Effects of Soil Nail Head Arrangement on Loose Soil - Report Example

Soil Nailing and Current Practices Student Name Professor's Name, Student's Main Institutional Affiliation Student's Secondary Institutional Affiliation Abstract: Slope strengthening model has been applied in determining the effectiveness of the current soil nailing practices. Of particular interest is the soil nail distribution, soil nail design and saturation levels. These parameters aid in the establishment of the effects of surcharge loading as a means of getting the distributed load. Numerical predictions are also carried out to gain parameters for authoritative declaration of the suggested field test results. The pullout experiment is carried out on a 14° angle slope as field test at while varying the surcharge load distribution and saturation levels of the soil. Keywords: Soil Nailing, Grouting, pullout, limit equilibrium, embedded bond slip interface 1. Introduction Soil nailing is an established method of natural slope stabilization that has become widely used in the building and mining industry. The increased usage of this technique according to Wei and Cheng (2010) is based on the structural advantages that emanate from the passive inclusions that are meant to produce a coherent structure and hence restriction of soil movement which is the primary objective. Several nailing approaches have been put into action since the inception of this technique. These have often been differentiated from the old techniques which have since ceased usage with the major one being ground anchoring. Continued research and extensive study on soil nailing techniques practically show that there is a lot to be uncovered especially with the emergence of such technologies such as the limit equilibrium and strength reduction methods. The rationale of this research study is actually biased towards the establishment of the technological advantages that are attached to the current soil nailing practices. To begin with, the last two decades have seen an eruption of technologies that have been considered as advanced by such researchers as Juran and Elias (1991) who lay a platform for this discussion. This has also elicited debates in various fields due to the need to standardize this method in a bid to make this practice safe and sound for field applications. In particular the Australian Standard (2002) has been on the forefront of this important research as civil engineering projects being undertaken need proper vetting so as to avoid accidents both natural and manmade. As a standard requirement, soil nailing practice needs to advance both technologically and practically. The importance of these two factors has been realized prompting analytical investigations to be carried out in order to ascertain the important developments that have been achieved so far. Zhou et al. (2009), indicate some of the advancements that have been achieved in the geotechnical field towards the achievement of this important goal as the increased use of contemporary improved materials. These materials possess unmatched resistance against shear failure even under deployment conditions such as extreme loading. Such instances that have been constantly studied as justifications to the existing methodologies dwell on failure characteristics including slipping, static liquefaction and washout. This research paper thus seeks to examine the soil nailing technique in order to create an appreciation towards the importance of the latest technologies. The second objective of study was to investigate the contributing design factors that can be crucial in the development of this important practice. This is made possible through the study of contributory variables established in a laboratory simulation of the soil nail pull out resistance in comparison to numerical resistance through various soil nail angle design shift. The shift in angle is attributed to relative strength achievable from depending on the slope's prevailing gradient. Finite element simulation sets in as a perfect analysis method for the soil nailing techniques since it's cheap and can be carried out from the convenience of a simple laboratory. 2. Background of Study The maximum tension in sloping soil is due to lateral stress relief that in turn exposes the nails to tensional force. Inclining the nails horizontally usually results to bending and shear forces that add towards the stability limit state of the slope. According to Pedley et al. (1990) in Morris (1999), these investigations have been clarified in laboratory tests aimed at ascertaining stress conditions between two zones keeping into consideration that the nail is pulled towards the slope face. The pull out capacity of the slope zone has been often used to describe the strength of the soil nailing methodology keeping in mind that the bond capacity has to be realized in these exercises. According to Wei and Cheng (2010), the bending effect is a major contributory factor towards the establishment of a reliable soil nailing technique as much as most designers have neglected it in the past. The cement grout is said to determine the reliability of the bending action in providing for shear resistance