite (CDG) fill. The results show that the soil dilatancy has a significant influence on the soil nail pull-out resistance.

An Innovative Laboratory Box for Testing Nail Pull-Out Resistance in Soil

Abstract This paper introduces a new soil nail pull-out box with full instrumentation for investigating the soil nail pull-out shear resistance under different controlled conditions. The new box has overcome some limitations of previous pull-out boxes developed by other researchers, the first author, and his co-workers. The box has the following new features: (a) comprehensive transducers are installed, (b) an extension chamber is provided to house an extension part of the soil nail to keep the nail length inside the box constant throughout the testing, (c) a waterproof front cap is placed to cover and seal the soil nail head so that back water pressure can be applied to accelerate the saturation of the soil, and (d) a pressure grouting apparatus is constructed to make it possible to investigate the influence of the cement grouting pressure. The pull-out box can be used to study influences of (a) hole drilling process and stress release, (b) degree of saturation of the soil, (c) cement pressure grouting, (d) overburden pressure, etc. Two copies of such a new soil nail pull-out box have been constructed to accelerate the testing program. A series of soil nail pull-out tests has been conducted using the two boxes. This paper presents details of the new pull-out box, from design, boundary effect analysis, and instrumentation to setup. Typical results from soil nail pull-out tests are presented and discussed.