only if special exemption is obtained26 and should be inspectedweekly.

It should be noted that trenches may in some circumstancesconstitute a confined space and appropriate safety measuresshould be applied (see Table 40.4).

The legal responsibilities for safety cannot easily be subcon-tracted and the Swan Hunter case27 clarified further duties forproviding information. Contractors working alongside othercontractors (whether subcontractors, tertiary contractors orindependent contractors) have a duty to provide sufficientinformation so that each employer is aware how his operationsaffect others and how the operations of others affect his ownemployees. This duty applies not only downwards from maincontractor to subcontractor but upwards also.

40.7 Trenchless pipelaying

In urban areas the use of trenchless techniques can reduce to alarge extent the disruption caused by trench construction andalso allow obstructions such as rivers, railways, major roads,etc. to be negotiated conveniently. The techniques currentlyused may be roughly divided into microtunnelling (formation ofa new bore smaller than man-entry size) and conventionaltunnelling (larger than man-entry size). The lower limit of man-entry size is generally accepted to be about 900 mm diameter.This is the smallest diameter in which a man can work effect-ively (albeit with difficulty). Where pipes are to be inspectedonly, men can enter down to 600mm diameter if properprecautions are taken.

Since 95% of the UK sewer system is 900 mm diameter orsmaller, microtunnelling is frequently used for sewer refurbish-ment as well as new construction.

40.7.1 Microtunnelling design

In recent years there has been a considerable emphasis on thedevelopment of microtunnelling techniques, and methods areavailable for producing bores from 50 to 900mm diameter.

Figure 40.10 indicates the current techniques, which are basedmainly on pipe-jacking methods.

Little research has been done on the vertical loads exerted bythe ground on pipes installed by jacking. It is usually assumedthat a slight overbreak of soil around the pipe may occur whichwill create a soil stress system similar to a Marston narrowtrench condition. Therefore, soil load can be estimated by theequation by Young and O'Reilly (page 40/7) where Bd = effec-tive width of trench. This assumption is likely to be conservativein stable soil.7 Other soil theories may also be used; in particularTerzaghi's theory for buried pipes28 which is based on assump-tions similar to those used by Marston.

The loads on the pipe due to ground surcharge may also becalculated using the methods derived for pipes in trench.

The pipes must be strong enough to cope with the axial loadsproduced by the jacking operation as well as the external andservice loads. Experience shows that pipes which are designed tocope with the jacking forces are generally adequate for theground loads. Once the ground forces have been established, thepipe strength can be checked either using the bedding factormethod (a value of 1.9 is generally accepted for pipes installedby jacking) or by using elastic theories of ring compression.29

The jacking loads arise mainly from friction. This varies withdifferent ground conditions from 5 to 25 kN/m2 although moreextreme figures have been known. Sands and gravels causehigher values than cohesive soils and friction can be reduced bylubricating the outside of the pipe using bentonite slurry orother methods.

Jacking pipes of different materials and joint design vary inthe load accepted in end bearing, the different materials avail-able including steel, concrete, clay and GRP. Angular variationsarising between pipes during installation tend to reduce thefailure load and many types of joint have been tried to reducethis danger.28 The jacking forces must be resisted at the thrust pitby a thrust wall or a heavy foundation. These are designed byconventional soil mechanics methods.

40.7.2 Site investigation

A proper site investigation is an indispensable requirement. It is

Table 40.4 Safety checks for confined spaces

Before work starts:(1) Check ground conditions for hazards and sources of gas such as organic strata, refuse, sewers, gas mains, industrial

pipelines and the interaction between carbonate and acid (particularly in chalk, limestone and greensands)(2) Ensure personnel are fit and properly trained(3) Ensure breathing apparatus, lifelines and safety equipment is available. Define the procedures for contact with the

emergency services(4) Check gas monitoring equipment is available and working

Before entering:(5) Check atmosphere for oxygen deficiency and explosive or toxic gases(6) Check arrangements for ventilation(7) Use breathing apparatus if the atmosphere is dangerous(8) Check arrangements for entry and egress and lighting are adequate

While working:(9) Continuously monitor the atmosphere

(10) Ensure space is properly ventilated(11) Do not smoke(12) Ensure direct communication is maintained between everyone involved in the work(13) Ensure all equipment is maintained in good order and properly used

In emergencies:(14) Do not enter the space without proper equipment or without an attendant at the entrance. A lifeline and harness should be

used when entering and pulling out victims.(15) Do not attempt to purge dangerous atmospheres with pure oxygen which will cause an explosive hazard

Figure 40.10 Trenchless pipelaying techniques for new pipelines

essential to choose the right construction method for theconditions, since there is no access to the end of the bore and thetunnelling machine may be lost if unacceptable ground condi-tions are encountered. If this happens it will probably require ashaft or second tunnel to be dug to recover the machine. Inextreme cases the machine and the original tunnel must beabandoned.

