By Ernest K. Schrader
Builders of roller-compacted-concrete dams need to consider several design elements before beginning construction. These elements include choosing the best location, determining the need for leveling concrete, deciding the overall configuration of the dam, and designing to minimize the effects of features embedded in the dam.
Roller-compacted concrete (RCC) offers a range of economical and safe design alternatives to conventional concrete and embankment dams. And while the same basic dam design concepts apply, there are several unique considerations for RCC dams.
Some important considerations to address before proceeding with detailed final designs include but are not limited to: the basic purpose of the dam, the owner’s requirements for cost, construction schedule, appearance, watertightness, operation, and maintenance. A review of these considerations guides selection of several key components, including location, the use of leveling concrete, the basic configuration of the dam, and how to deal with conveyor supports. To fully capture the advantages of rapid construction using RCC technology, the overall design should keep construction as simple as possible.
Choosing the best location
Foundations that are suitable for massive internally vibrated concrete dams also are suitable for RCC dams with similar properties. However, because of the low cost, construction techniques, and material properties of RCC, this type of dam can use a wider base and special design details to accommodate foundations that would otherwise be unsuitable. To build a well-constructed RCC dam on unique foundations, proper attention to certain details is crucial. These details include the width of the structure, isolation of monolith joints, foundation shaping (including use of steps), footings for the delivery system, and use of leveling concrete. Foundation considerations for RCC dams are discussed more fully in previous HRW articles.1,2
Considering leveling concrete
One particular foundation consideration worth noting is the use of “leveling” concrete. Builders of some RCC dams have used leveling concrete to cover the foundation and provide a smooth base for the RCC. For other RCC projects, builders have started with RCC directly on the foundation. Each approach has merit, with each being more or less suitable to different conditions. There are considerations for and against the use of leveling concrete.1,2 These include:
- Leveling concrete simplifies the start of RCC placing and its initial production rate, but makes construction more time-consuming and costly;
- On a foundation with substantial undulations and slopes, the leveling concrete will be very thick in most locations, requiring forced cooling;
- Leveling concrete typically has long-term stiffness (modulus) values of 20 Giga-pascals (GPa) to 35 GPa (3 to 5 million pounds per square inch), with low creep. If placed on a foundation with lower tensile mass modulus, this concrete creates added restraint and higher thermal stresses;
- Any type of RCC mix can be placed directly onto foundation rock without leveling concrete. This is accomplished by first spreading a thin layer of high-slump bedding mix onto the rock, then spreading the RCC over the bedding and compacting it while the bedding is still fresh;
- If the foundation is relatively poor and would deteriorate from exposure before placing RCC, it is common to use shotcrete or a thin “mud mat” to seal and protect it; and
- Where shotcrete is placed against abutments and the foundation still tends to deteriorate, or where the abutment could not be cleaned to sound material before shotcreting, grout pipes have been used to ensure a seal between the shotcrete and foundation.
More details on the use of leveling concrete for RCC dams are available in previous HRW articles.1,2
Determining the configuration of the RCC dam
RCC dams can be built with straight or curved axes, vertical or inclined upstream faces, and downstream faces varying from vertical to any slope. The design type chosen, proposed height, and foundation characteristics strongly influence the basic dam cross section.3,4
The overall design of an RCC structure must balance the use of available materials, selection of structural features, volume and strength requirements for different-sized dam sections, and proposed construction methods. Each factor must be considered in light of the others. For example, a particular dam section may require certain shear strength for stability. However, available materials may not be capable of providing this strength or the construction method may not ensure sufficient lift-joint quality to provide the strength. In these situations, changes to the mix design, construction method, or section structure may be the solution.
Small dams and those on soft or soil foundations require special design considerations for differential settlement, seepage, piping, and erosion at the downstream toe. These dams usually require use of one or more special measures — such as upstream and downstream aprons, grouting, cutoff walls, and drainage systems. Figure 1 shows the basic configuration for a low dam on a weak foundation, including soil.
