The usbr manual on small dams

Liberal reference is made to design standards that have been developed as guides for Bureau engineers. Design and construction procedures for rockfill dams have changed over the last two or three decades and continue to do so. The chapter gives a good general background for the design of rockfill dams; however, the designer should also refer to the literature on the subject.

This chapter has also been revised to address concerns for concrete dams of any height. Sections on material properties and foundation considerations have also been added. More complete discussions are now included for forces acting on the dam, requirements for stability, and stress and stability analyses.

Discussions addressing the analysis of cracked dams have been clarified and expanded to include analysis during an earthquake. Also, a general iterative approach for cracked dam analysis, applicable for static and dynamic conditions, is now included. Join Co-production practitioners network.

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An RCC gravity section was designed to the basic same dimensions of the old timber crib struc- ture, i. Rebuilt Bear Creek showing timber planks being installed along downstream vertical face of new Figures 1. Army Corps of Engrs. Prices have not been adjusted to present day costs. Ease of access to mates. It is important during the development of the the site is important to the foundation and geologic inves- preliminary investigation program to keep in mind the tigation program.

Not only will access impact both the degree in which these issues need to be defined, and cost and scheduling of the foundation investigation pro- plan the investigation program accordingly. The hazard classification will establish the These circumstances might influence the engineer to framework for hydrologic studies, and the results of the increase use of geophysical methods in place of conven- hydrologic studies will establish the type and size of the tional drilling.

The size of the spillway can weigh heavily crossing and require access from each abutment. A small capac- Topographic features affecting project layout and ity spillway can be economically combined with either a design include steepness of abutments and valley shape. A large spillway These topographic features do not preclude the construc- can more easily be incorporated into a gravity dam at less tion of an RCC dam, but rather influence the method of cost than an embankment dam and is commonly a key RCC delivery from the production plant to the placement factor supporting the selection of an RCC dam.

Typically, surface and layout of project features. This layout requires favorable topography at sion works during construction and location of permanent one of the abutments and adds significantly to cost. The outlet facilities. A broad flood plain adjacent to the stream elements of a hydrologic study are outside the scope and course allows for placement of a diversion channel and the mission of this document.

The remainder of this chapter outlet works with an invert elevation similar to the stream will focus on site and foundation exploration. A narrow flood plain may require significant excavation of the abutment to set the 2. The excavation may include both earth and rock.

For an RCC dam, it may be Site exploration starts with the acquisition of available advantageous to locate the conduit or conduits in a trench information such as topographic, soil and geologic map- cut into the foundation rock or up against one of the rock ping, property boundary surveys, and previous abutments.

In this way, RCC can be placed efficiently engineering reports. Following review of the available without interference. Identification of groundwater This type of preliminary information can strengthen corre- depths is also very important. If the dam is to bear on a non-rock foundation, At this point in the investigation process, the pre- extreme care must be taken in assessing the foundation ferred dam type is generally unknown; however, some conditions.

Important information regarding a non-rock effort should be made to identify the location, type, and foundation includes soil type, origin, density, stratigra- approximate quantity of available earth materials and phy, uniformity, permeability, strength, and consolidation concrete ingredients that are located close to the site.

Once the dam type has been established, a more detailed Often times, the foundation exploration program is borrow source evaluation is warranted. The initial phase is used to charac- Depending on the level of information required for terize the depth and type of foundation materials the site investigation, review of available mapping geol- characteristic of the dam site, and to identify the location ogy, soils, and topography and a site visit may be and quantity of borrow if natural aggregates are avail- sufficient for preliminary purposes.

For many prelimi- able or identify potential quarry sites. The scope of the nary investigations and screening level site assessments, initial phase can vary, but might typically include borings this level of investigation may be sufficient.

However, spaced at between and ft 30 and 90m along the foundation explorations will be prudent or necessary if proposed axis. Standard penetration testing and sam- subsurface conditions are likely to be either difficult to pling of the overburden soils should be performed. Bedrock should be cored to depths up to the proposed hydraulic head of the dam. Where rock is known to be of adequate quality, boring techniques capable of sample 2. The ideal foundation for an RCC dam is competent Potential aggregate borrow areas can be investigated bedrock at shallow depth, but small RCC dams have been with both borings and geophysics, followed by confor- located directly on non-rock foundations.

