The OTO geotechnical group will feature a series of blogs discussing soil settlement concerns and mitigation. Topics will include forensic studies and remediation alternatives for existing building settlement and damage, as well as providing geotechnical engineering solutions for new construction to mitigate settlement concerns.
Part I: Soil Detectives! Assessment of Settlement of Existing Foundations – Ashley Sullivan, PE
The geotechnical engineers at OTO spend a good portion of their time providing geotechnical engineering solutions to mitigate potential settlement for new structures. In addition, we are often called in to assess situations where structural damage has already occurred due to the settlement of an existing building. Our role in these situations is to determine whether foundation settlement is a cause of the structural damage, and more importantly, what was the cause of the foundation settlement. We then will provide alternatives to mitigate on-going settlement and allow the structures to be productively used. We work closely with owners, structural engineers, architects and sometimes real estate agents. These are always interesting projects, since they allow us to put on our detective cap and practice forensic geotechnical engineering.
Often times, the OTO geotechnical engineer is not the first phone call, in that the client has already reached out to a structural engineer or architect. The structural engineer will often assess the aboveground building components such as columns and beams to determine whether these load-bearing components are sized correctly and functioning properly. If the structural components appear to be adequate, the team may start to look at the foundations and ground conditions. This is where OTO can be a valuable asset to the project.
Once our services are engaged, we first gather as much information as is readily available regarding the history of the building and likely subsurface conditions. We look for information regarding construction (year built, materials), type of damage observed (cracks, doors and windows that won’t close, leaning walls, etc.), and timelines (immediate/sudden settlement, on-going settlement over long time span, etc.). We also discuss any changes in site conditions, such as increased building or fill loads, or recent nearby construction work. Before we leave the OTO’s office, our geotechnical engineer will put some thought into anticipated soil conditions. We will access OTO’s database of soil boring and test pit information to review conditions at any nearby sites, review both on-line and OTO’s library of published soil and bedrock geology maps, along with historical Sanborn Fire Insurance and USGS topographical maps. With our experience and the help of published/public information, we often can take an educated guess as to what soil conditions are anticipated at the particular site.
Shortly after receiving the initial call, we normally perform a site visit to obtain a firsthand look at the problem area. We typically review topography and look for indications of fills, changes in drainage (sink holes, soft ground), or slope instability/erosion (bent tree trunks, surficial slips). At that time, we determine the best approach for investigations, such as the type and approximate locations for invasive testing and/or a settlement monitoring program. Investigations may include test pits, soil borings and a review of existing subsurface utilities and drainage. A monitoring program may include the installation of points installed on a building and nearby ground surface, which will be to be surveyed periodically over time to determine trends in the amount and rate settlement.
Many times, the test pits or soil borings with accompanying laboratory tests quickly reveal the cause of the problem. Some examples of potential causes include:
- A pocket of peat, soft clay or loose, non-engineered fill that has compressed under the building load.
- A buried layer of decomposed organics, trash or other deleterious material that has compressed over time, and will further degrade over time.
- A soft compressible layer of fine grained soil that has consolidated under the weight of the new structure or fill loads.
- Wet, loose, granular soils indicate a possible “wash out” condition due to a drainage pipe break and the introduction of water into the soil matrix.
We then continue the investigations to determine the nature and extent of the unsuitable conditions. After the assessment is complete, the geotechnical engineers can start the next phase of the evaluation, “The Fix”, which will be discussed in a future blog post.
Do you have a building that is settling? Contact Ashley Sullivan at 413-276-4253 or firstname.lastname@example.org to see how OTO can help!