A representative example of the problems associated with the excavation and support of tunnels in karst ground is presented. It is a peculiar case in terms of heterogeneity and spatial distribution of zones of poor geotechnical quality, requiring the need to define, preferably in the study phases, adequate site investigation, suitable design procedures, efficient construction techniques and appropriate ground treatment. The difficulties associated with the instability of the karstified ground, and the presence of cavities, wholly or partially filled with soils of low cohesion, are discussed via retrospective analysis. The solutions adopted to solve the problems encountered during the tunnel construction enabled a systematic approach, useful for new construction projects in limestone terrains of medium to high karstification.
The paper presents the case study of the Ampurdán tunnel that suffered an unexpected partial collapse during construction, due to the weathering of the claystone groundmass, after excavation and wetting by infiltration water. To overcome the problems encountered, the finite elements model was used to understand the behaviour of the tunnel and surrounding ground, to determine the geotechnical properties that lead to failure, allowing to choose suitable procedures for the construction of the tunnel. The parametric study performed simulated the deformations measured in situ and related with the tunnel collapse. The geotechnical parameters used for the weathered claystone, when compared with the ones of intact situation, correspond to a wide range of reductions between 8% in the apparent density and in the effective friction angle, up to 40% in the effective cohesion and 56% in the Young modulus.
During the construction of a highway in the north of Portugal, it was verified that in a few places the compaction control of the road fill materials presented unexpected results with the surface moisture density gauge (SMDG). The water content was lower than measured by oven drying, and the density and compaction grade were much higher than when using the sand cone. In an effort to understand the reasons for this unexpected offset, the road fill material was tested in situ with two different brands of SMDG, extra sand cone tests were performed and samples were collected at the test sites and tested in the laboratory to determine the water content by oven drying. The local geology, mineralogy, geochemistry and radiation level of the fill materials were studied. It was concluded that the incorrect results were caused by the presence of chemical elements in the road fill materials, mainly manganese, chlorine, cadmium and boron, which are able to capture the neutrons released by the SMDG, delivering a false low water content. An index to evaluate the susceptibility of the soil to present a false low water content (FLoW) is proposed.Despite the reliability of the SMDG in most fill control, it must always be used with great care, after crosschecking its results with other direct control tests.
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