A competent mation test pits. If both RCC and earth dam options are foundation can be defined as one that is capable of sup- under consideration, additional borrow area investiga- porting the structural loads imparted by the dam, without tion for earth fill materials should be included. However, excessive deformation, sufficiently impermeable to pre- for small volume RCC dams, concrete aggregates usually clude significant seepage under the dam, and resistant to come from an established nearby quarry.

Preliminary dissolution and erosion caused by seepage. The degree of investigation findings, in conjunction with hydrologic competence required for a foundation is also a function of and topographic information, are usually sufficient for a the size of the dam. Adequate load transfer and structural decision regarding the dam type. However, even for small type, and preliminary siting and project layouts prepared, gravity dams, most foundations merit some treatment to a second phase investigation program can be developed enhance performance.

The second stage program will focus The intent of a foundation exploration program is on characterizing both the physical and mechanical prop- to assess the character of the foundation materials and erties of the foundation materials both laterally and with define the type and extent of modifications required to depth, as well as identifying and quantifying the type and improve the foundation to effectively provide the above extent of improvements that may be incorporated in the noted conditions.

Foundation investigation techniques design. For an RCC gravity dam founded on bedrock, the for RCC dams do not materially differ from other dam foundation investigation would include additional core types. Programs focused on concrete gravity dams, borings into the bedrock at about a 50 to ft 15 to 60m however, tend to focus more on rock characteristics spacing and in-situ rock pressure testing.

Rock cores because this is the common bearing strata for a gravity would be tested in the laboratory for physical and dam. Investigation tools commonly include soil bor- mechanical properties. Geophysical surveys and test pits ings, rock corings, test pits, and associated field testing.

Information of importance regarding the overburden For non-rock foundations, the boring spacing soils includes gradation, statigraphy, depth, origin, and would be the same, between 50 and ft m. Soil mechanical properties. Even if the dam is to bear on a sampling should be continuous and sufficient quanti- rock foundation, overburden soils information is ties of samples obtained for laboratory testing.

Also important to considerations relating to excavation, undisturbed samples should be recovered for testing, diversion, dewatering, backfilling and disposal. Important information regarding bedrock includes rock Additional quantities of material can be obtained with type, origin, stratigraphy, variability, jointing, and test pits. Testing of proposed aggregate sources would usually include gradation, organic content, plasticity fines frac- tion , specific gravity, absorption, resistance to abrasion, weathering, and chemical attack, as well as alkali-aggre- gate reaction potential.

As previously noted, dams requiring relatively small volumes of RCC usually less than 5, to 8, cu yd [3, to 6, m3] are usually built using aggregates obtained from off-site commercial quarries. Most com- mercial quarries can provide gradations, specific gravity and absorption values as well as the results of durability testing. Further, information on site investigations can be found in references 1, 2, 3 and 4.

Curtain grouting is should investigate the site and determine what, if any, used to reduce horizontal permeability, and therefore, foundation improvements or treatments are required to seepage potential. Consolidation grouting is used to produce a properly functioning structure.

The layout and depth of equal to the height of the planned dam are considered grout holes, grout proportions, and grouting proce- to be of the greatest importance. The foundation condi- dures are a function of the type of bedrock and tions in this area have the greatest effect on the ability character of the jointing and fracturing. For additional of the foundation to withstand short-term or long-term information on grouting, see references 2, 3, 4 ,5 and 6.

The bedrock surface can be improved by the appli- Foundation investigation and identification of cation of dental concrete. Dental concrete is used to fill appropriate treatments are just as important as the voids, cracks, fissures, overhangs, and other discontinu- design of the gravity dam section itself. Nearly all failures related to concrete dams have occurred through the foundation material rather than through the concrete in the dam.

However, design engineers need to carefully study several factors before placing a Sound rock foundations are considered most suitable concrete dam on a low modulus rock or non-rock foun- for an RCC dam of any height. Rock foundations are dation material. The principal considerations are preferred because these materials possess high bearing differential settlement of the structure, seepage, uplift capacity, low settlement potential, and have a high pressure distribution, piping potential, and hydraulic ero- degree of both erosion and seepage resistance.

