BEHAVIOUR OF NAILED OPEN CUTS

Abstract

Large-scale industrialisation and shortage of land space has caused underground constructions e.g. parkings, shopping malls, storage tanks etc. The underground construction requires special skills and it may involve blasting and use of heavy machinery. These methods may cause extra cutting ofground, which though may not be possible at some sites. Soil nailing is a newly developed technology by which a clear underground cut may be made in-situ without involving extra space on ground and without causing damages to nearby structures. The technique is used to stabilise in-situ both natural and steep cut slopes. In urban areas, the technique is also used to construct temporary or permanent retaining structures to support the ground close to neighbouring structures that are sensitive to deformation with appropriate measures to reduce ground movement. The scope of the present work includes i) Development of a pseudo-static theoretical analysis to examine the stability of open cuts, ii) Validation of theoretical analysis by small scale and large scale model tests under static loading conditions and iii) Application of the developed methodology inan actual field problem. Many investigators (Gassier and Gudehus, 1981, 1983; Schlosser, 1982; Juran et al., 1988, 1990; Juran and Elias, 1992; Raju, 1996; Gupta, 2003) have proposed methods for investigating the stability of vertical/nearly vertical excavations. In each method, the assumed geometry of the slip surface is based on observations on either small scale model tests or full scale structures. The methods vary in the geometry of the assumed in failure surface, the definition of the factor of safety and the forces assumed to act on the active zone. The methods proposed by Juran et al., (1990), Raju (1996) and Gupta (2003) are based on moment equilibrium approach assuming the rupture surface as log-spiral, meeting the ground level at 90°. The formation of log-spiral rupture surface is supported by earlier investigators (Jewell, 1989; Plumelle and Schlosser, 1990; Juran et al., 1990) including the observations of Raju (1996) in his small scale model tests and trench tests. Chen (1975) showed that log-spiral mechanism is kinematically most admissible because soil particles on the slip surface move outward from the stable zone and not into it as the unstable soil wedge moves. In the present investigation, a pseudo-static analysis has been developed for analysing the stability of inclined nailed excavations located in seismic areas. Both horizontal and vertical seismic coefficients have been considered in the analysis. The approach is based on moment equilibrium method considering log-spiral failure mechanism. A uniformly distributed surcharge has also been considered on the top surface of the cut. A parametric study has been made considering i) Height of cut, H( 6 m, 8 m, 10 mand 12 m); ii) inclination of face of cut with vertical, a ( 0°, 5°, 10° and 15°); iii) diameter of nail, d as 25 mm; iv) length of nail/ height of cut , L/H as 0.8; v) yield strength of nail, fy as 4.15 x 105 kPa; vi) friction coefficient,/* as 0.5; vii) Nail inclination with horizontal, 8 (0°); viii) cohesion, c (0, 10 kPa, 20 kPa); ix) <{> (25°, 30° and 35°); x) seismic coefficient, ah (0, 0.05, 0.1 and 0.15); xi) seismic coefficient, av (= Oh/2) and xii) surcharge intensity on ground, q(0, 80 kPa and 120 kPa). Results have been presented in non-dimensional form and arranged in tabular form to give directly the iv number of nails for desired factor of safety. It was noted that if the nail diameter is chosen other than 25mm, say x mm, the values of factor of safety given in the tables may be multiplied by x/25. Variation in the results by this simple correlation was found less than 8%. In actual field problems, the average value off* lies between 0.5 to 0.65. Itwas found that the values of factor of safety increased by about 20% if f* was used as 0.65. To be on the safe side, tables have been prepared for lower value of f* i.e. 0.5. Another important finding of the analysis is that the horizontal nails gave the maximum factor of safety. However, if due to ease in construction, inclined nails are to be provided, their inclination may be limited to inclination of excavation face. To