Intro
If you’re planning a retaining wall project and already know that global stability matters, you’re ahead of the game. But the next key question is: Do you really need a soils report—or can you move forward with default soil parameters?
The answer depends on several factors, including wall height, soil type, site layout, and the loads behind your wall. In this guide, we’ll help you understand when soil testing is necessary, what tests are appropriate for DIY vs. professional evaluation, and what tools and techniques you can use to make smart, informed decisions about your site.
Why Soil Testing Matters
Segmental retaining walls (SRWs), especially those reinforced with geogrids (i.e., MSE walls), rely heavily on the properties of the retained and foundation soils. Soil parameters like unit weight, friction angle, and cohesion influence grid length, spacing, facing connection strength, and global stability.
Without accurate soil information, default values may lead to overbuilt (expensive) or underbuilt (unsafe) walls.
When Soil Testing Is NOT Required
You may not need soil testing if your site meets all of the following:
- Wall height: Less than 4 feet
- Site conditions: Flat grade, no nearby slope or surcharge (e.g., vehicles, structures)
- Soil type: Clean, well-drained granular soils (sand, gravel)
- Foundation: Uniform and firm; no fill, organics, or soft pockets
You can often use default design values like:
- Internal friction angle (ϕ): 27°–30°
- Unit weight (γ): 115–120 pcf
⚠️ Warning: If you’re unsure of what kind of soil you’re dealing with, or if you see evidence of fill or clay, don’t rely on default assumptions.
When Soil Testing IS Needed
Once your project involves greater height, surcharge loads, or questionable soils, testing becomes more important. Use this quick chart:
| Wall Height | Site Conditions | Soil Type | Recommended Tests | Who Can Do It |
| < 4 ft | Flat, no surcharge | Clean granular soils | None or visual/manual testing | DIY |
| 4–6 ft | Minor slope or light surcharge | Silty sands, low PI clays | Field compaction, plasticity tests | Contractor / Advanced DIY |
| 6–10 ft | Driveway, slope, poor soil | Clay or undocumented fill | Atterberg Limits, Proctor, Direct Shear | Soil lab or geotech |
| > 10 ft | Surcharge from structures/slopes | Expansive, mixed soils | Full geotechnical evaluation | Licensed Geotech Engineer |
⚠️ Red Flag: If your wall is taller than 6 ft and you’re dealing with clay, undocumented fill, or adjacent structures—testing is essential.
DIY Field Tests and Tools
You don’t need a full lab setup to get useful soil data. Here are some practical tools and tests:
1. Pocket Penetrometer
- Estimates strength of clay (unconfined compressive strength)
- Cost: ~$25–$60
2. Atterberg Limits (DIY version)
- Take a sample of your foundation soil using a soil probe
- Roll soil into 1/8″ threads for plastic limit
- Shake a jar of soil/water to see liquid limit
- Indicates soil plasticity and potential expansiveness
3. DIY Soil Sieve Kit
- Classifies soil gradation (sand, silt, clay)
- Cost: ~$100–$150
- Example: EISCO Set of 6 Test Sieves
4. Hand Compaction Field Test
- Squeeze a ball of moist soil. If it crumbles, it’s sandy; if it sticks together and is slick, likely clay.
5. Field Density Testing (Sand Cone or Balloon)
- Compare field compaction to Proctor values (if available)
- Cost: ~$250–$400
- Example: Sand Fling Funnel Density Cones
6. Undrained Shear Strength Test
- Uses a hand shear vane tester to measure undrained shear strength in cohesive soils (typically clay)
- Cost: ~$100–$200
- Example: AMS 59033 Pocket Vane Shear Test Kit
Lab or Professional Testing
For larger or more complex walls, these lab tests provide reliable data:
- Standard Proctor (ASTM D698): Determines optimum moisture content and max dry density
- Modified Proctor (ASTM D1557): Heavier compaction curve for load-bearing areas
- Direct Shear (ASTM D3080): Determines soil’s internal friction angle and cohesion
- Atterberg Limits (ASTM D4318): Identifies shrink/swell potential and clay plasticity
- pH / Sulfates / Resistivity: Needed for walls with galvanized steel reinforcement or chemically aggressive soils
Who Can Perform These Tests?
- DIY or Contractor: Basic screening tools and field testing
- Local Soil Lab: Accept small soil samples for basic classification, compaction, and strength tests
- Geotechnical Engineer: Required when wall height, loading, or poor soil conditions pose a structural risk
Pitfalls to Watch For
- Don’t trust undocumented fill—even compacted fill may contain organics or debris
- Clay soils can lead to expansion, shrinkage, or slope failure
- Under-compacted soil can cause wall deformation over time
- Global stability issues may not be visible from wall face movement alone
How Soil Properties Affect SRW Design
Soil properties directly control:
- Required grid length and spacing
- Facing connection strength
- Global stability and settlement
- Drainage and backfill compatibility
Even though some SRW software tools (offered by block suppliers) allow default values, CMHA’s TEK Notes and Design Manual emphasize using real, site-specific values for:
- Walls > 6 ft
- Any wall near a slope or structure
- Non-granular soils
References
- CHMA SRW-TEC-008-12
Conclusion
Soil testing may sound like a hassle, but it’s one of the best investments you can make when building a segmental retaining wall. Whether you’re a contractor scaling up to taller MSE walls or a homeowner tackling a big slope, soil testing gives you the confidence your wall will last.
Bottom line: Know your soil before you build your wall.