Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

•

Sump pumping stations should contain

two

pumps,

and

each should

be

capable

of

accommodating

all

normal

operating conditions.

• The

pumps must

be

capable

of

passing solids

of the

size commonly encountered.

For an

example,

in a

wastewater pumping station,

the

sump should

be

capable

of

passing

a

50-mm

(2-in.)

solid.

• The

operating conditions should

be

close

to the bep

to

avoid damaging vibration

and

radial thrust.

•

Large rates

of flow

should

be

able

to

enter

the

sump

through

a

grated

opening.

Avoid sumps

sealed

with

a

solid cover plate.

If

all of the

above rules

are

followed,

the

sump

pumps

will have

a

capacity

of 34

m

2

/h

(150

gal/min)

or

more, because

the

requirements

to

pass large

solids

and

to

discharge

to a

higher elevation dominate

all

other

considerations. However,

as the

pumps will

not be

required

to

operate very

often

(leakage

and

other con-

tributions under most conditions

are

quite small),

the

sump

itself need only

be of

sufficient

size

to

accommo-

date

the

installation

and

removal

of the

pumping equip-

ment.

If a

grated sump cover

is

used, ample capacity

is

available

if a

pipe coupling comes

loose

or

breaks

and

large quantities

of

liquid enters

the

room.

Hoisting

Equipment

Electrified

hoisting equipment

is

recommended

for all

but

the

very smallest stations. Large items

of

equip-

ment

or

subassemblies

from

large units

are

awkward

to

move

and

place.

As the

persons charged with this

work

are

typically unskilled

in

using hoisting equip-

ment,

specify

crane trolleys, bridges,

and

hoists

with

no

more than

the

following

speeds:

Recommended

maximum

Motion

speed,

ft/min

Hoist

10

Trolley

20

Bridge

30

Note

that

the

above speeds

are

considerably slower

than

those provided with commercially available

cranes intended

for

operation

by

skilled

personnel.

The

added

costs

for

slower-speed motors

and

larger gears

is

justified

for

safety.

For a

rated capacity

of

more than

3600

kg (4

tons),

specify

an

inching hoist feature

to

lift

or

lower

the

maximum load

at no

more than

1

ft/min

for

the

precision alignment needed

for

couplings.

Cranes

in

wastewater pumping stations

are

sub-

jected

to

varying concentrations

of

hydrogen

sulfide.

Such cranes should

be

constructed

of

corrosion-resi-

stant

material,

and all the

equipment should

be

explo-

sionproof

if

located

in the wet

well area within

an

enclosure. Power

to all

electrical

devices must

be

sup-

plied

by

festooned cables.

Cranes should have

full,

vertical access

to

heavy

equipment

in

lower rooms through removable cover

plates

or

supports

for

equipment

located

overhead

(such

as

motors).

Do not

expect that items

of

equip-

ment

can be

jockeyed under

an

opening

from

an

equipment

position

using

tie-back

tackle

or

"coffin

hoists"

—

a

dangerous practice. Design

the

doors

to be

wide enough

and

high enough

for

trucks

to be

driven

under

the

crane

for

removal

of the

bulkiest equipment.

Do not

assume equipment

can be

placed

on

wheeled

carts moved

by

hand outside

the

structure

for

mobile

cranes

or jib

cranes

to

hoist onto trucks. Cranes

require

rigid

frames

or

diagonal bracing

to

withstand

lateral

and

longitudinal forces.

Specify

all

cranes

and

hoists

to

conform

to the

standards

of the

Crane Manufacturers Association

of

America (CMAA),

and

specify that

the

installation

must

be

tested

and

licensed

by the

local state entity

providing this type

of

service.

In

addition,

the

specifi-

cations

should include crane operation

and

safety

training

to be

conducted

by the

crane vendor.

For

small stations,

lifting

eyes

(or

hooks) over

the

equipment

are

feasible (although awkward

and

some-

what

dangerous)

for all

items capable

of

being

lifted

by

manually operated portable

hoists.

Design

and

specify

lifting

eyes

to be

strong

and

substantially

anchored into

the

overhead structure,

as

shown

in

Fig-

ure

25-2.

Be

sure

to

design adequate headroom

for the

hoist

and the

machinery

so

that dismantling

is

unnec-

essary.

The

design maximum capacity should

not

exceed

900 kg (2

tons)

and

should preferably

be no

Figure

25-2.

Lifting

eye or

hook

for a

maximum

capacity

of

2

tons

and a

desirable

capacity

of

half

a

ton.

more than

25% of

that capacity.

Lifting

eyes

are

com-

pletely inadequate

for

medium

and

large stations.

Monorails, which

may be

adequate

in

medium-

sized stations,

are

often

supported

by

bolts

in

concrete

roof slabs. Design

the

bolts

and the

slab

to

resist

impact

as

well

as

static

loads.

Overloading

the

anchor-

ages

is

frequently

a

problem that violates OSHA

requirements (see ANSI

B30.2.0,

B30.ll,

B30.16,

and

B30.17).

Avoid

the use of

threaded inserts (into

which

bolts

are

screwed) whether cast

in

place

or

drilled.

Instead,

use

bolts with adequate mechanical

anchorage.

An

alternative

is a low

superstructure with

a

large hatch

in the

roof

for

access

by a

mobile crane

parked outside (see CHMA standards).

