Moser A.P., Folkman S. Buried Pipe Design

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of a masonry arch. The second purpose is to provide added structural

capacity or support to assist the damaged pipes to sustain structural

loads. The third purpose provides a seal against infiltration and exfil-

tration. Finally, the fourth purpose is to replace missing pipe sections.

In robotic repair, robots are used to structurally repair isolated

defect areas in pipelines. First, robots are used to grind the defect area

exposing a clean and smooth surface. Then these grooved areas are

injected with epoxy-based resins which bond to surrounding host pipe

creating a structural and a permanent barrier impervious to interior

or exterior chemicals or objects. Robotic point repair is used either as

standalone or as a precursor to other renewal methods. As a stand-

alone, robotic point repair is used to repair radial, longitudinal, and

spider cracks. The process also lends itself to repairing broken joints,

slip joints, open joints, protruding service connections, recessed service

connections, roots, and other foreign objects that are usually found in

collection pipeline systems.

The robotics process uses the epoxy resin as the final structural fix.

The epoxy bonds to the pipe medium and permanently seals the wall

from further infiltration of outside material (soil and/or water). Also,

due to the epoxy hardness and structural adhesion, a repair to the pipe

wall stops the occurrence of further cracking with respect to the loca-

tion repaired.

Robotic repairs are carried out by an operator manipulating the

robotics functions by remote control with the aid of a closed circuit

television. As a first step, the robot is positioned at the defect area and

is surveyed for the best starting position. Chemical grouting is carried

out if any infiltration of water is present. The operator then begins to

grind out the crack(s). This accomplishes two goals. One, the crack is

cleared of all foreign material and stopped from further cracking due

to the groove cut. Second, the groove created gives a larger surface

area to inject the epoxy resin. The second step is to fill the void area

with the epoxy. This is carefully accomplished making sure that the

groove is fully filled and flush with the pipe wall.

Grouting, like sliplining, is one of the oldest methods of pipeline

renewal. In recent years, there have been new advances in products

and equipment for many grouting applications.

Chemical grouting is normally used for pipe joints in sewer lines and

manholes to seal active leaks or to curtail leaks using joint testing

equipment. Sometimes, slight circumferential cracks, small holes,

slightly cracked pipe joints, and other minor areas of structural damage

can be successfully sealed using chemical grouts. In most applications,

however, chemical grouting is used in structurally sound pipelines.

Several types of chemical grouts are currently available. Each type is

for a specific application and requires a specific method and equipment.

Pipe Installation and Trenchless Technology 559

Some grouts are best suited to repair pipe joints, while others are

designed to provide an impervious barrier to groundwater leakage on

the exterior of the pipe joint or manhole structure. In both cases, the

use of chemical grout is to fill large open voids.

By design, chemical grouts have minimal compressive strength and

where large voids and loss of structural support may exist, it must be

supplemented by other structural repair methods. Chemical grouting

will, however, curtail the loss of soil and backfill material through

leaking pipe joints and manholes, prolonging the life of sewer line and

preventing the formation of larger void areas caused by continued

leakage. All chemical grouts are applied under pressure after appro-

priate cleaning and testing of the joint or other locations where grout

is going to be applied.

In the spray-on lining method, a thin mortar lining or a resin coat-

ing is sprayed onto pipes. This is a well-established technique. Spray-

on lining systems use either epoxy resins or cement mortar to form a

continuous lining within the host pipe. These systems result in

improved corrosion resistance and hydraulic characteristics. However,

except for shotcrete and gunite, such systems have little value in

enhancing the structural integrity of the pipe.

For worker-entry pipes, diameters between 36 and 142 in (900 to

3600 mm), structural reinforced sprayed mortars (shotcrete or gunite)

are effective and widely used for renewing pressure pipes and gravity

sewers. Acid resistance mortars have been used in industry as linings

in tanks or as mortar bricks. Development of mechanical in-line appli-

cation methods (centrifugal and mandrel) has established mortar lin-

ing as a successful and viable rehabilitation technique for sewer lines,

manholes, and other structures. Specialty concretes containing sulfate

resistant additives such as potassium silicate and calcium aluminate

have shown greater resistance than typical concrete to acidic attack on

sewer pipes and manhole structures. As with any other trenchless

pipeline renewal, the pipeline must be thoroughly cleaned and dried

before a renewal method can be applied. If the lining is carried by

machine or carts and applied manually with a trowel, the application

distance is limited by the length of the hose available and the distance

between valves, bends, tees, etc.

