Getting Technical: Information Required in a Geotechnical Investigation Report
Geotechnical reports are like a road map for a project: They try to tell you what's ahead, what to expect and what to watch out for. Without them, workers would be flying blind.
Much importance has been attributed to geotechnical reports, primarily because of their ability to steer a project in the right direction, whether it is the planning of the route or selection of machinery and construction material. As important as a foundation is to a building, a geotechnical report is to a construction project. Delays in construction schedules, excess cost, disputes and claims commonly occur due to problems below the surface.
Managing these risks is made easy by referring to geotechnical investigation reports carried out by geotechnical engineers for the concerned project. However, these tests are carried out at specific intervals along the proposed route and it is possible that there is moderate to significant variation in actual subsurface condition. Retaining the geotechnical engineer for the duration of the project can be useful in managing associated risks. (There are numerous ways to conduct geotechnical investigations. Learn more in The Geotechnical Investigation: Which Method Should We Use?)
Uniqueness of Geotechnical Reports
Geotechnical investigation for trenchless pipeline projects is undertaken to perform investigative assessment for a specific client, purpose and type of pipeline project. Each investigative report is unique and is non-transferable to another project unless so specified by the engineer. To get the most out of a geotechnical report and to avoid costly overruns, it should be studied thoroughly and, if needed, clarified with the engineer responsible for the report.
The report is made based on factors specific to projects such as the nature and scope of the project, objectives and goals of the client, site situation and utility location. Even in the case where site location is unchanged but there is a change in the nature of the project, ownership or design team, the geotechnical report is considered obsolete.
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Variation in Trenchless Methods
A good geotechnical report is never brief, but goes into elaborate details for the specific project so that no details are missed. When it comes to trenchless projects, horizontal directional drilling (HDD), horizontal auger boring (HAB), microtunneling and pipe jacking are some of the methods used to install pipelines. All these methods vary in the way they bore, for example directional boring is carried out at an angle from the surface while other methods utilize entry and exit pits dug to the depth of pipe ingress.
HDD uses drill bits and rods while HAB uses augers, microtunneling uses cutter heads and pipe jacking and ramming utilize jacking forces to jack pipes into the ground. All these methods require contractors to understand subsurface conditions thoroughly to use the right type of drill bit or cutter head. (To learn more, read The Right Drill Bit for Soft, Medium & Hard Ground Conditions.)
Contents of an Effective Geotechnical Report
With the different methods utilized in trenchless technology, it is easy to see why a good geotechnical report is so important. A geotechnical report will include the following:
This presents the base or thread of the investigation and the important conclusions derived from it.
This section includes the overview, background, scope and purpose of the project, and summary of the investigation, including research data in subsequent subheadings.
This section describes all site features that are relevant to the project, including conclusions arrived at and methods recommended. This section should include:
- Location and surface conditions – Land details should be provided for identifying the property, including address and parcel numbers. Description of adjacent properties and their current use should be mentioned. Surface features such as buildings, wells, vegetation, geological features, hazardous areas, etc., near the site should be specified.
- Geological setting – This section should provide a summary of local stratigraphy, occurrence of groundwater and regional geology.
- Subsurface soil conditions – Subsurface soil condition can be ascertained by carrying out tests in the soil by taking borings, digging test pits, lab testing of soil, chemical testing and field instrumentation. Each soil or geologic unit should be described with respect to its properties relevant to the conclusions and recommendations. Exploration logs should be provided for the client so that variations across the site can be assessed. Changes in subsurface soil composition and structure such as encountering rocks and unconsolidated soil should be specified.
- Groundwater – The occurrence of groundwater is very important in geotechnical investigation. The variation in level and flow of groundwater should be noted and the possibility of change from that which was encountered at the time of the investigation should be assessed.
- Soil Contamination – If any contamination is encountered in subsurface samples, it should be described, and an environmental assessment, if any has been performed, should be referred to. If the knowledge regarding the nature and extent of contamination is limited on the part of the investigator, it should be indicated clearly in the report.
Discussions and Conclusions
Geotechnical issues related to the project should be discussed here along with alternative approaches for the project. The basis for the approach recommended should be clearly described in this section. The information provided in the previous sections should be elaborated here. Constraints in the recommendations should also be clearly explained. Other sub-topics that should be discussed in this section are: stability of slope, protection of excavation face, seismic factors, sequence for carrying out site work, necessity for retaining structures, etc.
Design and construction recommendations should be provided in logical sequence and complete detail along with expected variations in actual conditions, safety factors, quality control, testing and monitoring. Design recommendations should also be provided for site grading and earthwork, temporary shoring and retaining walls, rockeries, reinforced soil structures, etc.
Figures and Illustrations
This section should include vicinity map, site plan, exploration logs, laboratory test data, cross sections and typical details such as visual aids, graphs and figures.
To ensure that a trenchless project utilizing HDD or any other method is designed accurately, the geotechnical investigation should be carried out as close as possible to the planned bore path, preferably deeper than the recommended depth. The soil testing and sampling should be taken at regular intervals along the bore path to get as much detail as possible. The bore sampling and geotechnical report can help determine the following information:
- Standard penetration test (SPT) for unconsolidated geology – SPT is carried out to obtain a representative sample of the strata for classification, identification and moisture and density tests, and to measure relative density and consistency of the soil.
- Unconfined compressive strength (UCS) for rock samples – This test is undertaken to classify the rock samples on the basis of their strength.
- Mohs hardness for different rock samples – This test gives a rough estimate of the resistance of a rock sample to abrasion from a substance of defined hardness.
While it's always possible for the unexpected to occur, a thorough, detailed geotechnical report helps to minimize risks and uncertainties, making it an invaluable tool for any trenchless project.