Moser A.P., Folkman S. Buried Pipe Design
Подождите немного. Документ загружается.
permanent material, such as chemically treated wood, for blocking.
Store small pipe in racks according to length and size. Store pipes
larger than 2-in diameter by stacking them with spacing strips placed
between each row. Withdraw pipe from the top rows.
Shoring and bracing
Use proper shoring and bracing to prevent cave-ins while vaults or
similar openings are under construction. Proper shoring cannot be
reduced to a standard formula. Each job is an individual problem and
must be considered under its own conditions. Federal and state or
provincial standards list specific recommendations for shoring of exca-
vations. The worker should take the following general precautions:
1. Do not take chances that may lead to injury.
2. Either use tight sheet shoring to guard against the caving in of
sandy soil or loose material when the depth of the excavation
exceeds 5 ft, or cut back the bank to the proper slope. Keep
shoring at or near the bottom of the ditch as it is excavated and
follow with bracing to ensure safety. Trench shields are also
acceptable as a protective system. A trench shield does not protect
the environment, only the worker.
3. The placement of shores will depend on the type (classification) of
soil encountered. Local, state or provincial, and federal laws man-
date the distances and sizing of shoring support systems.
4. Extend shoring of any type below the excavation bottom when-
ever possible, and brace it thoroughly using timbers, wedges, and
cleats, or a pipe/screw-jack combination. Place all bracing at right
angles to the sheeting or uprights and rigidly wedge, bolt, or cleat
it to prevent movement. Hydraulic units are being used in many
types of utility-trench construction.
5. Use only full-sized lumber that is assessed to be sound and
straight.
6. Install the upper braces or screw jacks first, and remove them
last for best protection.
7. Also consider excavation dimensions, soil stability, variable weather
and moisture conditions, proximity of other structures, weight and
placement of soil and equipment used on the job, and sources of
vibration when choosing the type of shoring to use, if any. The deci-
sion must rest with the engineer or foreman in charge.
8. Use hydraulic jacks temporarily only, and replace them with
properly sized screw jacks or solid bracing.
Pipe Installation and Trenchless Technology 529
Hard hats
Workers should always wear hard hats, especially subsurface workers.
Lifting
Personnel should not be required to do heavy lifting that may cause
injury; use mechanical lifting devices to raise, lower, or suspend heavy
or bulky material when working in trenches, manholes, or vaults.
Safe distance
Keep a safe distance from other workers to avoid striking them with
tools.
Ladders
Use ladders where required. Do not jump into an excavation.
Adequate means of trench exit
Provide an adequate means of trench exit, such as a ladder or steps.
Locate it so no more than 25 ft of lateral travel is required. Extend the
ladder from the bottom of the excavation to at least 3 ft above the
ground surface.
Edge of excavation
Do not place excavated material closer than 2 ft from the edge of an
excavation.
Falling tools
Falling tools are a danger to workers in the trench. Keep all tools,
working materials, and loose objects orderly and away from the exca-
vation shoulder.
Keep open traffic lanes clear
Keep tools, equipment, and excavated material out of open traffic
lanes. Continually remove pebbles and small stones from, or prevent
them from lodging on, a hard-surface roadway where tires may pick
them up and throw them.
Posting barricades and warning signs
Provide and maintain all necessary barriers, watchmen and flaggers to
protect workers, vehicles, and pedestrians.
530 Chapter Eight
1. Place advance warnings, instructional signs, barricades, and
delineators well ahead of the construction area to warn motorists
and pedestrians of the area and safely take them through or past
it. All such protection devices must meet the appropriate federal,
state or provincial, or local specifications for size, shape, color, and
placement.
2. Protect the work area with barricades, barriers, or planks to pro-
vide a safe working space. If necessary, use flaggers to direct and
slow down traffic. When used, place trucks or air compressors
between the work and the traffic.
3. During periods of reduced visibility, use adequate lighting on all
barricades.
4. When no work is in progress, place adequate barriers, barricades,
flashing lights, and signs to warn and divert traffic. Use reflect-
ing tape on all barricades.
5. In winter, divert traffic, if necessary, from streets covered with
surface ice resulting from a main break until sanding or scarify-
ing restores safe driving conditions.
6. All personnel should wear protective clothing including hard hats
and high visibility traffic vests.
Debris in excavation
If the walls of an excavation contain glass, wire, or other sharp objects,
carefully remove them.
Heavy rains or freezing weather
When resuming excavation after heavy rains or freezing weather,
inspect all banks for cracks. These may indicate earth movement and
the probability of cave-in.
Cave-ins
Frequently inspect the sides and rim of all open excavations to guard
against cave-in. Operate earth-moving equipment from a position that
will not imperil personnel or property by a cave-in due to vibration,
stress, or dead weight.
