Special Feature

Inspection/diagnosis and Repair/renovation Technologies for Conduits
Shigeru Yamaguchi and Kenshi Okumura†
Abstract
This article introduces maintenance and management technologies for communication cable conduits,
focusing on technologies for repairing and renovating corroded metal pipes accommodating opticalcommunication cables.

1. Introduction
The total length of NTT’s communication conduits
is approximately 630,000 km. Of this total, the mainline conduits that connect between manholes accountfor about 590,000 km. The breakdown by conduit
type is shown in Fig. 1. A recent inspection of about
20,000 km of conduits found that up to 25% of conduits were in poor condition, making the installationof cables difficult. Metal conduits had a higher defectrate than vinyl conduits. In metal conduits, defects
are mostly caused by rust, as shown in Fig. 2. Rustbuildup inside the conduit was the number-one cause
preventing cables from being laid. The photographsin Fig. 3 of the inner surfaces of metal conduits takenwith a pipe camera compare conducts in good condition with corroded ones.
The proportion of defective metal conduits tends toincrease over the years. An increase in the defect rate
may decrease NTT’s ability to respond quickly to service requests or may increase maintenance and management costs. Therefore, we are developing costeffective and timely inspection and diagnosis technologies as well as repair and renovation technologies for appropriately maintaining and managing
existing conduits to allow them to continue to be used
for a long time to come.

2. Inspection and diagnosis technologies forconduits
A pipe camera and its related equipment are shownin Fig. 4. It is utilized for an inspection before cablesare installed into a conduit or an inspection for preCast iron pipes
37,000 km(6.3%)

Rigid vinyltubes
244,000 km(42.6%)

Others (old anddeteriorated conduits)
3000 km (0.5%)

Steel pipes
303,000 km(51.6%)

Fig. 1. Types of main line conduit.
Broken4%

Defective joint
4%

Bent
10%
Corroded52%
Flattened20%† NTT Access Network Service Systems Laboratories
Tsukuba-shi, 305-0805 Japan
E-mail: k.oku@ansl.ntt.co.jp

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Clogged withearth and otherforeign material
10%

Fig. 2. Causes of defects in metal conduits.

NTT Technical Review

Special Featureweak conduits correspond to concrete conduits or trough conduits constructed from thelate 19th century to the first half of the 20thcentury.) Technologies for repairing empty
conduits in various conditions without theneed to dig a trench are shown in Fig. 5. Of
those, we describe the TM (thick membrane)lining and thin film lining technologies in thisConduit in good condition
Corroded conduit
section.
Fig. 3. Pipe camera images of the insides of metal conduits.
(1) TM lining
In TM lining, a resin film about 3 mm thickis formed on the inner surface by inserting a resin linPipe camera
ing material into an existing old and degraded conduit(Fig. 6). Since this method can inexpensively renovate conduits that were previously unusable because
Monitor,of insufficient strength, it is used to provide space forvideo deck
installing optical fiber cables. The technology isbased on municipal technology used for repairing
sewage pipes. A resin hose is inserted by high-pressure air or water or a flattened shape-memory resin
hose is pushed into the conduit. Then, the inserted
hose is hardened by heat from hot water or steam.
(2) Thin film lining
Fig. 4. Pipe camera equipment.
A thin film lining can repair and extend the working life of corroded metallic conduits other thanventive maintenance. A pipe camera designed with aexcessively old ones. After rust has been removed, ahead small enough to be used during the installation
resin material is sprayed on or a hose is used to lineof multiple cables has been developed and introduced
the inner surface, forming a thin paint film or a resinfor practical use.
film less than 1 mm thick.
3. Underground conduit repair technology

3.2 For cable conduits
We are adapting repair and renovation technology
used for empty corroded tubes to develop repair technology for corroded metal conduits in which cables
have already been installed.
(1) Washing
We are researching a method of removing rust on
the inner surface of cable conduits by using a water

3.1 For empty conduits
Some defective conduits that do not contain any
cables cannot have cables installed because of rust or
earth intrusion or because they are old and weakbecause they were constructed a long time ago andhave insufficient resistance to earthquakes. (Old andExpansion

Water jet

Old anddeteriorated
conduits

Pipe camera

Metalconduits

Flattened

Bent,
twistedPartiallyrusted

Fully
rustedLinear correction

Cause of defect
Cloggedwith earth,etc.

