The information found within a geotechnical report communicates site conditions. The data, analyzed by geotechnical engineers, helps site designers accurately plan their building design and commence their underground planning accurately. Essentially, without the information found within the report, site engineers would be forced to make assumptions about the conditions of the soil beneath the surface and estimate code minimums for building purposes without taking into consideration any problem areas.
Estimations are conservative figures and costs can increase substantially if potential issues are not addressed prior to the planning and design process. Commissioning a geotechnical report is the best way to avoid problems that may arise due to adverse soil conditions at a project site. While each report may vary on some of the information found, a comprehensive report would address these key areas.
General Inquiries of a Geotechnical Report
To begin a geotechnical report, engineers must be armed with a scope and purpose of the investigation. A succinct scope of work allows the engineers to have a basis for the conclusions and recommendations made within the report. A scope and purpose summary also allows the surveying engineer to notate survey restrictions they encountered due to the narrowness of the scope of work. In addition to the scope of work, the engineer needs access to any previous tests completed on the survey site. This information, along with any previous field explorations, will help with their analysis of the planned construction site. The initial scope and purpose summary should include a general area description. The report should also include all dimensions of the intended project, any retaining walls, utilities, grading and any planned site updates. Engineers would then use this information to test the site for structural stability.
Geotechnical Report Embankment Planning
Draining is an issue at any site. Engineers must address whether or not existing drainage conditions are adequate for the construction planned. If drainage is found to be inadequate, the report should include recommendations to address problems in order to avoid future issues with the foundation. In addition to drainage issues, the presence of springs or excessively wet areas on a property can pose construction problems. The engineer must address these wet areas by noting possible seasonal variations in water flows or levels, and how these changes could impact construction. The final report should include current groundwater levels.
The summary of the scope of work for the geotechnical report includes any erosion protection plans. However, it is up to the geotechnical engineer to determine if these methods are sufficient for the project based on the completed survey. If not, the report should indicate any additional recommendations. With an embankment, there is always the question of stability. An evaluation of the overall stability is based on a minimum of F.S. = 1.25 for side slopes and 1.30 for end slopes. The report must include recommendations based on the strength of the foundation soil relative to the proposed embankment.
Landslides pose significant risks to land development. All landslide issues should be addressed within the geotechnical survey. The report should also recommend preventative measures to be considered as well as needed corrective actions. Engineers may excavate an active landslide to correct problematic areas. The overall report should pay particular attention to any alternatives to slide excavation based on safety factors and estimated costs.
Because landslides are often the result of ground over-saturation due to heavier than usual rainfall, engineers must consider annual groundwater evaluations. These same measurements, taken for embankment consideration and coupled with the soil profile and soil shear strength, are analyzed to determine landslide potential. Backslope stability is another question to consider during site evaluation. Construction recommendations to eliminate risks of destabilization as well as any post-construction actions are part of the geotechnical report.
Construction uses retaining walls as a way to support soil mass to keep the ground at variant levels along the sides of the property. Areas below ground level, including basements, will incorporate the use of a retaining wall. Therefore, a question of soil strength is raised to determine if there is sufficient support for the wall. In these instances, engineers would conduct a survey of the terrain conditions including lateral strength.
Because ground water directly impacts the soil’s strength, the report would consider the overall groundwater concentration and seasonal elevations. Engineers use a particular formula to determine the exact percentage of moisture in the ground to create optimal strength situations. Building plans include options for proper drainage in the event of heavy rains and the presence of excess water. However, the geotechnical report would also address this situation, especially if the proposed plan is inadequate.
Structural Foundation Planning
There are several options to be considered relative to the structural foundation of any project. In order for geotechnical engineers to determine the best plan for a particular set of site conditions, the scope of work must include information on foundation requirements. During the study, engineers would then determine if the designs are adequate or would otherwise suggest necessary revisions.
Spread footing foundations are wider at the bottom in comparison to the load-bearing foundational wall it supports, which allows weight to be distributed over a greater area. In many regions, this type of foundation is an adequate support system. However, the engineer may find that, due to ground conditions, it is inadequate and may suggest other alternatives. Additionally, the surveying engineer may recommend elevation adjustments instead of using an alternative foundational structure. Included in the report are settlement estimates and an estimate of time for completion.
A driven pile foundation is common for industrial structures. The types of piles used include precast pressed concrete, steel H-piles, pipe and timber. The use of structural elements, as well as the kind of pile, depends on the ground conditions at the site. Geotechnical engineers determine if the site is suitable for a particular foundation type and if it is feasible for the needed bearing loads. Geotechnical reports would indicate if a minimum pile elevation is necessary, as well as estimate any eventual settlement. Detailed plans and ground surveys help address other issues such as surface obstructions, excavation needs and foundation drain planning. Engineers must take into account any existing adjacent structures to negate the potential for damage to these structures and the surrounding area.
Drilled shaft foundations are a cost effective way to ensure foundational support. A shaft is penetrated deep in the soil and then filled with concrete to form pillars. The geotechnical survey determines if the shaft lengths and diameters would support proposed loads. Soil data helps to determine the lateral load capacity. In the event of obstacles, such as boulders, the engineers may suggest an alternate foundational support structure.
Geotechnical reports do not only describe the condition of the soil at the proposed building site but also list any proposed improvements needed to prepare the site for construction. Wick drains or prefabricated vertical drains provide a path for water found in soft compressible soil. Engineers include data on the coefficient of consolidation for the duct as well as the length and spacing necessary for this option to work.
Road construction uses a lightweight filling material. This material helps to reduce loads, both horizontal and vertical, and simplify foundation designs. In the event a filler is recommended, engineers include material properties, compressibility, permeability and drainage requirements for the site. Depending on the ground conditions, geotechnical engineers may suggest the use of stone columns sunk into the ground. These columns improve the terrain by adding density to the soil and reinforcing the earth through the creation of a stiff composite soil mass. Reports suggesting stone columns include column dimension, settlement characteristics, spacing, bearing capacity and permeability.
Grouting ground improvements can improve the strength of the soil through the high-pressure injection of cement grout. This grouting method compacts the soil as the grout column expands. Alternatively, the grout material may be injected into the earth using the jet grout method. This method allows cuts and mixes with the soil to form uniform soil-cement columns. Recommendations for a grout process include the exact method to use, settlement limitations and quality control of the mixture.
To provide a narrow scope and purpose summary to the geotechnical engineer, site managers must allow for an area at the end of the report for questions. Only by making educated assumptions about the various techniques with which issues can be solved can the surveying engineer adequately test the site.