Pullout Resistance Of Soil Nails Research Articles

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.

An Innovative Post Grouting Technique for Soil Nails

This study presents an innovative technique of executing soil nails called sectorized post grouting (SPG). The most utilized technique of soil nail grouting is gravity grouting, with the literature reporting advances in pressurized grouting. Although obtaining higher pullout resistance of soil nails, pressurized grouting, mostly done in single-stage grouting, does not compensate for exudation and its use in higher nail lengths is difficult. Thus, a technique has been developed that compensates for exudation, with easier application in lengthier nails. The technique was qualitatively assessed to evaluate its surface roughness and later applied in seven real soil nailing works, where it could be quantitatively assessed. The results show that sectorized post-grouted nails obtained greater pullout resistances than gravity grouted and single-stage grouted nails. Although similar improvement was found in tube-à-manchette (TAM) grouted nails, this method presents lower economic efficiency than sectorized post grouting. The pullout resistance results obtained in this study can be utilized in future soil nailing works executed utilizing SPG.

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.

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.

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.

Experimental Study on the Influence of Drillhole Roughness on the Pullout Resistance of Model Soil Nails

Pullout resistance of cement-grouted soil nails is a key factor affecting the safety conditions of retaining walls, slopes, and excavations. The mobilized frictional resistance at the interface between soil nails and surrounding soils depends on various parameters such as saturation ratio, water content, grouting under gravity or pressure, etc. The interface roughness condition between soil nail and soil is a critical factor but difficult to control in both laboratory and field owing to technical difficulties in creating a rough drillhole surface. The present study focuses on the pullout behavior of model soil nails with different interface roughness conditions in the laboratory to establish quantified correlations between frictional resistance and roughness angles of internal drillhole surface. Cross-sectional shapes of internal drillhole surfaces were created using four plastic rods with various shapes of external threads (four different values of roughness angles). Measured peak values of pullout resistance of soil nails with rough drillhole surface were compared with the soil nails with smooth drillhole surface. The present test results were verified with a previous analytical model, which considered the soil dilation to be an important factor influencing the pullout resistance of soil nails. Test results also indicate that the peak pullout resistance increases almost linearly with an increase in roughness angles. In addition, the pullout resistance values decrease approximately linearly with an increase in pullout displacement after peak pullout resistance is approached. The soil-nail diameter values of the soil nails with rough drillhole surfaces expanded substantially after being pulled out of the ground. This leads to a substantial increase in pullout resistance of soil nails.

Comparative study on pullout behaviour of pressure grouted soil nails from field and laboratory tests

Pullout resistance of a soil nail is a critical parameter in design and analysis for geotechnical engineers. Due to the complexity of field conditions, the pullout behaviour of cement grouted soil nail in field is not well investigated. In this work, a number of field pullout tests of pressure grouted soil nails were conducted to estimate the pullout resistance of soil nails. The effective bond lengths of field soil nails were accurately controlled by a new grouting packer system. Typical field test results and the related comparison with typical laboratory test results reveal that the apparent coefficient of friction (ACF) decreases with the increase of overburden soil pressure when grouting pressure is constant, but increases almost linearly with the increase of grouting pressure when overburden pressure (soil depth) is unchanged. Water contents of soil samples at soil nail surfaces show obvious reductions compared with the results of soil samples from drillholes. After soil nails were completely pulled out of the ground, surface conditions of the soil nails and surrounding soil were examined. It is found that the water content values of the soil at the soil/nail interfaces decrease substantially compared with those of soil samples extracted from drillholes. In addition, all soil nails expand significantly in the diametrical direction after being pulled out of ground, indicating that the pressurized cement grout compacts the soil and penetrates into soil voids, leading to a corresponding shift of failure surface into surrounding soil mass significantly.

Experimental study on the pullout resistance of pressure-grouted soil nails in the field

The pullout behaviour of cement-grouted soil nails, particularly in field conditions, is not yet fully understood. In this study, a series of tests was conducted to evaluate the pullout response of grouted soil nails in a field slope. A new innovative grouting packer system was developed to control the grouted length and maintain the cement grout pressure of the grouted part. By using the grouting packer system, a total of 10 soil nails placed at different soil depths were grouted with different pressures in the field. The pullout results of present field tests and a number of past laboratory tests indicate that the apparent coefficient of friction (ACF) decreases with the increase of overburden stress even though grouting pressure is applied. In addition, when the overburden stress is unchanged, the obtained ACF values in the field tests appear to increase almost linearly with the increase of grouting pressure. After the soil nails were completely pulled out of the ground, the surfaces of the soil nails and surrounding soil were examined. It is found that the water content of the soil samples at the soil–nail interfaces decrease substantially compared to the water content of soil samples in drill holes. Measurement results also show that the failure surfaces of soil nails shifted about 16 mm on average into the surrounding soil due to the application of grouting pressure.

