Repairs and Waterproofing of New Tunnels and Pipelines

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General

This Sub-section deals in a general way with repairs and waterproofing which are sometimes needed to new tunnels and pipelines and arising from incidents during construction. It does not cover the much larger subject of the renovation of old sewers which may be brick lined tunnels and pipes of concrete or clayware. A few references on this latter subject are included in the Bibliography at the end of this section; the total amount of published material on sewer renovation which has appeared during the past ten years is quite staggering.

The materials and techniques which have to be used in the waterproofing of tunnels and pipelines are in many ways different to those adopted for waterproofing structures such as reservoirs and sewage tanks. Tunnels and pipelines must inevitably be waterproofed on the inside and against the inflow of water, while, as discussed earlier in this section, repairs, etc., to water-retaining structures can often be carried out on the water face.

Tunnels and large diameter pipelines are usually driven or laid through water-bearing ground and the inflow of water, particularly in the case of tunnels, can be very considerable. Most consulting engineers realize this and accept a certain amount of infiltration. Many papers have been written about tunnels and tunneling, and also about main intercepting sewers, but it's rare to find this basic problem of watertightness discussed, and details given of the amount of infiltration accepted by the designers. An exception to this is the paper by Haswell on the Thames Cable Tunnel, read at a meeting of the Institution of Civil Engineers in London on 17 February, 1970.

In the case of tunnels, the amount of infiltration will depend on many factors including the pressure of the ground water, the type of lining, whether single or double skin, the materials and techniques used for caulking the joints between the segments and how leaks elsewhere are dealt with. Haswell in his paper quotes acceptance figures of 5 liters/m and 32 liters/m in different sections of the tunnel.

In addition to the tunnel lining it's usual to pressure grout behind the lining as work proceeds. The joints in cast iron segments are usually caulked in lead while with precast concrete segments, the caulking material is normally a cement based compound.

None of these are 100% effective and so additional waterproofing is required on a certain percentage of the joints. In addition, there is always some leakage at intermediate places. To seal these leaks against the inflow of water, ultra rapid setting compounds have to be used. There are many proprietary materials on the market. The older ones were usually based on Portland cement with an admixture of gauging liquid to promote an almost instantaneous set. Portland cement and HAC in about equal proportions will give a flash set. In recent years organic polymers, such as epoxide, polyurethane, polyester, and acrylic and styrene—butadiene resins, have been introduced for this waterproofing work. Sometimes they are used in addition to the older materials. The formulators of some of the polymers can produce compounds with special characteristics tailor-made for specific site conditions.

Underground pipelines for main sewers are almost entirely reinforced concrete; asbestos-cement was also used in the UK for diameters up to about 120 m. Asbestos-cement pipes up to 2 m diameter are made and used on the Continent and the USA. Both types of pipeline have flexible joints formed with rubber rings. When correctly installed these form a watertight joint, but if the rings are displaced during pipe laying or grit gets between the rings and the pipe, leakage will occur. It is usually at joints in the pipeline and the junction between the pipes and manholes that infiltration is most likely to occur.

With large diameter pipes, 900 mm and over, men can enter the line and inspect and repair the joints. When there is a considerable inflow of water, repair can be difficult and time consuming. It is impossible to remove the jointing rings and to correct their position. The joint has therefore to be made watertight by the insertion of a sealing compound into the narrow space between the end of the spigot of one pipe and the inside of the back of the socket of the next. Sometimes due to excessive ground movement, or incorrect laying, this space may be extremely narrow in one half of the joint and very wide in the other. Such conditions require the concrete to be cut back so that there is an adequate space for the sealant.

In one particular job where about one thousand such joints had to be made watertight, the prescribed dimensions of the sealing groove were 30 mm wide and 40 mm deep. The main items of work that had to be done were:

(a) The joints had to be cut out and prepared to the dimensions given above.

(b) The inflow of water then had to be completely sealed off by means of an ultra-rapid setting compound.

(c) On completion of (b), the joint surfaces had to be prepared to ensure maximum bond with the selected sealant.

(d) The sealant, a specially formulated flexible polyurethane com pound, was inserted and troweled off flush with the inside of the pipe.

Figures 9.16 and 9.17 show the infiltration through a joint, and work in progress to seal the joint.

With pipelines having diameters less than about 900 mm, the tracing of infiltration points and other defects can only be carried out by closed circuit television. However, it's not possible to assess by photographs just how much damage has been done to a pipeline by chemical attack, in terms of the thickness and soundness of the remaining concrete.

9.6.2 Pipelines Constructed by Jacking

During the past 20 years, the use of pipe jacking techniques to lay large diameter pipes without excavating trenches has increased considerably. In common with all construction methods, it has advantages and disadvantages. A disadvantage is that damage can be caused to the pipes during the jacking; this usually arises from the pipes diverting from the prescribed line and level due mainly to changes in ground conditions.

FIG. 9.16. Infiltration through joint in concrete pipeline

The damage thus caused is usually at the joints, and unless it occurs near the leading pipes at the end of a thrust, the only practical solution is to repair the pipes from the inside. In pipes manufactured for jacking, the joints are normally of the spigot and socket type and are within the thickness of the pipe wall. The damage is mainly spalling and cracking at or close to the joints.

Where the spalling is minor it can be repaired using an epoxide mortar, particularly at the arrisses of joints. For larger areas of damage, the better solution is to use a cement/sand mortar or a cement-rich, 10 mm coarse aggregate concrete, both being modified with the addition of 10 liters of styrene—butadiene emulsion to 50 kg cement. The repair technique should be as previously described in this guide. Cracks can be dealt with as previously described and as appropriate in the circum stances.

There is sometimes a tendency for resident engineers to order the concrete in the pipe wall to be cut back behind the reinforcement. This is not necessary unless the concrete is damaged or otherwise unsound; no useful purpose will be served by removing concrete which probably has a compressive strength of 50—60 N/mm and a water/cement ratio of about 0

During jacking, particularly on a curve, the gap at the joints which is there to allow for some degree of flexibility, closes up around part of the circumference of the pipe. It is important that a reasonable gap width be maintained for the complete circumference of the pipe, of say 10 mm.

FIG. 9.17. View of joint in Fig. 9.16 after initial sealing

Any significant reduction in this width should be rectified by careful cutting with a disc. The new exposed surface should be given a brush coat of epoxy or polyurethane resin.

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Updated: Monday, April 12, 2010 19:29