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521

Pipe Installation and

Trenchless Technology

Introduction

This chapter briefly discusses a number of the more common require-

ments of installation, omitting precise details that vary in individual

installations. Also included are some safety aspects of pipeline con-

struction; however, a general treatise on safety is outside the scope of

this text. The use of trenchless methods for installing pipe and reha-

bilitating pipe is becoming more common and some information on

techniques used is included in this chapter.

The construction of a pipeline depends on many controlling factors,

including pipe materials, trench depth, topography, soil conditions, and

operating conditions. The properties of the soil being excavated and

soil used as backfill in the pipe zone are particularly important. How

the pipe is handled and installed can have huge effects on its external-

load carrying capacity and can be a controlling factor in the design of

the pipe. How the pipe supports the loads from handling, soil cover,

and water must be determined when the pipe installation is designed.

If the cover or other external load on the pipe is high, the degree and

uniformity of bedding support can have a substantial influence on the

required pipe strength.

Transportation

Delivery of the pipe to the job site is usually considered part of the

installation process. Requirements for packaging, stowing, restraining

Chapter

pipe during transit, unloading, and handling during the installation

process are all important considerations. Transporting by railroads, on

water via ships, and by trucks all present complications, but can be

overcome if given the proper considerations in advance of shipping.

Most pipes shipped by truck are carried on flat-bed trucks and trailers

directly to the job site. Often damage is done to the pipe by tie-down

equipment that is overly tensioned. One-time handling between ship-

per and customer will often avoid damage encountered by multiple

loadings and unloadings. Whether the pipe is delivered directly to the

jobsite or placed in temporary storage areas, care should be taken to

place the pipe so it can be reached for movement to the trench with as

little extra handling as possible. Also, every precaution should be taken

to prevent damage to the pipe. Pipe ends are particularly vulnerable to

damage from impact or point loading that may result from contact with

construction equipment, rocks, or other obstacles on the ground.

When nesting smaller diameter pipe inside larger pipe, the nested

pipe will usually be padded to protect pipes from damage. Stacked

and/or nested pipe should be prepared with sufficient stringers so that

high concentrated loads are not applied to a single bearing point.

Pipe should, at all times, be handled with equipment designed to

prevent damage to either the inside or outside surface of the pipe. Care

should be used in loading and unloading so as not to damage the pipe.

Equipment to be used for handling pipe includes nylon straps, wide

canvas or padded slings, wide padded forks, and skids designed to pre-

vent damage. Unacceptable items include cables, hooks, narrow forks,

unpadded chains, sharp edges on buckets, and metal bars. The place-

ment of pipe along a rough right-of-way could damage the pipe.

Necessary support to the pipe should be supplied. The pipe may be laid

on sandbags, mounds of sand, wood blocks (padded if necessary), or

other suitable supports to protect the pipe. Supports should be about

one-quarter length from each end. It is usually not acceptable to allow

pipes to roll or fall from the truck to the ground.

Trenching

If the pipe-zone bedding and backfill require densification by com-

paction, the width of the trench at the bottom of the pipe should be

determined by the space required for the proper and effective use of

tamping equipment. Where the sides of the trench will afford reason-

able side support, the trench width that must be maintained at the top

of the pipe is the narrowest practical width that will allow proper den-

sification of pipe-zone bedding and backfill materials, regardless of the

depth of excavation. The space between the pipe and trench wall must

be wider than the compaction equipment used in the pipe zone.

522 Chapter Eight

Minimum width shall be not less than the greater of either the pipe

outside diameter plus 16 in or the pipe outside diameter times 1.25,

plus 12 in. The effect of the trench width on the performance of the

pipe is dependent on the type of pipe and is discussed in Chap. 3.

Safety considerations are of the utmost importance. Where possible,

sloping the sides of the trench above the top of the pipe to the ground

surface may be desirable if costs associated with sheeting and bracing

can be reduced. Specially designed equipment may enable the satis-

factory installation and embedment of pipe in trenches narrower than

specified above.

