The geotechnical report is a tool used to communicate the site conditions and design and construction recommendations to the site design, building design, and construction personnel. Site investigations for building design projects have the purpose of providing specific information on subsurface soil, rock, and water conditions. Interpretation of the site investigation information, by a geotechnical engineer, results in design and construction recommendations that should be presented in a project geotechnical report.
The information contained in this report is referred to often during the design period, construction period, and even after completion of the project (resolving claims). Therefore, the report should be as clear, concise, and accurate as possible. Both an adequate site investigation and a comprehensive geotechnical report are necessary to construct a safe, cost-effective project. Furthermore, engineers need these reports to conduct an adequate review of geotechnical related features (i.e. earthwork, retaining walls, foundations, etc.).
While the geotechnical report content may vary by project size and producing agency, all geotechnical reports should contain certain basic essential information, including:
- Summary of all subsurface exploration data, including subsurface soil profile, exploration logs, laboratory or in situ test results, and ground water information;
- Interpretation and analysis of the subsurface data;
- Specific engineering recommendations for design;
- Discussion of conditions for solution of anticipated problems.
If a geotechnical report is required by the building department or has been utilized for similar projects adjacent to, nearby or in the same subdivision as the project, the client should ensure that it is provided to the engineer prior to beginning the engineering process.
Without the use of a geotechnical report, engineers will have to make assumptions for the design criteria of the onsite soils. Unfortunately, the building department is usually not willing to recommend design criteria for the soil thus leaving the engineer to assume code minimum criteria apply. Code minimums are often very conservative and may increase construction costs significantly.
By deciding not to obtain a geotechnical report, clients are ultimately assuming responsibility for any adverse soil conditions that may be encountered. They also assume the risk for any resulting defects in the structure that are related to such soil conditions.
Geotechnical concerns including expansive soils, potential for erosion, lavacap, bedrock, mudflow, soils with a high liquefaction potential, a high water table, frost heave potential, etc. may be encountered. These could significantly increase the project costs if they are discovered during construction. Typically, the client bears all additional engineering or construction costs incurred as a result.
Although we do not perform geotechnical analyses at NorthStar Engineering, we feel that they are the best method for determining the properties of the onsite soil and recommend a geotechnical report for every project (where financially feasible).