Geotechnical investigation

Summary

Geotechnical investigations are performed by geotechnical engineers or engineering geologists to obtain information on the physical properties of soil earthworks and foundations for proposed structures and for repair of distress to earthworks and structures caused by subsurface conditions; this type of investigation is called a site investigation. Geotechnical investigations are also used to measure the thermal resistance of soils or backfill materials required for underground transmission lines, oil and gas pipelines, radioactive waste disposal, and solar thermal storage facilities. A geotechnical investigation will include surface exploration and subsurface exploration of a site. Sometimes, geophysical methods are used to obtain data about sites. Subsurface exploration usually involves soil sampling and laboratory tests of the soil samples retrieved. Geotechnical investigations are very important before any structure can be built, ranging from a single house to a large warehouse,

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https://en.wikipedia.org/wiki/Geotechnical_investigation

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The lack of reliable methods for evaluation of soil abrasivity and revealing the effects of different parameters on wear rate are nowadays considered as a deficiency in geotechnical investigation during feasibility study, design and construction phases of tunneling projects with the use of tunneling boring machine (TBM). The subject is recently attracts a broad international attention and focus. The background of existing standard test methods are reviewed and wear types occurrences in TBM tunneling are discussed in this paper. A new soil abrasion testing system is developed which is called Soil Abrasion Testing Chamber (SATC) and the results of soil abrasion tests are compared with results of the commonly used tests such as: Cerchar, LCPC and SAT tests. Some of the most influencing factors including presence of water, bentonite slurry, soil particle size, quartz content, water pressure and confining chamber pressure are considered for the use of the new devise. The test results indicate that the soil abrasivity tends to increase with the increasing of soil particle size, bentonite slurry, soil pressure and quartz content. The effect of water on abrasivity varies for different types of soil. The soil abrasivity decreases with the increase of water pressure. The internal friction does not seem to have any influence on the wear rates measured with the new proposed apparatus. (C) 2015 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2015

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Concrete is a most important geomaterial as it is used for a large part in our buildings and infrastructures. According to Planetoscope (2012) its production is about 6 billion m3 per year (190 m3 each second) which makes it the most used manufactured material in the world. The alteration of concrete, tightly associated with the durability and security of our infrastructures, depends on its primary composition but also on a wide range of environmental factors (mechanical solicitations, freezing-thawing cycles, alkali-aggregate reactions, etc.). Hence, the contribution of geological or geological-related analysis to understand concrete alteration processes is required because this material is mainly composed of aggregates made of different minerals and rocks, the type and quality of which are important due to their influence on concrete behaviour. In this study, high resolution X-ray Computed tomography (X-ray micro-CT) was used to image 3D different types of concrete cores in order to characterize their respective state of alteration. The global alteration index (GAI) developed by Christe et al. (2010) for natural cataclastic rocks was applied to the segmented X-ray CT images of these concrete cores and compared to the results of microstructural analysis on thin slices and of compression tests. Also, an internal attack of concrete by hydrochloric acid was carried out in laboratory to simulate artificial alteration through carbonate dissolution and its process was monitored along time by X-ray micro-CT imaging (4D monitoring). Our first results show that X-ray CT imagery enables, without any destruction of the specimens, to characterize concrete internal features in terms of macroporosity, highly microporous cement paste, standard cement paste or aggregates. In particular, initial entrapped porosity as well as cracks are easily detected, characterised and quantified before and after mechanical testing. Petrographic analyses on thin sections enabled to verify the physical meaning of the X-ray CT-based detected features, such as the highly microporous cement paste, the opening of the microcracks and the detachment halos around aggregates which all act as weakening parameters. Despite the limited number of samples, a coherent relation between the GAI and the compressive strength of the concrete specimens is observed as the concrete compressive strength clearly decreases when the GAI increases. In this case, it logically means that the concrete with a higher porosity is less resistant. Moreover, the 4D monitoring of the acidic attack test led to dynamically show how the carbonated structure of the concrete was progressively altered. These preliminary results demonstrate that GAI, based on XRCT imagery analysis, can be used to evaluate the degree of alteration of concrete and could thus lead to estimate its strength. In more general terms, X-ray CT analysis opens new perspectives to relate the quality of concrete with its mechanical properties. This method could probably be further applied to a wide range of problems related to the inspection and maintenance of concrete infrastructures.

2014

Analysis of TBM tunnelling performance in faulted and highly fractured rocks

Erika Paltrinieri

The thesis focuses on the study of the performance of TBMs in highly jointed and faulted rocks. Despite the great production rates recorded in favourable ground conditions, the TBM advance can be significantly slowed down in limiting geological situations such as extremely fractured and fault zones. After an introduction about the major issues resulting from tunnelling in bad ground conditions, a brief review of some TBM performance prediction models is reported and the most common parameters adopted for evaluating the TBM performance are identified. The geotechnical characterisation of fault zones is very difficult due to their heterogeneous nature (i.e. weak and strong rock components). This particular aspect is highlighted through a literature review about the geological/geotechnical description of fault rocks. In order to investigate possible relationships between difficult rock mass conditions and TBM performance, data of several tunnel projects are collected in a database by including information from the field, laboratory tests and literature. Preliminary analyses are carried out in order to identify correlations between TBM performance (i.e. penetration and advance rate) and rock mass parameters (e.g. rock strength, fracturing degree, etc.). Although some trends are identified, the scattered results confirm the difficulties in predicting the machine performance in complex geological environments. In order to obtain a more complete geotechnical description of disturbed zones, a classification system for highly fractured rock masses and fault zones is developed. Four âfault zoneâ classes are identified by considering the fracturing and the weathering degree of the rock mass. Furthermore, in order to analyse the response to mechanical excavation of the identified classes, a set of numerical simulations are run with the aim to investigate the TBM performance at the cutter-scale. The data recorded in the TBM-performance database are then analysed according to the new classification system. For each âfault zoneâ class, a reduction rate for selected TBM parameters is defined with respect to the tunnelling performance observed in good ground conditions. The results of these analyses are of great relevance as they allow to successfully quantifying the effect of altered rock mass conditions on the TBM behaviour. The results obtained in the previous steps are used to carry out probabilistic analyses of the tunnel construction time and costs by means of the Decision Aids for Tunnelling (DAT). The DAT are a software package that evaluates the influence of uncertainties related to both geotechnical conditions and excavation process on the final tunnel construction time and costs. In this framework, the TBM advance rate reductions previously defined for each âfault zoneâ class are input in the simulations. A reliable estimation of the effect of degrading geological conditions (i.e. faulted and highly fractured/crushed rocks) on the tunnel construction process is obtained. The results of this research provide useful insights regarding the TBM performance reduction in bad grounds, with respect to the âordinaryâ tunnelling conditions. The study highlights the need to better characterise, from a geomechanical point of view, the highly fractured and faulted rocks. This can be done by considering the parameters representative of the degraded state of the rock mass, instead of those commonly used for estimating the TBM performance in good rocks.

EPFL2015

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2014

Analysis of TBM tunnelling performance in faulted and highly fractured rocks

Erika Paltrinieri

EPFL2015