analysis. Such models have been found to be important when it comes to the establishment of shear behaviour models. The use of piles in the study of soil nails with regard to their geometry is not a new phenomenon owing to the similarity in soil bond. This has often been observed in such areas with clayey soils where bond resistance is high prompting several soil nail designs to advance in complexity. Skempton (1959) in Morris (1999) has demonstrated the average in-situ theory for use in shear strength of a clay soil sample through empirical corrections as: (1) Where: is shaft friction, is shear strength of the slope, And is coefficient of friction. This acts as a guide towards stress analysis for soil samples whose effective behaviour is within the clayey soil limit. But since the frictional angle is known to greatly affect the soil behaviour, then equation 2 below describes the relationship given a pile angle ø. (2) Where: is shaft friction, is the effective cohesion, is effective stress on the horizontal plane, And is effective friction angle for the disturbed soil. This equation is based on the assumptions that the changes in properties are based on the load against soil. These properties have also been demonstrated by embedded bond-slip models that aim at steel-grout failure with a possibility of major strengthening thereafter. Zhou et al. (2009), further indicate that nail modelling can be achieved as a function of axial resistance and plain strain. The assumption made in this case is that the nail displacements are compatible with soil displacements as a result of axial load. In a pullout experiment carried out by Su et al. (2010), there is a clear indicator that a three dimensional analysis is necessary for complete property simulation. This may however not be the case with this research which shall dwell greatly on two dimensional analyses. Worth noting from Su et al. (2010) is the constrained dilatancy of the soil nail arrangement that was set for improvement of the existing methodology. The developments in this research prove that the overburden pressure is a coincidence to the surrounding soil pressure changes. This should not be observed when coming up with soil nail designs as seen in cases prior to this research. These researches eliminate the obsolete basis of design thereby emerging with simple but effective approaches of soil nail design. Zhou and Yin (2008) also developed a mathematical model towards the improvement of soil nails based on the normal stress brought about by soil dilation and normal stress distribution. This is achieved through the three dimensional analysis of forces around the nail using the following relationship: (3) Where: is normal stress, is post-installation normal stress, is normal stress due to dilation, And is normal stress due to bending of the soil nail. Instituting mathematical models for the sake of these analyses is a major move intended at improvement of the soil nail arrangement. Simulation of measured data is found to be important in another model by Zhou (2009) who capitalizes on the numerical model for back-analysis of field tests conducted to ascertain the elasto-plastic behaviour of water pore diffusion. Numerical analyses using various modelling approaches have been found to offer predictions towards the interface characteristics for maximum nail force. Staging a comparison between the predictions and field data gives and embedded nail technique that is usually denoted by the bond-slip interface model. 3. Tests and Discussion Construction of a temporary slope for the purpose of a pullout experiment was carried out by filling a slope with clayey soil mixture. This was set at an average gradient of 14° and a height of 3m. The width and length of this experimental setup was 3m and 12m respectively with strong gravity control walls on the sides of the slope. a) Loose fill slope designed for the purpose of numerical analysis data collection. Using a proctor standard test, the dry density was measured and the moisture content for the soil sample established to be 16.1%. The compaction level of the sample was done to reach a maximum of 80% within this slope confinement. Further reinforcement was deemed unnecessary owing to the makeshift nature of the experimental setup. Drilling a 75mm diameter hole was carried out for the purpose of inserting a 20mm round bar with ribs. Cement grout was filled ensuring an adoption of two independent beams i.e. the independent head and grillage. The experiment was carried out for surcharge and surcharge with and without wetting in that order in accordance to trends set by Zhou et al. (2009). The basic assumptions of this methodology were that the surcharge had to be unsaturated for the initial experiment to be a success. This was meant to establish the shear strength and the volumetric behaviour of the experimental setup as described by Zhou et al. (2009). For the purpose of this field research the loose fill was considered to be a porous fill which adopted the multiphase material behaviour. Installing two inclinometers to check for horizontal deviations was also done with three monitoring sensors attached to different areas of the surcharge stages. The