For non-worker entry pipes, diameters between 4 and 36 in (100 to

900 mm), the lining is sprayed directly onto pipe walls using a remote

controlled traveling sprayer. The lining materials include concrete

sealers, coal tar epoxy, epoxy, polyester, silicone, urethane, vinyl ester,

and polyurethane. These linings are intended to form an acid resistant

layer that protects the host pipe from corrosion.

The link sleeve method of pipeline renewal uses a sleeve to correct

localized structural damage. Spot repairs can be conducted with this

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method on pipe diameters ranging from 6 to 110 in. For diameters

between 6 and 24 in, a stainless steel sleeve wrapped in polyethylene

foam is used. This sleeve and an inflatable sewer plug are placed over

the damaged area. With the aid of a TV camera, the plug is inflated

until the sleeve lock is in place. The plug is then deflated and a visual

inspection takes place.

There are different proprietary techniques for this method. Before the

operation, the pipe must be thoroughly cleaned and inspected by TV to

identify all the obstructions such as displaced joints, crushed pipes, and

protruding service laterals. The operation then continues according to

instructions from the system manufacturer and type of application.

The application of point CIPP is for pipelines that are structurally

sound, but may contain isolated pipe lengths that have structurally

failed. The materials used in point CIPP repair are the same as regular

CIPP methods with more than two decades of proven performance. Point

CIPP installation involves pulling the resin-saturated fabric liner and an

inflation hose through the existing sewer line. The alignment of the liner

is closely monitored by a CCTV camera positioned in the sewer line. Once

the liner is aligned in the proper position, the inner hose is inflated via a

combination of air and water pressure causing the liner to regain its orig-

inal circular shape. Hot water is introduced and recirculated within the

CIPP pipe. The hot water accelerates the curing of the fabric liner in a

tight fit against the existing sewer line wall. Table 8.1 presents a sum-

mary of pipeline renewal capabilities. It should be noted that during the

last five years the capabilities of these technologies have increased and

design engineers and potential users of these methods need to keep

abreast of new capabilities and new technologies that may have come to

the market. Such factors to be considered for each method include design

methodology; applicable size range; type of material (indicative of chem-

ical resistance); types and degree of disruption (degree of excavation

required) ; method of installation, total footage installed (or number of

installations); time required for installation; and whether or not by-pass

pumping is required (especially for large diameter pipes).

Trenchless construction methods (TCMs)

Horizontal directional drilling (HDD).

Horizontal directional drilling

(HDD) involves a steerable boring system for both small and large

diameter lines. In most cases, it is a two- or three-stage process. The

first stage consists of drilling a small diameter pilot hole along the

desired centerline of a proposed line. The second stage consists of

enlarging the pilot hole to the desired diameter to accommodate the

product pipe and pulling the product pipe through the enlarged bore-

hole. This method is so termed because of its unique ability to track the

Pipe Installation and Trenchless Technology 561

562

TABLE 8.1 Comparison of Different Trenchless Pipeline Renewal Methods (Najafi, 1994)

Diameter range, Maximum

Method mm (in) installation, m (ft) Liner material Applications

CIPP

Inverted in place 100–2700 (4–108) 900 (3000) Thermoset resin/fabric composite Gravity and pressure pipelines

Winched in place 100–1400 (4–54) 150 (500) Thermoset resin/fabric composite Gravity and pressure pipelines

Slip-lining

Segmental 300–4000 (12–158) 1700 (5600) PE, PP, PVC, GRP Gravity and pressure pipelines

Continuous 100–1600 (4–63) 300 (1000) PE, PP, PE/EPDM, PVC Gravity and pressure pipelines

Spiral-wound 100–2500 (4–100) 300 (1000) PE, PVC, PP, PVDF Gravity pipelines only

In-line replacement

Pipe bursting 100–800 (4–32) 100 (300) PE, PP, PVC, GRP Gravity and pressure pipelines

Pipe removal Up to 900 (36) 100 (300) PE, PP, PVC, GRP Gravity and pressure pipelines

Close-fit pipe

Modified cross section 100–400 (4–15) 210 (700) HDPE, PVC Gravity and pressure pipelines