A full topographical survey is required including the accuratelocation of all existing buried structures. The soil investigationshould extend to a generous depth below the invert of the pipe.Boreholes and trial pits should not be located exactly on thecentreline of the pipe to avoid weakening the ground and theyshould be p-operly grouted up and backfilled for the samereason. Soil testing is required to ascertain the description,density and strength of the ground. Cobbles and boulAers are aparticular hazard and small-diameter boreholes may be inade-quate for identifying such conditions.

Information on groundwater levels and permeabilities mustbe provided. Poor conditions may need treatment before thetunnel excavation starts. Suitable techniques might includegroundwater lowering with well points or deep wells, groutingor ground freezing.

The detection of existing services may be done by a variety ofmethods. Magnetic field location is commonly used. Passivelocators are used to locate buried conductors by detecting thenatural magnetic field surrounding it. The method is quick andsimple but only locates. It is not appropriate to trace or identifya buried service. If this is required, an active locator should be

PIPE DIA (m)

used. This comprises a transmitter, which applies a signal ofknown frequency to the pipe or cable and a receiver which istuned to the signal and used to trace the service. The method canbe time-consuming and requires a skilled operator but givesgreater accuracy than the passive technique.

Non-metallic pipes can be traced by the use of small self-contained transmitters known as sondes. These are movedthrough the pipes using rods or high-pressure water or towingcables, and the signal is traced from ground level.

Other location methods are available including ground-prob-ing radar, seismic reflection techniques and the detection ofvariations in static magnetic fields. Also dowsing methods havebeen used with success.

A common fault of existing service maps is lack of accuracy.The digitization of records and production of digital mapsbased on Ordnance Survey maps is more accurate and is gainingfavour. This should improve the situation in the future and easethe exchange of information between utility companies andothers.

40.7.3 Ground movement

The installation techniques for microtunnels may be classified as'convergent', where the volume of excavation exceeds thevolume of the pipe installed, or 'expansive' where the opposite isthe case.

In the convergent case, ground may be lost due to face andperipheral encroachment (similar to overdig in larger tunnels),

Small-diameter tunnelsMicrotunnels

(1) Percussive moling

(2) Thrust boring

(3) Horizontal drilling

(4) Pipe driving

(5) Remote controlled tunnelling

(6) Pipe jacking

(7) Tunnelling in shield

(8) Headings

Figure 40.11 Microtunnelling with a bentonite slurry shield. (Courtesy: lseti Poly-Tech Inc.)

Operationboard

Power

3—stagemolemeister

Lasertransit

Pitbypass unit

Thrustpit

Slurrydischargepump

Slurrychargingpump

Slurry separation equipment tank

consolidation due to changes in drainage patterns, and localinstabilities during driving. Because of the small diameter of thehole the absolute volume lost tends to be small and, hence, thepotential movement is usually limited compared to man-access-ible tunnels.

Expansive techniques include percussive moling, pipe drivingand on-line pipe replacement and may disturb adjacent buriedpipes or cause ground heave at shallow depths. This problem isdiscussed by Howe and Hunter30 and O'Rouke.31

40.7.4 Pipe jacking with slurry shields

These machines are essentially miniaturized versions of shieldsused for conventional tunnelling and employ a full-face cuttinghead within a shield (Figure 40.11). The cutter can be movedrelative to the shield, allowing it to exert a constant pressure onthe ground irrespective of the rate of advance of the pipe. Thegroundwater pressure is balanced by flooding the cutting facewith water or a bentonite slurry. In many machines the slurry isused to transport the spoil back to the surface.

The ability to balance soil and water pressure gives goodcontrol of ground movement at the face. The alignment of themachine is monitored by using a laser beam focused on a targeton the shield. The target is observed by an operator at groundlevel via closed-circuit TV and he can adjust the alignment byremote control using steering jacks within the shield.

The shield is jacked forward from the jacking pit leading apipe string through which its power and slurry lines run. As newpipes are added to the pipe string the power and other lines must

be broken and reconnected. One manufacturer avoids this byusing a temporary lining behind the shield which has a recess toaccommodate the lines. The temporary lining is replaced by thepermanent pipe in a second-stage operation.

Slurry shield machines can operate in most soft groundconditions although the cutter type and the slurry separationsystems may need adjustment for different materials which cancause practical difficulties in variable ground conditions. Thesize of cobble which can be excavated depends on the size andtype of machine used. One manufacturer has machines whichcan crush stones measuring up to about one-third of thediameter of the shield. In ground containing large stones it willbe necessary to install a larger machine which can cope with theexpected conditions.

40.7.5 Pipe jacking with steerable borers

This category covers a wide range of machines. The steeringmay be by means of jacks on the shield or by rotating a specialcam behind the articulated cutting collar. Alignment is moni-tored by visual surveying from the launch pit or by using a lasersystem observed by closed-circuit TV. Spoil is usually removedback to the launch pit by screw conveyor which also forms thedrive shaft. Water or slurry may be used to cool the cutting headand assist in spoil transport. Other systems form a pilot holewhich is then reamed out by passing spoil forward to thereceiving pit.

