GIS in ENGINEERING GEOLOGY

INTRODUCTION: 
A full three-dimensional GIS system is obviously the best system to handle engineering geological data and analyses. For regional studies, this may lead to very extensive databases and consequently long calculation times, although this may become unimportant with further development of computer power. In regional studies, if property distributions are not important and the geology is relatively simple, a 2.5D system may be sufficient.

A full 3D-GIS system is necessary for site-specific analysis in which either an accurate representation of the sub-surface is required and/or property distributions are required, or where the geology is more complicated. Figure shows an example of a sub-surface property model for a tunnel project and Fig. shows the application for a dam project. Geographical Information Systems, and in particular 3D-GIS, are able to offer considerable help to the engineering geologist however, it does not add these qualities in itself. The quality of the output is directly related to the quality of the input and the quality of the manipulation that is done with the data, e.g. “rubbish in” is still “rubbish out”. Another point that should be considered is that GIS software is complex and not always user friendly. Hence, it is often time consuming to use the programs and this extra time is certainly not always justified for all type of projects. Some professionals, probably it should be questioned whether these are professionals, cut down on time consuming operations by using simple programs, for example, 2.5D instead of the more complicated 3D programs, and by using simple calculation routines when more complicated but better relations are known. Obviously, GIS used in this way can result in lower quality results than traditional methods.

Quality of Published Information and Limitation of Liability: Maps, whatever their character, contain such information as was available at the time of their compilation, which is often out of date at the time of publication and later use. Information that contains an interpretation of available data, give those opinions based on the understanding of the engineering behavior of the ground of that time, which is not necessarily that of the time of later use. If information expresses opinion, which is virtually always the case with geology interpretations, the question arises as to who is responsible for the quality and accuracy of that opinion. The types of maps which are published by government agencies in any one country depend, therefore, upon the social, political and legal systems in that country because if engineering works are planned on the basis of the information provided by these maps, there must be some understanding as to who is responsible for the validity of the data they present.

An Aid to Engineering Geological Mapping: As an aid to engineering geological mapping the author developed the system discussedbelow. Regional scale engineering geological mapping must have purpose. The rockmass classification described below is aimed at distinguishing between those rockmasses which pose no particular problems for general civil engineering to be conductedon or in them from those which will give problems.

Factors in the PRI:

The factors considered in the Problem Recognition Index are:

1.       Layer strength

2.       Layer uniformity

3.       Discontinuity spacing

4.       Uniformity of surface weathering profile

5.       Material sensitivity to weathering or alteration

Layer Strength Rating (LS): Rock strength is an obvious parameter of considerable significance in most forms ofengineering in rock. Ratings assigned vary from 10 for strengths less than 1.25 MPa to 100 for strengths greater than 200 MPa. However, some allowance must be given for anisotropy. In the field, layer strengths are measured in two orthogonal directions, which, in bedded rocks, would be normal and parallel to the bedding, using a Schmidt hammer or by geological hammer blows.