13-08-2013, 04:55 PM
Tunnelling Technology (CE:5416) SITE INVESTIGATIONS
Tunnelling Technology.pptx (Size: 4.92 MB / Downloads: 164)
Introduction
Tunnel construction is governed by the ground and hence site investigation is vital to obtain ground characteristics and geotechnical parameters.
Knowledge of the ground conditions plays a key role in the choice of construction technique, and hence the success of a tunnel project.
It is important to realize that the ability to influence the project outcome (in terms of cost and schedule) is easier earlier on in the project programme and much more difficult at a later stage, and the site investigation results can be a key influence on the early decisions.
In many respects the site investigation for tunnelling projects is similar to other civil engineering projects and thus general textbooks and standards should be consulted.
Site Investigation During a Project
For any given project there are a number of different types of site investigation:
Preliminary investigation,
Design investigation and
Control investigation
These may be carried out during different stages of the project and have varying objectives.
The main focus of the preliminary investigation is to assess the general suitability of the site and compare different alignments, with due consideration to third parties.
The main aim of the design investigations is to provide information required for the design of the tunnel, including the construction method.
In addition, control investigations may be required during the construction or execution of the project, and include, for example, checking ground characteristics and groundwater conditions.
Desk study
The desk study is a very important stage of any site investigation, which, if done well, can save considerable time, and hence money, later on in the investigation process.
The aim of the desk study exercise is to assess the conceptual model developed for the tunnel scheme using all the available records of the area where the proposed scheme is to take place.
Desk studies cover all aspects of the site, including current usage, overlying and adjacent structures, historical usage and geology.
It is important that the desk study highlights any issues that could affect the health and safety of personnel during the subsequent site investigation and also the construction of the project.
It should also provide as much information as possible to aid the planning of the subsequent stages of the site investigation, which in the case of tunnelling projects is usually the
location, depth and type of boreholes.
Ground investigation
A tunnel is commonly a composite structure made up of the tunnel lining and the surrounding material.
The surrounding material not only has a loading function, but is also the medium in which a void is created with the help of the supporting role of the surrounding material.
Without this supporting role of the ground an economical tunnel design would not be possible.
To make a judgement about the stability of the tunnel, the characteristics of the ‘building materials’ must be known (both the tunnel lining and the surrounding ground material).
There is difficulty in determining the ground parameters particularly when there are faults, inhomogeneity and weathering.
Laboratory and field experiments can be carried out to give an indication of the soil and rock stability, which can be used to give some idea of the ground stability.
Field investigations Non-intrusive methods
Intrusive methods allow the inspection and testing of the ground itself, they are normally restricted to discrete locations.
Non-intrusive methods can be used for determining additional information about the ground and include geophysical methods.
Geophysical methods can be used to obtain information over a relatively large area, and hence can be used to help locate boreholes, provide information about the nature and variability of the subsurface between existing boreholes.
The interpretation of the output from these methods is not easy and usually requires a borehole(s) to correlate results.
Some of the more appropriate geophysical methods for tunnel projects are:
Points should be noted related to the borehole investigation
The borehole positions should be shown accurately on the proposed plans for the tunnel and the ground level at each borehole position must be recorded.
The majority of information from the investigatory boreholes is derived from cores from the whole depth. This allows undisturbed samples of the rock to be obtained and tested.
If the boreholes are very deep, it is sometimes preferred to get cores only at specific sections, not for the whole depth.
LABORATORY TESTS
After conducting in situ sampling it is possible to visually inspect these samples, describe the material in accordance with appropriate standards and carry out testing in the laboratory.
It should be noted that the results from the samples tested only provide information on the sample itself and engineering judgement is essential to translate this information into ground characteristics.