Richard Gernant, P.E., our lead Geopier® design engineer in Iowa, shares primary ways that wind project foundation soil reinforcing can benefit from planning ahead for groundwater before, during and after ground improvement.
There are two conditions where groundwater can affect the performance of a wind turbine foundation. The first is during construction and the second is over the design life of the completed structure.
In areas with shallow groundwater levels, like many areas of Iowa, groundwater levels can be expected between three and five feet from the surface — well above the base level excavation for a typical wind project foundation pad.
Other states are different, of course, but if your geotech study discovers deeper groundwater tables during bore hole measurements or cone penetration testing, it may not be the whole story. Review of local, superficial soil mapping may reveal a potential for high-water levels or perched water conditions, both occurring during seasonal fluctuations or abnormally heavy rains. We design our ground improvement solutions based on the high-water level noted in the geotechnical site analysis rather than observed groundwater conditions at the time of testing.
Regardless, once site conditions are considered, you still need to manage seepage from both a constructability standpoint and long-term stability for your Rammed Aggregate Pier® reinforced foundation soils. Here are methods to consider in your geotech design and construction plans before, during and after ground improvement.
Choose Open-Graded Aggregate
In many cases, even when installing reinforcing elements below groundwater levels, Geopier reinforcing elements can be constructed quickly with very little influence of water infiltration during installation. However, if we are dealing with clay soils layered with water-bearing sand seams or pockets, temporary casing can be used to control water inflow until the piers are in place. Once installed, the elements will perform as intended. However in some cases water can’t be controlled by casing or there can be a risk of seepage through the Geopier reinforcing elements. We can solve this concern with Geopier elements built with open-graded or free-draining aggregate materials, which are just as strong and allow water to flow on its normal course.
This solution also helps manage the risk of upward pressure and seepage that can loosen densely compacted aggregate in the upper portion of the piers and weaken the matrix soils. This solution is the simplest method for helping to maintain the subgrade for foundation pad construction.
Now, if piers are constructed with well-graded or less permeable aggregate material, and the site experiences upward pressure and hydraulic flow, the contractor can remediate the issue by removing the upper six to 12 inches of well-graded material and replacing it with open-graded material. It is an extra step the contractor will have to go through, but in most cases this will stabilize the surface prior to construction of the foundation pad.
Another issue with foundation construction, of course, is water seepage into the excavation site, which may be left open to the elements for several days or a week. A successful method for protecting the subgrade during foundation pad construction is to install a mud mat, which is basically concrete slurry. A mud mat will protect the reinforced foundation soils from surface water and creates a good working surface for foundation pad installation.
It is important to get your mud mat in place as soon as possible after ground improvement, though, to avoid surface rains or seepage that can soften the foundation subgrade. If the mud mat is not installed in a timely manner and water ponds in the excavation, it may require removal of six to eight inches of subgrade and replacement with open-graded materials and/or a thicker mud mat.
Trenching and Pumping
As a further best practice to retain a dry excavation site, especially when water has a high potential to flow into the excavation, a trench may be placed around the perimeter of the pad site with pumping to divert water until the foundation is constructed.
Once the pad is constructed, with proper backfilling and compaction, then groundwater can stabilize at normal levels with little if any future effects on foundation performance.
Some or all of the methods we’ve listed here can be considered and implemented to obtain a competent, reinforced soil subgrade for your wind project foundation, as well as improve long-term performance regardless of water levels.