Overhanging bank
If it is absolutely necessary to work above an overhanging bank, use
a safety belt and a lifeline. Have a helper nearby to assist in an
emergency.
Pipe Installation and Trenchless Technology 531
Other utility lines
To avoid striking electric or telephone conduits, gas lines, or other sub-
structures, locate other utility installations before starting work.
Protective clothing
Require workers to wear adequate eye, ear, and foot protection when
using a jackhammer or when exposed to flying particles or falling objects.
Machines
Workers should always be aware of locations of running machines (back-
hoes, trenching machines, etc.). Workers should keep clear of the sweep
path and try never to turn their backs toward the working machine(s).
Work breaks
Take work breaks, rests, etc. at designated locations away from the
excavation.
Trenching machines
The following rules apply equally to all mechanical devices used to dig
trenches and/or make excavations including various types of
trenchers, backhoes, buckets, scoops, and similar pieces of equipment.
1. Operators should always wear hard hats.
2. Never attempt to oil or grease a mechanism or repair or adjust
any moving part of a trenching machine while it is in operation.
Only qualified personnel should operate a trenching machine.
3. Guard all moving parts. Before starting the conveyor, make sure
that no person is endangered by it.
4. To remove obstructions from the conveyor mechanism or buckets,
stop the machines.
5. Be alert for falling material that might roll from the conveyor.
6. When practicable, drop dirt between the excavation and the high-
way to act as a barrier.
7. Cautiously fill gasoline or diesel tanks. Keep spout in metallic
contact with the machine to prevent static sparks from bridging
the gap and igniting the vapors. Do not smoke. Keep proper fire
extinguishers available when refueling construction equipment.
Use only approved containers when storing flammables on the job
site; clearly mark and define storage areas.
532 Chapter Eight
8. Use flags by day and flashing lights or flares by night to warn the
public of the trenching machine and its operations. Liberally use
these precautions on all highway or street work. Plan the warn-
ing system before the work is started.
9. Operate the machine vertically to prevent undercutting the
trench walls.
10. When loading or unloading trenching machines or other heavy
equipment from truck beds, lowboys, or other conveyances, pro-
vide suitable skids and ample blocking to prevent movement of
the conveyance.
11. When manually lifting or lowering pipe in an excavation, use two
or more rope slings looped under the pipe and handle from each
side of the excavation. To prevent a heavy pipe from pulling work-
ers into the excavation, anchor one end of each rope sling to a
massive object such as a truck.
12. When aligning pipe in the excavation, either manually or
mechanically, keep hands and fingers away from ends of pipe and
other substructures that could crush.
13. Govern crane operations only by the signals of a qualified
worker.
14. Never try to catch and hold a length of pipe that slips from a
crane or hoist sling.
15. Be alert to unsafe excavation sides when measuring, testing, or
inspecting pipe in place on an excavation bottom.
16. When cutting sections of pipe, keep feet in the clear and use ade-
quate blocking, chocks, or pipe vises to prevent pipe movement.
Wear safety goggles.
17. Keep tools and appliances in good condition for handling, cutting,
threading, or treating pipe. Always use the right tool for the job.
18. Do not let tools or materials become stumbling hazards where
pipe is being handled.
19. Avoid shortcuts and makeshift methods that may increase the
hazards of handling pipe.
Blasting operations
Only authorized and experienced employees may use explosives. These
employees must conduct blasts in accordance with nationally recog-
nized good practices. Always heed the following principles for avoiding
accidents when using explosives:
Pipe Installation and Trenchless Technology 533
1. Train these people properly. Handle explosives carefully and with
respect. The fewest possible people should handle explosives to
reduce the risk of accident. Choose only those with good judgment
to handle explosives.
2. Have the explosive manufacturer’s technical representative
instruct the field crews in all blasting practices.
3. Rigidly enforce all safety regulations.
4. Do not use a two-way radio near blasting areas, as it might pre-
maturely detonate a charge.
5. Open kegs or cases of explosives only outside and away from the
magazine.
6. Use wooden, rubber, or fiber tools to open cases of explosives.
7. Cut the fuse long enough to extend at least 2 ft beyond the collar
of the hole to allow time to get safely away. The minimum length
of a safety fuse is 36 in.
8. Use a standard cap crimper, making sure that the cap is securely
fastened to the fuse.
9. Under wet conditions, thoroughly waterproof the joint between
fuse and cap.
10. Always keep the fuse free of kinks.
11. Use sufficient stemming to protect explosives from the end spit of
a fuse or flying matchheads.
12. After a blast, permit only an experienced powderman to work in
the area until it is definitely proven to be safe.
