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Structural and Architectural Design of Pumping Stations - 04...
Chapter - 04 : Structural Analysis and Design...
The foundation materials encountered may be a determining factor in the siting and layout of a pumping station. In some
areas, the measures required to provide a proper foundation for the structure may be prohibitive and may dictate relocation
of the plant site. Sufficient soil sampling and testing should be done prior to selecting a site so that the type and
extent of foundation work required can be estimated. Investigations, including sampling and testing, should be performed
in accordance with the provisions of EM 1110-1-1804.
( a ) Soil Foundations.
For structures founded on soil, a determination of soil type, shear strength, cohesion, internal friction angle, and unit
weights in dry, moist, and submerged (or saturated) conditions must be made for each material to be used in backfill or
embankment sections and for each material in the foundation. From these parameters, the allowable foundation bearing value
will be determined. Also from these parameters, structure and embankment settlement and slope stability for excavation and
embankments will be assessed. The results of the settlement analyses will be used by the structural engineer in designing
discharge piping connections and the low flow and discharge culverts. These designs should be coordinated between the
geotechnical and structural design elements. Therefore, contact between these design elements should be established early
and coordination maintained throughout the design process.
( b ) Rock Foundations.
Where small structures are to be founded on rock it will usually be unnecessary to make comprehensive rock tests. However,
early in the design process sufficient coring and testing must be accomplished to determine the load carrying capacity of
the foundation material, and to identify any faults, seams, or other potential problem areas. For large structures, a
comprehensive program of foundation exploration must be initiated early in the design process so that sufficient foundation
information will be available for use in the facility siting studies. This exploration program should progress from a
general investigation of various sites to an in-depth investigation of the finally approved site. Pumping station
substructures are generally formed of reinforced concrete having compressive strength of from 2,500 to 3,000 psi, and the
proportioning of the structure for allowable base pressure is controlled by the compressive strength of the soil foundation
material. However, for structures founded on rock, the compressive strength of the foundation material may be greater than
the bearing strength of the substructure concrete. In these instances, the structure base must be proportioned so that these
pressures do not exceed the strength of the concrete. Also, in some instances the foundation rock may be fractured or
contain seams which could shift or compress under loading, causing movement of the structures above. In these instances,
the structures may be founded on drilled caissons with the foundation grouted to preclude underseepage.
( c ) Pile Foundations.
If the foundation materials do not have sufficient bearing capacity to sustain the imposed structure loads, and if other
stabilizing methods are impracticable or unfeasible, foundation piles may be required. The piles may be of wood, concrete,
or steel, but the use of wood piles should be restricted to those locations where the pile cut-off elevation is below the
minimum ground water level. Design loading for piles and pile lengths required to sustain a given loading should be
verified by driving and loading test piles in accordance with the provisions of EM 1110-2-2906. For small plants requiring
foundation piles, the cost of pile load tests may be prohibitive. In these cases, conservative values may be assumed for
pile design and load tests may be omitted. Large horizontal loadings are sometimes imposed on pumping stations and
appurtenant structures. When these structures are founded on piles, they must be designed to withstand this horizontal
loading. Battering the piles is one effective technique for this purpose. Vertical piles can also be used if documented by
appropriate analysis. The method used in designing the pile foundations will generally be dictated by the size of the
structures and resulting size of the supporting pile group. For nominally loaded structures requiring small pile groups,
conventional pile design methods may be used. For large structures involving extreme horizontal loading, more detailed
analysis and design methods may be required, as discussed in EM 1110-2-2906.
( d ) Foundation Alternatives.
If investigations indicate that the foundation materials are incapable of sustaining the imposed loads without failure or
unacceptable amounts of settlement, a variety of alternative compensatory measures may be taken. Some of the possible
alternatives are :
(1) Provide footings outside the lines of the substructure walls.
(2) Excavate and replace unsuitable material to a sufficient area and depth to provide a stable foundation on good material.
(3) Employ in-situ foundation improvement methods such as dynamic compaction, vibro-replacement, in-situ densification,
and preloading and drainage using wick drains.
( e ) Ground Water Control.
Management of ground water during construction and under operating conditions is often a sizeable task. During construction,
the ground water level must be lowered enough to allow the work to proceed. This is a particular problem for pumping
stations because they are usually located in low lying areas to facilitate water intake. Under operating conditions, it may
be necessary to suppress the ground water level to keep uplift pressures within acceptable limits. Ground water control is
usually accomplished by relief wells from which water is pumped to lower the ground water level. Another problem related to
water handling is the seepage of water beneath the structure. Measures to lengthen the path of this underseepage and thus
reduce its effects on structure stability include the placement of a concrete cutoff wall or construction of a monolithic
structural key to some depth beneath the structure foundation elevation and near the face of the structure at which seepage
4-2. Primary Structural Components...