Depth of trenches in city streets may be governed by existing utilities

or other conditions. Where no other requirement is provided, the mini-

mum cover should be generally selected to protect the pipe safely from

transient loads where the climate is mild and should be determined by

the depth of the frost line in freezing climates. The profile should be

selected to minimize high points where air may be trapped. With favor-

able ground conditions, excavation can be accomplished in one opera-

tion; under more adverse conditions it may require several steps.

The trench bottom should receive careful attention and adequate

provisions for maintaining grade. Typically, the trench bottom is exca-

vated to a depth of at least 2 in and more typically 4 in below the estab-

lished grade line. The bottom is brought to grade with material in

which all stones and hard lumps have been removed. This bedding

material should be firm, stable, and uniform along the pipe. In some

soils, this bedding under the invert can be achieved by raking the

trench bottom with the backhoe teeth to loosen the soil. The bedding is

then brought to grade by the workers in the trench.

If excavation requires blasting such as in hard rock, the sharp rock

in the bottom of the trench may cause damage to the pipe. In such

cases, the trench bottom should be excavated 6 in below grade and a

bedding of crushed rock or sand should be used to establish grade.

For unstable foundations, the foundation material should be

removed to a sufficient depth. This should be done under the direction

of a soils engineer. Excavate to a depth as required by the engineer and

replace with a foundation of ASTM Class IA, Class IB, or Class II

material (see Chap. 2). Use a suitably graded material where condi-

tions may cause migration of fines and loss of pipe support. Place and

compact foundation material. Control of unstable trench bottom con-

ditions may be accomplished with the use of appropriate filter fabrics.

Place pipe and fittings in the trench with the invert conforming to

the required elevations, slopes, and alignment. Provide bell holes in

pipe bedding, no larger than necessary, in order to ensure uniform sup-

port. Fill all voids under the bell by working in bedding material. Also,

excavation for sling removal should be provided to permit removal of

Pipe Installation and Trenchless Technology 523

the slings without damaging the pipe. In special cases where the pipe is

to be installed to a curved alignment, maintain angular “joint deflection”

(axial alignment) or pipe bending radius, or both, within acceptable

design limits. Minimize localized loadings and differential settlement

wherever the pipe crosses other utilities or subsurface structures, or

whenever there are special foundations such as concrete capped piles or

sheeting. Provide a cushion of bedding between the pipe and any such

point of localized loading. If trench sidewalls slough off during any part

of excavating or installing the pipe, remove all sloughed and loose mate-

rial from the trench.

The primary function of trench boxes, sheeting, and bracing is for

safety to prevent a cave-in of the trench walls or areas adjacent to the

trench. In noncohesive soils combined with groundwater, it may be

necessary to use steel sheet piling to prevent soil movement.

Continuous steel sheet piling can be installed so that it is relatively

watertight and, if necessary, dewatering with trench-bottom sump

pumps can be undertaken.

In some soil conditions, it is economical and practical to use a pre-

fabricated unit that is at least as long as one section of pipe. The units

are called laying shields, trench shields, or trench boxes. Such a box is

pulled forward as the trenching and pipe laying progresses. These

movable supports should not be used below the top of the pipe zone

unless approved methods are used for maintaining the integrity of

embedment material. They protect workers from sloughs and cave-ins.

They do not support the trench walls. Before moving such a device for-

ward, place and compact embedment soil to sufficient depths to pro-

vide necessary support for the pipe. As the shield is advanced forward,

the placement and compaction of the embedment soil at the rear of the

device should be completed.

The design of the system of supports should be based on sound

engineering principles of soil mechanics and the materials to be used,

and the design must comply with applicable safety requirements.

Normally, supports are left in place unless otherwise directed by the

engineer. Sheeting driven into or below the pipe zone should be left in

place to preclude loss of support of foundation and embedment mate-

rials. If sheeting is to be removed, especially heavy sheeting, consider-

ation must be given to the additional soil loads that may be

transferred to the pipe. Make sure that the pipe, foundation materials,

and embedment materials are not disturbed by support removal. If

pulling leaves voids, fill all voids with same materials and compact to

required densities.

