What is Engineering Geology
Engineering Geology is an applied discipline of geology that relies heavily on knowledge of geologic principles and processes. It is an interdisciplinary profession in which an engineering geologist works closely with a civil engineer and with it’s ability of geological judgment he can respond to the needs of the civil engineer. Engineering geologic practice centers on conducting investigations. It is the means by which the engineering geologist applies geologic skills to solve an engineering problem. It requires that engineering geologist be, i) a competent geologist, ii) able to translate geologic findings into forms applicable to engineering needs iii) capable of providing sound judgment and finally iv) he must be competent enough to take independent decisions.
The Role of an Engineering Geologist
The main role of an engineering geologist is to conduct investigations for the suitability of the site for civil engineering works. The engineering geologist applies his knowledge and geologic skills to identify and provide solutions for engineering problems. The geologic data collected by an engineering geologist from the proposed site must be thoroughly analyzed and interpreted to give the most realistic geologic conditions of the proposed site. For this certain characteristics are the prerequisite for a geologist who conducts such engineering geologic investigations. According to Burwell and Roberts (1950) the main characteristics of an engineering geologist are; i) his competency in the field of geology, ii) his ability to translate geologic findings into such forms that can directly be applied to engineering works, iii) his ability to take decisions and to provide sound judgment and finally iv) his temperament in terms of tact, levelheadness and practicality.
An engineering geologist must have essential skills and thorough knowledge in structural geology, geomorphology, hydrogeology, petrography and all major streams of geology, which relate to the engineering problems. An engineering geologist must have competency in general geologic knowledge while developing specialized skills important in solving engineering geologic problems. For example, the knowledge required identifying an active fault or shear zone is the same regardless of whether the geologist is conducting a dam feasibility study or trying to explain the tectonic history of a mountain range. The results and findings of an engineering geological investigation must be transformed by an engineering geologist into terms applicable to the engineering project. This means that description and interpretations of rock and soil conditions at a site need to be stated in terms understandable to an engineer. Besides, an engineering geologist must also understand the engineering aspects such as design criteria, structural details and other relevant features of the engineering structure. For example, an engineering geologist carrying investigations for a dam project must understand thoroughly the basic design of the structure, likely loads transmitted to the foundation and other relevant information about appurtenant structures of the proposed dam.
An engineering geologist can make a successful investigation for a proposed site only when he makes sound judgment and is capable enough in taking proper decisions. Engineering projects are developed on the basis of economic, political, engineering and scientific considerations. Therefore while making judgment an engineering geologist must keep these factors in his mind. For example, if a given dam site is not suitable from engineering geological point of view, it may be made suitable by adopting certain remedial measures, but this may affect the economy of the project. Thus, while taking decision an engineering geologist may consider an alternative dam site with less engineering geological problems. However, while taking final decisions he must consider economic, political, engineering and scientific considerations for alternative sites.
Engineering Geological Investigation
Engineering geological investigation is often considered as synonymous with drilling, sampling, mapping and other geological fieldwork. However, in true sense it is to deal with all data/information collected from the secondary sources and the primary data generated pertaining to the suitability of the proposed site. The important objectives of the engineering geological investigations are;
The purpose of a site investigation is to evaluate the impact of construction on existing site conditions and of existing site conditions on proposed construction. Another purpose is to anticipate what can be expected during construction besides to develop criteria for design and construction. Broadly viewed engineering geological investigation incorporates all of the aspects of the scientific method. The first element is formulating a hypothesis for testing. Formulating question to be answered for a proposed site. In addition scope of the study is decided and possible methodology is worked out. The next element followed is the data collection, which involves the previous data/information available in the form of published data and the data to be collected from the field. The next element is to interpret the data collected in the light of the scope of the study for the proposed site. This element is followed by applying analytical techniques. Finally, the last element of investigation is to communicate the findings to the user in a simple and adequate manner.
Phase I – Formulating an Investigation
This involves formulating the investigation by identification of the question to be answered by the investigator. In simple words defining the objectives of the investigation. In this phase an engineering geologist has to know details or the amount of information needed for the proposed structure. An engineering geologist in this phase has to decide the area to be investigated, the time limit to complete the investigation and the duration for the investigation.