The primary structural components of a pumping station are the substructure, operating floor, superstructure, crane runways,
and discharge facilities.
( a ) Substructure.
The conventional pumping station substructure includes the sumps and water passages required to conduct water to the pump
intakes. The structural components comprising the substructure include the sump floors and base slabs for the water
passages, the outer walls of the structure, and the sump separator walls. The sump area components are generally analyzed
as a frame extending from the foundation to the operating floor. The forebay area is similarly designed assuming a frame
extending from the foundation to the top of the side walls or to the top of the exterior forebay deck. For both of these
analyses, care must be taken to assure that the assumed degree of fixity at the frame joints reflects as nearly as possible
the actual behavior of the structural components under critical design loading conditions. For some pumping stations, other
areas will require detailed structural analysis, such as the intake/trashrack deck, the discharge chamber if constructed
integrally with the pumping station, dewatering sump areas required in some installations, and retaining wall or flood wall
sections constructed monolithically with the pumping station.
( b ) Operating Floor.
The primary interior structural floor is the operating floor. The electrical and mechanical water handling and control
equipment is mounted on this floor, subjecting it to the dead weight of the pumping and control equipment, and the hydraulic
thrusts generated during the pumping operation. The design of the operating floor is complicated by the presence of the
necessary hatchways, pump openings, etc., which interrupt the continuity of the structural floor. The layout of this floor
for spans over sump walls, location of machinery, and location and size of openings, is a coordinated effort involving
hydraulic, electrical, mechanical, architectural, and structural requirements. The floor is usually designed as a system of
beam sections and slabs laid out about the various openings and spanning across the supporting walls below. The sump layout
determines the location of these supporting walls and the location of the pumps on the floor. This layout is also a
ordinated effort involving input from mechanical, hydraulic, and structural requirements to arrive at the optimum
arrangement for each plant. Once the general layout and loading configuration for the operating floor are determined, the
design of the structural elements can be undertaken. These elements may be designed assuming the floor to act independently
of the supporting wall sections below, or the operating floor, supporting walls and sump floor may be designed as a
continuous frame. The assumptions made will be dictated by the relative size of the components and the general configuration
of the plant structure, and must be consistent with the way the structure is expected to behave under the design conditions.
( c ) Superstructure.
Most pumping plant installations will be of the indoor type. This means that an enclosure is provided for the equipment and
personnel areas in the plant. This enclosure must be sufficiently tight to protect the equipment from the elements and
sufficiently durable to be economically maintained. It must also withstand the loading conditions given in paragraph 4-4.
Pumping station superstructures are commonly constructed of reinforced concrete, or concrete masonry unit and/or brick wall
sections. In structures of brick or concrete masonry, a separate framework is usually provided inside the outer enclosure
to support the bridge crane. It is often economical to incorporate this framework in the structural wall and/or roof section
to provide additional strength and support; however, with larger cranes, the operating forces may dictate that the crane
support framework be separated from the wall sections so these forces will not be transmitted to the superstructure walls.
( d ) Crane Runways.
As prescribed in EM 1110-2-3105, indoor type plants are usually equipped with bridge cranes for equipment removal and
handling unless other workable and economical means can be used. The runways for the bridge crane may be mounted on
structural steel or reinforced or prestressed concrete beam sections supported on structural steel framework, on reinforced
concrete column or haunch sections, or on ledges formed in reinforced concrete. Generally, only in larger installations
with reinforced concrete superstructures will the walls be large enough to support the crane loads.
( e ) Discharge Facilities.
The facilities incorporated into a pumping station for discharge across the protection line can be of various types and
configurations. A station located on the protection line will usually discharge directly, either by pumping into open water
or into a discharge chamber constructed monolithically with the pumping station. This type of installation requires the
least amount of discharge piping, but is subjected to maximum hydraulic loading from the discharge side. Also, if a
discharge chamber is constructed in the pool, it must be gated and designed for maintenance access to the gates. This
access can be provided by periodic unwatering under full external hydraulic load, or by an arrangement that allows the
gates to be removed for maintenance. Pumping stations not located on the protection line require extensive discharge piping.
