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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Design Guideline 7 Soil Cement

7.1 INTRODUCTION

In areas where high quality rock is scarce, the use of soil cement can provide a practical countermeasure alternative for channel stability and scour protection. Soil cement has been used to construct drop structures and armor embankments, dikes, levees, channels, and coastal shorelines. Soil cement is frequently used in the southwestern United States because the limited supply of rock makes it impractical to use riprap for large channel protection projects.

7.2 DESIGN GUIDELINES

The following design guidelines reflect guidance in information provided by the Pima County Department of Transportation in Tucson, Arizona (Pima County DOT) and the Portland Cement Association (1984, 1986, 1991). Typically, soil cement is constructed in a stair-step configuration by placing and compacting the soil cement in horizontal layers (Figure 7.1). However, soil cement can be placed parallel to the face of an embankment slope rather than in horizontal layers. This technique is known as plating.

Photograph of 30 year old stair step facing along an embankment on Bonny Reservoir Colorado.
Figure 7.1. Stair step facing on Bonny Reservoir, Colorado after 30 years (PCA).

7.2.1 Facing Dimensions for Slope Protection using Stair-Step Method

In stair-step installations soil cement is typically placed in 8 ft (2.4 m) wide horizontal layers. The width should provide sufficient working area to accommodate equipment. The relationship between the horizontal layer width (W), slope of facing (S), thickness of compacted horizontal layer (v), and minimum facing thickness measured normal to the slope (tn) is quantified by the following equation:

Equation 7.1: horizontal layer width W = minimum facing thickness t subscript n times the square root of (slope of facing S squared plus one) plus (thickness of compacted horizontal layer v times slope of facing S) (7.1)

As illustrated in Figure 7.2, for a working width, W, of 8 ft (2.4 m), a side slope of 1V:3H (1V:(S)H), and individual layers, v, of 6 in. (150 mm) thick, the resulting minimum thickness, tn, of facing would be 24 in. (620 mm) measured normal to the slope. Bank stabilization along major rivers in Pima County, Arizona is constructed by using 6 in. (150 mm) lifts of soil cement that are 8 ft (2.4 m) in width and placed on a 1V:1H face slope.

When horizontal layer widths do not provide adequate working widths, the stair-step layers can be sloped on a grade of 1V:8H or flatter toward the water line. Sloping the individual layers will provide a greater working surface without increasing the quantity of soil cement.

7.2.2 Facing Dimensions for Slope Protection Using Plating Method

On smaller slope protection projects a single layer of soil cement can be placed parallel to the embankment. In this technique, known as plating, a single lift of soil cement is applied on slopes of 1V:3H or flatter (Figure 7.3).

All extremities of the soil cement facing should be tied into nonerodible sections or abutments to prevent undermining of the rigid layer. Some common methods used to prevent undermining are placing a riprap apron at the toe of the facing, extending the installation below the anticipated degradation and contraction scour depth or providing a cutoff wall below that depth.

As with any rigid revetment, hydrostatic pressure caused by moisture trapped in the embankment behind the soil cement facing is an important consideration. Designing the embankment so that its least permeable zone is immediately adjacent to the soil cement facing will reduce the amount of water allowed to seep into the embankment. Also, providing free drainage with weep holes behind and through the soil cement will reduce pressures which cause hydrostatic uplift.

7.2.3 Grade Control Structures

Grade control structures (drop structures) are commonly used in Arizona to mitigate channel bed degradation (Figure 7.4). The location and spacing of grade control structures should be based on analysis of the vertical stability of the system. Toe-down depths for soil cement bank protection below drop structures should be deepened to account for the increased scour. Some typical sections of soil cement grade control structures are shown in Figure 7.5.

7.3 SPECIFICATIONS

In addition to application techniques, construction specifications are equally important to the use of soil cement for channel instability and scour countermeasures. Important design considerations for soil cement include: types of materials and equipment used, mix design and methods, handling, placing and curing techniques. The following list of specifications reflects guidance in the Pima County Department of Transportation's guidelines on applications and use of soil cement for Flood Control Projects (Shields et al. 1988).

Portland Cement. Portland Cement shall comply with the latest Specifications for Portland Cement (ASTM 150, CSA A-5, or AASHTO M85) Type II.

Diagram of typical soil cement slope protection using the stair step method.  Diagram shows: slope of 1 vertical to 3 horizontal; 2.4 meter step overlap length; layer thickness 150 millimeters to 230 millimeters; layers toed down below existing ground; maximum water level two layers below the top of embankment
Figure 7.2. Typical section for soil cement slope protection (stair-step method).

Photograph of soil cement embankment protection with the plating placed parallel to the slope. The tightly packed soil cement plating appears as a grid over the embankment slope.
Figure 7.3. Soil cement placed in the plating method parallel to the slope (PCA).

Photograph of soil cement bank protection and drop structures in Laughlin Nevada. Photograph shows post construction with no flow in the trapezoidal channel and multiple drop structures.
Figure 7.4. Soil cement bank protection and drop structures in Laughlin, NV (Hansen and Lynch 1995).