Phase II – Data Collection
Data collection involves data collection from secondary sources i.e., the published data and the data collected from the field.
Secondary Data Collection
Data, documents, records, maps and other related materials that are collected from various sources, to be utilized partly or fully as supporting data or information for the proposed investigation, is known as secondary data. Secondary data are in existence prior to the beginning of any investigation. Secondary data forms an important part of any investigation and no investigation can be completed successfully without this data. There may be a variety of these secondary data and may depend upon requirement and the nature of the proposed investigation. However, some common type of secondary data required for engineering geological investigation are;
Data in field is collected through exploration, sampling and testing. Depending upon need both surface and subsurface exploration method may be employed.
Surface and sub surface explorations;
Sampling and Testing
Sampling and Testing starts simultaneously with surface and sub-surface exploration. Sampling and testing is done to characterize the materials and the conditions present. Sampling produces qualitative and quantitative results. Identifying the various lithology while drilling is qualitative result while, measuring the fractures in core recovered from drilling is quantitative result. Testing is done in-situ and in laboratory on representative samples. It is a decision to be made by the engineering geologist whether to perform an in-situ test or have to conduct a laboratory test.
Phase III – Interpretation of Data
An engineering geological investigation will be incomplete without proper interpretation of data collected from the field and the test results obtained from the representative samples in the laboratory. The interpretation of data must be carried out keeping in mind the scope of work and requirements. The results thus obtained are used for making conclusions and recommendations.
Measurement and scale
Relationship of data to scale and measurement is important while making interpretation of required data. For this it is wise to understand different scale of measurements;
Making Data Manageable for Analysis
The raw data should be converted to a manageable form. Appropriate analyzing technique is applied to the refined data. The results thus obtained must satisfy the scope of investigation. For example; the joint data collected from a rock slope exposure has to be plotted on a stereo net and analyzed to workout the preferred orientation of the joint planes. Later a kinematic check is done to identify the possible mode of failure. In this case the scope of the work is to workout the stability condition of the slope. To make an effective interpretation reorganization of results is done which involves preparation of tables, graphs, profiles and other representations that permits comparison.
Applying Analysis Techniques
Once the data are in a manageable form, analysis will become simple. Maps may reveal the presence or absence of some important factor. From cross sections information on sub surface conditions may be worked out. However, in some other cases mathematical or statistical analysis techniques are to be applied. Mathematical analysis techniques are usually based on some theoretical concepts applicable to the condition being analyzed. For example determination of slope stability condition by applying ‘Limit Equilibrium’ technique is a mathematical approach. The Infinite Slope analysis technique is based on mathematical relationship that applies principles of physics to soil or rock mass. These mathematical techniques often involve simplifying assumptions to make the formula manageable (Morgenstern and Sangrey, 1978)
Statistical techniques used in analysis are also based on mathematical theory, but this theory is not specific to a particular process. Many statistical techniques used in engineering geological analysis help to identify the degree of correlation between factors of interest. Thus statistical analysis may help in predicting or assessing certain factors, which can not be known or measured directly. For example the occurrence of debris flows is an outcome that is difficult to predict because of a lack of long-term records in the area. However, a correlation can be made between rainfall intensities and debris flow occurrences. Thus, by using statistical approach a correlation can be made between rainfall intensity and debris flow, which may further help in estimating the debris flow in a particular area.
The mathematical and statistical techniques are based on certain assumptions. Therefore, while making final interpretation of data it must be kept in the mind and accordingly the recommendations should be made.
Conclusions and Recommendations
The last step in interpretation is developing the conclusions and recommendations for the investigation. At this stage the analyzed results are transformed into the form, which are applicable for engineering use. For example if the investigation is being made for the suitability of the foundation for arch gravity dam, the conclusions and recommendations made after the investigation must address the suitability of the abutments and the foundation conditions. In conclusion it must be clearly indicated whether the rocks in the abutments are competent enough to withstand the lateral thrust exerted by the impounded water in the reservoir. What remedial measures, if required, are to be provided to make the foundation and the abutments suitable for the arch gravity dam.