This piping may be installed over, through, or under the protection line as required by the specific situation. The
structural design of this piping, its supports, appurtenant gate structures, and discharge structures can be undertaken only
after the coordinated plant arrangement has been determined, incorporating input from hydraulic, mechanical, and structural
elements. All piping inside the pumping station should be of ductile iron, and discharge piping will usually be of ductile
iron, steel, concrete pressure pipe, or cast-in-place reinforced concrete. Whether the pumping station is located on the
protection line or not, it is often necessary to provide a low flow gravity discharge structure. This structure will usually
include an intake headwall with bulkhead slots, a gravity discharge conduit through the protection line, a gate structure
near the discharge end of the conduit, and a headwall and stilling structure at the conduit outfall. There are many
variations on this arrangement including combination of the various components of the pumping station and pump discharge
system and the components of the low flow discharge system. Plant arrangements involving innovative facilities or
arrangements should be thoroughly reviewed from a construction and operations standpoint during the planning and layout
stages to assure constructability and to facilitate operation and maintenance over the life of the project. These unique
features and arrangements should also be thoroughly coordinated with higher authority.
( f ) Miscellaneous Structural Items.
There are, in any pumping plant, various miscellaneous items which must be addressed by the structural engineer. These
include retaining walls, channel lining and slope protection slabs, and gates, flap valves, and bulkheads and their
associated guides and mountings. These items generally constitute a small portion of the total project cost and are not
usually designed until late in the design process. However, they should be accounted for in all estimates of project
construction costs either separately, as in the case of relatively large concrete retaining wall sections, or in general
terms, as in the summary "miscellaneous metal" cost item for gate guides, etc.
4-3. Structural Loads...
( a ) Soil Loading.
Lateral soil loads for stability analyses and determination of base pressures should be computed by the method in EM
1110-2-2502. In many instances, the design of vertical walls below grade and wing walls and retaining walls will be greatly
affected by wheel loads or other surcharge loads on the ground surface. These loads should be considered in structural
stability calculations and in detailed structural design as appropriate. They should be derived based on the heaviest piece
of machinery likely to be placed on the fill during construction or operation and maintenance of the facility.
( b ) Hydrostatic Loads.
For the portion of the hydrostatic loading not included in the soil load calculations (water above the ground line) the
conventional triangular distribution of water pressure with depth should be used. The water surface elevation will depend
on the hydrologic situation at each site and must be coordinated prior to beginning structure design. A station located on
the protection line will usually experience larger differential hydrostatic loads than one located inside the protection
line. If a discharge chamber is used, the hydrostatic loading under unwatered conditions may be more severe with the
chamber located in the discharge reservoir. The hydrostatic loading on a station inside the protection line will be
related to the hydraulic gradient between the free water surfaces on the discharge side of the protection line and at the
pumping station intake. This gradient is affected by the presence of foundation drains, the proximity of the station to the
protection line, and the type of protection used (levee or flood wall).
( c ) Uplift.
The uplift experienced by a pumping station will vary with its proximity to the protection line. Stations located on the
protection line will generally be subjected to larger hydrostatic loads and correspondingly larger uplift pressures than
those located inside the protection line. The uplift forces used in the structural design may be derived from actual field
data, but more commonly will be based on an assumed flow path relating the head at the discharge side of the protection
line to that at the pumping station intake. This relationship is usually assumed to be a straight line variation with the
uplift at the pumping station assumed to be that portion of the gradient envelope intercepted by the vertical projection of
the structure base as shown in Figure 4-1(a). The full uplift may be modified by incorporation of maintainable foundation
drains into the site design, However, the uplift reduction may not exceed 50 percent of the difference between the full
uplift head at the pumping station intake and that at the point of the drain (Figure 4-1(b)).
( d ) Seismic Loading.
Seismic investigations and design should be performed in accordance with the provisions of ER 1110-2-1806. As a minimum, an
investigation should be performed to determine the types and extent of defensive design measures which may be economically
justified for the project to resist the effects of seismic events. These measures may include arrangement of the facilities
to minimize seismic damage, use of flexible couplings on discharge conduits, and restricting the height of structures to a
minimum to reduce the effects of earthquake motion. A seismic coefficient analysis, using the minimum coefficients specified
in ER 1110-2-1806 should be used to calculate sliding and overturning stability for all structures subject to earthquake
loading. In addition, a dynamic response analysis is required in high seismic hazard locations as specified in ER
1110-2-1806 to determine areas of high stress within the structure. The seismic forces for the components of the pumping
station include the building components, fixed operating machinery, and other fixed equipment should be calculated using
the procedures of TM 5-809-10. In the stability analyses, water inside the structure, confined between structure walls
placed perpendicular to the direction of earthquake acceleration, is treated as part of the structural wedge, as is any
saturated or moist earth mass bearing vertically on any projecting structure footing or sloping exterior wall face. Free
water above the ground surface and above a structure footing or sloping exterior face Is not included as part of the
structural wedge. Seismic forces for inclusion with static forces from earth and water impinging on the sides of the
pumping station are computed in accordance with the provisions of EM 1110-2-2502. For structures having sloping exterior
walls, or footings extending outside the structure walls, the force wedges used for structure stability analysis will
originate at a vertical plane projecting upward from the outer edge of the structure footing or wall at the foundation.