Steerable borers can deal with a wide range of groundconditions by selecting an appropriate cutting head. Auger bits

Air supplyhose threadedthroughnew pipesections

New pipesinsertedbehingimpact mole

Impact mole shattering existingpipe and enlarging hole

Guide cable fromwinch

Figure 40.12 Impact mole used for upsizing an existing sewer

can be used in soft strata while rock roller bits can be used inharder materials. Various methods are employed for dealingwith water pressure including short pitch augers or pressurizingthe face with water or compressed air.

40.7.6 Pipe jacking with non-steerable augersThere are less complicated tools than those described abovealthough they do incorporate similar features. The non-steeringauger uses a simple cutting head rotated by an auger screw. Therotational power and thrust for the auger is provided by a driveunit at the launch pit. The cutting head is difficult to controlaccurately once boring has progressed a few metres and oftenalignment is not monitored between the launch and receivingpits during excavation. Large deviations may occur if the boreobliquely crosses hard strata. In long drives steady bearingshave been installed in intermediate pits and reasonable accuracyachieved. A wide range of ground conditions can be excavatedbut problems may arise in water-bearing sands and gravels.

40.7.7 Pipe rammingThis technique, also known as pipe driving or pipe pushing, isanalagous to pile driving. It normally utilizes a steel pipe whichis driven into the ground from a launch pit using percussiveloading at the trailing end. Small-diameter pipes are drivenclosed-ended but larger diameters are left open and cleaned outusing augers or water jets.

Pipe ramming can be carried out in most types of ground andthe percussive action tends to break up cobbles and otherobstructions. There is little control of accuracy once the drive isunder way and obstructions can cause deviations. The finalaccuracy depends on ground conditions and lengths of drive donot usually exceed 30 m.

40.7.8 Impact molingThis technique relies on displacing and compacting soil to forma void in the ground. The mole is usually powered by com-pressed air which causes a hammer piston to strike a chisel-headed anvil. This pierces the ground and the tool movesforward. The machines are generally reversible to aid recovery ifrefusal conditions are encountered.

Moles can be used to make pilot holes for subsequentenlargement or alternatively they can tow in pipes while formingthe bore if polyethelene or plastic pipes are used (Figure 40.12).

Impact moles can operate in most materials and are able tobreak up isolated stones or boulders although these may causesome deviation in line. In very soft conditions the mole tends tosink under its own weight and follows a downward curvingtrajectory. In shallow bores the mole may curve upwards.Therefore to prevent ground heave, and to control accuracy, aminimum cover of 9 times the diameter should be used. Accur-acy depends on ground conditions and is typically about 1%.

Impact moles are also used widely in replacing and up-sizinggas and sewer pipes (see below).

40.7.9 Directional drillingThis method employs techniques used in drilling oil- andgaswells, and is particularly suited to the construction of rivercrossings, etc. A surface-mounted rig is used to drill a pilot holefrom one side of the river to the other using a down-the-holemotor at the end of drill rods. The motor is powered bybentonite slurry pumped through the drill rods. The returningslurry cools the drill bit and pushes the cuttings back to thesurface. The curved profile is achieved by steering the ,drill rodsusing a special fitting mounted behind the motor.

Figure 40.13 Directional drilling, (a) Stage 1: pilot hole drilledby advancing the drill string and overdrilling with the washover pipein stages of approximately 80 m; (b) stage 2: pilot hole completedwhen both drill string and washover pipe exit on opposite bank;(c) stage 3: drill pipe is removed, barrel reamer connected towashover pipe which is in turn connected to the pipeline pullinghead by a swivel joint; (d) stage 4: the barrel reamer is pulled backand rotated by the drill rig positioning the non-rotating pipeline intothe formed hole

40.8 Man-accessible tunnels

Man-accessible tunnels are defined as having diameters greaterthan 900 mm which is the smallest in which a man can effect-ively work.

A wide range of excavation and lining methods are availabledepending on ground conditions and operational require-ments.32 In soft ground timbered headings, segmental liningsand pipe jacking are the principal construction methods used.

Excavation is usually done by hand in timber heading. In

The pilot hole is cased with a steel pipe called a washover pipewhich is then used as a draw string for a reaming tool when thepilot hole is complete. The reaming tool is a circular cutterwhich is attached to the leading end of the washover pipe. Therig then rotates the pipe and pulls it back through the pilot hole(Figure 40.13). A further washover pipe is towed behind thereaming tool. Bentonite slurry is used together with the secondwashover pipe to stabilize the enlarged hole. The permanentpipe is then pulled into place using the second washover pipe.Further reaming and smoothing tools may be used in thisoperation to ensure a proper fit for the permanent pipe.

Drilling rig

Wash pipeDrill pipe

Drill bit

Drilling rig Drill bit

Barrel reamerPipeline

Swivel joint

Wash pipe

Wash pipe

Drilling rig

Pipeline

Swivel jointBarrel reamerWash pipe

segmental tunnels and pipe jacking hand excavation may beused but more usually a variety of mechanized methods areemployed, including backhoe excavators within a shield, andfull face tunnelling machines. The latter include bentonite slurryshields and mechanical earth pressure machines intended tominimize ground loss and movement at the excavation face.