The

lifting

effort

of

cranes

or

truck-mounted booms

actually

used

in

lifting

submersible pumps should pref-

erably

not

exceed

about 150%

of the

weight

of the

pump

and

must

not

exceed

the

allowable strength

of

the

discharge elbow

and its

eccentrically loaded

connection

to the

concrete base.

In at

least

one

known

accident,

the

discharge elbow

and

pipe were pulled

up

with

the

pump, thus leaving

the

owner with

a

very

expensive repair job. Anchors

for

discharge elbows

should

be

buried deep enough

to

develop

the

ultimate

tensile strength

of the

anchors,

and the

anchors should

be

surrounded with

a

strong cage

of

reinforcing bars

to

prevent

the

gradual cracking

and

deterioration

of

con-

crete

caused

by

vibration

of the

pump.

Compressed

Air

Systems

Air

compressors, particularly

the

positive displacement

type,

can be

high-maintenance items.

Be

conservative

about

estimating

air

demand, then double

it for

leakage

at

joints, etc. Select compressors

on the

basis

of at

least

10

times

that

figure. Be

generous

when

calculating distri-

bution

system losses,

and

avoid selecting

a

compressor

at

the

extreme limit

of its

range.

For

example,

do not

select

an

80 ft

3

/min, 100

lb/in.

2

compressor

for an

application

where

the

calculated

figure is 75 ft

3

/min at 90

lb/in.

2

Furnish

at

least

two

compressors

and

plenty

of

receiver capacity.

All

distribution piping should con-

tain

sediment

and

water traps

at low

points.

If it is a

large system, consider using terminal receivers

in

addi-

tion

to the

control receivers

at the

compressor location.

Instrument

air

systems require

nonlubricated

com-

pressors

and

some means

of

removing water vapor

from

the

compressed

gas

stream. Refrigerated dryers

have

worked well

for

this purpose

as

long

as the

pip-

ing

system will

not be

exposed

to

freezing

tempera-

tures.

If

freezing

temperatures

may be

encountered,

use

heat reactivated desiccant type downstream

from

the

refrigerated dryer

to

provide

a

pipeline dewpoint

of

-73

0

C

(-10O

0

F)

(see also Section

20-3).

Water

Systems

Usually

two

water systems

are

required

in a

pumping

station:

one to

provide

a

pressurized supply

for

shaft

seal purge

and

lubrication,

and the

other

for

house-

keeping purposes. Usually

the

seal

water supply

requirements (small demand, high pressure)

can be

satisfied

by

using

a

pair

of

regenerative turbine pumps

(two

are

required

for

redundancy) operated

on the

pressure

in a

hydropneumatic tank.

To

avoid

air

bind-

ing,

use a

diaphragm-type tank

for

control

and

stor-

age.

The

multistage

centrifugal

pump

is an

alternative

to

the

regenerative turbine (see Section

11-10).

Housekeeping water supply requirements

of 2 to 4

L/s (30 to 60

gal/min)

at 600

kPa

(90

lb/in.

2

)

gauge

pressure

or

more)

can be

satisfied

by

providing start/

stop controls

at

each utility station.

An

overriding timer

can be

provided

to

shut

the

pump down

after

a

timed

interval (say,

15

min),

thereby avoiding

the

possibility

the

pump

may be

operated against

a

closed valve.

State regulations

for

protection

of

potable water

systems against

backflow

require

at

least

the

installa-

tion

of a

reduced pressure principle

backflow

preven-

tion device

on the

water supply

to the

station premises.

Some states require

an air

break tank

for

absolute pro-

tection. Locate these

in an

area with good

free

drain-

age to the dry

well. When these devices

fail,

they

frequently

release

considerable

quantities

of

water.

Alarms

Alarm

functions

should

be

from

devices independent

from

controls systems, because

the

problem could

very

well

be

caused

by a

malfunctioning control sys-

tem. Alarms should

be

configured

to

provide warning

of

abnormal system behavior before

the

problem

becomes

a

crisis.

For

example,

in a

two-unit system

designed

to

provide ample performance with

one

unit

in

service,

furnish

the

following

features:

• Use a

broad-band alarm switch with

two

pairs

of

contacts.

• Set the

alarm

to

initiate

at a

value less than that

selected

to

start

the

follow

unit.

• Set the

second pair

of

contacts

to

open

at a

value

below that which will shut down

the

follow

unit.

•

Wire

the

switch contacts

to

maintain

the

alarm con-

dition until

the

second

set of

contacts have opened.

•

Provide

the

system with

a

manual lead/follow

selec-

tor

switch.

With

the

above arrangement,

if the

lead

unit

should

fail,

or an

unusual demand

is

placed upon

the

system,

an

alarm

will

be

initiated.

The

second unit will operate,

but it

will

not be

possible

to

cancel

the

alarm condition until

the

controlled variable

is

within

the

range

of the

lead unit.

The

alarm

condition serves

as a

reminder

to the

operating

staff

that

something

is

wrong

and

encourages investigation.

An

example

of

this arrangement, portrayed

as the

controls

for

a

sump pumping station,

is

shown

in

Figure 12-4.

25-9.

Instruments

and

Control

A

good basic rule

is to

avoid

too

much instrumenta-

tion because

it

increases costs

and

adds complexities

that

can

compromise

the

entire system.

Typical

Minimum Instrumentation

and

Control

Every

pump should have

a

pressure gauge

on the

dis-

charge

piping.