When top of sheeting is to be cut off, make the cut 1.5 ft (0.5 m) or

more above the crown of the pipe. Leave rangers, whalers, and braces

in place, as required, to support cutoff sheeting and the trench wall in

524 Chapter Eight

the vicinity of the pipe zone. Timber sheeting to be left in place is con-

sidered a permanent structural member and should be treated against

biological degradation (for example, attack by insects or other biologi-

cal forms), as necessary, and against decay if above ground water. A

note of caution: certain preservative and protective compounds may

react adversely with some types of rubber ring gaskets and certain

thermoplastics, and their use should be avoided in proximity of the

pipe. All applicable local, state, and federal laws and regulations

should be carefully observed including those relating to the protection

of excavations, and the safety of persons working therein.

Dewatering

Groundwater can be a serious hindrance during excavation, pipe lay-

ing, and backfilling. If properly planned for in advance of construction,

difficulties associated with groundwater can be minimized. Maintain

water level below pipe bedding and foundation to provide a stable

trench bottom. It is important to ensure the ground water is below the

bottom of cut at all times to prevent washout from behind sheeting or

sloughing of exposed trench walls. Where feasible, the trench should be

dewatered until the pipe has been installed with the prescribed bed-

ding and backfill has been placed to a height at least above the ground-

water level. For dewatering smaller volumes of water, the trench may

be over-excavated and backfilled to grade with crushed stone or gravel

to facilitate drainage of water to the point of removal. Dewatering a

large amount of groundwater will require the use of a well-point sys-

tem consisting of a series of perforated pipes driven into the water-

bearing strata and connected to a header pipe and pump. Control

running water emanating from drainage of surface or ground water to

avert undermining of the trench bottom or walls, the foundation, or

other zones of embedment. Provide dams, cutoffs or other barriers

periodically along the installation to preclude transport of water along

the trench bottom. If needed, well graded materials, along with perfo-

rated underdrains, can be used to enhance transport of running water.

The gradation of the drainage materials should be selected to mini-

mize migration of fines from surrounding materials. Backfill all

trenches after the pipe is installed to prevent disturbance of pipe and

embedment.

Pipe Installation

The pipe trench should be kept free from water that could impair the

integrity of bedding and joining operations. While placing pipe in the

trench, slings should be used and the pipe should not be dragged along

Pipe Installation and Trenchless Technology 525

the bottom of the trench. It should be supported by the sling while

preparing to make the joint. Pipe should be laid to lines and grades

shown on the contract drawings and specifications. The pipe must

never be struck with the excavating bucket or other equipment acci-

dently or on purpose to drive the pipe to grade. Such impact loads will

damage the pipe wall, the interior wall, or coating. Such damage is

often not visible on the outer surface but will result in eventual pipe

failure.

Pipe is normally assembled in the trench except under the most

unusual conditions. Pipe that has O-ring rubber gaskets as seals

must be assembled section by section in the trench. Smaller-diame-

ter pipe, joined by welding or couplings, may be assembled above

ground in practicable lengths for handling and then lowered into the

trench by suitable means which allows progressive lowering of the

assembled run of pipe. If the method of assembling pipe above

ground prior to lowering it into the trench is used, care must be

taken to limit the degree of curvature of the pi pe during the lower-

ing operation so as to not exceed the yield strength of the pipe mate-

rial and/or damage the lining or coating materials on the pipe. Pipe

deflection, at any joint, should be limited to the manufacturer’s rec-

ommendation during the lowering operation. Work normally should

proceed with the bell end of the pipe facing the direction of laying.

The bell and spigot should both be thoroughly cleaned and lubri-

cated in accordance with the pipe manufacturer’s recommendations

before the spigot is inserted in the bell. Following assembly, the pipe

joint should be checked to determine that the proper insertion depth

has been achieved. Most manufacturers have a mark on the spigot

end. This joint should be mated so this mark is just at the bell, not

inside the bell. Also, a thin metal feeler gauge should be used to

ensure that proper gasket placement exists. A gasket that has been

rolled out of its groove is called a fish-mouthed gasket and such a

joint will leak.

Making the Joint

For elastomeric seal joints, verify that pipe ends are marked to indicate

insertion stop position and ensure that pipe is inserted into pipe or fit-

ting bells to this mark. Push spigot into bell using methods recom-

mended by the manufacturer, keeping pipe true to line and grade.