Seismic forces due to water above ground acting in the same direction on opposite sides of a structure are calculated by
the Westergaard approximation.
( e ) Wind Loading.
Wind loads should be applied according to the provisions of ANSI A58.1. These loads should be applied in conjunction with
other loads as prescribed in paragraph 4-4. Wind loads will also be applied to the appurtenant structures as applicable.
( f ) Floor Loads.
The structural support system for the operating floor should be designed for dead loads including the weight of the pumps
in their operating locations plus a minimum live load of 100 pounds per square foot. Since the pumping equipment may be
removed for repairs, the floor area must be designed to support the heaviest work piece anywhere it might be placed on the
floor. The machinery loads, for both service and maintenance conditions, should be furnished by the pump designer. The
service loads will include the machinery weight plus the weight of the water column for most pumps, with a 50 percent
increase in water column weight to account for dynamic effects. However, for some pump arrangements the pump motor and
pump impeller are supported at different levels. For this arrangement, the floor supporting the motor must carry the full
downward hydraulic thrust under operating conditions in addition to the weight of the rotating element and the motor. The
pump support must carry the weight of the impeller and water column which will be partially offset by the upward hydraulic
thrust against the pump casing. All personnel areas inside the pumping station should be designed using the applicable
minimum dead loads given in ANSI A58.1. Table 4-1 gives minimum uniformly distributed live loads. For areas not covered in
this table, refer to TM 5-809-l. The live loads indicated in Table 4-1 may be reduced 20 percent for the design of a girder,
truss, column, or footing supporting more than 300 square feet of slab, except that, for pump room and erection floors, this
reduction will be allowed only where the member under consideration supports more than 500 square feet of slab.
"Figure 4 - 1 : Typical structure uplift derivation"...
"Table 4 - 1 : Minimum uniformly distribured live loads"...
( g ) Stairway and Landing Loads.
Stairways and landings should be designed using the live load given in Table 4-1 unless special loading in excess of this
amount is indicated.
( h ) Roof Loads.
Roofs should generally be designed for dead load, live load, and either wind or seismic loading, whichever is the more
critical for the plant location. In certain localities live load produced by snow accumulation must be considered. Snow
loads should be determined and distributed according to the provisions of ANSI A58.1. Snow load is not included in the
minimum design live loads indicated for roofs in Table 4-1. Roof live loads from Table 4-1 and imposed snow loads are per square foot of horizontal projection.
( i ) Crane Runway Loads.
Crane wheel loads should be treated as live loads in the design of crane runways and maximum wheel loads should be computed
from the weight of the crane and trolleys plus the rated live load capacity of the crane. The load should be placed in the
position that produces maximum loading on the side of the runway under consideration. The design load should include
allowances for dead load, live load, impact (for power operated cranes), longitudinal forces, and lateral forces. In
addition, crane stops at each end of the runway should be designed to safely withstand the impact of the loaded crane
traveling at full speed with power off, and the resulting longitudinal forces should be provided for in the design of the
crane runway. Acceptable allowances for impact, longitudinal forces and lateral forces are as follows :
- Impact : 10 % of maximum vertical wheel loads for cranes over 80 ton capacity. 12 % to 18 % of maximum vertical wheel
loads for all others.
- Longitudinal forces : 10 % of maximum vertical wheel loads (applied at top of rail).
- Lateral forces : 10 % of trolley weight plus rated crane capacity (3/4 of this amount to be distributed equally among
crane wheels at either side of runway and applied at top of rail).
( j ) Moving Concentrated Live Loads.
Medium to large pumping stations may be designed with forebay and discharge decks to accommodate trucks and heavy cranes for
handling and transporting stoplogs and gates and for disposal of trash raked from intake trash racks. These decks should be
designed for dead load plus the worst case live load considering the minimum uniform loading from Table 4-1 or the weight of
the heaviest piece of equipment (truck crane, tractor trailer, etc.) fully loaded. Load distribution for truck loading
should be made in accordance with AASHTO "Standard Specification for Highway Bridges." It may be advisable to place load
limit signs at the entrances to these deck areas where these load limits are the controlling factor in the design.
4-4. Loading Conditions and Design Criteria...