It is the

"stethoscope"

that

is

used

to

check

the

condition

of the

pump

and

force main

or

transmission

pipeline

—

conditions

such

as

worn impel-

lers, blockage

in the

force main, deposits

in the

trans-

mission pipeline,

and

partly clogged pumps.

The

gauges

can

be

permanently mounted (which

is

convenient

but

costly

if

there

are

many pumps)

or

portable

and

attach-

able

with quick-connects (which

is

less expensive, more

easily

calibrated,

and

less subject

to

wear,

but

less con-

venient

and

less

apt to be

frequently

used).

A

good

gauge

installation

is

shown

in

Figure 20-6.

Other instruments

and

controls include

the

following:

•

Incoming power

feeders:

voltmeter, ammeter,

and

watt-hour

meter;

the

watt-hour meter readings

may

differ

somewhat

from

those

of the

utility's meter.

•

Motors: elapsed time meter;

on

large motors

[>400kW

(>300

hp)], individual ammeters

and

thermal

protectors.

•

Engines: instruments

and

controls

as

recommended

by

the

manufacturer

for the

particular engine,

and

its

auxiliary systems such

as

fuel,

cooling system,

and

storage tanks.

• Wet

well: level sensors (bubbler

or float

switches)

with

low-

and

high-water pump controls

and

low-

and

high-

water

alarms.

• Wet

well storage tank: level sensor (pressure trans-

ducer,

capacitance,

or

ultrasonic) with high

and low

levels

and

alarms.

• Dry

well: water level sensor

for flood

alarm.

•

Boosters: pressure switches.

•

Water

wells: pressure switch, automatic timer,

or

on-off

switch actuated

from

a

central station.

•

Chlorination equipment:

if

necessary.

•

Flowmeter

(optional):

choose reliability

and

simplic-

ity

and

allow enough room

for

installation according

to

maker's specifications. Most

flowmeters

require

a

straight entrance

of 15

pipe diameters,

and

even mag-

netic

flowmeters

require

2 to 5

pipe

diameters.

•

Multiple

pumps: controllers

to

start

one at a

time

on

a

"first

off-first

on"

basis; some experts object

to

automatic duty rotation unless

the

number

of

starts

per

hour would otherwise

be

excessive. Automatic

alternators

add

unnecessary complexity. Wear

can

be

equalized

(if

desired)

on a

periodic basis

by

operating personnel.

•

Variable

speed: controls

and

instruments

as

required

by

manufacturer.

Automatic

Control

Entirely manual control

is not

economically

efficient.

Properly designed automation

•

Reduces operator workload

•

Optimizes operational

efficiency

•

Avoids potentially dangerous occurrences.

Automatic

controls

can be

anything

from a

simple

float

switch

that turns motors

off and on to a

micropro-

cessor

or

even

a

computer. Standardized

(or

"off-the-

shelf)

microprocessors

are

efficient,

effective,

reliable,

and

relatively inexpensive. Like computers, they

are

get-

ting

better

and

less expensive. Microprocessors,

in

par-

ticular,

have several advantages over simple switches

even for

simple process controls such

as the

level

in a

wet

well

or

storage tank (see Section

20-1

1

and

Chapter

21 for

additional discussions

of

automatic controls).

Recording

Recording instruments vary

from

simple, circular chart

recorders

to

strip chart recorders that document many

operational variables

at

once, either

at the

station

or at a

centralized location. Consult

the

owner

to

determine

whether recorded data

are

needed. Unless

the

data will

be

used, recorders should

be

omitted. However, consider

using digital data storage (tape cassettes) that

can be

recycled,

or

consider digital recorders that automatically

produce

a

properly annotated chart when abnormal con-

ditions occur. Some information could

be

invaluable

if a

malfunction

occurs

in an

unattended station.

Complex

Instrumentation

Sophisticated instrumentation

can

take many forms,

such

as: (1)

local monitoring

and

control,

(2)

local

control with central monitoring,

(3)

local analog con-

trol with digital monitoring,

and (4)

distributed digital

control.

The

total initial costs vary

from

about

$300,000

to

about

$1,000,000.

The

total

cost

per

month

(including amortization, energy

for

instruments

and

control,

and

salaries) ranges

from

about

$20,000

to

$40,000.

Thus, sophisticated control instrumenta-

tion

is

very expensive (see Chapter 21).

25-10.

Structural

Design

Many

parameters

differentiate

the

design

of a

pump-

ing

station

from

other commercial

or

industrial-type

structures.

The two

most important are:

• The

structure must

be

watertight

• The

structure will

be

exposed

to a

potentially corro-

sive

environment.

Pumping

stations,

as

with

most sanitary structures,

are

usually required

to be

continuously operated

for

their

entire service

life

of 30 to 50

years. Hence, durability

and

serviceability

are

overriding considerations. Conven-

tional concrete structures

are

designed

to

crack

—

the

very

assumption upon which strength design

is

based.

Although

it is

virtually impossible

to

design convention-

ally

reinforced concrete structures without cracks, crack

depth, width,

and

number

can be

reduced

by

modifying

the

design procedures

of ACI

Building Code

318.

When

cracks

are

reduced,

the

permeability

of the

concrete

is

also reduced

and the

durability

of the

structure

is

increased.

