The Elements of Site Investigation

Site investigation is a process of site exploration consisting of boring, sampling and testing so as to obtain geotechnical information for a safe, practical and economical geotechnical evaluation and design. Generally it is an exploration or discovery of the ground conditions especially on untouched site.

In other words the main purpose of site investigation is to determine within practical limits, the depth, thickness, extent and compositions of each subsoil stratum, the depth and type of rock, the depth and composition of groundwater, the strength, compressibility and hydraulic characteristics of soil strata required by geotechnical engineers. Sometimes it is also known as geotechnical investigation.

The Importance of SI

• To study the general suitability of the site for an engineering project.
• To enable a safe, practical and economic design to be prepared.
• To determine the possible difficulties that may be encountered by a specific construction method for any particular civil project.
• To study the suitability of construction material (soil or rock).

Why Have To Perform SI?

Obviously, it is a part of geotechnical processes in preliminary stage.

Lack of geotechnical processes will lead to a:

• Failures where many case histories are available.
• Significant delay and increase in construction costs when the design has to be revised or amended.

Generally the elimination of the site investigation will not safe the cost of the project thus it only comprises from only 0.1% to 5% of the project cost. In fact most frequent claims in civil engineering contracts are on the basis of inadequate SI or obstructions resulting in extra costs which could not reasonably have been foreseen by an experience contractor.

Wok Procedure for SI
Steps of work involved in site investigation:

1. Desk study to collect all the relevant data and information,
2. Reconnaissance of site works,
3. Planning program after reviewing the above,
4. Ground or soil exploration includes boring, sampling and testing,
5. Laboratory testing (also field if necessary),
6. Preparation and documentation of SI report,
7. Engineering design stages,
8. Review during construction and monitoring.

Planning of SI Works
Surface Investigations:

• Site inspection to assess general site condition if there is any anticipated problems that might arise during the construction later on.
• Usually the engineer is required to inspect the site to appreciate actual site and ground problems with particular reference to terrain, vegetation, swamps, water runoff, stratigraphical formations where it is exposed.

Sub-surface Investigations:

• Ground or soil investigation by means of boring, sampling, testing, and etc. And also as to determine the stratigraphy and pertinent properties of soil underlying the project site.

Site Reconnaissance
A reconnaissance is a preliminary examination or survey of a job site. First step is to collect and study any pertinent information already available. After collecting and studying the data available, the engineers should visit the site in person, observe thoroughly and interpret what is seen. Results of reconnaissance help to determine the scope of subsequent soil exploration. It is important to locate any underground utilities to assist in planning and carrying out subsequent subsurface exploration.

A few generalizations of reconnaissance:

• Details on the ground surface for an early observation,
• Topographical characteristics, e.g. flatland, hilly, swamps or pit area,
• The possible location of the ground water tables (GWT),
• Interviewing the local residents for further information,
• Taking a lot of photographs of the proposed site.

Steps of Soil Exploration
Soil exploration in site investigation consists of:

• Boring: Refers to drilling or advancing a hole in the ground. The test would include hand auger, motorized hand boring (wash boring), deep boring (rotary drilling), and/or trial pits.
• Sampling: Refers to removing soil from the hole. The samples can be classified as disturbed or undisturbed sampling. Disturbed samples are usually used for soil grain-size analysis, determination of liquid limit, specific gravity of soil as well as compaction test and California bearing ratio (CBR). The undisturbed samples are collected at least every 1.5 m and if changes occur within 1.5 m intervals, an additional samples should be taken.
• Testing: Refers to determining the properties from the soil. The test can be perform either at laboratory or at field. Laboratory testing would normally be moisture content, sieve analysis, liquid limit, compaction test, CBR and so forth. Field test would include Standard Penetration Test (SPT), Cone Penetration Test (CPT) and Vane test.

Record of Soil Exploration
It is important to keep complete and accurate records of all data collected. Boring, sampling and testing are often costly but it is necessary in every site investigation. A good map giving specific locations of all boring should be available. All boring should be identified and its location documented by measurement to permanent features. And all pertinent data should be recorded in the field on a boring log sheet.

Soil data obtained from a series of test boring can best be presented by preparing a geologic profile:

• Arrangement of various layers of soil,
• Ground water table,
• Existing / proposed structures,
• Soil properties data (e.g., Standard Penetration Test values).