The most desired properties obtained from a foun- There are numerous solutions for non-rock founda- dation rock investigation for a low RCC dam are tions requiring improvement for seepage.

The main 1 compressive strength, 2 shear strength, and 3 per- potential problem can be caused by relatively high meability. In non-rock foundation materials to withstand this seep- addition to these critical material properties, the degree age gradient. Most of these solutions focus on and orientation of jointing and fracturing is extremely lengthening the seepage path, causing a reduction in important to the designer.

Special attention should be the seepage gradient. Design features such as upstream given to identifying potential sliding planes in the and downstream aprons, upstream earth berms usual- foundation rock, especially those that "daylight" down- ly using fine grained material , cutoff walls, grout stream of the dam.

In this 3. The project initially consisted of a 16 ft 4. In , a ft 70m high masonry dam was constructed downstream, which caused the crib dam to be inundated at times. The owner and its consultants determined that the old deteriorated crib dam would have to be replaced with a more durable structure, which could withstand frequent overtopping and complete submergence.

A small RCC dam was determined to be the best design solution to meet operational, cost, and construction schedule requirements. The Cedar Falls Dam is located in a valley where glacial deposits are up to ft m deep. It would not be practical to try to found the concrete dam on Figure 3. An earth dam could not be safely inundated and thus was not considered A 20 ft 6. A filter and drain system consist- rated a number of defensive measures to produce a safe, ing of a uniformly graded gravel with a geotextile fabric well-performing dam.

A downstream slope of 0. A typical design cross-section for this small RCC dam is shown in Figure 3. Figure 3. While higher than the 50 ft 15m limit for small RCC dams, it is a sion of the soil foundation due to rain runoff, or if this good example of an RCC dam founded on a variable non-overflow section ever overtopped.

The geology at the site was such the prepared soil foundation for the monolith section. The that along ft m of the left abutment, conditions top of the drainage trench is visible along the down- varied from soil, with STP blow counts of 7 to 20 to hard stream portion of the foundation.

This portion of the dam was divided into three sections, called monoliths. One section was founded on hard rock, the middle section on over 30 ft 9m of soils, and the other on partially weathered rock.

Each section was analyzed as a stand-alone gravity section. The only connection between each monolith was an mil thick geomembrane joint material that was heat welded to the membrane attached to the rear side of the precast concrete upstream facing panels.

For the sections supported on soil and partially weathered rock, a detail involving an upstream concrete cutoff wall, a concrete starter footer, the geomembrane, and a drainage trench accommodated differential settle- ment while still maintaining seepage control in this area.

Post construction surveys indicated that the 46 ft 14m high monolith founded on soil settled about 2 in. The design called for the RCC test section to be constructed along the downstream toe of the monolith founded on soil. The tailwater loads at the overflow spillway Considerations relating to stability of small dams section are a function of the flow conditions over the spill- are most commonly assessed using a two-dimensional way and the depth of backwater.

When the depth of analysis. When there is signifi- cussed further and incorporated in the example cant discharge over the spillway, the impingement of problem. The remaining forces are discussed in detail in high velocity spillway flows on the tailwater can produce references 1, 2, 4, 7, 8, and Figure 4.

To accommodate applied loads, and approximately where they act. Several references including refer- 4. The dead load is the weight of the structure plus any Uplift Section 4. For computational purposes, the dead load is being analyzed.

The unit weight of RCC is largely dependent on the specific gravity of the 4. Loads on typical gravity section to occur along any horizontal or near horizontal plane 4.

The method of RCC Earth pressures will exist if sediment accumulates or construction results in near-horizontal lift joints posi- soil backfill is placed against the face of the concrete tioned usually every 12 in. For locations where the soil backfill is submerged, height of the structure. If there are no drains within the buoyant unit weight should be used to calculate earth structure that can reasonably be depended upon to pressures.

