مقایسه ظرفیت تحمل شالوده نواری بر روی شن با ژئوسل و با فرم های صفحه ای تقویت ژئوتکستایل  Comparison of bearing capacity of a strip footing on sand with geocell and with planar forms of geotextile reinforcement

۱٫ Introduction Geosynthetic materials have been widely used in geotechnical engineering applications for, e.g., longer-lasting road construction layers, stable embankments over soft soil and expedient access over soft ground. An additional, possible, use would be to improve the bearing capacity of footings, but, at present, this application is made difficult because of the limited knowledge on the load-settlement behaviour of footings on reinforced soils. To investigate such applications, researchers have undertaken many studies to investigate how best to arrange effective reinforcement. For example, Yoon et al. (2004), Ghosh et al. (2005), Patra et al. (2005, 2006) used model tests to study the influence of different types of reinforcement on the bearing capacity and settlement of the footing. They confirmed the beneficial effect of reinforcement on the enhancement of bearing capacity and reduction in settlement of footing. Hufenus et al. (2006) carried out full-scale field tests on a geosynthetic reinforced unpaved road to investigate the reinforcing effect on the bearing capacity and its performance on a soft subgrade. The various geosynthetics used for this reinforced unpaved road were found to have a relevant reinforcing effect only when used under a thin aggregated layer on a soft subgrade. El Sawwaf (2007) investigated the behaviour of strip footings on geogrid reinforced sand over a soft clay slope. Test results indicated that the inclusion of geogrid layers in the replaced sand not only significantly improves the footing performance but also leads to a great reduction in the depth of the reinforced sand layer that is required to achieve the allowable settlement. Moghaddas Tafreshi and Khalaj, (2008) performed an experimental study to investigate the beneficial effect of geogrid on the deformation of small diameter pipes and on the settlement of the soil surface when subjected to repeated loads that simulated vehicle loading. They reported that the percent of vertical diameter change and settlement of soil surface can be reduced significantly by using geogrid reinforcement. Although planar geotextiles and geogrids have most often been studied, several investigations have also highlighted the beneficial use of geocell reinforcement in the construction of foundations and embankments. Rea and Mitchell (1978) and Mitchell et al. (1979) carried out a series of small-scale laboratory tests on footings supported over sand beds reinforced with square shaped paper grid cells and observed different modes of failure. Shimizu and Inui (1990) carried out load tests on geotextile wall frames filled with sand overlying soft soil. Cowland and Wong (1993) reported a case study of the performance of an embankment supported on a geocell mattress over soft clay. Jenner et al. (1988), making use of slip line theory, have proposed a methodology to calculate the increase in bearing capacity due to the provision of geocell mattresses at the base of the embankment resting on soft soil. Krishnaswamy et al. (2000) carried out a series of laboratory model tests of earth embankments constructed on a geocell mattress supported over a soft clay bed. Dash et al. (2001a, b) investigated the reinforcing efficacy of the geocell mattress within a homogeneous sand bed supporting a strip footing. Dash et al. (2003, 2004) also reported load test results from a model circular footing supported on geocell reinforced sand overlying soft clay. In all of the above studies, the beneficial ability of geocell constructions to improve the bearing capacity of footings is reported. Madhavi Latha and Murthy (2007) through tri-axial compression tests have observed that geocell is a superior form of reinforcement than the planar one. Sireesh et al. (2009) carried out a series of laboratory scale model tests on a circular footing supported by geocell reinforced sand beds overlying clay bed with a continuous circular void. They reported that substantial improvement in performance can be obtained with the provision of geocell mattress, of adequate size, over the clay subgrade with void and beneficial effect could be obtained when the geocell mattress spread beyond the void at least a distance equal to the diameter of the void. Wesseloo et al. (2009) have studied the stress–strain behaviour of soil reinforced with single and multiple geocells. They reported geocell reinforcement owing to its three-dimensional configuration arrests the lateral spreading of the infill soil and creates a relatively stiffened mat that redistributes the footing pressure over wider area, on the underlying poor soil, thereby giving rise to enhanced load carrying capacity. In most of these studies, researchers have reported the results of foundations supported by planar or three-dimensional geosynthetics (geocells) separately, whereas a comparison of planar and geocell reinforcement with regard to effectiveness and economy is likely to be more important in practice. At present, only a single experimental test has been reported in which a geocell (this type of geocell used was hand-made from geogrid) and a planar geogrid reinforcement arrangement were compared (Dash et al., 2003). Hence, in the current research, and in order to develop a better understanding of the geocell reinforcement concept, a series of different laboratory, pilot scale tests were performed to evaluate the bearing pressure and settlement of a strip footing supported by reinforced relatively dense sand with either geocell (formed of geotextile) or with planar geotextile reinforcement. The overall goal was to demonstrate the benefits of geocell, with the detailed objective of this study being to compare the performance of geocell reinforcement systems and planar reinforcement systems that had the same characteristics and the same mass of geotextile reinforcing material (see Table 4). The various parameters studied in this testing program include the width of reinforcement, the number of planar layers and height of geocell reinforcement below the footing base, the details of which are presented in a later section. It should be noted that only one type of geocell and planar reinforcement, one footing width, and one type of sand were used in laboratory tests. It is recognized that the results of this study may be somewhat different to full-scale foundation behaviour in the field, although the general trend may be similar.