The profile was prepared with data obtained from the boring, sampling and testing of each borehole from selected points.

Geotechnical Report Guideline
Here is the comprehensive guidelines for Site Investigation report:

• Table of Contents: Summery of content details included in the geotechnical report.
• Executive Summary: Brief to the point summary not exceeding one page of findings and design recommendations.
• Terms of Reference: Outline terms of reference and scope, identify requesting source. Find out geotechnical requirements from the project manager, structural engineer or the geometric designer at the beginning of the assignment and keep track of changing requirements thus terms of reference.
• Background Information/Review of Existing Data: Provide site description. Describe, topography and geology (in terms of engineering significance and engineering properties), seismic ground motion data, lab data, ground water and drainage information. Provide location map, National Topographic Series 1:50,000 map reference, e.g. 92B/12, Longitude and Latitude, Universal Transverse Mercator coordinates if possible. Provide plan profile where applicable, site history if available.
• Site Investigation: Describe what is needed in light of existing information, provide specific rationale for the scope and methods of site investigation to make it possible for reviewers to assess the adequacy of the investigation. Describe what was carried out. Show location of test holes or pits or geophysical lines if any. Include field observations at the site, soils and existing conditions.
• Laboratory Testing: List the tests done and present the results using standard format.
• Evaluation and Analysis: Discussion of the site investigation and laboratory test results and their implications on the proposed facility or the stability of the site investigated. The seismic assessment should be provided. Describe analyzes performed, assumptions, parameters and methods used (use two methods for analyzing slope stability or calculating bearing capacities where practical). Provide foundation or slope design information in terms of both static and dynamic (seismic) design if required and state what safety factors are in place. Provide anticipated range of settlement for foundations and fills, and Factor Of Safety of fill. Apply your field observation of the site conditions and existing foundations if any, on your choice of foundation type.
• Sand and Gravel Sources / Disposal Areas: Provide legal description, status (Crown, lease, etc.). Describe potential sand and gravel sources, tested or estimated material properties and projected quantities. Describe investigation methodology. Provide recommendations on waste or surplus material disposal areas.
• Design Recommendations, Including the Design of Pavement Structures: Point out possible foundation and construction difficulties, effects on the existing adjacent structures and suggest methods of overcoming these difficulties, recommend the preferred type of foundation, describe why and suggest possible alternatives (value engineering) where possible. Refer to findings of field investigation, lab test findings and analyzes the results. Point out that the geotechnical engineer should be given appropriate opportunity to review the geotechnical aspects of the completed design prior to construction.
• Discuss Predicted Effects of The Recommended Work on The Environment (water quality, etc.): Provide recommendations on mitigation measures. Provide specifications and special provisions for construction contract. Provide cost estimates for the recommended work.
• Literature References: Provide a list of references used in the preparation of the report.
• Appendices: Correspondence Soils and rock core logs (make sure standard disclaimers are included with the logs in contract drawings), test hole location plan, design profile for new roads, pit development plan, drawings, plan and profile, and also photos.
• Quality Control of Work: Reports must be signed and stamped both by the author and the reviewer. It is the responsibility of the author and the reviewer to determine the appropriateness and accuracy of input data and the correctness of the computed results. Use of computer programs does not free the Professional Engineer or the Professional Geo-scientist from this responsibility.

Summarizing
Scope of site investigation works when planned by different engineers tend to be varied because there are an infinite number of conditions to be met and the process of planning also leaves many areas where individual judgment and experiences must be applied. It is also impossible to attempt to provide an exhaustive step by step guidelines applicable to all possible cases. It should be realized that there is a possibility that any site investigation may leave some area unexplored or overlooked. The main risk in foundation design is the uncertainty involving in predicting soil conditions which may change with environment. The more site investigation the more it will reduce the margin of uncertainty but the time and cost requirement will be exorbitant.

Therefore the extent and the cost of site investigation should be such that risk is at an established acceptable level to the designer and also comply to the accepted code of practice.

Read More Constructive Posts:

• Video: Geotechnical Investigation at Site
• Foundation In GeoTechnical Perspective

Tagged as: Geotechnical

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