monitoring sensors were used to check the deviations in spread of load with respect to head of soil. The three stages of slope models were approached amicably to establish the deformation patterns that exist in the soil nailing technique of soil or slope stabilization. Considering that the two inclinometers were installed at two different positions, the two dimensional simplifications of the observed deformation was found to be necessary since the level of experimentation was still at the preliminary stage. According to shear failure criterion by Mohr-Coulomb, induced plastic deformation could only be carried out when the surcharge pressure had reached a significant level of appreciation. The deformations observed from the two inclinometers indicated a bulge-shaped mechanism for depth of approximately 1.7m and beyond. This was attributed to the increase in surcharge pressure below the slope crest thereby indicating a major shift in stabilization forces (Zhou et al., 2009). Apart from deformation patterns, improvements should also be carried out on nail force distribution as observed from this study. This was approached through the modelling approach above whose analysis further indicated the patterns as those observed by Su et al. (2010) for peak pullout. Basing on their argument, the three models in study by this research utilized three strain gauges installed in various positions of the slope as a matter of ascertainment of force distribution. These sensory nodes were able to reflect the desirable tuning levels for adoption of soil nail capacities for slope strengthening. Compressive forces achieved from this system's deepest strain gauge indicated that the only improvement for soil nail arrangement is to adopt embedded bond-slip arrangement. This is one of the most used techniques which were disregarded by earlier techniques. Surcharge loading on the upper slope shows a larger nail force which implies a greater movement within the soil nail arrangement (Su et al., 2010). Another point that was noted during this experiment is the projected moisture content of the slope. The study towards achievement of a numerical model for this factor is a welcome move that is supposed to improve the slope strengthening approach. Engaging the soil moisture coefficient in this study eliminates doubt that emanates from unseasoned designs that may be as a result of unsaturated soils. This can well be depicted through increasing soil moisture which in turn redistributes the surcharge load applied. Therefore there is a direct proportional relationship between the surcharge load distribution, moisture content and soil nail arrangement. This model however works in an equilibrium state with a steep decline at the point of no return. The establishment of this among other factors is considered as improvements towards achievement of better soil stabilization models and techniques. The figure below indicates a typical graph that was obtained by Zhou et Al. (2009) in the same type of experiment. a) Effects of moisture saturation on slope stability (Zhou et Al., 2009) 4. Conclusion The current soil nailing practices studied by this research focus on soil nail arrangement, effects of soil saturation levels and the soil nail design. The field test results are applied in computation of modelling approaches necessary for the improvement of this practice. The mechanical behaviour has been established for numerical predictions of nail slope traits for embedded bond slip interface. Soil nails are found to increase the stability of a slope owing to the redistribution of the surcharge load. The model does not however establish the effects of suction on soil strength thus this factor should be studied in future researches. 5. References Australian Standard, (2002), ‘AS 4678: Earth Retaining Structures’, Standards Australia, Sydney, Australia. Juran, I. & Elias, V., (1991), ‘Ground Anchors and Soil Nails in Retaining Structures, Foundation Engineering Handbook, Van Nostrand Reinhold, pp. 868-905. Morris, J., (1999), ‘Physical and Numerical Modelling of Grouted Nails in Clay’, University of Oxford, UK, Su, L., Yin, J., & Zhou, W., (2010), Influences of overburden pressure and soil dilation on soil nail pull-out resistance, Computers and Geotechnics, vol. 37, pp. 555-564, viewed 15 March 2013, Elsevier Science Direct. Wei, W. B. & Cheng, Y. M., (2010), ‘Soil nailed slope by strength reduction and limit equilibrium methods’, Computers and Geotechnics, vol. 37, pp. 602-618, viewed 15 March 2013, Elsevier Science Direct. Zhou, W. & Yin, J. (2008), ‘A simple mathematical model for soil nail and soil interaction analysis’, Computers and Geotechnics, vol. 35, pp. 479-488, viewed 15 March 2013, Elsevier Science Direct. Zhou, Y. D., Cheuk, C. Y. & Tham, L. G., (2009), ‘An embedded bond-slip model for finite element modelling of soil–nail interaction’, Computers and Geotechnics, vol. 36, pp. 1090-1097, viewed 15 March 2013, Elsevier Science Direct. Zhou, Y. D., Cheuk, C. Y. & Tham, L. G., (2009), ‘Numerical modelling of soil nails in loose fill slope under surcharge loading’, Computers and Geotechnics, vol. 36, pp. 837- 850, viewed 15 March 2013, Elsevier Science Direct. Read More