Phase IV – Investigation Communication
The final step in an investigation is communication. It is obvious that without transferring the findings in a useable format to the user the investigation will serve no useful purpose. The communication process transfers the investigation results to the user for action. The user may be individual, a company manage or a government board. Since the users may be different the results findings of an investigations has to be communicated in different formats. Table 1.1 demonstrates a sample checklist for an investigation.
Types of Investigation
There are two basic types of investigations: regional studies and site investigation.
Regional studies are usually undertaken to provide information for land-use planning and other resource management. It is useful for broad decision making purposes required in regional planning. Maps or inventories are the typical products of regional studies. However, such studies may lack the detailed required for the engineering design. Regional studies define capabilities for various land use activities. Engineering geology identifies geologic factors in the affected area that are either a constraint or a resource. Constraints are factors that make one site less suitable for a use than another. A resource is a geologic material necessary to land use activities. Geologic constraint and resource are found in both the natural and man altered environment.
The purpose of the site investigation is to evaluate the impact of construction on existing site conditions, and of existing site conditions on proposed construction, to anticipate what can be expected during construction, and to develop criteria for design and construction based upon determined site specific physical parameters.
There are three distinct stages of site investigation: i) preliminary investigation that supports a feasibility stage of the project, ii) detailed investigation which provides information necessary for project design and construction planning and iii) implementation studies permit changes in expected conditions that are incorporated in design modifications and evaluation of completed work, thus supporting the actual construction of the project.
Studies in support of project feasibility usually address geologic factors that are important in determining: i) the relative suitability of alternative sites or project designs, ii) the extent of detailed investigation required for construction and iii) geologic information required for basic cost estimates. Preliminary studies are related directly to the key criteria for a particular project, generating information needed to complete the project and to prepare environmental documents or application for project approval by government bodies.
Most the preliminary information required for a site investigation can be obtained from: i) published maps and ii) previous investigation reports. From these maps and reports following information may be obtained;
The municipal departments may provide information on;
Site exploration is the intensive investigation of the proposed site. The results obtained from the site explorations are incorporated into the final design and construction of the project. Site exploration for any engineering site basically involves two important objectives. The first is determining and interpreting surface and subsurface conditions that influence design and construction. The second is evaluating the behavior characteristic and engineering significance of earth materials present or those intended for use in construction, this applies for explorations such as roads. Explorations along a proposed road alignment would include areas that require special designs. The areas, which are, wet needs proper drainage, stream channels needs bridges or large fills and rock outcrop requires blasting these may be factors identified during exploration stage. The evaluation of earth materials would include both the materials along the proposed alignment and the sources of construction material. Along the proposed road alignment, the behavior characteristics of the materials may influence the height and steepness of cut slope and the amount of excavation needed. Sources of earth material for constructing the road could include rock aggregate used to surface the road. The ability of the aggregate to withstand repeated wetting and drying would be one of the characteristics evaluated to determine a suitable aggregate source.
During site explorations the engineering geologist is required to plan explorations in such a way so that he is able to provide information on surface and sub-surface geological conditions, the probable adverse conditions and possible remedial measures to overcome the likely problem. The information should also include the ways to improve the design or to decrease the cost of the project.
Strategies For Exploration
Because of complexities of geology and the variety of functional demands, no two engineering sites are alike. It is therefore difficult to give hard and fast rules about the required intensity of explorations. However, some rules which may help in the planning of explorations are;
There is a misconception that once the construction begins the engineering geologist has no further role in the project. According to Terzaghi, the investigations must be continued during the construction stage. It was referred to as “learn as you go method”. The continued investigations may help in determining the geological conditions encountered during project construction, which may differ from what was expected during exploration stage. This is called “changed condition”. Continued investigation during construction stage will serve as the basis for changing the project design to avoid major problems in project performance. Investigation during construction stage may also provide information for specifications for project, which are important for contract inspections.