Drawdown 62–600 (3–24) 300 (1000) HDPE, MDPE Gravity and pressure pipelines

Roll-down 62–600 (3–24) 300 (1000) HDPE, MDPE Gravity and pressure pipelines

Point-source repair

Robotic repairs 200–760 (8–30) N/A Epoxy resins/cement mortar Gravity

Grouting N/A N/A Chemical grouting Any

Link sleeve 100–600 (4–24) N/A Special sleeves Any

Point CIPP 100–600 (4–24) 15 (50) Fiberglass/polyester, etc. Gravity

Spray-on lining 76–4500 (3–180) 150 (500) Epoxy resins/cement mortar Gravity and pressure pipelines

Manhole renewal Any N/A Spray-on lining, profile PVC, Sewer manholes

CIPP, cast-in-place

location of the drill bit and steer it during the drilling process. The

result is a greater degree of precision in placing the utilities.

Horizontal direction drilling method uses a survey system to locate

the drill head. There are a variety of survey systems which have been

patented by different manufacturers. The choice of a particular system

will largely depend on the type of job, the site conditions and accessi-

bility, operator skill, finances available, etc. There are three classifica-

tions of the HDD method; namely, mini horizontal directional drilling

method (Mini HDD), midi horizontal directional drilling method, and

maxi horizontal directional drilling method. (see Table 8.2). For more

information on HDD, refer to references at the end of this chapter.

Major advantages. The major advantage of HDD is its steering capa-

bility. In case of obstacles being encountered, the drill head can be

guided around the obstacle. The system does not require bore pits and

only three workers are required to conduct the operation. The method

is capable of installing pipes and conduits from 2 to 60 in and up to

200

ft deep.

Major limitations. Since the cutting head consists of a drill bit, the sys-

tem can cut through existing utilities unless all the existing utilities

are clearly located and physically (using hand potholing or vacuum

excavation) exposed before starting the boring operation.

Pipe ramming method. The basic procedure consists of ramming a steel

pipe through the soil by using a device, generally air powered, attached

to the end of the pipe. In this method, the tool does not create a borehole,

rather, it acts as a hammer to drive the casing pipe through the soil. Once

the casing pipe is installed and spoil is removed from inside with air or

hydraulic pressure, a product pipe can be installed inside the casing. The

product pipe can be used for water, sewer, electric, gas, or any other util-

ity. The most common application for this method is horizontal auger bor-

ing is road, highway, and railroad crossings.

Major advantages. The pipe ramming method is an effective method

for installing medium to large diameter pipes. The versatile pit sizes,

varying lengths of pipe that can be installed, and the ability to handle

almost all types of soil conditions make this method a practical and an

economical technique for installing pipes. This method does not

require any thrust reaction structure as the ramming action is due to

impulses induced in the pipe by the percussion tool. The pipe ramming

method is also multi-functional. A single size of pipe ramming tool and

the air compressor can be used to install a wide variety of pipe lengths

and sizes. Ramming can also be used for vertical pile driving, angular

ramming, or pipe replacement.

Pipe Installation and Trenchless Technology 563

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TABLE 8.2 Comparison of Main Features of Typical HDD Methods (Najafi, 2005)

Type Diameter Depth Drive length Torque Thrust/pullback Machine weight Typical application

Maxi 24–60 in 200 ft 6000 ft 80,000 ft-lb 100,000 lb 30 ton River, highway crossings

Midi 12–24 in 75 ft 900 ft 900–7000 ft-lb 20,000–100,000 lb 18 ton Under rivers and roadways

Mini 2–12 in 15 ft 600 ft 950 ft-lb 20,000 lb 9 ton Telecom and power cables, and gas lines

Major limitations. The major limitation of the pipe ramming method is

the minimal amount of control over line and grade. Therefore, the initial

setup is of major importance. Also, in the case of obstructions, like boul-

ders or cobbles, especially with small diameter pipe, the pipe may be

deflected. Therefore, sufficient information on the existing soil condi-

tions must be available to determine the proper size of casing to be used.