To

achieve these ends,

ACI

350, (Concrete

Sanitary

Engineering Structures), produced

in

1983,

should

be

used instead

of ACI

318

for

pumping station

concrete design.

As

compared with

ACI

318,

ACI

350:

•

Lowers

the

allowable tensile stress

of

reinforcing

by

requiring either

the use of

working stress design

or

by

requiring higher load

factors

for

strength design

•

Emphasizes serviceability requirements

and

lower-

ing

of the "Z"

factor

•

Suggests guidelines

on mix

design components

and

proportions

•

Increases minimum reinforcement requirements

•

Provides guidelines

on

limitations

for

construction

and

expansion joint spacings

•

Provides guidance

on

concrete coating

use and

selection

•

Provides discussion

and

guidance

on

design

for

impact

and

dynamic loading.

Abbreviations

and

Definitions

Consult

ACI

Building Code

318

for a

more complete

list

of

abbreviations

and

definitions.

Those

given here

are

primarily

for the figures in

this chapter.

A

8

Area

of

tension reinforcement.

A

vf

Area

of

shear

friction

reinforcement

(in.

2

),

b

Breadth.

C

3

A

Tri-calcium

aluminate.

/

c

Specified

compressive

strength

of

concrete

(lb/in.

2

).

/

y

Specified yield strength

of

reinforcement

(lb/in.

2

).

h

Height

or

total thickness

(in.).

€

d

Development length; equals

(in.

=

€

db

x

modification factors).

€

db

Basic development length

(in.).

U

A

factor (multiplier) used

on the

live

load

for

ultimate strength design.

V

n

Nominal shear strength

(lb/in.

2

).

V

u

Factored

shear

force

at a

section

(Ib.).

Z A

distribution

factor

for flexural

reinforcement.

See

text.

(|)

Strength reduction factor.

Ji

Coefficient

of

friction.

1

k/in.

2

=

1000

lb/in.

2

=

6.9 MPa =

6,900

kPa

1

k/in.

=

1000 lb/in.

=

0.175

kN/mm

1

in.

2

= 645

mm

2

.

The

factor

Z is

defined

by ACI

3

18

as

Z=J

5

(^A)

1

/

3

(25-1)

where/

s

is the

computed tensile stress

in a

steel reinforcing

bar,

d

c

is the

depth

of

cover over

the

bar,

and A is a

cross-

sectional area

of

concrete equal

to 2

d

c

x bar

spacing.

Geotechnical

Considerations

The

purpose

of

most wastewater pumping stations

(raising

the

hydraulic grade line

to

minimize pipe

depth) means that these structures

are

usually

founded

quite

deep

—

frequently

below groundwater.

The

importance

of

obtaining good

geotechnical

informa-

tion prior

to

designing

the

structure cannot

be

overem-

phasized.

A

geotechnical engineer should

be

consulted,

and the

following information

and

recom-

mendations should

be

solicited.

•

Safe

construction slopes

for

excavation

•

Design high groundwater level including seasonal

and

tidal

effects

•

Recommended

dewatering

methods

•

Feasible

foundation types

•

Characterization

of

site soils

•

Active, at-rest,

and

passive lateral soil pressures

•

Seismic loadings including dynamic soil loading

(if

UBC

indicates site

is in

Zones

3 or 4)

•

Corrosivity

of

soil

or

groundwater (water-soluble

sulfates

and

chlorides)

•

Soil

unit weight

and

buoyant soil unit weight

The

determination

of

type

of

excavation should

be

based upon both cost

and

feasibility. Open cutting

with

a

2:1

slope

(2

horizontal

to 1

vertical)

is the

cheapest method where

it can be

used,

but it is

usually

impractical

in

areas with high groundwater,

or

where

there

are

adjacent structures

or

property

line

restric-

tions. These

and

other factors

may

require sheeted

excavation with bracing

or

tie-backs.

Buoyancy

can be a

major factor

in the

determina-

tion

of

both foundation types

and

structural section.

The

groundwater level

may be

subject

to

seasonal

or

tidal

variation,

and the

level

at the

time that

the

geo-

technical investigation

was

performed

may be

inap-

propriate.

The

design high groundwater level should

be

used

in

designing

the

structure

for

buoyancy.

The

factors

that

can be

used

for

resisting buoyancy

are

• The

weight

of the

structure

• The

uplift

resistant

of

piers

or

piles

• The

buoyant weight

of

soil acting

on the

footing.

The

latter

two

factors require evaluation

by the

geo-

technical consultant.

The

proportion

of

soil mobilized

by

the

footing

to

resist

uplift

can be

determined

by

tak-

ing

the

buoyant weight

of the

prism

of

soil over

the toe

of

the

footing

and

adding

the

effect

of the

soil shearing

stresses acting

on

this prism.

A

simpler

and

more com-

mon

practice

is to use the

buoyant weight

of the

soil

contained

in an

envelope

defined

by the toe of the

foot-

ing and a 30°

projection

(from

the

vertical)

to the

ground

surface. When conventional spread footings

are

used,

it is

usually more

economical

to

generate

uplift

resistance

by

extending

the toe of the

footing than

to

thicken

the

walls

or

slab. Depending upon

the

confi-

dence

in the

design high groundwater level,

a

factor

of

safety

of

1.25

to 1.5

should

be

used

for

buoyancy.

A mat

foundation

is

usually

the

preferred founda-

tion

type unless conditions exist that would preclude

its

use or

make

the mat

foundation cost-prohibitive.