Soft ground tunnels are invariably lined and the choice oflining depends on a number of factors including ground condi-tions, operational requirements and the construction method tobe used. The relationship of these factors is complex and thefinal choice should be based on mutual agreement between theemployer and the contractor.

In hard rock the traditional method of excavation is drill andblast which has the advantage of coping with a wide range ofground conditions. Machine excavation can be much quickerand hence cheaper, but it is more sensitive to changing groundconditions which may be outside its range.

Linings for bores in hard competent rock may be unnecessaryfor structural reasons but could be desirable to provide asmooth hydraulically efficient surface. In this case an in situsprayed or poured concrete lining may be used. In less compe-tent ground the rock may be stabilized with rock bolts prior tolining with in situ concrete or a bolted precast concrete liningmay be used.

In many cases the primary lining supporting the excavationmay be too large for efficient hydraulic design and the tunnelmust be lined with a small carrier pipe or a channel invert.Timbered headings are invariably backfilled around the carrierpipe which must be designed for the loads arising.3

40.8.1 Site investigation

The main factor affecting construction cost of a tunnel is thenature of the ground. The site investigation will be similar innature to that for microtunnelling but usually will be moreextensive due to the larger works being constructed. Whenassessing suitable construction methods and the loads on thelining, more extensive in situ and laboratory testing will berequired. Full descriptions must be given for the strata togetherwith a geotechnical interpretive report since the ground struc-ture has increasing significance as the tunnel size increases.When large tunnels are contemplated, consideration should begiven to the possibility of constructing a pilot tunnel as aseparate contract. This will yield valuable information for thedesign and construction of the larger tunnel.

A tunnel lining is interactive with the surrounding soils in amanner which depends on their relative stiffnesses. Measure-ments in existing tunnels show that frequently the lining doesnot carry the whole overburden load.

The fundamental design problem is predicting the behaviourof the ground and the way it is varied by the constructionprocess. The current trend is towards designing flexible liningswhich will deflect to produce virtual equality of radial stress inthe ground, usually by the horizontal axis extending as thevertical axis becomes shorter. The process of design mustconsider the construction method and the operational require-ments of the lining, and then check that it is adequate for theground loads expected. Handling stresses in the lining elementsoften give the worst design conditions particularly in tunnels forshallow depths.

A variety of methods of designing lining thickness are avail-able.32-33 Empirical techniques based on field observations allowthe designer to estimate the likely ground loads and deformationof the lining when similar construction methods are used insimilar ground conditions. Other design procedures are basedon mathematical analysis and use closed form elastic solutionsor numerical methods. These require the soil behaviour to bemodelled in terms of its elastic, plastic and time-dependent

characteristics to predict the changes in the in situ groundstresses. It is difficult to determine representative values forthese factors and to assess the in situ state of stress of theground. Hence, the methods are frequently used to model arange of possibilities to assess the sensitivity of each parameterand allow an engineering judgement to be made on likely stableor unstable configurations.

Pipes to be installed by jacking methods are designed as rigidstructures as previously described for microtunnelling. Particu-lar attention must be paid to the leading pipe and the trailingpipes at intermediate jacking stations since these are repeatedlysubject to the full ram load during the jacking operation.

40.8.2 Headings

Headings are rectangular or square in section and usually dugby hand. The ground is supported by timber frames which areerected as the excavation progresses (Figure 40.14). The timbersizes are either estimated by rule of thumb or designed astemporary works with appropriate factors of safety.19 Thedimensions of the heading must allow sufficient working roomfor the construction of the carrier pipe and the compaction ofthe backfilling around it. The smallest practical size is about1.15 x 0.7 m clear between supports.

The technique requires experienced and skilled operatives andis relatively expensive. As a result its use is restricted to specialcircumstances.

Figure 40.14 Timbered heading

40.8.3 Steel liner plates

Steel liner plates may be used for lining small-diameter tunnelsin firm ground. Typically, four plates are used to construct thering and the plates are bolted on the longitudinal and circumfer-ential joints. The plates are back-grouted to ensure even bearingof the ground. Joints can be welded if necessary to prevent wateringress.

The finished lining behaves as a flexible pipe and is generallygiven a secondary lining.

40.8.4 Precast concrete segmental linings

These are available for either bolted or boltless construction.Bolted segments have flanges on all four sides and tend to beused in poor ground and where there are problems with wateringress (Figure 40.15). The segments are erected within theprotection of a shield and are back-grouted every ring or everyshift. The method is particularly useful in conditions requiringcompressed air and for hydraulic tunnels with a moderatepressure.

Where a smooth internal finish is required it is necessary touse a secondary lining. To avoid this, smooth bore, bolted

Head and side boards'driven successivelyfo rward as excavationproceeds

Head boards

Head tree'

Sideboards

Lininq timber

Figure 40.17 Boltless smooth bore concrete segmental ring.(Courtesy: Charcon Tunnels Ltd)

40.8.5 Triple segmental block (minitunnel system)

This proprietary system has been developed to provide smooth-bore tunnels in the range 1.0 to 1.3m diameter. The liningconsists of three segments which can be erected without the useof a former ring (Figure 40.18). Each segment has longitudinalV-shaped grooves inside and outside which function as stressraisers to ensure that the lining deforms and acts with forces incompression.