This design lacks a formed vertical upstream face because of the extra work and costs required. It is easier, cheaper, and faster to overbuild the dam at the upstream face, without forms. In addition, the extra mass of the dam resulting from this method of construction provides more safety within the RCC and may allow less stringent specifications or inspection.
Compressive and shear stresses in a low RCC dam are so small they are almost meaningless. However, if the structure will be subjected to overtopping, a reasonable level of bond between the top lift joints is necessary. This can be assured by using a bedding mix between the top layers of RCC, where uplift and/or negative pressures caused by the overtopping would result in tensile stress at lift joints that is greater than the average lift joint tensile strength. An appropriate factor of safety for this condition is usually 2.0 to 3.0.
Cement contents for very small dams usually are dictated by exposure conditions, mix workability, gradation of the available aggregate, quality of the mixing equipment, and degree of inspection. To account for these factors, small dams that may need a cement content of about 50 pounds per cubic yard for structural loads should instead have higher cement content, usually 150 to 300 pounds per cubic yard. For small dams with volumes of several thousand cubic yards, the cost for the extra cement is insignificant.
For low dams, a downstream slope of 0.9 horizontal to 1.0 vertical or flatter is suggested because it is easy to build with any RCC mix. Again, the extra RCC material involved is negligible.
The top width selected for a low dam should be the minimum that allows reasonable construction with small bulldozers, highway dump trucks, and rollers typically used for small projects. This can be as narrow as about 9 feet, but about 12 feet is a more conservative minimum width. The suggested minimum width of 12 feet is a more realistic width for permanent access (if required) after construction, and to comply with typical safety regulations.
As an alternative to overbuilding the dam using RCC, fill material can be used at the upstream face to steepen the slope, narrow the width of RCC, and save volume. Impervious fill used at the upstream face can also improve watertightness. When fill is used to steepen the RCC slopes of a dam on a non-rock foundation, consideration should be given to the increase in bearing pressure and sliding that this causes at the base of the dam because the RCC base is now spread over a smaller area. When placed at the downstream face of the dam, the fill hides minor seepage and protects the RCC from exposure.
Other RCC dams
Many RCC dam designers have used the basic gravity dam section with a vertical upstream face and constant downstream slope on a vertical face. The low cost of RCC often makes it reasonable to flatten the downstream slope of the dam and add more mass than is economically feasible with conventional concrete. This reduces foundation stress, RCC strength requirements, and lift-joint concerns. Reductions in cement content also result, with related reductions in unit cost and in thermal stresses.
Miel 1 Dam in Colombia is an example of an RCC dam where the downstream slope changes at different heights. Construction of the dam was accomplished using five different RCC mixes.
However, the possibility of using higher cementitious contents with higher strengths also should be investigated if the thermal stresses can be tolerated and the volume reduction offsets the increase in cost due to higher unit costs of the RCC. Influencing factors in this decision are the length of the dam, shape of the valley, cost and availability of cement and pozzolan, quality and production costs of the aggregates, and foundation quality.
A parapet wall can reduce costs of constructing larger dams by reducing the quantity of RCC. The wall also can act as a personnel barrier and curb. Added height or “freeboard” for overtopping waves is not necessary with RCC. Also, curving the top of the parapet wall outward can direct waves back to the reservoir. The wall can be a continuation of upstream precast panels, if that option is used to form the upstream face of the dam. A “breakaway” parapet (fuse plug) designed to fail during overtopping can be designed. This can allow water to flow over one side of the dam while protecting any downstream powerhouse or access road on the other side.
The width of the dam should be established after considering several factors, including:
- Cost of additional RCC and downstream vertical facing;
- Required width for access during operation and construction;
- Inertia (seismic loading) of the laterally unsupported top section of the dam;
- Effect of the mass on sliding stability due to the added confining load;
- Effect of the mass on the location of the resultant force for the section;
- Distribution of foundation stresses; and
- Possibility of causing tensile stress across downstream lift joints when the reservoir is empty.