Horizontal auger boring (HAB). The horizontal auger boring (HAB) is a

trenchless excavation technique used extensively throughout the

United States, primarily for road crossings. This method utilizes a

process of simultaneously jacking casing through the earth while

removing the spoil inside the encasement by means of a rotating flight

auger. The auger is a flighted tube which transfers spoil back to the

machine and has couplings at each end that transmit torque to the cut-

ting head from the power source located in the bore pit. The casing

supports the soil around it as spoil is being removed.

Major advantages. The major advantage of auger boring is that the

casing is installed as the borehole excavation takes place. Hence, there

is no uncased borehole which substantially reduces the probability of a

cave-in which could result in surface subsidence. Also, this method can

be used in a wide variety of soil types which makes it a very versatile

method.

Major limitations. The auger horizontal earth boring method requires

different size cutting heads and auger sizes for each casing diameter

which calls for a substantial investment in terms of equipment. This

method also calls for a substantial investment in terms of the bore pit

construction and the initial set up. In case of soils containing large

boulders, this method cannot be used advantageously because the size

of boulders and other obstacles this method can handle is limited to

one-third the nominal casing diameter.

Compaction method. The compaction method forms the borehole by

compressing the earth that immediately surrounds the compacting

device. Therefore, the soil is displaced rather than removed. The

method is restricted to relatively small diameter lines in compressible

soil conditions. These methods are classified as expansive installation

techniques since the earth surrounding the borehole is displaced by

the expanding effect. Spoil is not removed, but is compacted and is left

in place during the installation process. The most common compaction

method is percussion method as described below.

Percussion method. The percussion or the impact moling method, as

it is sometimes called, utilizes an underground piercing tool that is self-

propelled by a pneumatic or a hydraulic power source. The diameter

Pipe Installation and Trenchless Technology 565

and length of the tool vary by manufacturer, but they are all stream-

lined into a bullet or missile shape. Compressed air or hydraulic fluid,

transmitted to the tool through flexible hoses imparts energy at a blow

frequency of 400 to 600 strokes per minute to a reciprocating piston

located inside the nose of the tool. This action results in the tool pro-

pelling itself through the ground. These tools are effective in most

ground conditions from loose sand to firm clay.

The soil around the tool applies friction to the body holding it in

place when the piston returns on its back stroke. Thus, friction is nec-

essary for the proper operation of the tool. Without this friction, for

example, in very wet unstable soils, the tool will vibrate, but will not

move forward. Percussion tools vary in diameter from 1.75 in (45 mm)

to 7 in (175 mm). While all the tools claim a relatively high degree of

accuracy, some of the later developed small diameter units seem to be

more accurate. This has been credited to a properly designed diameter-

to-length ratio and weight distribution.

Percussion tools typically travel at a rate of 3 in (75 mm) per minute

to approximately 4 ft (1.2 m) per minute with travel speed being a

function of soil conditions and not a function of tool size. Although the

larger tools are more powerful, their speeds are not much different

than the smaller sizes because they must displace larger volumes of

soil. Most of the percussion tools have a reversal capability. This

enhances their capability drastically. In case of obstacles or when the

tool has swayed away from the desired course, the unit can be backed

out of the borehole. Before the development of this feature, the unit

would have been lost or required open excavation resulting in doing

what was being avoided in the first place and defeating the purpose of

boring. Since this method is for small diameter bores only, abandon-

ment does not cause any serious problems.

The typical procedures for the percussion method require the use of

boring and receiving pits. The bore pit size varies significantly and

depends on the depth of the bore and the size of percussion tool

selected. For stable ground conditions where a high degree of accuracy

is not required, the percussion tool is placed in the bore pit and col-

lared into the embankment by one person. However, when a high

degree of accuracy is desired, a launching platform is used. The plat-

form provides support for the tool so that it is aligned as required. A

sighting device is used to ensure alignment. Some manufacturers pro-

vide adjustable bearing stands so that vertical adjustments can be

readily made. After proper alignment has been obtained, the tool is col-

lared into the embankment, the power is applied, and the operation is

begun. The operation is monitored until the tool exits in the receiving

pit. The tool is then removed and the cable is attached to the air hose

and pulled back through the borehole. When rigid pipe is to be

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installed, the tool and the air hose are removed and the pipe is pushed

through the open borehole.