The segments are erected within the protection of a shieldwhich is used to dig an oversize hole. The overbreak is filled withgravel injected through an orifice in the rear of the shield tail.The gravel packs around the completed segment ring andsupports the ground. The main longitudinal and circumferentialjoints are sealed with a rubber bitumen strip compound duringerection and the gravel annulus may be grouted up to provideadditional waterproofing.

40.8.6 Pipe jacking

As with the microtunnelling technique, the pipes are forced intothe ground using hydraulic jacks at a working shaft (Figure40.19). A steel or concrete cutting edge is used at the leading endof the pipe string and miners working within the pipe excavate

segments have been developed and their use is generally knownas one pass construction (Figure 40.16).

Boltless segments also produce a smooth bore tunnel and areused in ground which has a reasonable stand-up time. Thesegments are erected on a temporary steel former ring usingbolted connections. After the annulus behind the concrete ringis grouted the steel former is demounted and the processrepeated (Figure 40.17).

Other smooth bore linings are of the expanded type intendedfor use in clay. The linings are erected on an erector arm or barsand expanded by using a wedge-shaped block or by cirumferen-tial jacks which form spaces to be filled with dry pack or specialblocks.

Grey cast iron has great strength and traditionally was usedwhere heavy ground loads were expected. More recently it hasbeen replaced by spheroidal cast iron which has a better tensilestrength. Cast iron has been used for both bolted and expandedsystems. The segments can be manufactured to a better toler-ance than concrete and, hence, produce a better seal againstwater ingress in bad conditions.

Temporary erectionformer ring

Cross section Section A-A

Cross - sectional plan Section A-A

Circle joint

Cross-jointDetail ofsolid key

Details ofcaulking groov

Section throughcircle joint

Section throughkey joint

Figure 40.15 Bolted concrete segmental ring

Groove to accept 'Hydrotite' sealing gasket

Caulking groove'Fastlock'dowels tocircle joints

Expanding fastener tolongitudinal joints

Grout hole

Interlocking steelloops at longitudinaljoints

Tee-box used forlifting purposes

Figure 40.16 Bolted smooth bore concrete segmental ring.(Courtesy: Charcon Tunnels Ltd)

Figure 40.19 Section through a typical pipe jacking operation

material as in a normal tunnel shield operation. The accuracy ofthe drive can be improved by using trimming jacks or steeringfins to adjust the attitude of the cutting shield. These also allowpipes to be jacked round bends.

The maximum length of drive can be increased by injectingbentonite or other lubricants to reduce the friction on theoutside of the pipe. Also intermediate jacking stations can bebuilt into the pipe string to increase overall jacking capacity andto restrict the length of pipe to be pushed by each set of jacks.

40.8.7 Ground movement

This is caused by ground loss during tunnel excavation and byadditional consolidation as the porewater pressure is reduceddue to drainage induced by the presence of the tunnel. Observa-tions of various tunnels in the past have provided a basis forestimating the magnitude of potential ground movementscaused by tunnelling.15-34 Efficient management and good work-manship are critical factors in reducing movement to a practi-cable minimum.

In soft, loose and very permeable ground some pretreatmentmay be necessary to limit the ground loss and to improve thesafety of the construction phase. Typical methods includedewatering, grouting and freezing.

The use of compressed air may be considered to overcometechnical difficulties during excavation in soft clays or water-bearing ground. It acts in three ways, by: (1) balancing the waterhead; (2) providing a direct reaction to field forces; and (3)drying the skin of the soil, thus increasing its effective stress.However, it also has significant physiological risks for opera-tives and must be regarded as a last resort.

40.9 Working in confined spaces

A confined space may be defined as a workplace which does nothave the benefit of natural ventilation. Consequently, thedanger exists that the atmosphere in such places may becomeeither deficient in oxygen due to the build up of gases orvapours, or hazardous due to concentrations of toxic or flam-mable gases or vapours.

The construction and maintenance of buried pipelinesinvolves working in a variety of confined spaces. Examples aremanholes, shafts, tunnels, pits, boreholes and, in some situa-tions, trenches. Precautions must be taken in these cases toensure that safe working conditions exist and the work beingcarried out does not give rise to hazards. Much specializedadvice is available2*-27-35-37 and tunnels and boreholes have Bri-

Gravel surround

Stress inducers

Injector

Injection protection hood

Shield

Figure 40.18 Mini tunnel system. (Courtesy: William F. Rees Ltd)

