If you’re getting ready to build a retaining wall, you’ll want to get a geotechnical report before you order lagging panels. These detailed documents include essential information about a site’s geological conditions, along with project design and construction recommendations.
But while geotechnical reports play a crucial role in construction work, they can get overwhelming if you aren’t sure what to look for. When you know how to read geotechnical reports, you’ll be able to make the most of the data included in these documents.
What To Look for in Your Geotechnical Report
While you’re looking through a geotechnical report, keep an eye out for:
Groundwater Levels
Before you can construct a retaining wall, you need to know the elevation of your construction site’s groundwater table. A high water table can increase soil saturation, leading to higher levels of soil pressure and additional drainage requirements.
Your geotechnical report can help by providing groundwater table elevation info for your work site. This report should also let you know whether your water table is fluctuating or constant, and if your area has perched water conditions (water in soil/rock layers above the groundwater table). While looking at this data, make sure you have access to projected rainfall levels for your area, too.
Lateral Earth Pressures
Geotechnical reports may include info on the equivalent fluid pressure (EFP) soil would exert on your retaining wall. EFP is a simplified way to describe the lateral load soil places on lagging panels by comparing it to an imaginary fluid that would create roughly the same amount of pressure. Typically, EFP is expressed in pounds per cubic foot.
The active earth pressure coefficient can also help you determine the lateral loads your lagging panels will have to deal with. This is a ratio expressing the horizontal to vertical effective stress of a given soil mass.
Borehole Logs
Geotechnical engineers use borehole logs to share valuable insights on a site’s excavation conditions. When you check these logs in your geotechnical reports, you’ll get information on soil layer depths and soil classes on your job site.
The Unified Soil Classification System assigns two-letter symbols to different soil classifications, making it easy to understand your soil properties, grain size and texture. As an example, the letter “G” describes gravel and the letter “P” describes poorly-graded soil, so the symbol “GP” refers to poorly-graded gravel.
SPT N-Values
A standard penetration test (SPT) is a widely used procedure for geotechnical investigation. When geotechnical reports include these test results, they can give you valuable information about your soil’s soil density, consistency, and risk of instability or raveling.
The SPT procedure includes the following steps:
- Drilling a borehole. At the start of this process, licensed geotechnical technicians or engineers must drill a borehole to their intended sampling depth. The sampler used in this test is a split-spoon sampler, which is attached to the drill rod.
- Dropping a hammer. After drilling the borehole, the people conducting the test drive the sampler into the ground. To do this, they drop a 140-pound hammer from a 30-inch distance. When the sampler reaches a depth of six inches, they repeat the process two more times for a total depth of 18 inches.
- Recording the number of blows. The number of blows needed to drive the sampler six inches into the ground is known as the “seating drive.” Meanwhile, the number of blows needed to drive the sampler the rest of the way to its total depth is called the “standard penetration resistance” (AKA “N-value”).
Soil Strength Parameters
When you’re going through your geotechnical report, look for these values:
- Cohesion (c). In geotechnical engineering, “cohesion” refers to the shear strength holding soil particles together without confining pressure. Cohesion is expressed in units of stress (either kPa or kN/m²).
- Friction angle (φ). A soil’s angle of internal friction is its shear resistance from the frictional resistance and particle interlocking between individual soil grains. This measurement is expressed in degrees.
With this information, you can predict your soil’s performance under stress with the Mohr-Coulomb failure criterion. This is expressed as:
τf = c + σ**·**tanφ
Here, τ is a material’s shear strength and σ refers to your soil’s normal stress.
Geological Hazards
Some soils are more likely than others to create problems for a retaining wall project. With that in mind, you should know when these materials are at risk of losing cohesion and falling off slopes – a process known as “sloughing.” Weak and organic soils are particularly susceptible to sloughing.
You should also be aware of the problems that come with constructing a retaining wall on collapsible soils. Collapsible soils can shrink in volume when they get wet or withstand heavy loads, which can easily cause trouble for structures built on these materials.
What Other Details Do You Need for Retaining Wall Construction?
As part of your retaining wall planning process, take time to consider your:
- Excavation depth. The right excavation depth for your retaining wall project will vary depending on the type of soil you want to build on. The worse your site’s soil drainage is, the deeper your excavation work will have to go.
- Soldier pile spacing. When you’re ordering lagging panels, make sure you’re getting the right panel size for your project. One major factor to consider is the span between your soldier piles, which will determine the length your panels must cover.
- Lagging panel material. Timber lagging panels can last a few decades under ideal conditions, but they can easily suffer from water damage, wood rot, and pest problems. On the other hand, the National Precast Concrete Association has stated that precast concrete has an average lifespan of 50 to 100 years.
- Surrounding structures. Your retaining wall needs to be ready for “surcharges,” or loads that add to your site’s standard earth pressure. Slopes and structures near your retaining wall can easily contribute to surcharges.
- Traffic conditions. As you might expect, vehicular traffic can easily add to surcharges. However, pedestrian traffic can also affect these loads.
Get the Right Lagging Panels for Your Project
There’s no denying that soil stability plays a huge role in any retaining wall project. But reliable lagging panels are just as important, so you should do your research before choosing a panel supplier.
American Precast Concrete Inc. goes the extra mile to deliver high-quality concrete lagging panels for residential and commercial retaining wall projects alike. If you’re ready to build a retaining wall you can count on for decades to come, contact us today!