The following loading conditions should not be regarded as a comprehensive list. In many instances, unique site specific
factors such as water conditions, station arrangement and location, pump type, pump and discharge arrangement, etc. will
dictate modification of some of these loading conditions to fit the specific site. The conditions described should be used
as a guide to the range of stability analyses required. The external structure forces and distributed loads should not be
factored for stability analysis, but may be subsequently factored when applied to the concrete members of the structure for
use in reinforcement design in accordance with EM 1110-2-2502. Design of the miscellaneous structures associated with the
pumping station (wing walls, headwalls, discharge piping, culverts, gate structures, etc.) should be based on the applicable
design water levels, earth levels, etc. for those structures, and their design load conditions should be adapted from the
basic loading conditions. See paragraph 4-7 for design loading and guidance to be used for these structures. Wind and snow
loads should be applied in conjunction with the basic loading conditions as applicable depending on the meteorological
condition at the site. Stability and stress criteria vary according to the nature of the loading condition imposed on the
structures. For the purpose of criteria application, there are three categories of loading conditions; usual, unusual, and
extreme. Usual conditions are defined as those related to the primary function of a structure and expected to occur during
its life. For pumping stations, all of the operating flood conditions should be considered usual. Unusual conditions are
those which are of infrequent occurrence or short duration. Construction condition, maximum design water level condition,
maintenance conditions, rapid drawdown condition, and blocked trash rack condition are examples of unusual loading for
pumping stations. Extreme conditions are those whose occurrence is highly improbable and are regarded as emergencies, such
as those associated with major accidents or natural disasters. For pumping stations, pumping station inundated and
earthquake conditions should be considered extreme. The basic loading conditions for design and their categories are listed
( a ) Construction Condition.
Pumping station complete with and without fill in place, no water loads. Unusual.
( b ) Normal Operating Condition.
Plant operating to discharge routine local floods over a range of exterior flood levels for which the pumps are operating
at approximately 100% efficiency. Usual.
( c ) Start-up Condition.
Station empty with water at pump start elevation or maximum pump level. Usual.
( d ) Pump Stop Condition.
Water below pump start elevation on intake side, levee design flood on discharge side. Usual.
( e ) High Head Condition.
Maximum design water level outside protection line, minimum pumping level inside. Usual.
( f ) Maximum Design Water Level Condition.
Maximum operating floods both inside and outside protection line, maximum pump thrust. Unusual.
( g ) Maintenance Conditions.
Maximum design water level inside with one, more, or all intake bays unwatered. Unusual.
( h ) Rapid Drawdown Condition.
Water at pump stop elevation, sumps unwatered. (Apply to stations inside protection line only.) Unusual.
( i ) Blocked Trash Rack Condition.
Five foot head differential across trashracks. Unusual.
( j ) Pumping Station Inundated.
Maximum flood levels inside and outside protection line, pumping station inoperative, foundation drains inoperative,
protection line intact. Extreme.
( k ) Earthquake Conditions.
Earthquake loading combined with normal operating condition. Extreme.
Analyses should be made for stability of structures against overturning, sliding, flotation, and foundation pressure.
( a ) Overturninq.
For overturning stability, all structures should meet the criteria given in Table 4-2 for percent of base in compression.
( b ) Sliding.
The resistance to sliding under various loading conditions will be analyzed according to EM 1110-2-2502. The result of this
analysis is expressed in terms of a sliding safety factor which is the ratio between the total shear strength available in
the soil-structure wedge system and the applied shear stress. The minimum sliding safety factors for various types of
loading are shown in Table 4-2.
( c ) Flotation.
The analysis of structures for stability against flotation should be performed in accordance with the procedure in Appendix
B. Required safety factors are given in Table 4-2.
"Table 4 - 2 : Stability criteria for pumping stations"...
( d ) Foundation Pressure.
In conjunction with the overturning analysis, the base pressures and foundation pressures for each loading condition should
be calculated and the maximum values compared with the maximum allowable values determined for the foundation material.
These maximum allowables should not be exceeded for any loading condition. The allowable values should be coordinated
between the geotechnical and structural engineers.
4-6. Design Stresses...
Allowable working stresses for structural materials will generally be as prescribed in EM 1110-1-2101, except that
reinforced concrete structures should be designed in accordance with the strength design method given in EM 1110-2-2502.
Working stresses for use in proportioning masonry structural components should be taken from TM 5-809-3. For earthquake
loading, design stresses should be evaluated in accordance with guidance given in ER 1110-2-1806 and TM 5-809-10.
4-7. Miscellaneous Features...
( a ) Discharge Lines.
Design of the pump discharge lines is based on the type of protection works, consideration of backflow effects, and
economics. There are two general categories of discharge piping, over the protection line and under or through the
protection line. The under or through type is more susceptible to backflow problems and should be avoided if possible.