Thrust pit structure to suitground conditions

Hydraulic jacks and spacersLoad distributing unitLoad

spreaderThrustwall Slider with jacks

for level adjustment

RiverThickwall reinforcedconcrete pipes to BS 5911, Part 120

Thrust shield incorporatingspecial steering devices

3-Segments

In case of fire

Carbon dioxidedry powder

Carbon dioxidedry powder

Carbon dioxidedry powder

Carbon dioxidedry powder

Foamcarbon dioxidedry powdervaporizing liquid

Prevention

Ventilation control

Ventilation controlVentilation control

Ventilation control

Ventilation control

Ventilation control

Ventilation controlVentilation control \

Cylinder care J

Ventilation controlVentilation control

Principal sources

Explosives/engines

Natural/enginesExplosives/engines

Freezing processes

Natural

Natural

NaturalLeakages \Leakages VLeakages JNatural/inducedSpillage

TLV* Explosive limitsLower Upper

(p.p.m.) (%) (%)

50 - -

5000 - -25 - -

5 - -

- 5.3 14

10 4.3 46

5 - -1000 2.2 9.5600 1.5 8.5- 2.5 81.0

- 1.3 7.5.

Hazard

Toxicflammableexplosive

AsphyxiantToxicExtremely toxicAsphyxiant

liquid causesburns

Explosive andasphyxiant

Toxic andexplosive

ToxicExplosive and

asphyxiant

AsphyxiantExplosive

Specificgravity

CO 0.97

CO2 1.53NO 1.04NO2 1.60N2 0.80

CH4 0.60

H2S 1.70

SO2 2.301.55 j2.10}0.91 J

N2

Gas

Carbon monoxide

Carbon dioxideNitrogen oxides

Nitrogen

Methane

Hydrogen sulphide

Sulphur dioxidePropaneButaneAcetyleneDeoxygenated airPetrol/diesel vapour

* Threshold limit value.

Table 40.5 Summary of the most commonly encountered dangerous gases in confined spaces

tish Standard requirements.38-39 The Construction Regulationsand Factory Acts also give guidance.

Work in confined spaces should be undertaken only bypersons properly trained for the job. Under the Health andSafety at Work Act 1974 the employer has a duty to providesuch information, instruction, training and supervision as isnecessary to ensure, so far as is reasonably practicable, thehealth and safety at work of his employees. The employershould also check that employees who are expected to work inconfined spaces are physically and mentally suitable.

Safe systems of work must be laid down and strictly observed.The minimum requirements will be:

(1) To test the atmosphere before entering the confined space.(2) To monitor the atmosphere while people are working.(3) To maintain contact between the operatives and an atten-

dant in free air. The attendant must be trained to carry outemergency procedures.

Adequate means of access and egress must be provided. Rectan-gular or oval holes of 458 x 407 mm and circular holes of407mm diameter are the minimum sizes laid down by theFactories Act.

Larger access should be provided where feasible so that rescueequipment can be used more easily. Permit to Enter and Permitto Work systems should be established.24 These must be admi-nistered by a responsible person who will keep records of thesafety measures taken and will sign a certificate accordingly.

Gas testing of the atmosphere may be done using simple,rugged equipment. Potential hazards can often be anticipatedby an intelligent assessment of local conditions. Works nearwaste tips are likely to encounter methane or hydrogen sulphidegenerated by the decomposition of organic material as well asother industrial compounds. Carbon dioxide, which is heavierthan air, may be produced by the action of acidic water onlimestone or other calcareous rock. It may also arise from theexhausts of nearby internal combustion engines and will bemixed with carbon monoxide. Table 40.5 summarizes the com-monly encountered dangerous gases. Oxygen deficiency canarise due to rusting processes or a fall in atmospheric pressurecausing deoxygenated gas to seep out of the surroundingground.

In sewers additional hazards arise due to the danger of beingswept away or drowned by fast-flowing streams. Dangerousconditions can arise quickly and may be noticed by increasingmovement of air or water through the sewer or the noise ofapproaching water. Attention should be paid to weather fore-casts as a preliminary precaution.

Sewers can also give rise to infection from a number of causesand all persons entering should be inoculated. Personal hygienemust be meticulous and any injury or abrasion should receiveproper medical attention.

Table 40.6 Definitions of remedial works in sewers

Confined spaces can rapidly become contaminated from thework processes being used. Often these processes are themselvesintended to avoid other hazards. For instance, ground freezingand chemical grouting for excluding water, and pipe and cablefreezing to isolate sections of pipe for maintenance or repair.Burning and thermal welding processes use up oxygen and giverise to toxic fumes as do many solvents and paints. Pipes whichhave contained flammable materials must be purged thoroughlybefore any work is carried out.

Dust and noise must also be regarded as dangerous contami-nants in confined spaces. Lighting must be efficient and provideillumination to at least 20 Ix using flameproof equipment and nomore than 24 V. An emergency lighting system must be avail-able for immediate use in the event of failure of the main system.

Rescue from confined spaces should not be attempted with-out proper equipment. The rescue team must be a minimum oftwo so that one person can attend at the entrance while the otherenters. Non-observance of these requirements is likely to makethe situation worse.

40.10 Rehabilitation of pipes andsewers

Trenchless methods for the rehabilitation of buried pipes offerthe advantages of reduced disruption and cost. The gas, watersupply and water disposal industries in particular have de-veloped new methods of construction, particularly for non-man-entry pipes, and further innovation can be expected.

