ABSTRACT:

Based on detailed geological-geotechnical investigations on the Blagovica-Konpolje motorway section (E-W corridor in Slovenia), the influence of time on the changes of the stress-strain in the physical behavior of Carboniferous-Permian mixed soft rock mass was defined. Significant volumetric and distortion deformations appeared after a long period of rainfall after unloading of the excavation. Finally, gradual structural degradation caused sliding of the altered or disintergrated soft rock mass as this soft rock mass took on the characteristics of the saturated residual soft soil. Most important in this process were pre-existing weak soft rock zones. As a result of the suction process of pore water pressure dissipation out of the micro-cracks (dilatation suction and dissipation), viscous creeping developed. The presentation focuses on numerical back analyses which were made at three scales: mineralogical petrography specimens at micro-scale, at the scale of borehole samples and at the scale of land-sliding slopes.

1. INTRODUCTION

Based on a detailed geological-geotechnical investigation of the Blagovica-Kompolje motorway section, Carboniferous-Permian soft rocks were divided into three typical layers with different strength and deformability parameters. The purpose of the investigation and analysis was to determine the input parameters for numerical analysis which were needed to specify support and retaining measures for the excavation of cutting slopes. Excavation for the foundation caused unloading and, along with heavy rainfall during autumn in 2002, activated a deep landslide in the building area of this motorway section. The landslide was successfully stabilized with an anchored pile wall, material exchange below the pile wall and a special drainage system. With detailed laboratory analysis, "in situ" examination and monitoring results, we have gathered enough data for the creeping analysis of the landslide. The analysis was conducted using a special method of calculation enabled by the software used. During each calculation step, the grid is updated and active pore pressures are recalculated. Geometric nonlinearities, occurring in deformations of this extent, are thus avoided. With the slowing of creeping, conditions were established to begin a planned restoration using an anchored pile wall. Technical observation is ongoing while the structures are operational.

2. GEOTECHNICAL INVESTIGATIONS

Prior to the motorway construction, geological-geomechanical investigations were first conducted in the preliminary project phase and later in the main project phase. For those sections where the route was planned to cut into slopes, additional investigations were conducted. During the motorway construction, the back slopes of the planned supporting and retaining structures became accessible. For the main project, technical observation and monitoring were called for. For determining rock mass characteristics, we engineering geologically mapped the slopes in detail during the excavations and catalogued bore holes, outcrops and piles. For that we used the new rock mass classification for mixed and soft rock mass by determination of the geological strength index GSI [1]. In selected bore holes, preasuremeter measurements ("in situ" measurements) were conducted and intact samples taken. During laboratory examination, we conducted triaxial consolidated undrained shear tests ("CU"-tests) of fifteen intact samples and nine compressibility and permeability oedometric tests.

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