Adding mass and width to the dam at the base by using a sloped upstream face may improve stability. An extra benefit is the downward vertical component of the reservoir load on the horizontal projection of the dam face. The designer must determine whether this causes tensile forces to develop or to become unacceptable at the upstream face in both the foundation and lower RCC lifts. Slopes up to about 0.1 horizontal to 1.0 vertical can be built for the upstream face of most RCC dams without noticeable effect on the cost, schedule, or construction practicality.
Tension at the upstream face of both RCC and conventional concrete gravity sections is a controversial issue. Each project should be evaluated for its own set of conditions. What may be acceptable for one location and type of mix may be unacceptable for a different location or mix. Designers and regulating codes in most countries consider that gravity sections should have little or no tension at the upstream face in the normal reservoir or normal operating condition. Minor tension is occasionally allowed for severe flood conditions. However, to accommodate the need for small but sustained tensile stresses on the order of a few percent of the compressive strength for the normal operating condition, it is reasonable to provide high cementitious content mixes or bedding mixes across lift joints near the upstream face. Allowing for softening of the foundation with a lower tensile mass modulus at the heel of the dam also will reduce this tensile stress.
It is reasonable to allow minor tensile stress for flood conditions at isolated areas of good inspection or special construction treatment, when necessary. For seismic conditions, tensile stress is usually unavoidable and allowed at up to 150 percent of the expected static tensile strength to account for the increased tensile strength under very fast loading. This increase is referred to as the dynamic increase factor (DIF). With this allowed stress, a factor of safety just greater than 1.0 is typically accepted — by essentially all international authorities and codes for concrete gravity dams — under earthquake conditions, with higher factors of safety for flood and normal load conditions.
Another feature that needs to be considered when designing RCC dams is galleries. Galleries should be minimized in RCC dams. The tendency to extend galleries beyond where they are needed raises costs, slows production, and results in lower overall RCC quality. In large open areas, such as at the base of a high dam section, galleries slow production by about 15 percent for the uncemented fill method of construction. Conventional forming slows production even more. In the upper portions of the dam, the decrease in production at the area of a gallery can be 50 percent or more, and the quality of placement may decline significantly. Where a gallery is needed high in a dam for uplift, an open graded rock drain of coarse aggregate should be considered. If placed about four lifts high, the drain can be excavated for access in the future if necessary.
The vertical distance between galleries is usually determined by the accuracy that equipment can drill from the floor of the top gallery to intersect the roof of the gallery below. This typically is about 100 feet for a gallery 6 feet wide, using rotary percussion drilling equipment.
A gallery high in a dam can be a point of weakness in a seismic event. Designers of low- and medium-height dams should consider overbuilding the dam enough so that galleries and drains are not even necessary. The unit cost of the RCC decreases, while construction, operation, and maintenance are simplified.
Using a bedding mix between lifts or a high cementitious content RCC is suggested upstream of galleries and between the first three layers in the area above and below the gallery floor and ceiling. This reduces seepage to provide watertightness, bond against uplift below the floor, and added sliding resistance against reservoir pressure at the upstream gallery wall.
A grout curtain can be installed prior to the RCC or can be installed afterward from a gallery. The gallery should be large enough to accommodate suitable production equipment, especially at interior corners and intersections.
Internal drains can be easily drilled using track-mounted rotary percussion equipment. Nominal 3-inch holes at spacings of 10 to 15 feet are adequate. These holes can be drilled with an accuracy of plus or minus 3 feet in about 120 feet. A very efficient way to drill these holes is immediately after placing the RCC lift that is the gallery floor. When a long gallery with holes starting at the same elevation is called for, it is effective to stop RCC placement for a day while several track drills drive onto the lift and drill the holes. The area is then cleaned and treated as a cold joint, and RCC placement resumes.
