Water Resources Frequently Asked Questions

Water Resources FAQs

• How is the thickness of soil-cement for upstream slope protection of embankments determined?

  Typical thickness design of soil-cement slope protection depends on the severity of the intended application and on the method of construction. Soil-cement can be placed for slope protection in horizontal lifts adjacent to the slope (stair-step) or parallel to the slope (plating). PCA guidelines for the stair-step and plating methods of construction are summarized below.
  
  Stair-Step Construction Method
  
  Soil Cement for embankment slopes exposed to moderate-to-severe wave action or high velocity lateral flows should be constructed using stair-step method of construction.  To determine the minimum thickness required, designers should consider the density of the proposed soil-cement and all possible loading conditions, including but not limited to uplift due to wave action and hydrostatic pressures that may develop during rapid drawdown.  The thickness would then be determined to insure that the soil-cement weight is heavy enough to resist all upward forces.  In determining the soil-cement thickness, designers should also include a safety factor in the form of an additional sacrificial amount of soil-cement to account for wear or loss of surface materials, which can occur especially if the lift edges are unformed or not compacted to a high density or the structure is exposed to severe freeze-thaw action. Based on the performance of soil-cement bank protection projects over the past 50 years, the majority of any surface wear and material loss will occur within the first couple years of service.
  
  On many projects, the required thickness based on structural analysis would be less then the thickness dictated by the embankment slope, lift thickness, and construction equipment.  Stair-step construction on most projects has been accomplished by equipment approximately 8 feet wide placing soil-cement lifts 8 to 9 feet (2.4 - 2.7 m) wide, 6 to 12 inches (150 - 300 mm) thick.  For slopes, ranging from 2H:1V to 4H:1V, the resulting thickness of soil-cement measured perpendicular to the slope would be between 2 and 2.5 feet (0.6 - 0.8 m).  (Figure 1). This thickness compares favorably to a typical rock riprap with bedding design. If desired, the overall thickness can be reduced by placing narrower widths. On some projects, conveyor belt or other placement methods were used to reduce the lift width to 6-ft. (1.8 m) or less.  
  
  Plating Construction Method
  
  Plating method of construction can be used on slopes 3H:1V or flatter on projects not exposed to severe applications.  Although this method has been used on a few projects where the slopes were as steep as 2.5H:1V, it is generally not recommended for this steep a slope because of the difficulty in placement and compaction. The steeper slope requires the use of lighter compaction equipment or additional cable supported equipment working from the crest to maintain stability of the vibratory roller.
  
  Similar to the structural requirements for stair-step construction, the required thickness for plating method is typically determined based on structural analysis considering all possible loading conditions and accounting for possible loss of surface materials during the desired service life of the structure.  The analysis should include but not be limited to hydrostatic pressure uplift due to rapid drawdown and wave action.
  
  For detailed information on soil-cement design and construction methods, refer to PCA publication EB203, “Soil-Cement Guide for Water Resources Applications.”

• How does the use of aggregates affect erosion and abrasion resistance in soil-cement applications?

  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.

• What are the different types of facing systems used for RCC dams?

  A variety of upstream and downstream facing systems have been used on RCC dams. Upstream facing systems have consisted of conventional concrete, precast concrete with and without membrane backing, exterior geomembrane plating and grout enriched RCC. Downstream facing systems have also been quite varied, including un-faced RCC, and conventional concrete placed concurrently with RCC or following RCC placement. The exposed surface of the downstream face is most commonly configured to have either a continuous slope or stepped surface. Stepped configurations are often used in the overflow section to provide the added benefit of energy dissipation for spillway flows.
  
  Although the type of facing system selected for a new dam is site specific and is based on a number of criteria, all successful facing systems for RCC dams have one feature in common – they do not impede the potential for high RCC placement rates. This should always be a consideration in selecting a facing system for an RCC dam. Selection of the facing system(s) for an RCC dam must also consider the intended purpose of the facility, operation and performance criteria, local climatic conditions, materials availability, dam size, owner preferences, and public perception. General selection criteria should be based on many factors including constructibility, watertightness, durability, appearance, and cost. The general selection criteria will and should differ between the upstream and downstream faces of the dam.
  
