Considerations for Identification and Evaluation
10 November 2020
Groundwater is found beneath the Earth’s surface in soil pore spaces and rock formation fractures. The flow of groundwater below the surface is a fundamental property that controls the strength and compressibility of soil impacting soil’s ability to hold up on structural loads.
When soil is saturated, the soil media takes on very specific physical characteristics due to the relative incompressibility of water. These characteristics come into effect below the groundwater-surface or table.
Groundwater tables can fluctuate with time. Changes in groundwater surfaces can be slow as they can change seasons, or they can be relatively rapid such as in tidal basins or stormwater detention basins. Groundwater pressure heads can exceed elevation heads and, in those cases, result in water flowing out on the ground surface as artesian flows or springs or swampy wetlands.
Whenever construction must take place below the water table or soil is used to retain water, groundwater affects the project by impacting the function and design of the facility, and the cost of its construction. Groundwater is a frequent cause of disputes between owners and contractors in construction projects.
Common groundwater issues during construction:
- Unstable subgrade
- Unstable excavation and water seepage
- Construction delays and cost overrun
Common groundwater problems after construction:
- Water leaks, wet basements, and mold growth
- Cracked and uneven floors
- Cracked and uneven walls
- Unstable slopes and retaining walls
- Delayed movements of foundations
How is groundwater identified and evaluated during planning and construction?
Reconnaissance: Using imagery interpretation and site visits to identify an overview of water table conditions, but often requires subsurface investigations.
Subsurface Investigation: Test borings and/or test pits to depths below the anticipated excavation will be required to define the groundwater depth and conditions including static, perched, and artesian conditions. Groundwater conditions can typically be visually observed in cohesionless soils (sands, gravels, and silty sands) because water can flow more readily through these types of soils. Groundwater conditions in cohesive soils (clay and silty clay) cannot be visually observed for water flow and often need to be tested in the lab. Flow velocities in clays can be less than 1 foot/year.
Due to the slow rate of flow in cohesive soils and wells, piezometers and other subsurface instruments may take days to months to record groundwater changes and pressure. When these changes are paramount to a design process, groundwater reading may require a “zero” volume change device such as a diaphragm transducer to read changes in groundwater head in a real-time environment.
The engineering team responsible for all the phases of a project, from initial planning and budgeting through final construction, needs to be aware of the potential impact of groundwater during design, construction and after construction, so their decisions will be effective.