To increase sliding stability of the section and offset some uplift pressures, use of a “fillet” in the upper part of the dam at the downstream face provides needed additional weight. On a high dam, it moves the resultant force of the entire dam section slightly upstream. On a low dam, it shifts the force downstream. The distribution of stress under the dam, amount or existence of tensile stress, and maximum compressive stress are slightly affected. The fillet also reduces the height of the section at the top of the dam.
A downstream toe extension can provide additional stability for a high dam where sliding stresses increase significantly with a minor addition in height. It adds both weight and total cohesion, but only in the bottom portion of the dam, where it usually is needed. The fillet increases the mass across the full length of the dam, including the upper portions of the foundation where it usually is not needed.
The fillet also adds to foundation bearing and RCC compressive stresses, whereas the toe extension reduces the bearing and maximum RCC stresses. The extended toe requires extra excavation and foundation preparation, but only in the deepest section of the dam and not for much of its length. It is possible that an extended toe in a high dam will result in tension across downstream lift-joint areas at the maximum height for an empty reservoir condition. This can be overcome by an early partial reservoir filling.
A “key” is an effective way of providing additional sliding stability when it is needed in the foundation but not in the RCC. Although adding the key near the upstream face may seem like a good idea because of its potential to act as a cutoff, the downstream location typically is better. If analyzed for local stresses, an upstream key of a high dam may have tensile forces that could negate sliding friction resistance because there will be little or no vertical stress at the key and a full reservoir. A downstream location has the benefit of maximum vertical confining stresses and the resulting friction. To minimize the width of the key (upstream-downstream) and assure the required load is transferred to the foundation without slippage across a weak RCC lift surface, bedding mix should be placed between RCC layers in the key, or a high paste content mix should be considered.
The key provides added foundation stability by extending the foundation failure plane and by the related horizontal component of the downstream foundation-bearing capacity. A relatively simple consolidation grouting program in the area downstream of the key may significantly improve stability. A key is usually needed only in the deeper portion of a high dam (if it is on medium- to poor-quality rock), at isolated locations where the foundation condition is bad, or for a medium-height dam on an unsuitable foundation.
When the bearing and sliding strengths of a foundation are poor, a conventional concrete dam usually is not economical. RCC can be a viable option. Using a low-strength and low-cost RCC with a parabolically curved downstream face is one approach.
A preliminary design with this concept was prepared for a tailings dam in Mexico on a foundation of clay and weathered rock. The dam was composed of large monoliths that could undergo significant independent movements caused by time-dependent consolidation of the foundations. Each monolith sat on its own excavated foundation, with steps in the foundation matching the location of monolith joints. The abutments were tied in with embankments that would undergo deformation as required. The foundation was so poor that a massive key was needed to provide sliding resistance and lower the bearing pressure. Because foundation restraint is minimal for this type of foundation and cement contents are low owing to the low strength requirements, thermal stresses are minimized.
However, the thickness of the key (distance from the downstream surface to the foundation under the key) should be analyzed as a cantilevered beam to assure that it will not break from the rest of the dam. If bearing pressures under the key can accept the added weight, a fill can be placed over a portion of the toe to offset some of the cantilever forces. The fill also provides extra sliding stability if it is extended downstream beyond the RCC key.
Regardless of which option is used to widen a dam base, a reduction in bearing pressure and maximum stress occurs in the RCC. Reduced strength requirements allow less cement, cost, and thermal stress. Stresses at the lower levels are closer to stresses higher in the dam, so fewer “zones” requiring different quality RCC at different heights are needed.
Although structural requirements for strength reduce to zero at the top of a dam, some minimum strength is needed for erosion and weathering protection, impermeability, and making the mix cohesive enough to be placed and compacted. The minimum RCC strength should be based on factors such as exposure conditions, function of the dam, risk level, and economics. There may be some disagreement, but minimum strengths at one-year values of about 1,000 pounds per square inch (psi) are usually considered acceptable for the mass.
