On Shaky Ground — Addressing Unsuitable Soils in Construction

One of the first steps in commercial construction is obtaining land that’s zoned for commercial use. Perhaps you’ve already determined your site location and can envision the finished product. But what happens when there are unsuitable soils on that land?

There are measures that can typically be taken to alter the site or the building’s foundation in the event of unsuitable soils, but it comes at a cost. How do you determine whether there are unsuitable soils? What types of unsuitable soils are there and what are some options? Here’s an overview.

Conduct a Soil Analysis

A geotechnical firm with soil engineering services can help a project owner determine the condition of the soil and whether any major issues need to be addressed prior to construction.

Their soil engineers will conduct a soil and site evaluation by boring deep into the ground to retrieve samples. The soil boring depth may vary depending on the size and height of your proposed building. A one-story commercial facility may only need to bore a few meters below the surface, whereas a multistory building may require much greater depths.

A soil engineer will also conduct percolation tests, analyze for soil contamination, check water levels, and assess other conditions. Once they’ve completed their inspection, they’ll provide a complete report including recommendations for amendments and how to construct the building’s foundation based on the soil bearing capacity.

Types of Unsuitable Soils for Construction

You never know what will be unearthed as part of a soil inspection. Best case, the soil engineer discovers an ideal ratio of silt, sand, and clay atop a solid layer of bedrock. But many regions of the country aren’t that fortunate.

Soil with high organic content might be great for growing crops but typically doesn’t meet the required bearing specifications for construction. Organic matter in soils breaks down over time, compromising the soil structure, and will need to be addressed.

High water tables are another common problem in low-lying areas. Even sites that may not seem likely to have high water levels might experience undesirable conditions during prolonged heavy rains or seasonally. Water can create hydrostatic pressure that may push against the foundation and permeate it, weakening a structure over time. This condition may be exacerbated in northern climates with freeze/thaw cycles, causing the captured water crystals to expand and contract.

Man-made problems can plague a building site as well, including soil contamination stemming back to an era when there weren’t as many environmental controls as there are today. Some examples of soil pollutants might include industrial waste, old backfill, dredging deposits, or coal slags that were dumped decades ago resulting in heavy metal contamination. These conditions are not only hazardous to the environment but often lack structural integrity and need to be remediated prior to construction.

What to Do if Your Site Has Unsuitable Soils

There are specific solutions that can be leveraged based on the type of unsuitable soil that’s discovered on your building site. The soil engineer along with the structural engineer will determine the best options, and your construction manager will provide cost comparisons. The degree of remediation required will help you and your construction manager determine its impact on your construction budget and timeline. 

Common techniques to address unsuitable soils might include:

Over Excavation. This remediation technique involves removal of unstable or contaminated soil and replacing it with suitable soil. Costs will vary depending on the amount that needs to be excavated, any contaminant remediation requirements, the distance that unsuitable soils need to be hauled, and the amount of replacement suitable soil such as rock, gravel, and sand. Depending on the situation, this technique may only be required where the building is going to be placed rather than the entire site, potentially saving hundreds of thousands of dollars.

Pile Foundations. This is a type of deep foundation system where a series of columns are placed in the ground to reach down to stable soils to support the structure. These piles can be concrete, steel pipe, timber, steel H, precast or composite materials.

Rammed Aggregate Piers. This is a ground improvement system that utilizes stone piers rammed and vibrated into the existing soil to stabilize it to a point that it will support the required bearing capacities needed for the proposed structure. If a high-water table is an issue, rammed aggregate is often the solution because it won’t deteriorate, whereas some piling may deteriorate.

Subsoil Drainage. To further protect against groundwater penetration and damage to a building’s foundation, other measures may need to be taken to create drainage and divert water away from the site.

Push or Helical Piers. Push piers are a series of hollow pipes pushed into the ground and anchored to the structure’s foundation with pier brackets. Helical piers are large auger or screw piers that are drilled into the soil and attached to the structure’s foundation with brackets.

Soil Injection. This solution may be a consideration during renovation of a structure or where construction is taking place near other structures. This solution may be ideal for areas that have fine granular soils. Probes are inserted to inject a chemical into the ground that will stabilize the soil to meet the required performance for the structure.

First Steps for Evaluating Your Soil’s Condition

There are many avenues for improving unsuitable soils, and your construction manager can help guide you through your options based on recommendations from reputable soil and structural engineers. Once the situation is known, it’s important to work closely with your construction manager to determine how each solution may impact the cost and timeline of your project. 

One of the most important factors in building any facility is ensuring a solid foundation. That foundation must not only include the land and materials you choose, but the team you partner with to coordinate every detail and determine the most cost-effective and durable solution to get the job done right. Contact the team at The Samuels Group to help ensure your project is built on solid ground.

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