The methods are divided broadly into those incorporating theoriginal pipe structure, e.g. relining, and those removing ordestroying the existing structure, e.g. pipe bursting with percus-sive moles. The choice of method is heavily influenced by thesize of new pipe required and the access available. In non-man-entry sizes work must be carried out either by remote methodsor by open trench.

40.10.1 Inspection and basic strategy

Before repair and refurbishing operations can be undertaken thepipe must be inspected. Manual inspection is generally preferredwhere possible but in dangerous atmospheres and small-dia-meter pipes closed-circuit television is used routinely. It shouldbe recognized that in such cases breathing apparatus or forcedventilation may be required when inserting and recovering thecamera.

The results of the inspection should be recorded in a standardmanner to reduce the subjective nature of assessing defects. Insewers the recommendations of the National Water Council40

should be used.After inspection, a repair strategy must be decided. In sewers,

Rehabilitation:

Maintenance:Repair:

Renovation:Reinforcement:Renewal:

Replacement:

All aspects of upgrading performance of existing sewers. Structural rehabilitation includes repair, renovation andrenewal. Hydraulic rehabilitation includes replacement, reinforcement, flow reduction or attenuation andoccasionally renovationMinor repairs not involving reconstruction of main sewer or alteration of dimensionsRectification of damage to the structural fabric of the sewer and reconstruction of short lengths but notreconstruction of the whole pipeline.Improvement of performance of sewer by incorporating the original sewer fabricProvision of an additional pipeline which, in conjunction with the existing sewer, increases overall flow capacityConstruction of a new sewer on or off the line of an existing sewer. The function and capacity of the new seweris similar to the oldConstruction of a new sewer on or off the line of an existing sewer. The function of the new sewer willincorporate that of the old but may also include improvement or development work.

DisadvantageAdvantage

Originaleffective pipediameterBrief descriptionTechnique

High-pressure equipment is expensive andrequires trained operators

Man-access necessary and existing sewermust be structurally sound. Infiltrationmust be controlled and flow must bediverted when working in the invert. Qua-lity control difficult due to poor workingconditions

May not be suitable where infiltration isactually flowing

Expensive particularly where large voidsexist. High flows must be diverted. Re-stricted number of specialists for non-man-entry system. Difficult to seal packerswhere pipe surface is irregular

Reduces cross-sectional area but may im-prove hydraulic performance to compen-sate. Problems may arise where lateralsoccur

Lateral connections must be re-madequickly. This could take a lot of time orconsiderable excavation in non-man-accesspipes and services could be disrupted.

Pipe may tend to float during grouting.Lead in trench disruptive.Circular cross-section only available

Can be used in all diameters of pipe. Insmall diameter pipes the work can bedone remotely and monitored byclosed-circuit TV

Minimal disruption to existing service.Materials and equipment inexpensive

Materials and equipment inexpensive.PFA and other additives can be addedto grout to improve properties. Can beinjected from within pipe or fromground surface

Can be used for man-entry and non-man-entry pipes. Fast gel time allowsmethod to be used where infiltration isoccurring. Low flows tolerated withinpipe. Remote method for small pipesallows joints to be air tested as workproceeds

Can improve structural stability, hy-draulic capacity and chemical resis-tance. Structural design will requireannulus between old and new pipes tobe grouted up. Existing pipe may thenform part of the structure

Quick and large-diameter bends may beaccommodated

All

All

0.9mupwards

All

All

0. 1-0.9 m

High-pressure waterjetting to removescale and encrustation

In small-diameter pipes cutting isdone by remotely controlledmechanical or high-pressurecutters operated under observa-tion by closed-circuit TV

Hand- or pressure-pointing usedto renew joints in masonry orbrick sewers where these havenot closed up

Cement grout injected to sealleaks, fill voids and strengthenground

Low-viscosity chemicals injectedinto ground generally to sealleaks. Some increase in soilstrength may be achieved

New pipes are inserted in existingpipes

After cleaning and proving thediameter of the existing pipe thenew pipe (usually polyolefin) isjointed at ground level usingbutt fusion welding and thentowed or pushed into the exist-ing pipe via a lead in trench

Cleaning(1) Scaling

(2) Removal ofprotrusions

Stabilization(3) Repointing

(4) Cementgrouting

(5) Chemicalgrouting

Pipe lining(6) Pipelining

generally

(7) Sliplining

Table 40.7 Pipe stabilization and relining methods

Generally needs working pit and largenumber of joints.

