Five Best Practices for Successful Cement Stabilization
Written by Dani Nygren, Director, Wind, & Sam Jorgensen, Director, Geotechnical & Structural Engineering
Before the construction of a wind or solar project can take place, civil engineers must confirm the project’s access roads can support the loads and frequency of expected traffic onto the site. When there is weak soil present, a common way to strengthen the subgrade for access roads is through cement stabilization. By completing cement stabilization, the roads are given a boost of strength by blending cement mixtures in with the existing soil – but it doesn’t come with a one-size-fits-all approach. Each site must be prepared and tested in a way that works with the existing environment (temperature, soil moisture, soil type) and the expected vehicle loads. To complete a successful cement stabilization project, follow these five best practices.
1. UP FRONT LAB TESTING
Up front lab testing may be performed before construction to assess the feasibility of cement stabilization and estimate the cement application rate(s). Lab testing can help determine if cement stabilization will achieve the desired subgrade strength for the project. This type of testing typically consists of mixing representative native soil samples (topsoil included if applicable) with various amounts of cement and at various moisture contents. The soil-cement sample is then compacted and tested for strength at several different cure times. Up front lab testing can provide valuable guidance before test strips and field performance determine the appropriate cement application rate during construction.
2. TEMPERATURE CONTROL
Similar to best practices for concrete placement, it is not recommended to cement stabilize soil if the air temperature falls below 40 degrees. When the temperatures drop, the curing process slows down and may reduce the overall strength gain of the stabilized subgrade, especially at temperatures below freezing. Although stabilizing frozen soil is not recommended, there may be case-by-case scenarios where cement stabilization is needed as temperatures occasionally dip below 40 degrees. In this case, it is important to get a risk evaluation and recommendation from your engineering team to understand the best approach.
3. MOISTURE CONDITIONING
Moisture plays a key role in a successful cement stabilization project. Prior to stabilizing, it is important to obtain a moisture reading of the in-situ, or existing, soil. This moisture reading should be taken the day of stabilizing to help determine the cement application rate and whether moisture adjustments are needed. If the in-situ reading is too dry (relative to optimum), water should be added during the mixing process. And if the in-situ reading is too wet, the soil should be dried prior to mixing, or additional cement may be added. Ideally, the moisture content of the in-situ soil is near optimum before mixing. Proper moisture conditioning will also help with uniform blending of the soil-cement subgrade. After the mixing and compaction is complete, the stabilized subgrade should be at or slightly above optimum moisture content for ideal curing.
4. UNIFORM BLENDING
Prior to mixing, it is important to ensure the cement is being spread evenly across the full width of the access road. Then, during the mixing process, a reclaimer should be used to pulverize the soil-cement mixture. An important factor of the process to watch closely is the reclaimer. If the reclaimer moves too quickly, the speed may need to be reduced to obtain a pulverized material. Even if the moisture is optimum, without the correct speed from the reclaimer, the soil-cement can mix unevenly and create clumps of soil coated in cement. In the situation of different soil types, such as fat clay, it can be challenging to mix and may require more stringent moisture control as well as reclaimer adjustments (i.e., speed reduction and multiple passes). In the end, a uniform blend of the soil-cement subgrade will help ensure strength gain throughout the stabilized section.
5. QUALITY CONTROL
Post stabilization testing is recommended to verify that the stabilized subgrade meets strength requirements per the design specifications. There are three common tests: compaction, strength, and proof-roll. The combination of these three tests ensures the quality of the road is being met.
Compaction testing should be conducted immediately after the final compaction of the stabilized subgrade using a nuclear density gauge. Meeting compaction requirements will give the stabilized section the greatest odds of hitting the maximum strength gain needed for a successful project.
Strength testing will confirm the stabilized section meets the design parameters for vehicle loading and the overall longevity of the road subgrade. The testing often involves dynamic cone penetration (DCP) tests or unconfined compressive strength tests performed on undisturbed samples collected/prepared in the field. Strength tests can show uniform blending and strength throughout the entire stabilized section (typically a 12” or 16” stabilized depth).
Proof-roll is often the final test and is used to locate isolated weak spots. Proof-rolls consist of driving over the stabilized surface (after passing strength tests) with a loaded water truck, or similar truck, and observing the response of the subgrade surface for rutting or pumping. The proof-roll is the only test that is performed on the entire stabilized subgrade, whereas both compaction and strength testing are performed at specific intervals.
Assessing the strength of the access roads to your project can prevent costly delays and ensure your project stays on schedule. With an intricate process like cement stabilization, it is recommended that you have a dedicated field member monitoring the process and quality control. Westwood has completed successful cement stabilization on wind and solar projects for over 10 years. Reach out to Westwood for assistance on cement stabilization testing, design, and onsite observation for your next project.