Protect the end of the pipe during homing and do not use excessive

force that may result in over-assembled joints or dislodged gaskets. If

the force required for insertion is excessive, the gasket is probably

rolling from its groove. In such a case, disassemble and clean the joint

and reassemble. Use only lubricant supplied or recommended for use

526 Chapter Eight

by the pipe manufacturer. Do not exceed manufacturer’s recommenda-

tions for angular “joint deflection” (axial alignment).

For solvent cement joints, follow recommendations of both the pipe

and solvent cement manufacturer. If full entry is not achieved, disas-

semble or remove and replace the joint. Allow freshly made joints to

set for the recommended time before moving, burying, or otherwise dis-

turbing the pipe. Make sure joining area is well ventilated.

For heat fusion joints, the process should be in conformance with the

recommendations of the pipe manufacturer. Pipe may be joined at

ground surface and then lowered into position, provided it is supported

and handled in a manner that precludes damage.

Thrust Blocks. ( See Chap. 4.)

Pipe-Zone Soil. (See Chaps. 2 and 3.)

Bedding and Backfill

Bedding and backfill should be brought to the specified density around

the pipe and to the specified height over the top of the pipe. In-place

tests of soil density should be made as required by the engineer. To

guard against loss of pipe support from lateral migration of fines from

the trench wall into open-graded embedment materials, it is sufficient

to follow the minimum embedment width guidelines found in ASTM

D2321. Maximum particle size should be limited to 3/4 inch or less.

This enhances placement of embedment material for nominal pipe,

sizes 8-inch and larger. For smaller pipe, a maximum particle—size of

about 10% of the nominal pipe diameter—is recommended. All back-

fill materials should be free of lumps, clods, boulders, frozen matter,

and debris. The presence of such material in the embedment may pre-

clude uniform compaction and result in excessive localized loads and

deflections.

When coarse and open-graded material is placed adjacent to a finer

material, fines may migrate into the coarser material under the action

of hydraulic gradient from ground water flow. Field experience shows

that migration can result in significant loss of pipe support and contin-

uing deflections that may exceed design limits. Significant hydraulic gra-

dients can arise in the pipeline trench during construction when water

levels are being controlled by various well-pointing methods or after con-

struction when permeable underdrain or embedment materials act as a

“french” drain under high ground water levels. The gradation and rela-

tive size of the embedment and adjacent materials must be compatible

in order to minimize migration. In general, where significant ground

Pipe Installation and Trenchless Technology 527

water flow is anticipated, avoid placing coarse, open-graded materials,

such as Class IA, above, below, or adjacent to finer materials, unless

methods are employed to impede migration such as the use of an

appropriate stone filter or filter fabric along the boundary of the

incompatible materials.

Embedment Density

(See Chap. 3 for details.) Embedment density requirements should be

determined by the engineer based on deflection limits established for

the pipe, pipe stiffness, and installation quality control, as well as the

characteristics of the in-situ soil and compactibility characteristics of

the embedment materials used. For the design of a particular instal-

lation, the project engineer should verify that the density he or she

specifies will produce the desired pipe performance.

The engineer should not specify densities higher than required.

Achieving soil densities that are much higher than required is a waste

of money and it is usually the taxpayers money. Specify what is

required and then have good field inspection to assure that the design

assumptions are met. The densification of the backfill envelope must

include the haunches under the pipe to control both the horizontal and

vertical pipe deflections. There are several methods used to achieve a

required density. These are listed in Chap. 3.

Safety Procedures for Construction

and Related Activities

Introduction

Safety measures have always been important to protect both workers

and the public. Safety has gained increasing attention in the United

States since the advent of the Occupational Safety and Health Act. It

is imperative that responsibility for safety be assigned to an authori-

tative person who has full knowledge of the rules, regulations, and

requirements of federal, state, and local agencies. A project can be shut

down, for what may seem to be even a minor infraction of the safety

act. This section is only a brief outline of some safety concerns associ-

ated with pipeline construction and in no way should it be considered

official, inclusive, or definitive.

Pipe storage

Keep pipe yards and walkways clean and orderly. Always block pipe to

prevent it from rolling or falling. Arrange and block each row of

stacked pipe to prevent it from rolling from the pile. Use reasonably

528 Chapter Eight