  For a comprehensive look at the various types of facing systems, general criteria for selecting a facing system, special site and structure considerations, and current trends in facing system construction refer to the PCA publication Facing Systems for Roller-Compacted Concrete Dams & Spillways, EB400.
  

   

• How slurry walls are used to remedy seepage problems of levees?

  Since the 1940s slurry cutoff walls have been used as seepage barriers to limit the horizontal flow of water and provide added stability for dams and the foundations on which dams rest..  The two most common types of non-structural slurry walls are referred to as soil-bentonite (SB) and cement-bentonite (CB). Both types require trenching and placement of SB or CB backfill to form the cutoff wall. In the SB method, bentonite-water slurry is poured into the trench during excavation to provide temporary side wall support.  After the trench is excavated to its required depth, a mixture of soil, bentonite and water is placed into the trench displacing the bentonite-water slurry.  In the CB method, cement is added to the bentonite-water slurry just prior to pouring into the trench.  In addition to serving as stabilizing fluid to maintain open trench during excavation, the CB slurry remains to set up and form the permanent cutoff wall.
  
  For several decades, slurry cutoff walls have been used to control seepage both through and under dams and levees. It has also been used as an effective barrier to reduce leakage from ponds. In early 1980s, U.S. Environmental Protection Agency began approving slurry cutoff walls in waste management applications to control flow of groundwater at contaminated sites.  With the recent heightened awareness of many inadequate and high-hazard levees, the technology is often considered for levee rehabilitation. Several projects have been completed in Sacramento, CA.  One of the levee stabilizing schemes is listed on the City of West Sacramento website. http://www.westsacfloodprotection.com/pdfs/CityLightsFloodEdition.pdf 
  
  Depending on project requirements, levee stabilization may require the installation of a cutoff wall to control groundwater seepage and prevent piping failures, adding materials on upstream and downstream sides to improve stability, and/or raising the levee to increase capacity.
  
  Parameters usually considered when designing a slurry wall for levees are permeability, strength and deformability. Strength usually is not the primary factor and most projects require less than 100 psi (I would think the required strength is on the order of only 50 psi in order to maintain a ductile wall)  compressive strength at 28 days for CB mixtures.
  
  For more technical information on slurry cutoff walls, the following publication is suggested:
  
  • Slurry Walls: Design, Construction and Quality Control, ASTM STP1129, available in electronic format at http://www.astm.org/DIGITAL_LIBRARY/STP/SOURCE_PAGES/STP1129.htm
  

   

• How are RCC mixtures for water resource applications proportioned using soils approach?

  As Roller-Compacted Concrete (RCC) increases in popularity, many professionals from diversified fields are designing RCC mixtures.  Because RCC is a dry mixture that is compacted like controlled granular fill and because after curing RCC behaves like concrete, there are two approaches to proportioning RCC mixtures: soils approach and concrete approach.  Generally, professionals with geotechnical background tend to follow the soils approach and professionals familiar with proportioning concrete mixes tend to favor the concrete approach. 
   
  The soils approach is outlined in the following steps:
  • Selection of aggregate;
  • Estimating a cementitious content range for trial mixes;
  • Performing a modified Proctor test on one RCC mixture containing mid-range cementitious content and determining the maximum dry density and optimum moisture content;
  • Molding and testing cylindrical specimens; and
  • Determining cementitious content based on test results and project requirements.
  
  First, a suitable aggregate should be selected.  Because aggregate delivered to the jobsite is the most expensive ingredient in RCC, it is always recommend checking for possible sources near the job site and, if justified based on project size and site geology, on-site mining and processing should be considered.  For most projects, designers select durable aggregate meeting ASTM C33 requirements for concrete aggregates.  Well-graded aggregates are desired to reduce segregation and achieve high density and strength.  
  
