DESIGN GUIDELINES
Chart 3: Flow Chart for Forebay Design
3.2 Structural Design .1 Structural Design - General
3.2.1.4 Design Loads Dead Loads
Dead loads mainly consist of structural weight, superimposed backfill, and weight of permanent equipment such as gates. Often preliminary design begins before actual structural systems are determined and material weights are known. In the absence of actual test data, the unit weight of structural concrete can be taken as 2.4 tons per cubic meter. Final design should consider actual material weights determined by laboratory testing, if available.
Geotechnical Loads Soil Loads:
Soil and silt loads that are described in design criteria should represent conditions that will be anticipated for the effective life of the project. Sedimentation studies can establish desanding basin and forebay silt or soil levels.
Soil loads can be modeled as active, passive, or at rest pressures sometimes presented as equivalent fluid pressure analyzed as a fluid load. The type of soil pressure used will depend on the application.
Often the decision will be whether to use active soil pressure or at rest soil pressure for structural or
stability calculations. In general, active soil pressure should be used against surfaces that will flex under soil loads, and at rest pressure against rigid surfaces. The amount of displacement/rotation laterally away from soil being required to produce active earth pressure conditions is quite small, and a flexible structure is always very likely to deform sufficiently for the active pressure to be developed.
A very rigid structure might shear along the base without active pressure being allowed to develop.
Passive pressure is mobilized by movement of the structure into a soil or rock mass. A typical application of passive soil pressure is in the design of anchor plates or blocks embedded in soil with a tension rod or cable oriented so that cables pulls a block against the soil. Engineering judgment is required to determine what is the most appropriate pressure for the application.
Saturated Soil Loads:
Saturated soil loading above the prelatic surface, the weight of water is subtracted from the saturated weight of the soil, and the soil loading coefficient is applied to the buoyant weight alone. The water load is applied without reduction factors.
Surcharge Loads and sloped Surfaced Embankments:
Renkine's theory addresses surcharged or sloped embankment loadings on structures.
Silt Loads:
Silt loads can be modeled either as a saturated soil or as an equivalent fluid. The US Bureau of Reclamation (USBR) gives a commonly used equivalent fluid load in Design of small Dams (USBR, 1974) to be used in the absence of test data.
Rock Loads:
Rock loads are not generally considered as a design load on the structure itself. In most cases, the potentially loose rock mass is stabilized with a separate, self contained system. Conditions that could require that protective measures be taken include highly weathered rock formations, fractures within the rockmass that could result in a bock of rock becoming detached from the parent block, or undercut rock formations.
Potential solutions for unstable rock formations are rock bolts, reinforced shotcrete, wire mesh cover over the rock face, pre-stressed anchors, and pressure grouted soil anchors. The type of stabilization will depend on the permanence of the solution and the size and type of rock mass to be stabilized.
References for these techniques include: Post Tensioning Manual, 1990; Foundation Treatment (Swiger, 1988); and rock Reinforcement Engineering Manual.
Typically the designer presents the existing topography and the excavation drawing in the contract documents. Rock stabilization required for safety during construction is usually the responsibility of the contractor. Because the designer has access to and knowledge of the site, the contractor may expect or otherwise ask for guidance from the geotechnical or structural engineer on methods to support a temporary cut slope.
Seismic Considerations:
At sites where seismic activity is anticipated, there is potential for soil structure interaction and liquefaction.
Hydrostatic and Hydrodynamic Loads Hydrostatic Loads:
Hydrostatic loading consists of the horizontal loads from the outer surface and inner surface water pressure. The water pressure is taken to be the weight of the water times its height.
a) Outer surface load: The outer surface hydrostatic pressure is the weight of the water times its height. This results in a triangular distribution of load on a vertical face. The hydrostatic pressure should continue below the soil level to the base of the structure.
b) Inner surface load. Water flowing inside the conveyance system will have the same force as it is in the outer surface walls depending on the height of the water inside the conveyance system.
c) Uplift, Rock foundations. Normal uplift on rock foundations without cutoff or drainage is often assumed to be the linear gradient from full headwater at the upstream end to full tail-water (if appropriate) at the downstream end.
d) Uplift, Soil Foundations. The uplift characteristic of a structure on a silt foundation can be found by performing a seepage analysis of the structure for each of the different loading conditions. Uplift will be equal water pressure determined by the results of this analysis. Two acceptable methods for determining the magnitude of uplift pressure are : the Creep Theory and the Flow Net Method. The creep method calculates the distance that a molecule of water would have to travel as it flows beneath a structure. The flow net method divides the flow under the structure into a number of channels of equal flow and into equal potential lines. The flow and potential are used to calculate the uplift pressures.
e) Other Water Loads to be Considered: The weight of water within or supported by the structure is included in the analysis as appropriate. For example, the normal case structural weight calculated for the stability calculation should include the weight of water within the structure.
• Weight of water in structure will depend on the load case; water may be treated as dead load for certain cases of stability analysis.
• Weight of water on lip or sill Hydrodynamic Loads:
The hydrodynamic loads are caused by accelerating water, wave, current forces and hydraulic transients, or water hammer effects.
Seismic Loads Structural Mass:
The seismic case considers the mass of the structure to be analyzed accelerated at the level of the design earthquake. Other related design components are base shear, and the base-structure amplification fundamental period.
Other Seismic Considerations:
a) Liquefaction. Liquefaction is the condition where the seismically induced pore pressure exceeds the strength of the soil, causing the soil to behave as a liquid. Material that is susceptible to liquefaction is unsuitable as foundation material, backfill, or embankment dam material. However, if the designer is faced with this condition, the foundation treatment should be referred to a qualified geotechnical engineer for evaluation. In general, material subject to liquefaction are loose, uniformly graded sands and silts.
b) Soil Structure Interaction. Structures of soil foundations have to consider the effects of soil structure interaction.
c) Dynamic Silt: another seismic soil consideration is dynamic silt or dynamic soil loading.
The dynamic silt may be treated as an equivalent fluid and loading, determined in the same manner as hydrodynamic loads.
d) Vertical Acceleration. Earthquakes generate forces traveling in all directions. The designer must develop an appropriate vertical inertial force based on the site, the structure, and local requirements.
Water Retention Structures:
Water conveyance system which is the part of the hydroelectric project components are generally designed with the same seismic criteria as the dam itself.
Seismic loadings should be selected after considerations of accelerations which may be expected at each project site as determined by the geology of the site, proximity to major faults, and seismic history of the region as indicated by seismic records. The seismic parameters for the design of Modi Khola HEP is considered to be 0.1 x g in the vertical direction.
Ice Loads
The exposed conveyance system in high mountains where ice is formed in canal, desanding basin and forebay, designer should consider ice loads to the respective structure. Ice pressure is created by thermal expansion of the ice and by wind or current drag. Pressures caused by thermal expansion are dependent on the temperature rise of the ice, the thickness of the ice sheet, the coefficient of expansion, elastic modulus, and strength of the ice. Wind drag is dependent on the size and shape of the exposed area, roughness of the surface, and the direction and velocity of the wind. Ice loads are usually transitory. A second aspect of ice loading is its contribution to vertical live loads. If exposed decks or platforms ice-up, in addition to snow, the weight of an assumed amount of ice should be included in the design.
Wind and Snow Loads
Areas which experience extreme loading should use the locally accepted code loadings.
Equipment Loads
Items to consider for equipment loads are:
• Gate loads – partially open gates, fully closed gates, vibration, down-pull, opening and closing forces, and hoists.
• Trashrack loads
• Clogged trashracks and resulting differential head
• Crane loads – impact, crane weight plus load
3.2.1.5 Construction Materials