Unstable soils, expected high settlements, expansive

soils,

or

high groundwater with

a

limited footprint

are

all

conditions that

may

lead

to

consideration

of

alter-

native schemes such

as:

•

Piers

•

Piles

•

Caisson.

A

caisson

is a

heavy structure consisting

of the

outer

walls

of the

pumping station built above ground

and

sunk under

its own

weight,

often

with

the aid of

water

jets.

By

excavating

soil

from

the

inside

of the

caisson,

a

controlled soil failure occurs under

the

bot-

tom

"cutting

edge"

accompanied

by a

subsidence

of

the

caisson.

After

sinking

the

structure

to the

required

depth,

a

concrete

floor is

poured

to

form

the wet

well

and

pump room

floor. A

caisson

can be an

economical

pumping station design

in

certain

soil

conditions

or

where adjacent structures

or

property lines

are a

prob-

lem. However, this type

of

construction

is

somewhat

inflexible

for

layout,

and it

must allow

for

some

larger-than-normal

tolerances

on

plumbness.

Design

Criteria

and

Analysis

Loads

The

following loads should

be

considered

in the

design

of

pumping stations:

1.

Lateral earth. Either at-rest

or

active lateral earth

loads should

be

applied

to the

below-grade

walls

of

the

structure. Values

and

criteria

for

determining

which

to use

should

be

obtained

from

the

geotech-

nical

consultant.

2.

Hydrostatic. Lateral loads

from

water

contained

in

the wet

well and/or

from

groundwater.

3.

Seismic.

In

addition

to

seismic forces applied

to the

above-ground

structure (dictated

by

UBC), hydrody-

namic

and

dynamic soil lateral loading should

be

eval-

uated

for

structures

in

Seismic Zones

3 or 4.

Dynamic

soil loading should

be

obtained

from the

geotechnical

consultant.

Hydrodynamic

loading should

be

calcu-

lated

as

described

in

Chapter

6 and

Appendix

F of

Nuclear

Reactors

and

Earthquakes

[6].

4.

Buoyancy.

The

buoyant forces created

from

the

volume

displaced

by the

portion

of the

structure

below high groundwater (see subsection

"Geotech-

nical Considerations").

5.

Surcharge.

If

traffic

or

construction vehicles

are

anticipated adjacent

to the

below-ground structure,

the

associated

lateral loads should

be

applied

to the

walls.

6.

Floor

live

load.

Such loading

is

usually higher than

that

specified

by

building codes. Depending

on the

intended activities,

a

live load between

4.8

kN/m

2

(100

lb/ft

2

)

and 4

times

as

much should

be

used.

Operation

and

maintenance activities involving

equipment break-down, replacement,

or use of

porta-

ble

gantry cranes

can

easily generate

the

higher loads.

7.

Equipment. Most large pumps

and

motors generate

dynamic

or

vibration loading

in

addition

to

their

own

dead weight.

These

dynamic loadings must

be

carefully

evaluated

to

ensure that

no

unforeseen

amplification

occurs

due to

resonant frequencies.

In

general, structural natural frequencies must

be at

least

25%

above equipment excitation frequencies.

These loads must

be

combined

so

that

the

result

is

a

conservative design

in

which both

the

construction

sequencing

and the

potential

future

conditions

are

rec-

ognized.

The

following

design load combinations

are

suggested:

•

Load combination

I:

full

lateral earth

+

surcharge

+

hydrostatic high groundwater

+

dynamic soil

+

floor

dead load. This loading condition should pro-

duce

maximum negative vertical moment

at the

base

of the

walls

and

maximum positive vertical

moment near

the

midpoint

of the

walls.

•

Load combination

II:

same

as

above

+ floor

live

load. This loading should produce maximum nega-

tive vertical moment

at the top of the

wall

as

well

as

maximum

negative moment

in the floor.

•

Load combination III: one-half lateral earth

+ low

lateral groundwater

+ floor

dead load

+ floor

live

load.

This loading should produce

the

maximum

positive

floor

moment. Using half

of the

lateral

earth load

is

rational, because

the

development

of

full

design earth pressures

may

take

a

long time

or

may

never occur

at

all.

•

Load combination

IV: no

lateral earth pressure

but

full

hydrostatic loading

from

the wet

well. Such

a

loading should produce maximum positive

and

neg-

ative

wall moments

in the

opposite

faces

to

that pro-

duced

by

soil

and

groundwater loading. This load

combination

is

meant

to

account

for the

hydrotest

of

the wet

well prior

to

backfilling

the

excavation.

Designers should determine whether this test

can be

performed

with

or

without

the top

slab

of the wet

well

and

note

any

restriction accordingly

on the

drawings.

Wall

Analysis

Pumping stations

are

often

quite deep

and the

loading

of

the

below-grade

walls

is

much greater than

in

con-

ventional buildings. With these high loadings,

one

way

to

limit wall thickness

is to

recognize

the

poten-

tial

for

two-way action (wall spanning both horizon-

tally

and

vertically).

If the

aspect ratio

of the

below-

grade wall

is 3 to 1 or

less, there

is

probably

an

eco-

nomic advantage

in

designing

it as a

two-way wall.