validate the analysis, a test program which includes i) Pull out tests; ii) Small scale model tests and iii) Large scale model tests had been framed. The purpose of pull out tests was to study the effect of nail diameter, nail length and surcharge intensity on the coefficient offriction (f) between soil and nail interface. The tests were performed in a box of size 0.4 m x 3.25 m x 1.0 m (L x B x H) using sand (SP, Dr= 70%). The nail diameter (16 mm, 25 mm and 32 mm), length ofnail (1.0 m, 1.5 m, 2.0 m, 2.5 mand 3.0 m) and surcharge intensity (0, 3.3, 6.6, 9.9, 12.7, 16.2, 20, 24, 28 and 32 kPa) were used in the tests. One of the main findings of these results was that if surcharge intensity was more than 15 kPa, the effect of nail diameter and length became marginal. For this case, the value of f* ranged between 0.5 to 0.6. However for very low surcharge intensities, the values of f* were found even more than unity. In field situations, the nails in general will have confining pressure more than 15 kPa. Two test setups had been designed and got fabricated to test the nailed excavation of 1.0 mheight and 2.5 mheight upto failure. In 1.0 mheight set up same sand as used in pull out tests (SP, Dr = 70%) was used. In one ofthe tests, stable vertical face excavation had been created using 9 nos.. 10 mm diameter tor steel bars of length 750 mm as nails. It had a factor of safety of 1.86 when no surcharge was placed on the surface of cut. The horizontal displacement of vertical face was noted by dial gauges on applying surcharge of known amount on the top surface of excavation. This surcharge intensity was increased till failure of cut occurred. Failure of cut occurred at surcharge intensity of 9.74 kN/m2. For these numbers and length of nails, appropriate values of f* for nails and surcharge intensity at failure, the factor of safety was determined using the proposed analysis and was worked out as 1.12. In another test, 6nails of length 750 mm and of 10 mm diameter tor steel bars were used for stable vertical cut (F.O.S. = 1.47). Amount of surcharge intensity at which this cut failed came 5.27 kN/m2. Analytically for these conditions, factor of safety was worked out as 1.10. In the large model test set up, stable open cuts were made of 2.0 mheight using 12 mm diameter tor steel bars oflength 1.6 mas nails placed at vertical spacing of0.4 m and horizontal spacing of 0.33 m. In other test 10 mm diameter nails of length 1.5 mand placed at vertical spacing of 0.4 mand horizontal spacing of 0.25 mwere used. Without any surcharge on the top surface of cut, the proposed analysis gave factor of safety for the two cuts as 1.97 and 1.89 respectively. The top surface of the cut was loaded with various intensities of uniform surcharge and failure of the two cuts occurred at 112.7 kN/m2 and 101.7 kN/m3 surcharge intensities respectively. The theoretical analysis for this situation gives a factor ofsafety as 1.01 and 0.95. The findings of model tests thus validated the proposed analysis. vi The methodology developed in the research program had been tried in M/s Hero Cycles Industries, Ludhiana, Punjab, India, where foundations of four steel columns carrying a load of about 120 kPa each were strengthened in-situ without disturbing the functioning ofcrane even for a moment for laying 0.3 mthick raft foundation of a HPH Annealing plant and 3.0 mbelow the foundation of said columns (thus at 6.0 mfrom the finished floor level). At this site, the soil consists of poorly graded sand (SP). The average dry density of soil was 15.5 kN/m3. The samples brought from the site were tested to determine shear strength parameters (c = 0 and <{> = 27.5°). It was proposed to use tor steel bar nails of 25 mm diameter and 2.1 mlength. To have the realistic design, in-situ pull out tests were also carried out on the proposed nails to obtain the value off*. The average value off* was worked out as 0.5. The height ofcut to be stabilized was 3.0 m. Using this data, the proposed theory suggested that 25 mm diameter tor steel bars of length 2.1 mprovided at horizontal and vertical spacing of0.3 mwill make the cut stable having a factor ofsafety equal to 2.23. This nailed cut performed satisfactorily with no measurable horizontal displacement during actual earthquake also. vn