Floor Bearing Characteristics of Jointed and Layered Rock Foundation


Dheeraj Kumar

Faculty, IBAT School of Management, KIIT Deemed University,
Kharagpur, India

Samir Kumar Das

Associate Professor, Department of Mining Engineering, Indian Institute of Technology, Kharagpur, India



Analysis of stability (mainly bearing strength and settlement) under a footing on regularly bedded, jointed and layered model rock mass is conducted using physical simulation technique and with FEM analysis. A simple method to estimate bearing strength of both intact and jointed rock mass under circular, square and rectangular footings is proposed.

In jointed and layered rock mass conditions, the floor bearing characteristics analysis were carried out by considering the parameters like orientations of joint and weak layer with respect to the direction of loading, variations in the joint sets (joint spacing) and the layer thickness (as a function of footing plate width). The location of footing plates with respect to joint and weak layer were also varied.

3D Non-linear FEM analyses of floor bearing characteristics (under the similar conditions as in the case of laboratory investigation) were carried out using ANSYS 6.1 FEM software package. Three dimensional numerical models were developed for different conditions of surface footing foundations using appropriate rock mass properties. The inputs required for the FEM modelling were imported from the laboratory results of the measurements. Rock mass were modeled as elastic-plastic with Drucker- Prager failure criteria for plane strain condition. The joints and the weak layers in the floor rock mass were modeled in order to enable separation of zones in the models. The footing settlements correspond to the maximum applied bearing pressure on floor strata (for different sizes and shapes of footing plates and also under varying anisotropy conditions of floor strata) as obtained from the experimental results and FEM investigations, were compared and the maximum deviation was observed as 32 % whereas the minimum was even less than 1 %. From FEM analysis it is interpreted that maximum stress concentration occurs at the tip of the footing plate all along the boundary. The stress concentration extends maximum to a distance 2 to 3 times footing plate width in all direction of floor strata. It is further interpreted that maximum vertical settlement occurs near to the vicinity of the footing plate. From the FEM analysis of jointed rock it is examined that there is a slight movement of the two blocks separated by the joint. The joint plane acts as the rupture plane.

Keywords: bearing strength, intact rock, jointed rock, footing, FEM

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