Even

if

walls

are

designed

as

two-way structures,

shear instead

of flexure can

often

become

the

control-

ling

parameter

for

determining wall thickness, because

walls

are not

traditionally reinforced

for

shear. With

deep walls, especially

in

high groundwater, this mind-

set can

result

in

extraordinarily thick walls.

If,

without

reinforcing, shear

criteria

results

in a

wall thickness

that

is

over

25%

thicker than

is

required

for flexure,

shear ties

in the

walls should

be

considered.

There

are

several ways that walls

can be

analyzed

as

two-way structures.

One way is to use a

structural

analysis computer program that permits element load-

ing

perpendicular

to the

element

face.

If

this program

is not

available,

use

Nuclear Reactors

and

Earth-

quakes

[6], "Rectangular concrete

tanks"

[7], Moody

[8],

and

Housner,

[9,

1O].

Engineers should

pay

special

attention

to the

sup-

port

conditions

at the

wall

perimeter.

As

discussed

above,

the floor

slab

may not be in

place

at the

time

the

structure

is

backfilled

or

hydrotested,

so

there

may

be a

free

condition

on the top of the

wall

for

some

loading conditions. Additionally, caution should

be

used

in

assuming

fixed

conditions where rotation

at

the

support could occur

and

result

in

underestimation

of

the

mid-height, positive moment.

Dynamic

Analyses

of

Buildings

The

presence

of

rotating machinery requires

an

analy-

sis of the

effects

of

dynamic loading

or

vibration

on

buildings

—

especially

of

equipment supported

by

ele-

vated

floors. The

principal

focus

of

this analysis

is to

determine

the

natural frequencies

of the

structural sup-

port system.

The

lowest

fundamental

natural

frequency

must

to

compared with

the

excitation

frequency

of the

machinery

to

guard against

the

potential

for

resonance.

As a

rule, resonance

is

unlikely

if the

fundamental

nat-

ural

frequency

falls

outside

a

range

of 0.5 to 1.5

times

the

operating

frequency

of the

equipment.

Most

finite

element structural analysis programs

now

have

the

capability

of

calculating

the

natural fre-

quencies

and

corresponding

mode shapes

of the

struc-

tural

support system.

These

programs

can be

used

to

find the

potential resonant conditions

or to aid in

detuning

the

structure

to

avoid resonance. Avoiding

resonance problems

by

addressing these issues during

design

is

considerably easier than trying

to

solve them

once they have arisen

in a

constructed facility.

An

extensive discussion

of

vibration theory

is

presented

in

Chapter

22.

ACI

350

Code

The use of

either working stress design

or a

modified

strength design

is

allowed

by ACI

350.

An

additional

"service

factor"

by

which

the

normal load

factors

in

ACI 318 are

multiplied

is

used

in the

modified

strength

design.

The

result

is a

larger, more heavily reinforced

section than

if

designed

per ACI

318.

However,

the

modified

strength design usually results

in a

somewhat

more economical section (thinner with less reinforce-

ment) than

one

designed using working

stress.

ACI

350

also prescribes

an

additional

12-mm

(V

2

-

in.)

concrete cover over

the

reinforcing steel, because

it

is

recognized that these structures

function

in a

potentially aggressive environment.

Crack control

and

serviceability (receiving greater

attention

in ACI

350)

is

accomplished

by

reducing

the

value

of the

reinforcing distribution factor,

Z,

from

a

maximum

of 25

kN/mm

(145

k/in.)

for

exteriors

to a

maximum

of 20

kN/mm

(115

kips/in.)

for a

normal

sanitary

exposure

—

even

down

to 17

kN/mm

(95

k/in.)

for

severe sanitary exposure.

The

practical

effect

is

that,

for a

given required area

of

steel, smaller reinforc-

ing

bars

at

closer spacing

are

required.

In SI

units,

for

example,

if the

required area

of

steel

in a flexural

sec-

tion

is

37.5

cm

2

/m,

the use of

number

35M

bars

at 265

mm

spacing would

satisfy

the

strength requirement,

but

d

c

is

71

mm, so Z =

22.2,

a

number that exceeds

the

maximum

of 20.

However,

for

number

3OM

bars

at

180

mm

spacing,

d

c

is 68 mm , so Z =

19.0 and, there-

fore,

the

second choice

for

reinforcement satisfies both

provisions.

In

U.S. customary units,

the

required area

of

steel

is

1.78

in.

2

/ft.,

and

although number

10

bars

at

8.5-in. spacing satisfies

the

strength requirement,

d

c

is

2.72

in.,

and Z =

118.4

—

more

than 115. Number

9

bars

at

6.5-in. spacing with

d

c

at

2.62

in.

yield

a Z of

103,

which

satifies

both requirements.

One

other modification

in ACI 350

aimed

at

crack

control

is a

significant

increase

in

minimum temperature

and

shrinkage reinforcing.

In ACI 350 the

minimum

reinforcing

is

correlated

to the

spacing

of

movement

joints

by

allowing less reinforcing where joints

are

spaced more closely.

The

primary

effect

of

this provision

would

occur

in

sections

where

flexure is not a

controlling

parameter.

As

shown

in

Figure 25-3, even with move-

ment

joints

at a 9-m

(30-ft)

spacing,

50%

more reinforc-

ing

steel

is

required than

in

nonhydraulic

structures.

Concrete

Mix

The

concrete

mix

design

for any

hydraulic structure

subject

to the

same environment

as in

pumping sta-

tions should

be

modified over that specified

for

build-

ing

construction. Design 28-day

compressive

strengths should

be

28,000

kPa

(4000

lb/in.