In most soil conditions, the compaction effect is sufficient to develop a

borehole with sufficient stand-up time to perform the necessary pipe

insertion. However, overall performance and accuracy of the unit are

functions of the type of ground and soil conditions. The factors that need

to be taken into consideration are soil type and degree of homogeneity,

the degree of soil compactness, soil moisture content and its ability to be

deformed and displaced, the critical depth (the minimum depth from

point of entry through the line of travel below the surface), the contour

of the ground above the line of travel, and the presence of obstructions.

The effectiveness of soil displacement depends on soil properties and

characteristics such as compressibility, grain size, and gradation. The

presence of a high water table may affect soil compressibility. Generally,

compressible soils such as unconsolidated soft silt or clay, mixed grain,

or well-graded soil with a high void ratio are most favorable for soil dis-

placement methods. Poorly graded or dense soils are difficult to pierce.

Special situations can be accommodated with modifications to impact

tools. Larger sizes permit harder bores to be completed (rocky condi-

tions). Pulling accessories help the softer bores (silt, sand) to be com-

pleted. Shale has always been manageable by piercing tools. Underwater

projects are possible by pulling pipe to eliminate water infiltration into

the tool body. Large diameter rocks are the only major stopping points.

Characteristics.

1. Type of pipe installed. Since the boring process is independent of

the pipe insertion process, any type of pipe or cable can be

installed in the borehole by these methods. These methods are

extremely popular for installation of electric, telephone, or cable

TV cables. These methods are also extensively used for the instal-

lation of gas pipe, sprinkler irrigation systems, and service lines

for water systems.

2. Pipe size range. The size of pipe that can be installed by this

method is restricted by the size of the borehole that can be bored

by this method. These methods are designed to accommodate

small diameter pipes and cables. These are typically limited to

6 in (150 mm) or less. In very favorable soil conditions, boreholes

as large as 12 in (300 mm) in diameter have been successfully

obtained by these methods.

3. Bore span. Even though 200-ft (60-m) bores have been success-

fully made with these methods without using any sensing or guid-

ing systems, the span length should be limited to 60 ft (18 m) with

40 ft (12 m) being the optimum. The limiting factor controlling the

Pipe Installation and Trenchless Technology 567

bore span is accuracy and not the ability of the tool to move

through the ground. Since the bore size is small, it becomes diffi-

cult to control it as the span increases. Without a sensing and

guiding system, it becomes very difficult to locate the bore head.

However, with the development of sensing and guidance sys-

tems, the ability of these methods has been enhanced substan-

tially. With the use of electronic sensing devices, it is possible to

know the location and alignment of the bore head, and with guid-

ance systems, it is possible to maneuver the tool in the desired

direction. This has substantially reduced the risk of obtaining a

non-usable borehole.

4. Disturbance to the ground. The two major problems that can

result from the use of these methods in terms of ground distur-

bance are ground settlement or heaving. Since the compaction

method compresses the soil around it and does not remove spoil,

it creates extra volume and heaving is anticipated in such cases.

However, when percussion tools are used in loose cohesionless

soils, the vibrations due to the percussion tool consolidate the soil

to a point where the volume loss due to consolidation may exceed

the amount of expansion. This may result in ground settlement.

Hence, the compressibility of the soil is an important factor which

should be considered for settlement purposes. A rule of thumb in

the industry is that the depth of cover should be 1 ft (0.3 meters)

for each in (25 mm) of bore diameter. Some manufacturers con-

sider this as conservative, but it is reliable especially when the

specific soil characteristics are not known.

5. Area requirements. The compaction methods require the use of a

bore pit. The bore pit size depends on the method selected, the

depth of the bore and the size of the tool. Bore pit sizes vary from

as small as 6 in (150 mm) wide, 36 in (900 mm) long, and 18 in

(450 mm) deep to as large as 10 ft (3 m) wide, 30 ft (9 m) long, and

15 ft (4.5 m) deep.

These methods require a minimum surface area because of

minimum equipment requirements. In most cases, the equipment

required outside the pit is either an air compressor or a hydraulic

power source.

6. Operative skill requirements. The operation is very basic and the

procedures and principles are very simple. Hence, no special oper-

ator skill is required. In case of obstacles being encountered or

other problems, the borehole is abandoned. Since these methods

are for small diameter bores only, abandonment does not cause

any serious problems.

7. Accuracy. When no sensing and guiding conditions are utilized,

the accuracy of installation will depend on the initial set up,

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