Fly Ash. The Portland Cement Association recommends that fly ash, when used, conform to ASTM Specification C-168.

Water. Water shall be clear and free from injurious amounts of oil, acid, alkali, organic matter or other deleterious substance.

Aggregate. The soil used in the soil cement mix shall not contain any material retained on a 1-1/2 in. (38.1 mm) sieve, nor any deleterious material. Soil for soil cement lining shall be obtained from the required excavations or from other borrow areas and stockpiled on the job site. The actual soil to be used shall be analyzed by laboratory tests in order to determine the job mix. The distribution and gradation of materials in the soil cement lining shall not result in lenses, pockets, streaks, or layers of material differing substantially in texture or gradation from surrounding material. Soil shall conform to the following gradation:

Sieve Size Percent Passing (Dry Weight)
1-1/2 in. (38.1 mm) 98% - 100%
No. 4 60% - 90%
No. 200 5% - 15%

The Plasticity Index (PI) shall be a maximum of 3. Clays with a PI greater than 6 generally require a greater cement content and are more difficult to mix with cement.

Clay and silt lumps larger than 1/2 in. (12.7 mm) shall be unacceptable, and screening, in addition to that previously specified, shall be required whenever this type of material is encountered.

Diagram in cross section of three types of soil cement control structures.  The first sketch is of a solid soil cement structure generally trapezoidal in profile with: downstream slope at 1 Vertical to 1 Horizontal; top 2.4 meters wide; Upstream slope, to normal bank protection toe down elevation, at 1 Vertical to 1 Horizontal; Vertical below that. The second sketch is of a core fill structure generally trapezoidal in profile covered with a solid soil cement layer. Sketch shows downstream slope at 1 Vertical to 1 Horizontal; top 2.4 meters wide; Upstream slope at 1 Vertical to 1 Horizontal.  The third sketch is of a rectangular solid soil cement grade control structure on solid soil cement rectangular footing. Top is 3.7 meters in width and upstream point rests 1.5 meters from most upstream point of base. Rectangular soil cement base is 1.5 meters thick and 9.8 meters wide. All are height h and have soil cement toe down for scour hole.
Figure 7.5. Typical sections for soil cement grade control structures (PCA).

Mix Design. The design requirements for the soil cement shall be such that it has a compressive strength of 750 psi (5170 kPa) at the end of 7 days unless otherwise specified. A 24-hour test shall be run to monitor the mix design on a daily basis. Experience has shown that 24-hour compressive strength results for moist cured samples are approximately 50 to 60% of the seven day strength (moist cured for six days and soaked in water for 24 hours). Once the design strength mix is determined, a 24-hour test shall be run using the mix to obtain a 24-hour compressive strength which will be used to monitor the daily output of the central plant. Seven (7) day samples shall also be taken for final acceptance. The amount of stabilizer thus determined by laboratory testing shall continue to be monitored throughout the life of the project with modifications as required for existing field conditions.

NOTE: The stabilizer is defined as the cementitious portion of the mix which may be composed of portland cement only or a mixture of portland cement and fly ash or other supplement.

The cementitious portion of the soil-cement mix shall consist of one of the following alternatives:

  1. One hundred percent (100%) portland cement
  2. Eighty five percent (85%) portland cement and fifteen percent (15%) fly ash by weight of stabilizer.

The ratio of replacement shall be one kilogram of fly ash to one kilogram of portland cement removed meaning one to one replacement by weight.

Mixing Method. Soil Cement shall be mixed in an approved central plant having a twin shaft continuous-flow or batch-type pugmill. The plant shall be equipped with screening, feeding and metering devices that will add the soil, cement, fly ash (if utilized), and water into the mixer in the specified quantities. Figure 7.6 illustrates a typical continuous flow mixing plant operation. In the production of the soil cement, the percent of cement content and the percent of the cement plus fly ash shall not vary by more than +/- 0.3% from the contents specified by the Engineer.

NOTE: Soil cement can also be mixed in place, although for most bank protection projects the central plant method is preferred.

Blending of Cement and Fly Ash. The blending procedure shall provide a uniform, thorough, and consistent blend of cement and fly ash. The blending method and operation shall be approved before soil cement production begins. In blending of the stabilizer, the percent of fly ash content shall not vary by more than +/- 0.50% of the specified content.

Scales are required at both the cement and fly ash feeds. An additional scale may also be required at the stabilizer feed.

Required Moisture. The moisture content of the mix shall be adjusted as needed to achieve the compressive strength and compaction requirements specified herein.

Handling. The soil cement mixture shall be transported from the mixing area to the embankment in clean equipment provided with suitable protective devices in unfavorable weather. The total elapsed time between the addition of water to the mixture and the start of compaction shall be the minimum possible. In no case should the total elapsed time exceed thirty (30) minutes. This time may be reduced when the air temperature exceeds 90°F (32°C), or when there is a wind that promotes rapid drying of the soil cement mixture.