However, a properly designed system is acceptable and may result in significant cost savings compared to the
over-the-protection line type. Discharge piping passing over levees should be of steel or ductile iron suitable for use with
dresser or other flexible couplings. The pipe should be supported by the embankment surface on the inside slope and crown of
the levee and buried in a trench on the discharge side, with adequate cover to protect it from damage or exposure by
erosion. It should be anchored to prevent flotation during high water. The anchorage can be concrete supports placed at
intervals along the length, a continuous concrete bedding, or other approved means. The principal loads imposed on the pipe
are positive and negative hydraulic pressures and external compressive pressure from fill material and vehicular surcharge.
EM 1110-2-2902 contains procedures for the design of conduits under embankment and backfill loading. Embankment settlement
should be considered in the design of the pipe joints. Over-the-levee type pipes are sometimes designed as siphons, using
the pumps to establish flow. This introduces an additional design loading consideration. At the levee crest, a negative
pressure of up to 1 atmosphere could occur. This load must be combined with the external compressive loads from fill and
water. Guidance for siphon design is contained in EM 1110-2-3105. Discharge pipes passing through or under the protection
line are pressure pipes and the internal hydraulic pressures are therefore greater than for the over-the-protection line
type. When the protection line is a levee, careful attention must be given to insure that no leakage or infiltration is
allowed in the pipe or joints which would affect the integrity of the embankment. The materials used in these pipes are
ductile iron, steel, concrete pressure pipe, and cast-in-place reinforced concrete. To prevent leakage, steel and
ductile iron pipe should be joined with flexible, watertight couplings, and concrete pipe should have alignment collars
and waterstops at each joint. Materials used for discharge piping should conform to CEGS-02724 N7. The piping materials
should be selected on the basis of strength, durability, and project life economics.
( b ) Discharge Conduit Gates.
A pressure discharge conduit from a pumping station through the protection line must be provided with an emergency closure
gate on the river side of the floodwall or levee to prevent backflow into the protected area in case of failure of the pump
or rupture of the conduit. For a levee type installation the gate usually will be in a well in the riverside levee slope,
accessible from the levee top. When the pumping station is integral with a flood wall, the discharge pipes usually discharge
into a surge chamber through flap valves. Stoplogs are usually provided at the end of each pipe and upstream of each pump so
that, in the event of a flap valve failure, flow can be stopped in order to prevent flooding of the plant. For a pressure
pipe under a flood wall, the gate will usually be in a well integral with the floodwall. A simple slide gate for the smaller
sizes, or wheel-type gate for larger sizes is suitable. On pressure pipes, gates should be designed with operators capable
of opening and closing the gates under all head conditions so that flow can be discharged after an interior flood in order
to prevent excessive pressure build-up on the gate. Well type gate structures should be constructed of reinforced concrete
and designed in accordance with strength design provisions given in EM 1110-2-2502. The design loading conditions will vary
with the placement and configuration of the gate structure. A gate well placed on the discharge side of a levee will
experience fill loading, uplift and vertical water loads, and possibly rapidly varying pool levels. In most instances it
will be required that the gate structure be unwatered for maintenance purposes. The top of the gate structure must be
designed to withstand gate operating forces. See EM 1110-2-3105 for further discussion of forces on the gate structure
induced by gate operation. In areas of high seismicity, defensive structural layout may dictate that the concrete mass
extending above the ground line be kept to a minimum. Restriction of the gate structure projection above the ground line
might also be of value in areas subject to high wind loads. These factors must be addressed early in the layout and design
process and the configuration of the gate structure must be set based on functional, economic, and technical considerations.
( c ) Trashracks.
All pumping stations should be provided with trashracks at the station intake. These racks are generally constructed of
structural steel and are either attached to the face at the forebay side of the structure or inserted into formed slots near
the intake face of the substructure. Trashracks should be designed for a minimum of 5 feet of head differential acting
toward the pumping station for small to medium sized plants. For larger plants, higher head differentials may occur. This
should be addressed in an early design conference and definite design criteria established.
( d ) Trash Removal.
The types of raking devices used to remove trash from the trashracks depends on the size of the plant, frequency of
operation, type and size of the pumps, and type of inflow facilities (pipe, open ditch, etc.). When a boom across the inlet
channel or other means is used to remove a large portion of the trash before it reaches the intakes, mechanical trash
removal devices may not be required. However, for most installations some positive means of trash removal should be
provided. This may be done by hand on very small plants, but for medium to large size stations, mechanical trash rakes
should be provided. These trash rakes are manufactured in a variety of configurations, each applying forces to the structure
in different ways and to varying degrees. Before final design of the intake area and trash deck is begun, the type of raking
system must be determined and these forces identified. In the design of both trashracks and trash raking equipment,
durability under adverse operating conditions and harsh environment must be considered. These items should be designed to
function dependably with a minimum of maintenance over the life of the station. For the design of various types of trash
raking equipment, see EM 1110-2-3105.
