Navigating Foundation Challenges: Understanding Unsuitable Soils in Construction
Let’s start our conversation about unsuitable soils by defining what is often thought as problematic by those impacted in the construction industry such as developers, general contractors, or engineers to name a few. The most common description that is used involves excessively wet, soft, loose, high in organic content, soils containing solid waste, or granular soil materials located below grade. In other words, soils that are not considered as acceptable for the necessary support of the proposed structures. We all know construction has its share of unforeseen conditions; however, gaining more knowledge about what may be involved in the process once unsuitable soils are discovered could help you create solutions which will hopefully keep impacts to the budget and schedule to a minimum. Issues such as foundation settlement, cracking, lift or extensive erosion can stall progress for all involved so let’s discuss further.
Geotechnical Exploration for Parcel Development
There is a high probability that if you are developing a parcel, a geotechnical engineer was brought on board to complete subsurface exploration in an effort to uncover soil conditions prior to project design and commencement. This will include gathering historical data and City records, performing soil borings to various depths, studying the characteristics of rock and soil formations, conducting laboratory tests, measuring the water table, and collecting samples that will then help the engineers analyze critical data. These findings from the field then generate a Geotechnical Report which is an effective way to get a professional’s opinion on whether the site is buildable prior to obtaining financing. Ideally, the right mix of silt, sand, and clay atop a layer of bedrock is present. Unfortunately, due to the limited number of soil borings performed and the unpredictability of subgrade soils, surprises can materialize during the excavation scope which will create unique challenges for the project team. These challenges are often not insurmountable but could cost significant time and resources to correct before installing the footings and foundations.
Additionally, soil with high organic content might be suitable for growing crops but typically doesn’t meet the required bearing specifications for commercial construction. Organic matter in soils breaks down over time, compromising the soil structure and will need to be addressed if uncovered. Another culprit for unsuitable soils is high water tables caused from low-lying areas, seasonal storms, or freeze/thaw cycles in colder climates. Prolonged water exposure can create hydrostatic pressure that could push up against the foundation and weaken it over time. One other major concern involving unsuitable soils is soil contamination stemming from a previous use of the property. We all know environmental guidelines were not as stringent in our nation’s history and this could have led to extensive pollution which may or may not be detected by a geotechnical engineer. Examples of human influenced land degradation are pesticides and fertilizers, sewage sludge, mining contamination, nuclear and manufacturing waste, asbestos, heavy metals, or solvents not to mention the potential damage done by leftover underground storage tanks (USTs).
Risk Mitigation and Early Due Diligence
This information is not meant to scare anyone off but merely to inform you of the risk associated with finding and developing the right parcel. Early due diligence is helpful and recommended to reduce risk during the civil phase of the project. Retaining an engineer, talking to the adjacent landowners and City planners, inspecting the impact areas during wet seasons, and monitoring the site throughout excavation will lower the risk that drastic counter measures will have to be taken from unsuitable soils. If such is discovered and unavoidable, mitigation efforts will likely include over-excavation of the building envelopes, replacement of the identified unsuitable soils, extensive compaction and dewatering efforts, soil injections, pier systems (Rammed Aggregate, Push, Helical, etc.), pile foundations or the use of various types of geotextiles.
Global Occurrence of Unsuitable Soils
Unsuitable soils are more common globally than you may realize, and we have dealt with the issue on occasion while consulting for VERTEX. Recently this came to light on a Midwestern high-rise project where unsuitable soils plagued the general contractor for approximately three months. There was an unusual amount of seasonal rain in this particular city combined with certain organic soils not detected during due diligence. Both of these setbacks resulted in round the clock dewatering during foundation installation and a significant amount of soils having to be replaced. Luckily, the substantial completion date was minimally impacted due to a quick response; however, the unsuitable soils led to a negotiated change order to remedy the situation. The project team was professional and timely in finding a solution while VERTEX was critical in monitoring the mitigation, discussing the recovery schedule during OAC Meetings, and communicating the risk to our clients.