Laboratory Experimental Study on Pullout Behavior of Mortar Grouted GFRP Soil Nails

A large-scale laboratory apparatus has been built to study the pullout behavior of mortar grouted glass fiber reinforced polymer (GFRP) soil nails. The axial strain along the nail length and the displacement of the nail head under different pullout loads are measured, and the ultimate pullout load under the overburden pressure is obtained. Then, the influence of the overburden pressure on the ultimate value of the interface friction force is investigated. The experimental results illustrate that the pullout behavior of mortar grouted GFRP soil nails is similar to that of mortar grouted steel soil nails, and the relation between the displacement and the pullout load can be described with the hyperbolic function. In addition, the dilatancy effect of the soil near the nail during pull out should be taken into account in estimating the pullout resistance of soil nails in dense fills.

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.

Uncertainties of Field Pullout Resistance of Soil Nails

A large number of field pullout tests on soil nails have been carried out to provide valuable information for enhancing the understanding of pullout resistance of soil-grout interface and for reliability evaluation of soil-nailed slopes. In this paper, a data set of 167 field pullout tests performed in 23 nailed completely decomposed granite cut slopes is used for a statistical evaluation of four factors influencing the pullout resistance of soil nails, namely overburden pressure, grout length, soil suction, and soil dilatancy. For the tests in which nails were pulled out, the measured pullout resistance is essentially independent of the effective overburden pressure. A bias factor r* is defined as the ratio of the measured pullout resistance and the calculated value using a design equation. The mean value of r* is 4.30 and the coefficient of variation is 47%. When the uncertainties in grout length, soil suctions around nails, and the soil shear dilatancy are considered, the mean value of r* can finally be reduced to 0.99. The quantification of the uncertainties provides a better physical understanding of the working mechanisms of soil nails.

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...

Influence of degree of saturation on soil nail pull-out resistance in compacted completely decomposed granite fill

The nail–soil interface shear strength is a key parameter in the design and stability assessment of soil nailing systems. A number of factors will influence the nail–soil interface shear strength. Among these factors, the degree of saturation (Sr) of the soil is an important one especially for permanent soil nail structures. To study the influence of Sron soil nail pull-out shear resistance, a series of laboratory pull-out tests have been conducted on soil nails in compacted completely decomposed granite (CDG) fill prepared to different Sr. The tests were conducted using two specially designed pull-out boxes (with same specifications). In the near-saturated tests, a high Sr(about 98%) was achieved using two special features of the apparatus: a waterproof front cap and back-water pressure pipes at the bottom of the pull-out box. Test results showed that the nail–soil shearing plane migrated outwards into the soil when the Srof the soil increased. Also, peak pull-out strengths of soil nails were strongly influenced by the Srof the soil. Among the tested Sr, the highest values of peak pull-out shear strength were obtained at Srvalues between 50% and 75%.

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.

Elastoplastic analysis of pull-out resistance of soil nails in dilatant soils

Soil dilation has significant effects on soil nail pull-out resistance. The dilation mechanism around a horizontal soil nail is analysed in this paper by treating the soil as a linear elastic–perfectly plastic material, using a non-associated Mohr–Coulomb yield criterion. Using this approach, closed-form solutions for calculation of the average compression stresses on the soil nail surface are obtained. This analysis theoretically explains a phenomenon commonly observed in experimental and field tests of dilation effects on soil nail pull-out resistance. It also explains why the apparent friction coefficient inclusive of the dilation effect is higher than the soil internal friction coefficient, but the apparent friction coefficient decreases with the increase of the soil overburden pressure and soil nail diameter. The conditions where the analytical approach can be applied are discussed. The theoretical predictions are compared with test results with good agreement, giving support to its validity. La dilatation des sols a des effets significatifs sur la résistance des clous de sol à l'extraction. Nous analysons dans cet exposé le mécanisme de dilatation autour d'un clou de sol horizontal en traitant le sol comme matière linéaire élastique parfaitement plastique et en utilisant un critère d'élasticité Mohr-Coulomb non associé. En utilisant cette méthode, nous obtenons des solutions de forme fermée pour le calcul des contraintes de compression moyennes sur la surface du clou de sol. Cette analyse explique théoriquement un phénomène communément observé dans les essais expérimentaux et les essais sur le terrain, celui des effets de la dilatation sur la résistance à l'extraction d'un clou de sol. Elle explique aussi pourquoi le coefficient de friction apparent inclusif de l'effet de dilatation est plus élevé que le coefficient de friction interne du sol mais le coefficient de friction apparent diminue à mesure qu'augmentent la pression de surcharge du sol et le diamètre du clou. Nous discutons des conditions dans lesquelles la méthode analytique peut être appliquée. Nous obtenons une bonne concordance en comparant les prévisions théoriques avec les résultats des essais, ce qui confirme la validité de cette méthode.