Behavior of Model Circular Footings on Silty Soils with Cellular Supports
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BEHAVIOR OF MODEL CIRCULAR FOOTINGS ON SILTY SOILS WITH CELLULAR SUPPORTS R. Gupta* Department of Civil Engineering, Thapar University 147001, Patiala, India [email protected] A. Trivedi Department of Civil Engineering, Delhi College of Engineering 110042, Delhi, India [email protected] *Corresponding Author (Received: April 14, 2009 – Accepted in Revised Form: November 5, 2009) Abstract An experimental study of the effect of silt and influence of cell confinement on the bearing capacity of circular footings on silty sand was carried out. Laboratory experiments on clean sand and sand containing silt up to 25 % were performed. Cells with different heights and diameters were used to confine the silty sand. The effect of proportion of silt in sand, cell diameter, cell height and the embedded depth of footing were studied. Initially, the response of a footing without cellular support was determined and then compared with that of footing with cellular support. The results indicate that the bearing capacity of circular footing can be appreciably increased by soil confinement. It was interpreted that such confinement resists lateral displacement of soil underneath the footing. It leads to a significant improvement in the bearing capacity of the footing. The cell–soil footing behaves as one unit for small cell diameters, while this pattern was no longer observed with large diameter cells. Results of model-scale footing tests show that bearing capacity decreases with fines. It is due to fact that on the increase of silt while density increases but increase in compressibility offsets the effect of density. The cell height, depth, and diameter that give the maximum bearing capacity improvement are presented and discussed. Keywords Silty Soil, Model Tests, Footings, Shallow Foundations, Bearing Capacity, Confinement ﭼﻜﻴﺪه ﺗﺎﺛﻴﺮ ﺳﻴﻠﺖ و ﻣﺤﻔﻈﻪ ﺳﻠﻮل ﺑﺮ ﻇﺮﻓﻴﺖ ﺗﺤﻤﻞ ﭘﺎﻳﻪ ﻫﺎي ﻣﺪور ﻣﺎﺳﻪ ﺳﻴﻠﺘﻲ ﺑﺮرﺳﻲ ﺷﺪ. آزﻣﺎﻳﺶ ﻫﺎ ﺑﺮ روي ﻣﺎﺳﻪ ﺗﻤﻴﺰ و ﻣﺎﺳﻪ ﺣﺎوي ﺗﺎ 25% ﺳﻴﻠﺖ اﻧﺠﺎم ﺷﺪ. ﺳﻠﻮل ﻫﺎﻳﻲ ﺑﺎ ارﺗﻔﺎع و ﻗﻄﺮ ﻣﺘﻔﺎوت ﺑﺮاي ﻣﺤﺒﻮس ﻛﺮدن ﻣﺎﺳﻪ ﺳﻴﻠﺘﻲ اﺳﺘﻔﺎده ﺷﺪ. ﺗﺎﺛﻴﺮ ﻧﺴﺒﺖ ﺳﻴﻠﺖ در ﻣﺎﺳﻪ، ﻗﻄﺮ و ارﺗﻔﺎع ﺳﻠﻮل، و ﻋﻤﻖ ﺑﺴﺘﺮ ﭘﺎﻳﻪ ﻣﻮرد ﻣﻄﺎﻟﻌﻪ ﻗﺮار ﮔﺮﻓﺖ. در اﺑﺘﺪا ﭘﺎﺳﺦ ﻳﻚ ﭘﺎﻳﻪ ﺑﺪون ﺗﻜﻴ ﻪﮔﺎه ﺳﻠﻮﻟﻲ ﺗﻌﻴﻴﻦ ﺷﺪ و آﻧﮕﺎه ﺑﺎ ﭘﺎﺳﺦ ﭘﺎﻳﻪ داراي ﺗﻜﻴ ﻪﮔﺎه ﺳﻠﻮﻟﻲ ﻣﻘﺎﻳﺴﻪ ﺷﺪ. ﻧﺘﺎﻳﺞ ﻧﺸﺎن ﻣﻲ دﻫﺪ ﻛﻪ ﺑﺎ ﻣﺤﺒﻮس ﻛﺮدن ﺧﺎك ﻣﻲ ﺗﻮان ﻇﺮﻓﻴﺖ ﺗﺤﻤﻞ ﭘﺎﻳﻪ ﻣﺪور را ﺑﻪ ﻣﻘﺪار ﻗﺎﺑﻞ ﺗﻮﺟﻬﻲ اﻓﺰاﻳﺶ داد. ﺗﻮﺟﻴﻪ اﻳﻦ اﻣﺮ اﻳﻦ اﺳﺖ ﻛﻪ ﻣﺤﺒﻮس ﻛﺮدن ﺑﺎﻋﺚ ﻣﻘﺎوﻣﺖ در ﺑﺮاﺑﺮ ﺟﺎﺑﻪ ﺟﺎﻳﻲ ﺟﺎﻧﺒﻲ ﺧﺎك زﻳﺮ ﭘﺎﻳﻪ ﻣﻲ ﺷﻮد. اﻳﻦ اﻣﺮ ﺑﻪ ﺑﻬﺒﻮد ﻗﺎﺑﻞ ﺗﻮﺟﻪ در ﻇﺮﻓﻴﺖ ﺗﺤﻤﻞ ﭘﺎﻳﻪ ﻣﻨﺠﺮ ﻣﻲ ﺷﻮد. ﭘﺎﻳﻪ ﺳﻠﻮﻟﻲ ﺧﺎك ﺑﻪ ﻋﻨﻮان ﻳﻚ واﺣﺪ ﺑﺮاي ﻗﻄﺮﻫﺎي ﺳﻠﻮل ﻛﻮﭼﻚ رﻓﺘﺎر ﻣﻲ ﻛﻨﺪ، در ﺣﺎﻟﻲ ﻛﻪ اﻳﻦ اﻟﮕﻮ در ﺳﻠﻮل ﻫﺎي ﻗﻄﻮر ﻣﺸﺎﻫﺪه ﻧﺸﺪ. ﻧﺘﺎﻳﺞ آزﻣﻮن ﻫﺎي ﭘﺎﻳﻪ ﮔﺬاري ﻧﺸﺎن ﻣﻲ دﻫﺪ ﻛﻪ ﻇﺮﻓﻴﺖ ﺗﺤﻤﻞ ﺑﺎ اﻓﺰاﻳﺶ ﻣﻴﺰان ذرات رﻳﺰ ﻛﺎﻫﺶ ﻣﻲ ﻳﺎﺑﺪ. اﻳﻦ اﻣﺮ ﻧﺎﺷﻲ از اﻳﻦ واﻗﻌﻴﺖ اﺳﺖ ﻛﻪ در ﺣﺎﻟﻲ ﻛﻪ ﺑﺎ اﻓﺰاSIDﻳﺶ ﺳﻴﻠﺖ ofداﻧﺴﻴ ﺘﻪ اﻓﺰاﻳﺶ ﻣﻲ ﻳﺎﺑﺪ، اﻣﺎ اﻓﺰاﻳﺶ در ﻗﺎﺑﻠﻴﺖ Archiveﻓﺸﺮدﮔﻲ ﺗﺎﺛﻴﺮ داﻧﺴﻴﺘﻪ را ﺧﻨﺜﻲ ﻣﻲ ﻛﻨﺪ. ارﺗﻔﺎع، ﻋﻤﻖ و ﻗﻄﺮ ﺳﻠﻮل ﻛﻪ ﺑﻴﺸﻴﻨﻪ اﻓﺰاﻳﺶ ﻇﺮﻓﻴﺖ ﺗﺤﻤﻞ را اﻳﺠﺎد ﻣﻲ ﻛﻨﺪ، اراﺋﻪ و ﻣﻮرد ﺑﺤﺚ ﻗﺮار ﻣ ﻲﮔﻴﺮد. 