رفتار شالوده نواری بر روی شن مسلح با منافذ تحت بارگذاری مکرر Strip footing behavior on reinforced sand with void subjected to repeated loading

۱٫ Introduction Underground voids located in the failure zone of the footing can cause serious engineering problem leading to instability of the foundation and severe damage to the superstructure. If the void is located below the footing at shallower depth, the consequence can be very costly and dangerous. They may occur as a result of settlement of poorly compacted trench backfill; natural caves, tunnels, pipes, water and gas networks and old conduits. Because of the population growth and increasing demand for extending the urban outspread to the areas that might have previously undergone mining operations, the mining cavities (voids and old conduits) are becoming a growing concern for geotechnical engineers dealing with foundation stability issues, especially above soft ground beds. Many researchers have studied the performance of footing on unreinforced soil with void under static loads [1- 4]. Badie and Wang [2] performed a theoretical and experimental analysis on a model footing above clayey soil to investigate the stability of spread footings situated above a continuous void. The results of this study implied that there is a critical region under the footing and only when the void is located within that critical region, the bearing capacity of the footing varies considerably with the void location. When the stability and load-carrying characteristics of footing are affected by void, various alternatives such as filling the void with competent material; using piles to transmit the load to an acceptable soil or rocks at the bottom of the void; and relocation of the foundation so that it is placed away from the void may be considered. Among these, the footing relocation is relatively easy and costly justified. However, it is only practical if sufficient space is available. Other alternatives may be considerably expensive or impossible and infeasible for the existing conditions. In recent decades, due to ease of construction and ability to improve load-carrying characteristics under static loads, geosynthetics reinforced soil has been widely of interest to geotechnical engineers in various applications [5-21]. Theoretical and experimental studies have been carried out on dynamic characteristics of shallow foundations supported on unreinforced soil to discover the role of load cycles on footing settlement [22-24]. For footings on reinforced soil under repeated loads, only a few relevant studies have been reported [25, 26, 27, 28, 29]. Das and Maji [30] and Das [31] conducted laboratory model tests and observed that under repeated low frequency loading, footings on geosynthetics reinforced medium dense soil experience less settlements than static loading. Moghaddas Tafreshi & Dawson [21] carried out a series of laboratory model tests on strip footings supported on 3D and planar reinforced sand beds with the same characteristics of geotextile under a combination of static and repeated loads. They indicated that substantial improvement in the footing system performance can be achieved with the provision of reinforcement and also for the same quantity of geotextile material; the 3D reinforcement system behaves much stiffer and causes less settlement than does the equivalent planar reinforcement system. In the case of footing supported by reinforced soil bed with a void under monotonic loads, several researches were carried out [32, 33, 34, 35]. Das and Khing [32] used a laboratory model test to determine the improvement of the bearing capacity supported by a stronger sand layer underlain by a weaker clay layer with a continuous rectangular void located below the centerline of the foundation. They reported that the bearing capacity is generally reduced due to the existence of a void and it substantially increases with only one layer of geogrid. Sireesh et al. [35] carried out a series of laboratory scale model tests on a circular footing supported by geocell reinforced sand beds overlying a clay bed with a continuous circular void. They reported that substantial improvement in the performance can be obtained with the provision of a geocell mattress, of adequate size, over the clay subgrade with void. Since footings subjected to cyclic loads are occasionally situated above the void, understanding the effect of the void on the footing performance and also the beneficial effect of the soil reinforcement in negating the decreasing effect of the void on the footing settlement is of great importance. Also, the above literature indicates that there is still a major lack of comprehensive studies on the behavior of footings on reinforced soil with void under repeated loading. In order to contribute to develop a better understanding of such studies, in this research, a series of laboratory and pilot-scale tests under monotonic and repeated loads were performed to evaluate the settlement of a strip footing above a void supported on reinforced relatively dense sand with planar geogrid reinforcement. The testing program was planned to investigate the response of footings constructed on reinforced sand and unreinforced sand with void and subjected to repeated loading. In particular, it is aimed to demonstrate the benefits of geogride reinforcement application over soil unreinforced conditions. The effect of the number of the reinforcement layers (N) below the footing base, the embedment depth of the void (H/D) on decreasing the negative effect of the void on footing settlement and also the ratio of repeated load intensity to applied static load, and the number of load cycles rapidity with which steady-state conditions arise are investigated.