Donatılı kil zemin üzerine oturan yüzeysel şerit temellerin taşıma kapasitesi

Abstract

Many researchers investigated the behavior of surface foundations constructed on reinforced sand (Omar et al. 1993; Khing et al. 1993; Yetimoglu et al. 1994; Das ve Omar 1994; Adams & Collin 1997). However most of the problematic foundation soils are of cohesive nature. Therefore in this study the effect of reinforcing cohesive foundation soils was investigated. Normally the cohesive soil excavated would be replaced by a granular fill. This means that improvement will be obtained due to the geosynthetic reinforcement but also because of the soil exchange. In order to see the effect of geosynthetic reinforcement alone, in this study the backfill was considered to have the same properties as the natural cohesive soil. The foundation behavior is assessed using limit equilibrium and finite element method (FEM). In this study the finite element code, Plaxis was used. A parametric study was conducted. Safety factors were calculated by using the "Phi-c reduction" method in the finite element analyses. In the "Phi-c reduction" approach the shear strength parameters tan  and c of the soil are successively reduced until failure occurs. The soil was modeled as Mohr-Coulomb model and the reinforcement as a linear elastic material. In the analyses, no specific interaction model between soil and reinforcement was used. The analyses presented in this study involve strip foundations on clay soil. The problem was analyzed under plane strain condition. The material properties were chosen in accordance with those in the literature to represent a clay soil. The soil parameters adopted were:  = 15 kN/m3; E = 25000 kPa (500 * c);  = 0.30;  = 0o;  = 0oand c = 100, 75, 50, 37.5, 25, 12.5 kN/m2. Footing thickness was chosen as 0.143 m and was placed directly on the surface without any embedment. Geosynthetic axial stiffness per unit width was selected as J=2000kN/m to represent a typical geogrid (Deb et. al. 2007). For the finite element analyses, vertical boundary was chosen to have only horizontal fixity and bottom boundary has both horizontal and vertical fixity. The size of the finite element mesh used was the same for all the analyses. It was chosen to be large enough to reduce the boundary effects to a negligible level. Factor of Safety (GS) for bearing capacities of unreinforced soil were calculated first by Limit Equilibrium analysis and then by FEM. The footing width (B=2.5, 5, 10 and 20 m), cohesion of the foundation soil (c=50 and 100 kPa), and surcharge load on footing (100 and 200 kPa) were varied to assess their influence on the safety factor of the shallow foundation. A good agreement was achieved between bearing capacity calculated using limit equilibrium approaches and finite element solutions. The finite element analysis gave factor of safety values directly proportional to cohesion of the clay soil as expected. In order to investigate the influence of the reinforcement on the factor of safety, a parametric study was conducted using different reinforcement configurations. Number of reinforcement layers, depth of the first reinforcement, vertical spacing of reinforcement layers, width of reinforcements and cohesion values of soil were chosen as variables in the analyses. Results obtained are summarized below: As the number of reinforcement layer increases (from one to six), the safety factor increases as well. Regardless of total number of reinforcement layers, the foundation width did not have a significant effect on the bearing capacity. The depth of the first reinforcement varied between 0.05B and 0.7B. The most efficient initial reinforcement depth was found as 0.4B. The vertical spacing between horizontal reinforcement layers (H) were taken as 0.025B, 0.05B and 0.1B. When six reinforcement layers were used, increase in the factor of safety values were calculated as: 7.5%, 12% and 17% for H/B values of 0.025, 0.05 and 0.1 respectively. The reinforcement lengths analyzed were L=B; L=2B; L=3B; L=4B and L=5B. The increase in bearing capacity after a length of L=3B was negligible. BCR and effective reinforcement depth were not affected when cohesion of the soil changed. Keywords: Strip footing, bearing capacity, Finite Element method, clay soil.Geosentetikler zeminin taşıma gücünü arttırmakta donatı olarak kullanılmaktadır. Bu çalışmada kohezyonlu zeminlerde geosentetik donatıların etkilerini daha iyi anlayabilmek için donatılı kil zemine oturan yüzeysel şerit temellerin taşıma kapasiteleri analiz edilmiştir. Hesaplamalarda sonlu elemanlar kodu Plaxis kullanılmıştır. Temel zemini Mohr Coulomb ve donatı lineer elastik malzeme olarak modellenmiştir. Analizlerde Phi-c azaltma metoduyla güvenlik sayıları bulunarak taşıma kapasiteleri hesaplanmıştır. İlk olarak donatısız kil zeminde temel genişliği, sürşarj yükü ve zemin kohezyonunun etkileri incelenmiştir. Nihai taşıma kapasiteleri hesaplanarak literatürdeki limit analiz sonuçlarıyla karşılaştırılmıştır. Daha sonra, donatının etkisini araştırmak amacıyla parametrik bir çalışma yapılmıştır. Donatı sayısı, etkin ilk donatı derinliği, donatılar arası düşey mesafe, donatı genişliği ve zemin kohezyonu parametre olarak seçilmiştir. Analizler sonucunda donatı sayısı arttıkça taşıma kapasitesinin 6 donatı için %17’ye kadar arttığı, etkin ilk donatı derinliğinin 0.4B (B=Temel genişliği) olduğu görülmüştür. Kohezyonlu zeminde donatının çalışmasının zemin kohezyonundan etkilendiği, donatı sayısı 6 iken taşıma kapasitesini kohezyon cinsinden 0.9c kadar arttırdığı görülmüştür. Donatılar arası düşey mesafe arttıkça taşıma kapasitesindeki değişimi gösteren eğrinin eğiminin dikleştiği ve taşıma kapasitesinin büyüdüğü görülmüştür. Donatı genişliğinin temel genişliğinin 3 katından büyük olduğu durumlarda taşıma kapasitesi sabit kalmıştır. Taşıma kapasitesi oranları (BCR) ve etkin donatı derinliği zemin kohezyonundan bağımsız çıkmıştır. Anahtar Kelimeler: Şerit temel, taşıma kapasitesi, Sonlu Elemanlar metodu, kil zemin