1. INTRODUCTION Barinadi, Yammuna and Ganga, where the river sands are obtained with varied proportions of non- In the Indo-Gangetic planes, the Indian subcontinent plastic silts. The authors have diverse experiences has vast deposits of silty sands along the bank of with the soil exploration of these deposits for perennial Himalayan Rivers namely Indus, Ghaggar, structural foundations [1]. As per soil classification IJE Transactions B: Applications Vol. 23, No. 1, February 2010 - 21 www.SID.ir systems, the sand and silt are coarse and fine the available solutions is to use side supports to the grained granular materials. These are obtained in excavation during construction. Due to the abundance as geological deposits in the earth crust. problems associated with the removal of these They occur with varied surface textures and shapes supports, they are provided as part of the ranging from angular to spherical with moisture in permanent structure. Accordingly it consists of two void space. In modern times, some of the granular parts; it is to deal with the structural analysis of the industrial byproducts deposited as structural fill footing if the side supports are used as end with common range of specific gravity, unit weight supports for the foundation [13]. Secondly, the and grain characteristics are often classified for effect of these supports on the lateral movement of sizes as sand and silts [2]. Over the past fifty years, the soil underneath the foundation is to be there were intensive attempts to characterize sandy investigated as the effect of the lateral confinement soil without fines [3-7]. However, there were on the bearing capacity of the silty sands. While scanty efforts to map the engineering behavior of there are several solutions for the first problem, silty sands. The authors observed that silty sands such as isolating the foundation from the side are largely found in earth crest in a low to medium supports but the effect of lateral confinement by density states with varied moisture. This material these side supports on the foundation behavior is supports structural rafts and deep foundations for not well understood. multistoried buildings, underground excavations, The experimental studies on bearing capacity of tunnels and pipeline. There is a need to characterize sands need to include the followings: this granular media as an engineering material. There is a better understanding of the anticipated (I) Effect of proportion of fines on the bearing behavior of clean sand under undrained monotonic capacity of silty sands. loading in terms of its initial state of stress, void (II) Effect of confinement on the bearing ratio and state parameters. There is a mixed capacity of silty sands opinion in the literature on the role of fines on In order to investigate the effect of confinement on stress strain behavior of silty sand [8-10]. It is bearing capacity of circular footing, the cells were difficult to anticipate why the behavior of silty fabricated in the laboratory. These were made of sand is contractile. The basis for deformations, mild steel plates of thickness of 0.94 mm and whether silty sand would be contractile or dilative having different diameters. The cells were open at and what kind of stress strain behavior is to be both the ends. It was modeled as a circular footing expected compared with that of clean sand needs supported on a