2

)

mini-

mum

for

concrete exposed

to

freezing

and

thawing

cycles

and

24,000

kPa

(3500

lb/in.

2

)

minimum

for

concrete

not

exposed

to

these cycles. Again,

the

goal

of

these modifications

is to

produce

a

more durable

and

watertight structure.

Cement

In

structures exposed

to

wastewater

or

wastewater

effluent,

the

cement should have

a

C

3

A

content

of

less

than

8%.

Where possible, therefore, ASTM

C

150,

Types

II or V

Portland cements should

be

used.

In

some parts

of the

country, requiring Type

II or V

cement

can

significantly

increase

the

concrete

cost.

Substituting

a

pozzolan, ASTM

C618

(fly ash),

for a

portion

of the

cement

so

that

the

total

C

3

A

content

does

not

exceed

5% can

offer

similar sulfate resis-

tance with Type

I

cement.

The

pozzolan should

not

exceed

20% by

weight

of the

cement plus pozzolan.

Engineers should

be

aware that

the use of

Type

V

cement

or the

addition

of fly ash

frequently suppresses

Figure

25-3.

Wall

shrinkage

reinforcement.

the

early strength gain

of

concrete

and is

therefore

not

always

popular with contractors.

The

long-term concrete

strength

is not

affected.

Additionally,

a fly ash mix can

sometimes

be

more

difficult

to

trowel

finish

than

a

nor-

mal

cement-only mix. These measures should

be

used

only

where they

are

warranted

for

sulfate

resistance,

and

other

mix

designs should

be

allowed elsewhere.

Admixtures

An

air-entraining admixture conforming

to

ASTM C260

should

always

be

specified. Additionally,

it is

frequently

desirable

to use a

water-reducing admixture and/or

a

superplasticizer

to

maintain

a

workable

mix

with

the

reduced

water-cement

ratio required

in

hydraulic struc-

tures.

These admixtures should conform

to

ASTM C494.

Because

of the

concern

for

watertightness,

a

water

reducer-retarder

is

sometimes specified

to

reduce

the

potential

for a

cold joint during

a

large concrete pour.

However,

the

retarding

effect

can

become

a

problem

for

contractors during cold

weather

—

air

temperatures

of

less

than

4

0

C

(4O

0

F).

A

concrete

mix

using

a

retarder

with

set

times

of a

couple

of

hours

in the

summer

can

increase

to 8 to 10

hours

on

cold days. When tempera-

ture becomes

a

problem, allow

for the use of an

alterna-

tive

mix

with

the

water reducer

but

without

the

retarder.

Mix

Proportioning

The two

most important parameters

for

producing

watertight,

durable concrete

are the

cement content

and

the

water-cement

ratio.

A

minimum cement content should always

be

specified

for

concrete used

in

hydraulic structures.

Guidelines

for

minimum cement content

are

given

in

ACI

350. These cement contents

frequently

produce

concrete strengths

significantly

exceeding

the

speci-

fied

24.000

to

28,000

KPa

(3500

or

4000

lb/in.

2

).

Bear

in

mind, however, that

the

high cement content

is not

only

for

strength

but

also helps produce

a

denser,

more chemically resistant concrete.

Controlling

the

water-cement

ratio

is

paramount

in

producing watertight structures. Generally,

the

less water

used,

the

less shrinkage cracking will occur

in the

cured

concrete structure.

The

maximum

water-cement

ratio

should

not

exceed 0.45.

Water-cement

ratios

of

0.43

to

0.38

are not

uncommon when water-reducing admixtures

or

superplasticizers

are

used. When

pozzolans

are

used,

the

water-cement

ratio should

be

calculated

as the

water

weight

divided

by the

cement plus

pozzolan

weight.

It

is

desirable

to use the

largest aggregate size prac-

tical

—

up

to 38 mm

(1.5 in.)

for

concrete

in

hydraulic

structures. Large size aggregate also helps

to

reduce

shrinkage.

As

aggregate size decreases,

the

cement

content should increase

and the

water-cement

ratio

should decrease.

The

maximum size

of

aggregate

allowed should

be

determined

by the

following:

•

One-fifth

the

width

of the

wall,

or

•

Three-fourths

the

clear spacing between reinforcing

bars,

or

•

One-third

the

depth

of the

slab.

Watertightness

In

addition

to the

modified

concrete design

and mix

design, other elements

are

required

to

produce

a

water-

tight structure. These

are

discussed

in

this section.

Waterstops

Waterstops (see Figure 25-4)

are

often

a

troublesome

source

of

problems because

an

amazing number

of

con-

tractors

and

workers

do not

realize that,

to be

effective

at

all, waterstops must

be

fully

continuous with

all

splices

carefully

welded

and not

simply lapped. Corner

splices

and

intersections should

be

shop made. Water-

stops between

floors and

walls should

not be

placed

in a

groove

but in an

upturned

key as

shown

in

Figure

25

-4a.

Waterstops should never

be

permitted

to be

bent

out

of

shape

by

reinforcement. Waterstops

can be

made

of

rubber dumbbells,

ribbed

plastic,

or

steel plate.