Early RCC dams used higher-strength mixes for the upstream and downstream regions and lower-strength RCC at the interior. This proved to be a more serious construction and inspection problem than anticipated. The practice is now generally avoided. In addition, other factors have influenced this trend, including the good field performance of low-strength RCC exposed at the downstream face under severe weather. If needed, RCC can be protected at the upstream face by constant immersion in the reservoir, an unbonded impervious membrane (with or without protective pre-cast facing panels), and conventional concrete placed using one of many possible techniques. The downstream surface can also be protected by using conventional concrete or grout-enriched RCC at the exposed face.
It is usually best to use one mix throughout an entire section for dams up to about 120 feet in height. Because of thermal and economic considerations, higher dams are usually separated into horizontal zones, with higher-strength mixes used in the lower part of the structure. Generally, these zones are 30 to 60 feet thick, with increases in strength of 100 to 500 psi per zone. For very large dams, and large dams with high earthquake loading, it usually is best to use a combination of horizontal and vertical zoning. The interior mass — for example, 25 to 75 percent of the total volume — typically will have the lowest strength requirements and the leanest mix. Higher strength typically is necessary at the base and in high-stress areas of the upstream and downstream faces. Earthquake loading also can result in high stresses at the toe and heel of the dam, as well as localized regions at about the upper third of the dam near the downstream face.
In addition to the higher compressive strengths needed for higher principal stresses in the lower portion of high dams, stronger mixes in the lower zones also provide additional lift-joint tensile strength, added cohesion, and usually a slight increase in friction. When a mix in a lower zone has adequate strength for compression but not for sliding stability, there are several options. These include increasing the mass or weight of the dam and widening the base. Increasing the paste content of the mixes is another option if it is economical and does not cause thermal cracking due to added heat from hydration and/ or a higher elastic modulus. Another option uses bedding between RCC layers. This dramatically increases cohesion and moderately increases friction. This technique is especially useful when “cold joints” occur in low-paste mixes.
Dealing with conveyor supports
Conveyor supports are a common embedded structure for an RCC dam. Many RCC dams are constructed using a conveyor system, with the conveyor being supported on posts within the dam. Before there is sufficient RCC to support the posts, they require a substantial footing. Without proper design considerations, these footings represent fixed rigid blocks protruding into the dam. For example, the vertical faces of these blocks could initiate cracking. One way to deal with the restraint from footings is to place them at the center of monoliths, or so that one face of the footing is flush with a monolith joint. Ideally, footings near the middle of a monolith should be circular, without corners. If the footings have corners, they should be rounded or chamfered. If the footings are located at monolith joints, square footings can be used.
Previous HRW articles contain more details on conveyor supports.1,2
Dr. Schrader may be reached at Schrader Consulting, 1474 Blue Creek Road, Walla Walla, WA 99362 USA; (1) 509-529-1210; E-mail: [email protected]
- Schrader, Ernest K., “Building Roller-Compacted-Concrete Dams on Unique Foundations,” HRW, Volume 14, No. 1, March 2006, pages 28-33.
- Schrader, Ernest K., “Roller-Compacted-Concrete Dams on Difficult Foundations: Practical Examples,” HRW, Volume 14, No. 2, May 2006, pages 20-31.
- Schrader, Ernest K., “Design and Facing Options for RCC on Various Foundations,” International Water Power and Dam Construction, Volume 45, No. 2, February 1993, pages 33-38.
- Nawy, E.G., Concrete Construction Engineering Handbook, Chapter 20, CRC Press, Boca Raton, N.Y., 2007.
Ernie Schrader, PhD, PE, is a consultant with more than 30 years of experience in roller-compacted concrete (RCC). He has been involved in more more than 30 RCC dams that are complete and operational, several under construction, and several undergoing design and feasibility studies. The projects range from the world’s highest to smallest RCC dams.