DESIGN GUIDELINES
Chart 3: Flow Chart for Forebay Design
3.2 Structural Design .1 Structural Design - General
3.2.1.6 System Analysis Stability
Architectural Form Liners:
Where the finished concrete surface will be textured, the designer may consider using form liners.
Rigid plastic liners are attached to the formwork prior to placing the concrete. Many of the same considerations as for metal forms should be used during design.
Leave-In-Place Forms at Construction Joints:
In complicated structures, there are sometimes benefits to staging concrete placement for blockouts, first and second stage concrete, pedestals, and where a vertical formed construction joint is used. In lieu of using a smooth form, brush finish or "green cutting" a leave-in-place form can be considered.
The material resembles light gauge expanded galvanized metal which s easily molded and cut with sheet metal shears. When concrete is placed against the leave-in-place form, cement paste and sand extrudes through the form to create a rough texture with approximately 1/4 –inch to 3/8- inch profile.
The leave-in-place form can remain in place or be removed.
Material Testing Requirements
All material and fabrication should be of the highest quality. Any damaged material should be rejected, removed, and replaced; the material and fabrication should meet the governing acceptance standards.
3.2.1.6 System Analysis
CL + (W-U) tan(φ) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ (3.96) ΣH
Where
C = cohesion
L = base length in compression W = sum of vertical-down forces U = sum of vertical-up forces tan(φ) = coefficient of internal friction
ΣH = sum of horizontal driving forces
Floatation analysis:
The floatation factor of safety is the ratio of the vertical-down forces divided by the vertical-up forces on the structure. The down-force may be comprised of the structures' dead weight, backfill material, and water above and contained within the structure. The up force may consist of uplift pressures on the structure and the vertical seismic component. Structures can be susceptible to floatation problems if little mass concrete is used for the structure.
Factor of Safety for Floatation = W/U Where:
W = sum of the vertical-down forces U = sum of the vertical-up forces Finite Element Analysis:
Numerical modeling techniques have been improving at a rapid pace. As the personal computer gets more powerful, the use of numerical modeling for hydraulic structure analysis will become more common. Finite element models can produce extremely accurate results, and are the preferred method to analyze dynamic response of structures in earthquakes in Seismic Zones; however, there are some drawbacks with this method. Limitations of finite element modeling include the expense of programs, difficulty in checking the results, and the requirement of the knowledge of the limitations of the modeling techniques.
Factors of Safety
In most cases, the FERC guidelines will govern; however, if the structure is under the jurisdiction of the US Army Crops of Engineers or the U.S. Bureau of Reclamation, there requirements may govern.
The FERC classifies its dams into one of three categories based upon exposure to loss of life and economic loss: Low, Significant and High. The definitions may be found in the FERC's Engineering Guidelines (1991).
FERC Recommended Safety Factors for Moment Equilibrium
Loading Conditions Factor of Safety Overturning Factor of Safety Sliding High Hazard Low Hazard High Hazard Low Hazard
Usual 3.0 2.0 3.0 2.0
Unusual 2.0 1.5 2.0 1.25
Extreme >1.0 >1.0 >1.0 >1.0
These factors of safety are applicable for determining the allowable unit stress of concrete and foundation material by dividing the ultimate stress for the material by the appropriate factor of safety (listed above) and determination of the sliding factor of safety for cases without extension state-of-art foundation exploration.
U.S. Army Corps of Engineer Criteria:
The U.S. Army Corps of engineer used several criteria of evaluation of hydroelectric projects. Existing concrete structures are covered under Engineering Manual 110-2-256 (USCOE, 1981). This divides the structure being investigated and the surrounding soil mass into a series of blocks, and analyzes it in a manner similar to the slope stability program. The limit equilibrium analysis has been incorporated into a computer analysis package (CSLIDE) Available from the U.S. Army Corps of Engineers. The
FERC Engineering Guidelines (FERC, 1991) reference ETL 1110-2-256 in its description of Internal Hydrostatic Load (uplift), and in its evaluation of sliding stability. The advantage with computer analysis is that the program searches for multiple planes of weakness within the structure and is, therefore, not limited to an assumed geometry of failure.
The factors of safety for major concrete structures in ETL 1110-2-256 are zero for normal static loading conditions (usual case and 1.3 for seismic loading conditions extreme case). The FERC Engineering Guidelines (FERC, 1991) stipulate that the factors of safety for sliding in the ETL 1110-2- 256 (USCOE, 1981) are based on the premise that extensive foundation exploration and testing has been conducted using sophisticate, state-of-the-art techniques. In order to use the factors of safety in ETL 1110-2-256 (USCOE, 1981), FERC geotechnical engineer should be consulted concerning the adequacy of he foundation exploration and testing program. In general, the factors of safety will govern. Flood walls and retaining walls are covered in EM 1110-2-2501 (USCOE, 1948), and EM 1110-2-2502 (USCOE, 196).
U.S. Bureau of Reclamation Criteria Loading Condition Factors of Safety
Major Structures Minor Structures
During Construction
Structure Completed and Equipment Operating
During Construction
Structure Completed and Equipment Operating
Normal 2.5 3.5 1.5 2.0
Extreme 1.1 2.0 1.1 1.5
Base Stress
The stresses in the base material will be determined as a result of the stability analysis. The allowable stress for concrete foundations is based on the mix design and concrete testing results.
Foundation
Rock Foundations:
a) Foundation Stresses: The allowable stress capacity of the foundation material is optimally determined by laboratory testing. References are available that give general indications as to the allowable stresses in various types of foundation materials.
b) Zones of Weakness and Stability. It requires that rock foundations be analyzed for stability if there is a potential for direct shear failure, or whenever sliding is possible along joints, faults, or shears. Sliding failure may result when the rock foundations contain either discontinuities or horizontal seams near the surface. Anchoring the rock mass or additional rock excavation may be required.
c) Zones of Weakness and Foundation stresses: In addition to stability, potential for overstressing the structure foundation exists if there are zones of weakness which allow for differential displacement of rock blocks on either side of the weak zones, or when the weak zone represents an excessive span for the bridge. Such zones should be strengthened during construction.
Soil Foundation:
a) Seepage. In addition to bearing capacity of a soil foundation, structures founded on soil need to be checked for the seepage potential. The structure requires that the foundation hydraulic gradient be checked for seepage potential using the weighted creep method. If the seepage potential exceeds the allowable level, then a flow net analysis can be used to determine the exit gradient at the toe. A safety factor greater than 2.0 against piping at critical points in the foundation is acceptable.
b) Pile Foundations. These structures are examined on a case-by-case basis. For conveyance structures, it is common to have a large component of load in the lateral direction. Items to be considered include the combined stresses in the pile from lateral and vertical loads, the stresses in the soil, and deflections. Conservative methods are available to make reasonable
estimates of pile lateral load capacities and deflections. Given the complexity of the situation, it is often advisable to use battered piles to account for horizontal loads. Computer programs are available for analyzing more complex pile group configurations. More accurate analyses are available for pile group systems that make use of computers.
Seepage Control Methods:
Various methods are available to control seepage beneath the structure. The, application depends on the type of foundation material and the anticipated effectiveness of the seepage control. For example, isolated injection grouting may be effective in fractured rock. The following seepage control methods are generally practiced:
• Cutoff apron
• Sheet pile, concrete, or other upstream or downstream cutoffs
• Downstream apron, with scour cutoff
• Grout curtains
• Drainage system for uplift relief
3.2.1.7 Component Analyses