Out of sight, out of mind? Not so much in civil engineering! One of the greatest risks to a civil engineering project is what lies beneath the surface. I recently heard of a civil engineering project that was delayed by 2 months due to needing more geotechnical investigation. A few weeks ago, I was in a preconstruction meeting for a project I am working on, and a contractor said, “You can never have enough geotechnical study on a site.” In this week’s blog post, we explore what a geotechnical evaluation is and why it is important to civil engineering projects.
Subsurface conditions – the characteristics of the soil, rock, and other unknowns beneath a site – represent a huge risk to a civil engineering project. For example, there could be a layer of rock that is thicker than previously thought, requiring additional demolition and increasing project costs. The soil could be sand instead of clay which requires a different foundation design and support, maybe there is buried debris that was disposed of over the years, or there could be environmental contamination that needs on-site remediation. Any one of these unknowns is a risk to the project schedule and the overall project cost.
The primary goal of the geotechnical analysis is to characterize the subsurface conditions and determine what type of soil and rock is on the site. This is crucial information that informs the foundational and structural design of the building or proposed structure. The geotechnical analysis can also be used as a tool to predict how the soils will react and the potential soil settlement resulting from the construction activities and final structure. An analysis of groundwater conditions is also included. For example, does the groundwater table need to be drawn down prior to construction? Is the groundwater contaminated? If so, how does the discharge need to be contained to meet environmental regulations? How will the groundwater table interact with the final structure? Ultimately, a geotechnical analysis should be conducted in the early design stages of the project and will be used to inform foundation design and identify complications before they occur.
Typically, the information for a geotechnical analysis is gathered by conducting soil-boring test holes with a large machine called a drill rig. The geotechnical engineers should develop a boring plan that includes several test holes in the site area. Water well records and aquifer maps, soil and geological maps, aerial photos of existing and historic site conditions, and any information from past projects on the site should be used to develop the boring plan. It is also important that the borings are drilled to a specific depth to make proper determinations about settlement potential – typically this depth is 1.5 times the depth of the deepest excavation required. Furthermore, samples should be collected at regular intervals along the boring depth – typically this interval is every 2 1/1 feet for the upper 10 feet of the boring, and then every 5 feet for the remainder of the boring – however, these depths may be more frequent depending on site conditions. Samples are collected and stored and sealed in jars to prevent moisture loss during transport to the lab for further analysis. Based on the initial set of borings and the resulting findings, geotechnical engineers may decide that additional borings are needed, especially if inconsistencies or unexpected results are noted. Furthermore, in areas at greater risk of seismic activity and earthquakes, a more detailed, in-depth geotechnical analysis may be required.
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Author: Matt Fanghella, PE, CFM
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