13. Burn empty explosives cases in the open to prevent them from
being used as fuel.
Storing explosives
Always purchase, possess, store, transport, handle, or use explosives in
accordance with local, state or provincial, and federal regulations. A
few rules follow:
1. Store explosives only in a magazine that is dry, well ventilated,
properly located, substantially constructed, and securely locked.
Keep the area within 25 ft of magazine clean and clear.
2. Prohibit smoking, carrying of matches, open lights, or other fire or
flame in or near a magazine or while explosives are being handled.
3. Store only explosives in a magazine; leave all other materials
outside.
534 Chapter Eight
4. Replace the cover on a partially used package or case of explosives.
5. Store all cases of dynamite so that cartridges lie horizontally.
6. Store blasting caps or electric blasting caps in a box, container, or
magazine separate from other explosives.
7. Protect blasting caps or electric blasting caps from the direct rays
of the sun.
8. Store fuse or fuse lighters in a cool, dry place away from any flam-
mable liquids.
Working in confined spaces
Underground structures, such as manholes and vaults, may have con-
taminated air. Workers have died in manholes contaminated by gas
from a leaking gas main or by methane from decaying organic matter.
Do not enter an underground structure without first assuring that the
air is safe. Follow these precautions:
1. Use proper tools for opening the manhole or vault and handling
the cover to prevent foot and back injuries.
2. Exercise every precaution to protect the work area from traffic
hazards. Barricades, signs, high-level warning devices, and lights
should meet local and state or provincial regulations to ade-
quately warn traffic.
3. Continually station an attendant at the manhole entrance.
Manhole entrants should wear a lifeline and harness.
4. Prohibit smoking in or about a manhole.
5. The attendant should be knowledgeable about safety procedures.
He or she should have immediate access to rescue respiratory
equipment and should maintain communication with the person
inside the confined space. A two-way radio is handy for obtaining
emergency help, if needed.
6. Train all employees working in or near confined spaces in proper
work procedures, confined space hazards, and rescue procedures.
7. Use approved equipment and methods to verify the absence of
harmful or toxic gases in an underground chamber before per-
sonnel enter. Do not consider safe any underground or confined
structure until it has been demonstrated to be free of harmful
gases and to contain sufficient oxygen to sustain life. Use an
approved device to determine oxygen deficiency and concentra-
tions of toxic or flammable gases. Periodically calibrate all moni-
toring or indicating equipment, and maintain records.
Pipe Installation and Trenchless Technology 535
8. Provide adequate and continuous ventilation to ensure sufficient
fresh air for personnel within a vault or manhole. When a blower
is used for this purpose, place the discharge end near the bottom
of the manhole to force the air up and out.
9. Prevent surface water or debris from accidentally entering the
vault or subsurface during work.
Trenchless Technology
Introduction
In modern American cities, piping services are complex and mar-
velous. But average city dwellers don’t know about buried pipes, could
care less, and simply take them for granted. They cannot contemplate
the consequences if these services were to be disrupted. Cities can
improve only to the extent that city service systems are improved.
Improvement is slow because buried pipes are out-of-sight and, there-
fore, out-of-mind to planners and to sources of funding. Without main-
tenance of the piping systems, cities can only deteriorate. Without
well-maintained cities, the quality of human life deteriorates.
Pipeline engineers and city managers are sobered by the present-
day reality of deteriorating pipe systems. Leaks in buried (out of sight)
sewer pipes are either overloading treatment plants or are charging
the soil and groundwater with contamination. The first thought is to
replace the pipes. But many sewers have served so long that they are
over-grown with streets and buildings. Excavation and replacement
become an unattractive remedy.
Among the alternatives to replacement by excavation, are trenchless
technologies. Small diameter gas lines are being jetted into place. Large
diameter traffic tubes and tunnels are being bored into place and lined.
Moles and directional drilling are evolving with remarkable success.
Might something be done to rehabilitate existing pipelines? In fact,
many sewer lines could handle increased sewage loads: (1) if ground-
water infiltration were eliminated by stopping leaks; and (2) if flow
rates could be increased by smooth-lining the pipes. Plastic pipe
inserts are successful and attractive. They can be inverted, folded, or
swaged, then inserted, inflated, and heated to thermoset the plastic.
Leakage is stopped. Plastic inserts provide resistance to corrosion and
to abrasion of sediment flushed along the pipe. Plastic inserts even
contribute significantly to the structural integrity of the conduit.
But plastic has lower strength and lower stiffness than do most of
the older traditional materials. So how do flexible plastic pipes hold up
under external water pressure? If leaks are stopped by inserting a
plastic liner into a deteriorated sewer pipe (casing), groundwater no
536 Chapter Eight
longer drains into the sewer pipe and the water table rises. Still the
casing leaks, so external water pressure must be resisted by the liner.