  The next step is to determine the amount of cement and pozzolan (if pozzolan is used).  On most projects, the cement is Portland cement Type I/II and the pozzolan is fly ash Class F or C.  In the soils approach, the cementitious materials are expressed as percentage of the dry weight of aggregate. A series of trial mixes using different cementitious contents are performed in a laboratory.  Based on experience and knowledge of the available aggregate, RCC strength, durability, and thermal requirements, an experienced professional can estimate a range of cementitious content for the trial mixes.   A modified Proctor compaction curve (ASTM D1557) is then developed using one RCC mix at mid-range cementitious content.  From this curve, the mixture optimum moisture content and maximum dry density are determined.
  Higher RCC wet density can be achieved when the mixture is compacted at slightly above the optimum moisture content.  Also to allow for evaporation in the field, a design moisture content of between 0.5 to 1.0 percent above optimum moisture content is generally selected for the RCC mix design.
  
  Next, trial batches are mixed and a series of test cylinders are made at different cementitious contents.  At least three cementitious contents should be used: a mid-range and 10 to 15 percent above and below mid-range. At least two cylinders should be molded for each cementitious content and for each test age.
  Several methods can be used to manufacture the specimens. These include the vibrating hammer (ASTM C1435), modified Proctor (ASTM D1557), or pneumatic tamper.  Generally, the vibrating hammer is the most commonly used method for the soils approach mix proportioning. All methods are discussed in detail in the references listed below.  
  After the specified curing period, the molded cylinders are tested for compressive strength (ASTMC 39) and/or split tensile strength (ASTM C496).  The cylinders can also be used to determine the density of cured RCC.  Strength tests are performed at varying ages, depending on project requirements.  The test results are then plotted to develop age versus strength curves and the cementitious content meeting the project requirements is selected from the curves. 
  
  References:
  1. Portland Cement Association Publication EB218, 2002, Design Manual for RCC Spillways and Overtopping Protection, Chapter 7
  2. Y. K. Choi and J. L. Groom, “RCC Mix Design-Soils Approach.” Journal of Materials in Civil Engineering, January/February 2001, pp. 71-76
  3. Portland Cement Association Publication IS541, “Roller-Compacted Concrete Density: Principles and Practices.” 2004
  4. Portland Cement Association Publication EB215.02, 2000, Roller-Compacted Concrete Quality Control Manual, pp. 7-18
  5. Portland Cement Association Publication EB225.01, 2003, Design Manual for Small RCC Dams, Chapter 8
  

   

• What are “Poorly Reacting Sandy Soils” and how could they be detected prior to use in soil-cement applications?

  In soil-cement construction, certain types of sandy soils are found that cannot be treated successfully with normal amounts of portland cement. These types of soils will experience lower compressive strengths then would be expected based on the gradation of the soil. Although a black or dark brown colored soil is a good indicator for the potential for a poorly reacting sandy soil, appearance alone should not be relied upon as the only indicator. Poorly reacting sandy soils, which are typically acidic and/or have high organic contents, have been encountered mainly in glaciated areas in the northern United States and in isolated areas in the Eastern and Southeastern coastal areas.  It was determined that organic content and low pH do not in themselves constitute a definite indication of poorly reacting sand. However,  sandy soil with an organic content greater than 2% or having a pH lower than 5.3 will generally not react normally with cement1. 
  
  Laboratory tests must be performed to detect the presence of these types of sandy soils. The standard ASTM-AASHTO wetting and drying2 and, freezing and thawing3 test methods may be used; however, these tests take more than one month to complete.  The PCA’s “short-cut” test procedure for sandy soils4 may also be used, but this procedure requires a 7-day compressive strength  to complete.  These procedures were developed in the 1950’s and have been widely used, even today, to determine adequate cement contents for durability soil-cement. 
  A quicker test method to detect poorly reacting sandy soils was developed by Robbins and Mueller5 in late 1950’s.. The test takes about one hour and involves determining the ability of the soil to immobilize or absorb calcium. High absorption indicates the cement may not react normally with the soil. To measure the absorption of calcium by a soil, calcium ions must be placed in contact with the soil grains. A standard saturated and carbonate-free solution of calcium hydroxide is used as the agent. When a standard solution of calcium hydroxide is mixed with a predetermined amount of soil, the amount of calcium actually absorbed by the soil can be determined by titrating the calcium remaining in solution. This amount of calcium, when compared with that available in the standard solution, provides a direct measure of the calcium absorption ability of the soil. A complete description of the testing procedure is given in reference 5.
  