The at-rest earth pressure coefficient should reduce internal pressures, the uplift pressure is general- be used. Typical values are shown in Table 4. The depth of section to full tailwater at the toe downstream end of silt used in the design of a new dam should be devel- the dam. If there are drains and their effectiveness can oped based on anticipated sediment accumulation over be verified, a reduction in pressure at the drain location time, tempered with experience and judgment.

If appre- can be provided to reduce the local internal water pres- ciable, the sediment depth for an existing dam can be sure. The amount of reduction is a function of the size based on hydrographic surveys. A dynamic analysis is required to quake vibration period usually taken to be 0. The load is converted into two forces: an inertial force of the dam due to its own mass, affect- 4. The applied loads are converted to net RCC gravity section.

The inertial force due to the mass horizontal and net vertical forces. These forces are of the dam is assumed to act through the center of grav- opposed by a normal and tangential foundation reac- ity of the section being analyzed. The seismic coefficient tion.

The magnitude and location of the foundation is the ratio of the peak earthquake acceleration to the reaction must result in the summation of all forces and acceleration of gravity. The seismic coefficient is dimen- moments being equal to zero no unbalanced force or sionless and is considered to be the same value for the rotational tendency that could produce an instability. These coefficients, which vary geographically, are summarized in Figure 4. Seismically loaded gravity dam, non-overflow monolith Figure 4.

The most common structural cial equipment. This assumes the aggregate in the RCC shape for a gravity dam is a truncated right triangle is crushed. If rounded river gravel is used for the RCC with the base and vertical upstream face forming the coarse aggregates, the limit for building a slope with- right angle.

The crest is formed by truncating this trian- out forms is about 0. For low dams with two gular shape to accommodate the RCC construction sloping faces 1. For higher RCC dams the width at the crest is construction purposes. This, then, means that for all usually between 16 to 20 ft 4. This dimension is needed to produce the desired section. Formed down- needed to allow equipment to pass in two lanes. From stream slopes of either RCC or conventional concrete the downstream edge of the crest, the section is taken are typically stepped.

Stepped spillways are both vertically many times to intersect the downstream hydraulically efficient and aesthetically pleasing. In this manner, the volume of concrete is reduced. However, a formed sloping downstream face can be This crest detail is termed the "chimney" section as constructed using conventional concrete placed concur- shown in Table 5.

The design of a gravity dam is performed through A survey of designs actually used for small RCC dams an interactive process involving a preliminary layout of shows that most of these structures have been designed the structure followed by a stability and stress analysis.

Table 5. This process is repeated until downstream edge of the crest. A variation of the gravity an acceptable cross-section is attained. Here, the base Analysis of the stability and calculation of the stresses width is established from the upstream edge of the crest to are generally conducted at the dam base and at selected satisfy structural stability requirements. The point where planes lift joints within the structure. If weak seams or this slope intersects the foundation is then connected with planes exist in the foundation, they should also be analyzed.

This pro- duces a stable section with less volume than Table 5. However, the actual downstream slope thus produced is 5. Thus, the downstream face for this used in concrete dam designs: section requires forming. For very low dams, less than 15ft [4. Usual loading condition—normal operating section tends to be all RCC with both the upstream and a. Headwater elevation at top of spillway crest downstream faces sloped as shown in Figure 5. For b. Minimum tailwater these low dams the cost of additional RCC associated c.

Uplift with the two sloping faces is more than offset by the sav- d. Silt pressure, if applicable ings from eliminating forming and facing materials. Unusual loading condition—flood discharge middle half of the base. For an extreme load con- a. Headwater at flood level dition, the resultant must remain sufficiently wit b. Increased tailwater pressure in the base to assure that base pressures are within c. Uplift the prescribed limits see Table 5. Bureau of Reclamation. Reclamation manual 2 v.

Jan van 't Hoff, Art Nooy van der Kolff. Concrete manual concrete manual. New reclamation manual for bahrain - Benjamin Millington, November 4th, Bahrain s Ministry of Works is set to launch a new dredging and land reclamation manual to guide all future reclamation. Bureau of reclamation ftp Search Reclamation Search input Submit. Reclamation Home. Reclamation Offices. There is no denial that books are an essential part of life whether you use them for the educational or entertainment purposes.