In

addition

to the

traditional rubber, plastic,

or

steel

strip—

type

waterstops, there

are

numerous

"second

gen-

eration" waterstops available. Rather than being

a

strip

that

is

cast into

the first and

then

the

adjacent pour, they

are

usually placed

on the

face

of the

joint between pours

by

adhesive

or

mechanically fastening. They rely

on

either adhesion

to the

concrete

or

expansive compression

to

produce

a

watertight joint. These types

of

joints

are

slowly

gaining acceptance. Their advantages

are

that:

•

They

are

often

the

only practical waterstop

for

tying

into existing structures with

a

watertight joint.

•

Some have

a

lower installed cost than traditional

strip waterstops.

Their disadvantages

are

that:

•

Those

that rely solely upon adhesion

may

leak

if

any

subsequent movement occurs

in the

joint.

•

Those that rely

on

their expansive properties must

be

confined

adequately

by the

concrete.

At

least

3

inches

of

cover must

be

provided

or the

expansion

of

the

waterstop

may

break

the

concrete.

• The

adhesives used

to

attach

the

waterstop

to the

first

pour

face

tend

to

perform poorly

in

cold

or wet

weather.

•

Some have

a

higher installed cost than traditional

strip waterstops.

Joints

As

discussed above, location

and

spacing

of

construc-

tion

and

expansion joints

affects

the

amount

of

crack-

ing

that occurs.

The

locations

of

these joints should

either

be

shown

on the

plans

or

parameters

for

their

placement

should

be

given

in the

specifications. Typi-

cal

details

are

shown

in

Figures 25-4

and

25-5.

Walls

exceeding

15 m (50 ft) in

length should have

construction joints spaced

at no

more than

12 m (40

ft).

It is

desirable

to

wait

at

least

5

days between pour-

ing

adjacent panels. This waiting period does

not

delay progress unduly

if

alternating panels

are

poured.

Walls

or

slabs longer than about

37 m

(120

ft)

should

have

expansion joints spaced

no

more than

31 m

(100

ft)

apart.

Slabs should

be

cast

in

panels

not to

exceed

12 m

(40 ft) in

length

or

1000 square

feet

in

area. Further-

more,

the

panels should

be

cast

in a

checkerboard pat-

tern with

at

least

3

days between adjacent panel pours.

An

exception

to

these rules

are

slabs that

can be

cured

by

water-ponding. Good success

has

been obtained

with

very large monolithic pours cured

by

ponding

water

at

least

50 to 75 mm (2 to 3

in.) deep

for at

least

6

days. Shrinkage appears

to be

minimized

by the

unlimited supply

of

free

water during

hydration.

If

concrete

is

placed

from

a

height,

a

tremie should

be

used

to

limit

the

free

drop

to 1.5 m (5 ft)

maximum

to

reduce

the

potential

(1) for

segregation

of the

con-

crete

and (2) for

dislodging waterstops

or

other

inserts. Pumping, however,

is

preferable

to

placing

concrete

by

tremie, because

the

result

is

likely

to be

more uniform, with

fewer

rock pockets.

Exterior

Coatings

for

Groundwater Exclusion

Exterior coatings

are

used

in

pumping stations

for two

reasons:

(1)

corrosion protection

and (2)

waterproof-

ing for

keeping interior walls dry. Many

of the

corro-

sion coatings

can

serve both purposes.

See

Section

25-

1 1

for the

application

of

coating

and

linings inside

the

wet

well.

The

three

main types

of

waterproof coatings are:

•

Sprayed, brushed,

or

rolled-on

coatings

•

Membrane systems

•

Cementitious crystalline waterproofing.

With

new

regulations

on

volatile organic com-

pounds,

the

sprayed

or

rolled-on coating

field is

chang-

ing

rapidly. Spray-on coal-tar coating

has

traditionally

been

the

low-cost damp-proofing used where ground-

water

was

either

not

present

or

only

intermittently

Figure 25-4.

Details

of

construction

joints

with

waterstops.

(a)

Upturned

key;

(b)

wall

joint;

(c)

expansion

joint

in a

base

slab.

present.

The new

higher-solids epoxies that have taken

their place

are of

higher quality

at

higher cost. Unless

there

is a

potential

for

corrosion, other methods

for

waterproofing

are

desirable.

Cementitious

crystalline waterproofing

is

sprayed

or

brushed onto

new

concrete prior

to

complete cur-

ing.

It

reacts with

free

water

and

forms

a

crystalline

matrix

that seals

the

pores

and

capillaries

of the

con-

crete. These systems perform quite well

in

most con-

ditions

at

relatively

low

cost.

Membrane

systems consist

of

sheets

or

panels

that

are

applied

to the

concrete surface

to act as an

impermeable

barrier

for

ground water. Some systems

actually

employ clay panels that swell with contact

to

water providing

an

impermeable barrier.

In

gen-

eral,

membrane systems

are

more expensive

but

more

effective

than crystalline waterproofing.

Details

This section contains miscellaneous aspects

of

details

common

to

pumping stations.

As

with

any

facet

of

structural

design, there

are

many variations

of

these

Figure 25-5.

Concrete

joint

details,

(a)

Shear

friction

joint;

(b)

horizontal

corner

bars

for

walls;

V

u

<

0

V

R

<

0.2

/^A

0

<

800

A

c

;

<|>

=

0.85

for

shear

and

tension;

V

n

=

A^f

y

\i;

\L

=

1.0

for

rough

surfaces.

Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

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