SEISMIC BEHAVIOUR OF SHALLOW FOUNDATION ON SLOPES

Abstract

Many recent earthquakes e.g. 1991 Uttarkashi; 1999 Chamoli; 2011 Tohoku; 2011 Sikkim; 2015 Nepal and 2016 Kumamoto earthquake taught us the importance of seismic analysis of slopes as well as that of buildings on slopes. Severe damage of buildings on hills are reported by several researchers. However, the construction of buildings on slopes is increasing day by day with the increase of population. Therefore, the focus of researchers is shifted to the development of the seismic design of foundations and buildings on hill slopes. Literature is available to examine the seismic bearing capacity of foundations and buildings on slopes under vertical centric loads but not for eccentric and inclined loads. The construction of buildings on slopes is still a challenging task for engineers. Many non-engineered structures are constructed without understanding the seismic behaviour of slopes and buildings situated on slopes. This study aims to develop a basic understanding about the failure mechanism of slopes and foundations on slopes. The static and seismic bearing capacity of foundations on slopes under different loading condition is examined using experimental and numerical methods. The influence of topographic amplification on hill buildings is also reported in this thesis. The foundation of the building located at the face of the slope is considered in the present study, which was not reported in the literature. The methodology and relevant formulation involved in this study are discussed. The Finite Element Limit Analysis (FELA) along with the Strength Reduction Method (SRM) is used to check the stability of the slope under footing loads in terms of Factor of Safety (FOS). The rigorous solution of FELA is further used to examine the static and seismic bearing capacity of embedded footing on slopes under eccentric and inclined loads. The soil is assumed to be perfectly plastic, following the Mohr-Coulomb failure criterion. While the strip footing of elastic material located on a slope is considered rigid with a rough base. The small scaled Physical Model Tests (PMT) are carried out to determine the bearing capacity of footing on slopes from load-settlement curves. The static and seismic response of footing on slopes obtained from PMT are compared with those obtained using Finite Element Analysis (FEA). In FEA, small strain based Hardening Soil (HS small) constitutive model is used for soil. The Digital Image Correlation (DIC) technique is used to measure the displacement and to visualize the failure mechanism of footing on slopes. The effects of slope topographic amplification on the response of footings and hill buildings are examined using FEA. In the next chapter, the seismic stability of slopes under foundation loads, considering the vertical centric footing load is checked. The pseudo-static method is considered to represent the earthquake load. The influence of slope angle, footing load, edge distance of footing, footing width and material properties of soil on the seismic stability of slopes subjected to foundation loads are examined. Further, the effect of location of footing and water table is also investigated. The results of this study indicate that the FOS of a slope subjected to footing load is initially almost constant, but after a certain load (reported as a threshold load), the FOS decreases rapidly with the increase of loading intensity. This threshold load is significantly influenced by the slope angle, footing width, material properties and seismic force. Based on the present FELA results, an empirical equation is proposed to estimate the threshold load. The footing near the slip surface at the crest of the slope yields the most critical location. For slopes subjected to footing loads, four modes of failure i.e. global, mixed, pre-local and local failure modes were observed. The effect of the location of water table on slopes under footing loads is discussed in detail. A case study of slope failures in Yamamoto, during 2011 Tohoku Japan Earthquake is presented. For the stable slopes, the construction of a building or a structure can be taken up. However, the design of foundations on slopes needs adequate guidelines and design charts, which are not yet available. The bearing capacity and settlement of the footing should be determined for given slope angle and loading condition. In the present study, the bearing capacity of embedded strip footing of width (B) on slopes with variation in eccentricity (e) and angle of load inclination () is examined. From the FELA results, it is found that the effective width rule is not appropriate for determination of bearing capacity of footing particularly when the footing embedment increases. In horizontal ground surface, the bearing capacity is significantly reduced after eccentricity ratio (e/B ≥ 0.1) for the vertical load. Further reduction is observed as the angle of load inclination increases. While for sloping ground, the bearing capacity of strip footing is decreased as the slope angle increases. Additional reduction of the bearing capacity of strip footing on slopes is observed with the increase of eccentricity and angle of load inclination. However, the bearing capacity always increases with the increase of angle of internal friction, cohesion of the soil and embedment of the footing. Finally, a series of bearing capacity factors are determined for footing on slopes under eccentric and inclined loads. From the Digital Image Correlation (DIC), it was inferred that the size of the failure wedge developed beneath the footing depends on footing width and relative density of soil, whereas additional effects of slope angle and edge distance are observed when the footing is located on the slopes. Further, the physical model tests are performed under one-directional excitation to examine the kinematic interaction between the footing and slopes. A very good agreement between PMT and FEA results are obtained. From both PMT and FEA results, significant Seismic-Slope Topographic Amplification Factor (S-STAF) is observed near the crest with the increase of slope angle. The seismic settlement of strip footing is increased with the increase of slope angle, footing loads, amplitude and frequency of the excitation. While the seismic settlement of strip footing on slope decreases with the increase of footing width and edge distance. The effects of slope topography, soil non-linearity and soil structure interaction in hilly areas are examined in the present study using 2D-FEA. The values of S-STAF is increased due to the consideration of non-linearity of the soil. The seismic performance of setback and stepback & setback buildings are investigated under three different ground motions. The seismic response of hill buildings in terms of inter story drift ratio is examined and it was found that the inter story drift ratio is underestimated without consideration of the soil slope. Considering seismic loading, the value of inter story drift ratio of stepback building is smaller than that of stepback & setback building. The significant effect of Topographic-Soil Structure interaction (T-SSI) is observed with the increase of slope angle. Further, the rocking and stress distribution beneath the footing on slope is also investigated and reported in this thesis. Finally, a field application of the current study is presented. The site considered is located along Tons valley, Dehradun, Uttarakhand, India. The soil properties are taken from the literature. First, the threshold load is determined using proposed equation as well as FELA. The FOS is checked from the present FOS vs. loading intensity curves for given slope and footing geometry. The bearing capacity of site is determined for vertical centric load. The bearing capacity is also reported within allowable eccentricity ratio, e/B=0.16 and angle of load inclination =100. As per IS 1893-1 (2016), this site is situated in the seismic zone V. The seismic bearing capacity of footing is also reported. The study reported in the thesis has much practical significance as many structures and buildings need to be designed and constructed on hill slopes.