silty soil, which is surrounded by a further explanation. The factor that controls the mild steel cell having same soil outside. The tests behavior of confined silty sands is a matter to be were performed first without cells (un-confined investigated. The natural sands contain varying case) below the footing and then with cells amount of fines, whereas the current knowledge of (confined case) and the results of ultimate bearing its engineering behavior is primarily based on capacity are compared. clean sands [11,12]. Further, the presence of fines in sand has an influence on the bearing capacity. The problem of bearing capacity of shallow foundations on granular 2. REVIEW OF PREVIOUS WORK soils has been studiedArchive for many years [3-7]. of SID However, an accurate solution capable of predicting The investigations on the void ratio of silty sands peak load carrying capacity for a wide range of soil reveal some intrinsic dissimilarity. A series of tests relative densities, effective stress conditions and were carried out on Brenda tailings sand to know foundation shapes within a practical context remains the maximum and minimum void ratios [14]. It has elusive owing to the presence of fines. been found that the maximum and minimum void The structural measures for foundations are ratios of silty sand decreased as silt content widely used in weak soil conditions to support increases from 0 to 20 %. Similar trends were column loads. Sometimes the excavation needs to observed for Navada and Ottawa sands mixed with be braced during foundation construction. One of non-plastic fines [15]. 22 - Vol. 23, No. 1, February 2010 IJE Transactions B: Applications www.SID.ir Lade, et al [15] explained the pattern of decreasing increase in bearing capacity could be achieved the maximum and minimum void ratios of silty with the provision of geocell in the foundation sand with increasing fine content. With increasing sand. Dash, et al [19] conducted the model test percentage of fines in a dense or loose sand matrix, results on a circular footing supported on a dense most silt particles initially occupy the voids among sand layer (relative density of 70%) overlying a sand particles. This represents the reduction in void soft clay bed which showed about a 6-fold increase ratio with increasing the amount of fines. Some silt in bearing capacity with the provision of geocell in particles, however, end up between the surfaces of the overlying sand layer. The higher performance adjacent sand particles. Such particles would tend to improvement due to geocell in the sand bed cause an increase in void ratio, as they do not occupy compared with that in the soft clay bed is attributed the natural void space left by the sand matrix. This to the mobilization of higher passive force at the process pushes sand particles apart. geocell walls and frictional resistance at the For a given overall void ratio, there is a fines geogrid-soil surface. content for which they completely (or almost The aim of present study is to model and completely) separate adjacent sand particles. An investigate the effect of cell confinement in silty easy way to determine the fines content for which sands on the behavior of soil foundation system.