Partialpainting

Thin film
liningTM lining

Fig. 5. Trenchless repair technologies for empty conduits.

Vol. 4 No. 6 June 200633

Special Feature

Boiler truck

Reversingmachine

Lining material

Metallic tube

Hardenedlining material

Existing conduit

Air or water pressure

Fig. 6. Concept of TM lining technology.

jet (Fig. 7). While the cable with the attached washing device head is held, a water jet is emitted from
nozzles attached to the head. Removed rust or otherforeign material is carried by the water flow to a manhole. We are now investigating the optimum nozzle
specifications and the influence of water jets on existing cables.

(2) Lining
We are investigating lining technologies to prevent
rust and create a space for installing cables after theinner surface has been washed (Fig. 8). A lining layer
Existing metal tube

Lining materialHotwaterHigh-pressure washing device

Washing headCable
Existing cable

Rust
Water supply hoseNozzle

Washinghead

Existing
metal tube

Red paint applied to inner surfaceof acrylic tube for easy recognitionof rust removal in images

Cable
Appearance during water jet washing

Fig. 7. Concept of cable conduit washing technology.

34400SM optical cable(a) Cable holding method

Cable

Water
supplyhose

40SM optical cable

Lining material

Hot wateror vapor

Cable(b) Cable in lining method

Fig. 8. Concept of cable conduit lining technology.

NTT Technical Review

Special Feature

Bridge baseConnecting joint

Repair tube (rigid vinyl or FRP)Remaining conduit

Cable
Outer tube(upper)

Innertube
Outer tube (lower)
Cross sectional view ofrepair tube (rigid vinyl)

Bayonet socket (FRP)

Fig. 9. Concept of bridge base conduit repair technology.

that creates a clear space for installing cables isformed using the same technology as existing tube
renovation technology (Fig. 8(a)). Alternatively,
space for installing multiple cables can be created by
the following method. Lining material is pushed into
the conduit accommodating the existing cable and isclosed with a fastener or other device. It is then
expanded to form a space and hardened with hotwater or steam (Fig. 8(b)). This method lets us fully
utilize the space remaining inside the tube. In thefuture, we will further investigate methods of closing
the lining material.
4. Repair technology for conduits attached tobridges
Metal conduits attached to bridges suffer fromsevere environmental conditions such as repeated dryand wet cycles, adhesion of anti-freezing material
used in winter, or accumulation of salt in seaside
areas, which can all lead to rusting or corrosion.
Recently, the degradation of metallic conduits located at the bases of bridges due to corrosion has
become a serious concern. The trend of defective
conduits by age based on past inspection data is thesame as seen with metal conduits deteriorating withtime. An overview of deteriorated metal conduit
repair technology that we are now developing, focusing on conduits located at bridge bases, is shown in
Fig. 9. A tube with a socket is inserted into the bridgebase and repair tubes connected to this socket connect
existing tubes to each other. For a cable-accommodating conduit, split-type repair tubes are used. Weare studying rigid vinyl and fiber reinforced plastic
Vol. 4 No. 6 June 2006

(FRP) as materials for the repair tube.
5. Future plans
We will further develop conduit management andrepair technologies that take into consideration lifecycle cost reduction, including cable conduit repair
technologies that are more economical and functional, and nondestructive inspection technologies formeasuring tube thickness reduction due to corrosion
from inside the tube.

Shigeru Yamaguchi
Senior Research Engineer, Civil Engineering
Promotion Development Project, Second Promotion Project, NTT Access Network Service Systems Laboratories.
He received the B.E. degree in civil engineering and the M.E. degree in science and engineering from Kagoshima University, Kagoshima, in1982 and 1984, respectively. He joined Nippon
Telegraph and Telephone Public Corporation(now NTT) in 1984. He is currently engaged in
R&D of non-destructive exploration technology
and maintenance management technology forcommunication infrastructure facilities. He is amember of the Japan Society of Civil Engineers.

Kenshi Okumura
Research Engineer, Civil Engineering Promotion Development Project, Second Promotion
Project, NTT Access Network Service Systems
Laboratories.
He received the B.E. degree in civil engineering from Ishikawa National College of Technology, Ishikawa, in 1983. He joined Nippon Telegraph and Telephone Public Corporation (nowNTT) in 1983. He is currently involved in R&D
of maintenance management technology foraging communication infrastructure facilities.

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