( e ) Trash Deck.
For some large plants, the trash deck may be designed for heavy vehicular traffic and can be used as a work area for a truck
mounted crane and trash hauling equipment. This arrangement might be used in conjunction with, or in lieu of, conventional
trash raking equipment. The method of trash removal and handling should be coordinated early in the design process, and
provision for removal of trash from the intake channel and from the trash deck should be considered as a fundamental part
of the station layout and design.
( f ) Contraction Joints.
Joints between separate monoliths on large installations, and between the pumping station and adjacent wall sections when
the pumping station is located on the protection line, should be contraction joints. Each joint should be constructed in
one plane and no reinforcement should be allowed to cross the joint unless required as dowelling for alignment. If alignment
dowels are used, they should be firmly fixed in the concrete on only one side of the joint. These joints should be made with
no initial separation between adjacent placements except as required near the concrete surfaces to prevent spalling of the
corners. This can usually be controlled by using V-grooves at monolith joints. However, in some cases such as a thin wall
section abutting the end wall of the pumping station, deeper separation may be desirable.
( g ) Construction Joints.
Reinforced concrete portions of pumping stations may be placed in segments, separated either vertically or horizontally by
construction joints. These joints are meant only to facilitate the construction process by dividing the work into manageable
units and should be arranged so they will not disrupt the continuity of the structure. In large placements, construction
joints can also serve to minimize crack formation. Reinforcing steel should pass through these joints, and surfaces should
be cleaned and scoured as necessary to provide good bond between the concrete placements. In very large mass concrete
placements having vertical joints between the first and second placements, it may be expedient to provide keys to assure
transfer of stresses across the joints. However, in normal construction this will be accomplished by reinforcement
and by bond between concrete surfaces.
( h ) Waterstops.
Waterstops across contraction joints are necessary to prevent leakage and obtain dry operating and working conditions. They
exclude water under head in the substructure and ensure weather tightness of the joints in the superstructure. Experience in
the use of molded rubber or extruded polyvinyl chloride (PVC) waterstops in joints of conduits and hydraulic structures has
proven the practicability and advantages of using these materials. Their superior performance under conditions of
differential settlement or lateral displacement make them particularly desirable. Metal waterstops may be used in structures
with dependable foundations, but may fail where a yielding foundation results in uneven settlement in adjacent monoliths. A
greater width waterstop is required in the substructure where large concrete aggregate is used and high water pressures
exist than in low-pressure areas or for sealing against weather only. Waterstops should be placed as near to the surface as
practicable without forming weak corners in the concrete that may spall as a result of weathering or impact, and should
create a continuous barrier around the protected area. All laps or joints in metal waterstops should be welded or brazed;
joints in rubber waterstops should be vulcanized or cemented together, and joints in PVC waterstops should be adequately
cemented or heat sealed. Waterstops in contact with headwater for structures founded on rock should terminate in a recess
formed by drilling holes a minimum of 18 inches deep into the rock, and should be carefully grouted in place. Occasionally,
double waterstops are desirable in pier joints and other important locations, to insure watertightness in case of failure
of one of them. For pumping stations located on the protection line, waterstops should be placed between the pumping station
and adjacent wall monoliths and should extend from embedment in the foundation, or attachment to a seepage cutoff wall to
the nominal top elevation of the protection line.
4-8. Appurtenant Structures and Facilities...
( a ) Gravity Drainage Structures.
A gravity drainage system may be constructed to carry normal runoffs through the protection line. It may be constructed
separate from the pumping station or integral with it. The system will consist of an intake structure, discharge conduits,
a gate structure, and a stilling basin.
(1) Intake Structure. Where the gravity drainage system is constructed separately from the pumping station structure, it
should include an intake structure arranged so that it can be closed off for maintenance of the conduit and for
emergencies. This is usually accomplished by stoplogs. Thus the headwall must be designed for the loads imposed by fill
placed behind it and for loads on the stoplogs. When there are existing outlet structures on a site or where site space is
limited, it may be economical to incorporate the intake for gravity drainage into the pumping station. This will require
special gating and careful hydraulic and structural planning and coordination among all affected disciplines throughout the
functional layout and design process.