Sketch of flow process in a soil cement mixing plant. Soil on conveyer from the stock pile has cement added to the conveyer from the cement storage silo via a surge hopper and vane feeder. The resulting conveyer material is combined in the pug mill mixer with water added. The pug mill delivers the soil cement to a storage hopper from where trucks can be loaded.
Figure 7.6. Schematic of continuous flow mixing plant for soil cement.(7)

Placing. The mixture shall be placed on the moistened subgrade embankment, or previously completed soil cement, with spreading equipment that will produce layers of such width and thickness as are necessary for compaction to the required dimensions of the completed soil cement layers. The compacted layers of soil cement shall not exceed 8 in. (200 mm), nor be less than 4 in. (100 mm) in thickness. Each successive layer shall be placed as soon as practical after the preceding layer is completed and certified.

All soil cement surfaces that will be in contact with succeeding layers of soil cement shall be kept continuously moist by fog spraying until placement of the subsequent layer, provided that the contractor will not be required to keep such surfaces continuously moist for a period of seven days.

Mixing shall not proceed when the soil aggregate or the area on which the soil cement is to be placed is frozen. Soil cement shall not be mixed or placed when the air temperature is below 45°F (7°C), unless the air temperature is 40°F (5°C) and rising.

Compaction. Soil Cement shall be uniformly compacted to a minimum of 98% of maximum density as determined by field density tests. Wheel rolling with hauling equipment only is not an acceptable method of compaction.

At the start of compaction the mixture shall be in a uniform, loose condition throughout its full depth. Its moisture content shall be as specified in the section on Required Moisture (above). No section shall be left undisturbed for longer than 30 minutes during compaction operations. Compaction of each layer shall be done in such a manner as to produce a dense surface, free of compaction planes, in not longer than one hour from the time water is added to the mixture. Whenever the operation is interrupted for more than two hours, the top surface of the completed layer, if smooth, shall be scarified to a depth of at least 1 in. (24.5 mm) with a spike tooth instrument prior to placement of the next lift. The surface after scarifying, shall be swept using a power broom or other method approved by the engineer to completely free the surface of all loose material prior to actual placement of the soil cement mixture for the next lift.

Finishing. After compaction, the soil cement shall be further shaped to the required lines, grades, and cross section and rolled to a reasonably smooth surface. Trimming and shaping of the soil cement shall be conducted daily at the completion of each day's production with a smooth blade.

Curing. Temporarily exposed surfaces shall be kept moist as set forth in the section on Placing (above). Care must be exercised to ensure that no curing material other than water is applied to the surfaces that will be in contact with succeeding layers. Permanently exposed surfaces shall be kept in a moist condition for seven days, or they may be covered with some suitable curing material, subject to the Engineer's approval. Any damage to the protective covering within 7 days shall be repaired to satisfaction of the Engineer.

Regardless of the curing material used, the permanently exposed surfaces shall be kept moist until the protective cover is applied. Such protective cover is to be applied as soon as practical, with a maximum time limit of 24 hours between the finishing of the surface and the application of the protective cover or membrane. When necessary, the soil cement shall be protected from freezing for seven days after its construction by a covering of loose earth, straw or other suitable material approved by the Engineer.

Construction Joints. At the end of each day's work, or whenever construction operations are interrupted for more than two hours, a 15% minimum skew transverse construction joint shall be formed by cutting back into the completed work to form a full depth vertical face as directed by the Engineer.

7.4 REFERENCES

Hansen, K.D. and Lynch, J.B., 1995, "Controlling Floods in the Desert with Soil-Cement," Authorized reprint from: Second CANMET/ACI International Symposium on Advances in Concrete Technology, Las Vegas, NV, June 11-14, 1995.

Pima County Department of Transportation Construction Specifications, "Soil-Cement for Bank Protection, Linings and Grade Control Structures," Section 920, undated.

Portland Cement Association, 1984, "Soil Cement Slope Protection for Embankments: Construction," Report PCA, IS173.02W.

Portland Cement Association, 1984, "Soil Cement Slope Protection for Embankments: Field Inspection and Control," Report PCA, IS168.03W.

Portland Cement Association, 1986, "Soil Cement for Facing Slopes and Lining Channels, Reservoirs, and Lagoons," Report PCA, IS126.06W.

Portland Cement Association, 1986, "Suggested Specifications for Soil Cement Slope Protection for Earth Dams (Central Plant Mixing Method)," Report PCA, IS052W.

Portland Cement Association, 1991, "Soil Cement Slope Protection for Embankments: Planning and Design," Report PCA, IS173.03W.

Sheilds, S.J., Maucher, L.E., Taji-Farouki, A.A., Osmolski, A., and Smutzer, D.A., 1988, "Soil Cement Applications and Use in Pima County for Flood Control Projects," prepared for the Board of Supervisors/Board of Directors.

Updated: 09/14/2011

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