For Homeowner, Architect
What is a Soils Report and When is it Required?
Feb 10, 2020
A civil engineer conducting an inspection of the soil to prepare a soil report.
A civil engineer conducting an inspection of the soil to prepare a soil report.

Have you been told that your construction project needs a soils report? Are you confused about the procedure?

The average cost of a soils report in California is $1,000, so you’re not alone in wondering what you’ll be getting for your money.

A soils report is the summary of a geotechnical investigation - an analysis of soil conditions at the site of a proposed building.

Safety is a soils report’s primary aim. It’s achieved by finding dangerous soil conditions and recommending design criteria that address them. Besides ensuring safety, a soils report may help save money on construction costs, as it will allow your structural engineers to design a foundation based on the report’s criteria, rather than erring on the side of caution and over-designing.

In California, only registered civil engineers may prepare soils reports. These professionals determine the scope of the investigation, the equipment required, and the need for additional studies. A qualified representative of the engineer must be present on site during all exploration activities.

Aside from overseeing exploration and reporting on the site’s soil conditions, the engineer must also recommend strategies for rectifying unsafe conditions to a point where the building’s structure is not at risk of damage.

When Is a Soils Report Required?

The California Building Code (CBC) sets minimum requirements that local authorities must follow when requesting preliminary soils reports and geotechnical investigations.

Per the CBC, preliminary soils reports are mandatory for all subdivisions creating 5 or more parcels, 5 or more condominiums, community apartment projects with 5 or more parcels, and conversions of dwellings to stock cooperatives with 5 or more units. If a preliminary report identifies soil issues which could cause structural damage to the building, the CBC mandates a full geotechnical investigation.

With active seismic fault lines passing through California, most local governments’ requisites for a soils report exceed those of the CBC. In most cases, soils reports are mandatory for all new construction projects.

Depending on site conditions, a soils report may identify expansive soils, high water tables, shifts in subsurface rock structures, as well as the soil’s response to earthquakes. It may also guide the design team in planning deep foundations. The soils engineer’s findings must be addressed in the design before a building permit is issued.

Workers drilling holes to obtain soil samples for a geotechnical investigation.
Workers drilling holes to obtain soil samples for a geotechnical investigation.

Expansive Soil

Expansive soils swell when saturated with water, and contract when they’re dry. The resulting changes in the soil’s volume cause the building built on it to lift and settle, leading to cracks in the foundation and other structural damage.

If a geotechnical investigation identifies expansive soils, the reporting engineer must propose a way to brace the building against the condition. Common strategies include post-tensioned foundations, replacement of expansive soils with imported fill material, deep foundations, and/or adequate drainage solutions.

Water Table

The water table is the plane under which water saturates the ground. Its level can rise during a heavy rainfall and fall in a drought. If the water table is above the level of a home’s basement or crawl space, it can exert tremendous hydrostatic pressure, causing leaks and flooding.

One aim of a soils report is to find the elevation of the water table. If it’s high, the engineer will have to address the potential leaks and flooding in the below-grade floors of a building. The design may include groundwater control systems and/or waterproofing to mitigate the effects of a high water table. Damp proofing may be an acceptable moisture barrier where there is no hydrostatic pressure.

Deep Foundations

Deep foundations transfer the load of the building below the subsurface layer, down to the bedrock or a layer of sufficient strength. Deep foundations are typically used if the bearing capacity of surface soils cannot support the building’s design loads. This can happen if the surface soil quality is poor or if the building is heavy.

Should a building’s design call for deep foundations, the CBC requires a soils report.

Rock Strata

If a geotechnical investigation finds fluctuations in the structure of the rock intended to support foundations, further analysis may be needed. The rock must be classified and tested for quality, compressive strength and load-bearing capacity. The engineer must order enough borings to test these characteristics and propose strategies for managing them to the design team.

Compacted Fill Material

Proposed building sites with soils of poor bearing capacity may need imported fill material. To avoid damage to foundations, the fill must be compacted before construction starts. If such material is over 12 inches in depth, the CBC requires the engineer to produce a soils report.

Controlled Low-Strength Material (CLSM)

Compacted fill is not the only option for replacing soils with poor bearing capacity. CLSM, also known as flowable fill, is a mixture of Portland cement, water and fine aggregate or fly ash, and is an alternative to compacted fill. Its self-consolidating and self-leveling properties make CLSM popular among contractors. Before it can be used to support foundations, however, a soils report with relevant details must be issued.

Seismic Design Categories (SDC) C through

For proposed buildings falling within SDC C, D, E and F, some local adaptations of the CBC require a geotechnical investigation to evaluate the following hazards:

  • slope instability
  • liquefaction
  • total and differential settlement
  • surface displacement due to faulting or seismically induced lateral spreading or lateral flow

Soils reports for sites with structures assigned to SDC D, E and F may be required to show additional data, which can be found here. Before structural design gets underway, check with your building authority whether a soils report is mandatory.

If your building officials require a soils report to evaluate hazards, our engineers can help - call us at (877) 704-5687 to learn more.

Exemptions From Soils Reports

Per the CBC, one-story, wood-frame and light-steel-frame buildings of Type V construction, with a floor area of 4,000 ft² or less are exempt from soils report requirements as long as they are not located in Earthquake Fault Zones or Seismic Hazard Zones. Instead, allowable foundation and lateral soil pressure values may be found in Table 1806.2 of the CBC.

As well, local building officials may waive the need for a soils report if enough data from adjacent sites is available. Conversely, they may use discretion to request a soils report where none required by the code.

Design Everest Can Help

If your site has challenging soil conditions, we can help. Our skilled civil engineers will find solutions that comply with recommendations from your soils report and get you your building permit ASAP. Our structural engineering team will design the foundation and structure around the site’s soil conditions to ensure your building’s structural soundness and safety.

During our 14+ years of operations, we’ve successfully completed over 4,000 projects in various soil conditions. We have the skills and experience to make your project a success. Call us at (877) 892-0292 for a FREE consultation and quote.

Sources:
[1] https://www.britannica.com/science/water-table
[2] https://www.concrete.org/tools/frequentlyaskedquestions.aspx?faqid=745
[3] https://www.kompareit.com/homeandgarden/developers-engineers-soil-testing.html
[4] https://up.codes/viewer/california/ca-building-code-2016-v2/chapter/18/soils-and-foundations#1803.5.12
[5] https://up.codes/viewer/california/ca-building-code-2016-v2/chapter/18/soils-and-foundations#1803.1.1.1
[6] https://up.codes/viewer/california/ca-building-code-2016-v2/chapter/18/soils-and-foundations#1803.5.3
[7] https://up.codes/viewer/california/ca-building-code-2016-v2/chapter/18/soils-and-foundations#1803.5.5
[8] https://up.codes/viewer/california/ca-building-code-2016-v2/chapter/18/soils-and-foundations#1803.5.6
[9] https://up.codes/viewer/california/ca-building-code-2016-v2/chapter/18/soils-and-foundations#1803.5.8
[10] https://up.codes/viewer/california/ca-building-code-2016-v2/chapter/18/soils-and-foundations#1803.5.9
[11] https://up.codes/viewer/california/ca-building-code-2016-v2/chapter/18/soils-and-foundations#1803.5.11

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