(2) Drainage Conduits. The drainage conduit should be designed according to the provisions of EM 1110-2-2902. The shape of
the conduit will be dictated by the height of the overlying fill and the hydraulic capacity and flow characteristics
required. A gravity drainage culvert should not generally be designed for pressure flow and should be gated near the
discharge end to prevent high reservoir water from flowing back into the protected area. All joints in the gravity conduit
should be sealed against seepage and infiltration. This may be done using flexible couplings for metal pipes, steel joint
rings with solid-ring rubber gaskets for concrete pressure pipe, or waterstops and seepage rings at each joint in
cast-in-place reinforced concrete construction. When a new facility which includes a gravity outlet system is being
designed, it may be desirable to provide two or more separate gravity outfall conduits. This will allow one conduit to be
dewatered for inspection and maintenance of the conduit and gate structure without completely stopping normal flow during
these operations. Common types of conduits used under various conditions of fill height, hydraulic requirements, facility
location and importance, etc., include corrugated metal with protective coatings, reinforced concrete, precast prestressed
concrete cylinder pipe, and cast-in-place concrete culvert. These will generally provide the most economical and serviceable
gravity drainage conduits. However, under certain circumstances other materials may be desirable because of special site
specific requirements such as the presence of deleterious chemicals in the soil or water. These other conduit materials may
include reinforced plastic masonry (RPM), fiber reinforced plastic (FRP), or certain high strength plastics for pipes in
smaller sizes. These types of pipe will usually be much more expensive than the more common types. Also, the performance
experience over time may be very limited for some of these materials. Use of specialized types of pipe must be closely
coordinated with higher authority and may require special testing as well as special placement procedures. Generally,
reinforced concrete pipe should be used for urban levees and other levees where loss of life or substantial property damage
could occur. Corrugated metal pipe (CMP) with protective coating may be used as an option on agricultural levees. When CMP
is considered as an option, a life cycle cost study should be done. Generally a minimum of one CMP replacement should be
assumed during the life of the project. For further guidance concerning the type of pipe for use in gravity outlet systems,
see Appendix C.
(3) Gate Structures. Gravity discharge conduits should be gated so they can be closed against high water in the discharge
area. The gates should be located on the discharge side of the protection line as near the conduit outfall as practicable.
They should be situated in a gate structure which extends upward over the conduit to a sufficient height to provide dry
access to the gate operators from the top of the levee under all operating conditions. This access may be provided by
walkway bridge or embankment. Gate structures are usually constructed of reinforced concrete. The types of forces on the
structure may vary, but will typically include hydrostatic and lateral earth pressures and uplift loading. Additionally,
the top of the structure must be capable of withstanding the forces imposed by the gate operator. The structures should be
designed to be unwatered to allow servicing of the closure gates. In certain circumstances it may be expedient to empty the
pump discharge piping into the gravity drainage gate structure, thus limiting the length of discharge piping required and
negating the need for construction of a second gate structure. This may offer particular advantages where a gravity outlet
gate structure already exists. Such an arrangement should be analyzed carefully to assure that the outlet piping and
stilling structure are adequate to handle the pumped flow. These layout procedures must be investigated and coordinated
among the design elements and with higher authority from the earliest planning stages.
(4) Stilling Basin. At the outlet of the gravity drainage structure, some means of dissipating the discharge energy and
protecting the surrounding bed and bank materials against erosion may be required. This may be accomplished by construction
of a headwall and stilling basin with block type energy dissipators. This is a special type construction and may vary with
each application. However, the design principles are fairly constant. The stilling structure must be designed to resist
hydraulic thrusts imposed by flowing water in addition to the normal horizontal earth, hydrostatic, and uplift
( b ) Retaining Walls.
Walls and footings or slabs of reinforced concrete required to retain fill as a part of a pumping plant installation should
be designed according to the provisions of EM 1110-2-2502. These features may be constructed as approach structures
immediately upstream of the pumping station or gravity discharge structure, as wing walls adjacent to these inlets, or as
simple retaining walls. They may be conventional T-wall sections or may be designed as U-frame structures. In areas of high
seismicity, defensive layout measures may dictate that high cantilever walls be avoided where possible and that special
treatment (alignment dowelling, etc.) be given to adjacent wall sections and walls abutting larger structures.
"Plate - 1 : Design network diagram"...
"Plate - 2 : Typical open forebay type pumping station with over the levee type discharge"...
"Plate - 3 : Typical open forebay type pumping station superstructure and trash deck"...
"Plate - 4 : Typical open forebay type pumping station superstructure"...
"Plate - 5 : Urban industrial installation with floodwall type flood protection"...
"Plate - 6 : Large urban pumping station superstructure, trash deck and operating floor"...
"Plate - 7 : Large urban pumping station superstructure, pump floor, forebay and intake sump area"...
"Plate - 8 : Large civil pumping installation, operating floor, forebay deck, superstructure and quarters area"...
"Plate - 9 : Large civil pumping installation, equipment floor, forebay, sump area and discharge area"...