Erosion of soil cement in water resource applications may be defined as the progressive disintegration of the material by water motion. Abrasion erosion can be defined as the wearing away of a surface by the action of water and waterborne particles.
Laboratory studies to assess the erosion and abrasion resistance of soil cement go as far back as 1942. The earliest tests were conducted at the Civil Engineering Department of Oklahoma A & M College (now Oklahoma State University). Additional studies were at PCA in early 1970s and at three Universities in United States and Canada in the early 1980s.
During these studies, soil cement samples made using different types of soils were subjected to:
- Standard ASTM D559 (wet-dry) and ASTM D560 (freeze-thaw) durability testing,
Compressive strength tests,
Water carrying gravel,
Breaking waves and impacting debris, and
- Water jet with varying water velocities.
Generally, these studies concluded that the erosion and abrasion resistance of soil cement exposed to water carrying waterborne particles can be significantly improved by using a coarser material as the aggregate or adding gravel to a finer soil. The erosion resistance can also be improved by increasing the cement content. These methods improve the strength of the soil cement. Similar to concrete, the strength of soil-cement continues to increase with time. As a result, soil-cement will continue to gain strength with age.
Because most soil cement mixtures in field applications contain very little coarse aggregate, the soil cement erosion resistance is in most cases controlled by the compressive strength of the cement paste. PCA recommends that adequate cement content for soil cement be determined based on durability tests or a minimum 7-day compressive strength that correlates to durability. Depending on exposure conditions, a typical design will require a minimum 7-day unconfined compressive strength of 750 psi (5.2 MPa). Some agencies have specified that an additional 2% of cement be added to account for construction-related variations.
Laboratory test results and field performance show that properly designed soil-cement can withstand the flow of clean water up to a velocity of 20 ft/sec (6 m/sec) with little damage. Also, soil-cement designed with adequate strength and durability, even without air entrainment, can resist the long-term erosion affects caused by wave action, flowing water and freeze-thaw cycles with little deterioration.
For higher flow velocities or water carrying abrasive waterborne particles, the compressive strength should be increased or different materials such as roller-compacted concrete be used. Means to increase the strength of soil cement exposed to more severe erosion conditions include modification of the mixture proportions, including increasing the cement content and/or changing to a coarser, more well graded aggregate. Significant improvement in strength and erosion resistance can be obtained by simply adding a gravel component of 20 percent or more to a sand of silty sand soil.