  Ways to mitigate the problem of poorly reacting sandy soils include the use of calcium chloride and other chemicals to effectively neutralize this adverse reaction6,7.  The use a small percentage of calcium chloride (CaCl2) on the order of 0.6 to 1.0 percent by weight of dry soil or blending normal reacting aggregate into the mixture have also been effective in minimizing the problem.  Research also shows that the addition of sodium chloride (NaCl) may also be effective.  
  
  Typically poorly reacting sandy soils are located in the top few feet of the surface. These soils should be stripped and (1) used on the job in applications other than soil-cement, (2) diluted with normally reacting soil or treated with admixtures as mentioned above, or (3) wasted.   The underlying normally reacting soils can be excavated, stockpiled and used in soil-cement. 
  
  References:
  1. M. D. Catton, “Research on Physical Relations of Soil and Soil-Cement Mixtures.” Highway Research Board Proceedings, 1940
  2. Methods of Wetting and Drying Test of Compacted Soil-Cement Mixtures, ASTM Designation: D 559; AASHTO Designation: T 135
  3. Methods of Freezing and Thawing Test of Compacted Soil-Cement Mixtures, ASTM Designation: D 560; AASHTO Designation: T 136
  4. Portland Cement Association Publication EB052.07S, 1992, Soil-Cement Laboratory Handbook, Chapter 6
  5. E. G. Robbins and P. E. Muller, “Development of a Test for Identifying Poorly Reacting Sandy Soils Encountered in Soil-Cement Applications.” Highway Research Board, Bulletin 267, 1960
  6. M. D. Catton and E. J. Felt, “Effect of Soil and Calcium Chloride Admixtures on Soil-Cement Mixtures.” Highway Research Board Proceedings, 1943
  7. T. W. Lambe, A. S. Michael, Za-Chieh Moh, “Improvement of Soil-Cement with Alkali Metal Compounds.” Highway Research Board, Bulletin 241, 1960
  

   

  
  

   

• How is cement content determined for soil-cement slope protection applications?

  For early soil-cement upstream slope protection applications an adequate cement content was determined using the standard wet-dry (ASTM D 559) and the freeze-thaw (ASTM D560) durability tests. Both tests take about a month to complete and tend to be expensive. Based on experience and laboratory tests, researchers found that for granular soils with the same cement content, the freeze-thaw test consistently produces greater weight loss than the wet-dry test. Therefore, for granular soils, good durability results could be obtained based on the freeze-thaw test alone. 
  
  Today, however, most cement content determinations use compressive strength as the basic design criterion. PCA developed a relationship between 7-day compressive strength and durability based on more than 1,700 sets of tests.  On the average, this research concluded that:
  
  • Approximately 87 percent of sandy soil samples that achieved 600 psi (4.14 MPa) compressive strength at seven days will pass the durability tests, and
  
  • About 97 percent the soils that achieved 750 psi (5.17 MPa) compressive strength at seven days will pass the durability tests.
  
  PCA’s compressive strength versus durability relationship resulted in the adoption of a short-cut mix design method that requires only a moisture-density test, sieve analysis, and a seven-day compressive strength test.  These tests can be completed in about a week and cost significantly less than the durability tests. It should be noted that the PCA methods were developed for soil-cement pavement base applications.  It is recommended that the cement content determined from these methods be increased by a 2 percentage points to account for more severe conditions typically encountered in water resource applications. It should also be noted that the short-cut method is generally applicable for sandy soils that do not contain more than 35 percent fines and are not highly plastic.  Soils that contain more than 35 percent fines or have plasticity indices higher than eight may still be suitable for soil-cement applications, but the cement content for these soils should be determined based on the strength and durability tests.