The conditions exist for buckling of the liner if external pressure is
increased. A typical scenario for failure is the following:
The empty liner floats up leaving a gap on the bottom where the
external pressure (head h) is greatest. The liner is flattened a bit on
the bottom because the perimeter shrinks under pressure.
Consequently, the radius of curvature is increased. Both the increased
radius of curvature and the loss of support, at the point where pres-
sure is greatest, are the conditions for buckling of the liner. If pressure
is increased, the liner will buckle. Because of plastic creep over the
long term, the perimeter shrinks even more over time and the condi-
tions for buckling worsen. What is the time to failure? What is the
decrease in failure pressure in fifty years—or a hundred years?
Tests at USU have given some answers to these questions. Failure was
defined as the maximum pressure when the liner is just on the verge of
buckling. Buckling is reversal of curvature. It is the result of instability
and might be initiated by a slight glitch (holiday) in the material of the
liner, by a slight deviation of the shape, or over a period of time.
Data from a report, “Long Term External Hydrostatic Pressure
Testing of Encased Insitupipes,” show that long-term failure pressure
is about half the short-term (quick-load) failure pressure. The half-ratio
of long-term to short-term failure pressures applied to all Insitupipes
test with approximately the same D/t ratio. With ample safety factor,
long-term design can be based on the half-ratio rule of thumb.
Except for an allowance for long-term plastic creep, the structural
performance and performance limits of plastic pipes are based on the
same generic properties required of all flexible pipes—including met-
als, composites, etc. Of course, pipe performance must not exceed per-
formance limits. We refer to performance limits rather than failure
because failure implies rupture or complete collapse. Performance lim-
its usually fall short of failure. Performance limit is usually defined as
excessive deformation of the pipe. Deformation includes rupture, buck-
ling, ring deflection, puncturing, denting, etc.
Design of pipe liners
For a pipe liner in a casing, internal pressure is usually of no concern.
Even if the liner inflates, it is confined by the casing like an innertube
in a tire. External pressure on the liner causes ring compression stress
of P(D
O
)/2t where P is the external pressure, D
O
is the outside
diameter, and t is the wall thickness. The ring compression stress must
be less than yield strength of the pipe wall. If steel has a yield strength
eight times as great as PVC, then the PVC pipe liner wall must be eight
Pipe Installation and Trenchless Technology 537
times as thick as the equivalent steel pipe liner wall. This can be
demonstrated by a section of pipe placed in shaped blocks and loaded
to crushing of the pipe wall.
Buckling of the pipe liner wall is more complicated. It depends upon
both the yield strength of the pipe wall and the pipe stiffness. But pipe
stiffness depends upon wall thickness, modulus of elasticity E, shape,
and the degree of confinement by the casing. Because there are so
many interactions, wall buckling of the liner is best found by experi-
ence, either from tests or from performances and failures in service.
Bases for evaluation of liners
Structural evaluation of liners must include wall strength to resist
ring compression and pipe stiffness to resist buckling. Both rigid lin-
ers (such as mortar liners) and flexible liners (such as plastics) must
meet the same requirements. Both must be designed for long-term ser-
vice. Long-term service includes deterioration, corrosion, abrasion, and
creep in the case of plastic liners. Long-term service for creep means
long-term, persistent pressure. Design by ring compression is based on
long-term strength. If pressure is only instantaneous, ring compres-
sion design is based on short-term strength. Short-term external pres-
sure may be caused by a sudden vacuum inside the liner. It should be
emphasized that testing is important in order to evaluate performance
and performance limits of liners.
Liners in broken casings
The question arises, do liners reestablish any of the original strength of
broken casings? More often than not, the soil backfill retains the casing
which continues to perform as a conduit. Of course, if horizontal soil sup-
port were lost, the pipe would collapse. Collapse could occur if sidefill soil
was fine enough to be washed into the pipe through the cracks leaving
voids on the sides of the casing. But what if the ring deflection of the cas-
ing were to increase—say due to increased surface loads or due to partial
loss of horizontal side support? The results of tests performed at Utah
State University show that for a typical Insitupipe installation in pre-bro-
ken pipes, the vertical soil load at any given pipe deflection is roughly one
and a half times greater for the casing with the Insituform lining than for
the casing with no lining. This is significant increase in strength in the
event that ring deflection increases. If there is no increase in ring deflec-
tion, at least the margin of safety is increased by roughly half.
Design specifications for plastic inserts should be based on proven per-
formance—a track record. In general, design specifications are either
procedural or performance. Procedural specifications spell out the details
of manufacture and installation. Performance specifications describe the
538 Chapter Eight