BEHAVIOUR OF ECCENTRICALLY OBLIQUELY LOADED FOOTINGS ON COHESIONLESS SOILS

Abstract

Shallow foundation is a very common type of foundation provided for many structures. In general, these foundations are subjected to a vertical load, a moment and a horizontal load. The resultant of these becomes eccentric-inclined load on the foundation. Examples of such structures, whose foundations are subjected to eccentric-inclined load, are retaining walls, abutments, columns and portal frames etc. For proportioning of such foundations ultimate bearing capacity, settlement, tilt and horizontal displacement are the main criteria. The proposed mathematical model is for predicting pressure-settlement characteristics of model footings subjected to eccentric-inclined load resting on Ranipur sand. In this method, separate equations were developed for each case. i.e., square footing (200 mm x 200 mm), rectangular footing (200 mm x 400 mm) and strip footing (100 mm x 600 mm) for calculating maximum pressure intensity pm, settlement corresponding to maximum pressure intensity Sem, and the constants m and C. Assuming suitable value of pressure intensity Pei, one can obtain the value of settlement Se using the following equation [ vs Se =Sem 1+4121-1 Pei In this way, plot of pet versus Se curve may be made for given eccentricity and inclination of load. Using the unique non-dimensional correlations of Se/So and Sm/So, with e/B and i/q5 developed by Agrawal (1986), a procedure has been developed to get the pressure-settlement and pressure-tilt characteristics of actual footings subjected to eccentric-inclined loads. In this method, ultimate bearing capacity of the footing (B x L) for given values of eccentricity width ratio (e/B) and inclination of load (i) is obtained using the following equation: 1 que,.— 2 yBNre' + yD N , .Aq Where, Ay = shape factor = (1-0.2 B/L); 0q = shape factor = 1.0 --I iii NYS, and Nye, = Agrawal's bearing capacity factors pressure-settlement characteristics of the footing (B x L) may be obtained using plate load test data and the following equation: Sf [ Bf ( Bp + 3012 Sr= Bp (Bt+30) Where, S/ and Sr are settlement of the actual footing and plate respectively, BI and Bp are width of the foundation and width of plate respectively, (cm). It may be noted that the above equation holds good for same pressure intensity. Therefore the pressure-settlement characteristics of the actual footing (B x L) may be obtained only up to the maximum pressure intensity (i.e. ultimate bearing capacity of the plate). The further portion is extrapolated by extending the curve smoothly up to the ultimate bearing capacity of actual footing (quo). For better accuracy, this can be done in more than one-step depending on the size of the footing. Divide the value of quei by a factor of safety say F1 to get the pressure intensity corresponding to the factor of safety Fi.Use the following correlations to get the values of Se, Su., and tilt (t) (Agrawal, 1986): olSAi,ielre +4 +A, B 0) A) SS„, B0 + B1 e ; So ) sin t S.— Se B/ _e / 2 Where, Se = settlement of the footing under point of application of load; So, = maximum settlement of the footing; So = settlement of the footing subjected to central vertical load. Ao, A1, A2, Bo' and B1 are the constants. It may be noted that Se, Sm, and So belonging to the same factor of safety. From pressure-settlement curve obtained for plate load test data, read the value of So corresponding to quo/F1, F1 is the same factor of safety as used in determining cid!. Repeat the same procedure for the other values of factor of safety i.e, F2, F3, .. etc. In this way, plot of per versus Se, Pe, versus S„, and Pei versus tilt (t) curves may be made for actual footings. iv