Relatively high loss of cross-sectional area.Pipe may tend to float during grouting.Lateral connections may cause a problem in

non-man-accessible diameters

Full over-pumping required during installa-tion. Site set-up cost is relatively high onsmall jobs. Monopoly supplier

Jointing is labour-intensive. Strutting oftenrequired during grouting operation

Jointing is labour-intensive. Support usuallyrequired during grouting. Care must betaken not to overstrain or otherwisedamage the barrier layer of resin

Segments are heavy. Jointing is labour-inten-sive. Not suitable for pipe clearances lessthan 1070 x 760mm

Similar to (c). Not suitable for pipe clear-ances less than 900 x 600 mm

Various materials may be used (e.g.IiPVC, reinforced plastic mortar, GRP,

GRC).Quick large-diameter bends may be

accommodated.Joints can be screwed, socket and spigot

or welded depending on material.Cost effective for short deep lengths.May be possible to vary shape of cross-

section with some materialsRapid installation using existing man-

holes. Bends and minor deformationsin the existing pipe can be accommo-dated. Thickness is approximately3mm minimum but can be increased to19mm. Smooth finish improves flowcharacteristics. Resins can be speciallyformulated to give chemical-resistantfinish

Variety of cross-sections available.Material easily cut to form connections

Deforms easily to suit cross-section. Highstrength : weight ratio

Variety of cross-sections available. Nostrutting required. Segments can sup-port earth load

Similar to (c)

All

0. 1-1. 2m

0.6mupwards

0.6mupwards

See note

See note

Lining pipes are jointed within aworking pit and then jacked ortowed into the existing pipe

After cleaning the existing pipe aspecial polyester felt linerimpregnated with polyesterthermosetting resin is insertedinto the pipe through an exist-ing manhole. The liner is in-itially inside out and unrolledinto the pipe by flooding withwater. The liner is specially tail-ored to suit the diameter andlength of the pipe. The resin isthen cured by circulating hotwater within the lined pipe

These methods are frequently usedfor non-circular man-accessiblepipes

Segments are usually produced intwo sections with lap joints.Connections are made usingbolts, pop rivets, self-tappingscrews, etc. The invert is laidfirst and bedded on mortar orwooden blocks. The crown isthen installed and centralizedwith wedges. The annulus isgrouted up and the laterals re-connected as work proceeds

Segments can be either one piecewith a single longitudinal jointor two pieces with lap joints

Segments are installed by layinginvert and then jacking crowninto position. Reinforcement istied and in situ gunite used tomake joints. Annulus then pres-sure grouted

Similar to (c) but lap jointed usingepoxy-modified mortar seals

(8) Sectionalliningpipes

(9) Inversion:polyesterliningcuredin place

(10) Segmentalrelining

(a) GRCsegments

(b) GRPsegments

(c) Preshotgunitesegments

(d) Precastresinconcretesegments

Dusty and difficult to supervise. Very depen-dent on operator skill. Must remove re-bound material from invert.Overpumping required

Non-structural. Laterals difficult to dealwith. Flow must be diverted

Cheap material and simple process pro-duces a jointless lining. Connectionsand changing cross-section easilyaccommodated. Access through exist-ing manholes

Existing manholes can be used for access.Suitable for a wide range of pipe dia-meters. Cheap material

0.9mupwards

All

All methods need thorough clean-ing of the pipe and dewateringof the pipe to prevent infil-tration

Standard gunite process used inman-accessible pipes to rebuildinside of pipe. Small-diameterrod or mesh required for struc-tural applications

The wet mortar mix is pumpedthrough a revolving mortar dis-penser on to the walls of thepipe. The lining machine istowed through the pipe at aspecified rate followed by rotaryor drag trowels which give asmooth finish

Similar to (12) and currentlyundergoing development

In situ coatings

(11) In situgunite

(12) Centrifugalcementmortarlining

(13) Polyurethaneor cold-curedresin

Table 40.7 Pipe stabilization and relining methods (continued)

the pipeline is likely to form only one part of a larger reticula-tion and changes in its flow characteristics may have adverseeffects elsewhere. The Sewerage rehabilitation manual** sets outthe requirements of a proper management strategy for sewerrehabilitation and gives definitions of the various terms to beused when describing the works (Table 40.6).

40.10.2 Methods of rehabilitation

Some methods of rehabilitation are based on microtunnellingtechniques while others are based on more traditional pro-cedures (Table 40.7).

The practical requirements common to gas, water and sewerpipes are: (1) the need to cope with existing flows; (2) the need towork through existing manholes wherever possible; and (3) theability to cope with lateral connections.

When relining gas and sewer pipes it is sometimes possible touse the annulus between the old and new pipes to maintainexisting service connections. The annulus is grouted up once theservice connections have been remade and the new pipe is onstream. In other cases bypasses and overpumping must be usedwith the possibility of interrupted services for the consumer.

Lateral connections present a considerable problem in sewers.They must be located accurately prior to any work being carriedout and all live connections identified. The position and angle ofentry must be recorded with great accuracy. If the connectionprotrudes into the sewer then it may be necessary to cut theprojection back even before the inspection phase can be com-pleted.

Various remote methods of reconnection have beeninvented,42'43 but local excavation is often the only practicalsolution. On occasions, relined sewers can be reconnected usinga 'down the drain' remote-controlled cutter observed by closed-circuit TV within the new sewer or using a cutter working withinthe new sewer. Accurate location of the cutting tool is requiredto ensure coincidence within the existing branch and severalspecialized devices are available to achieve this. Once the newpipe has been holed through, special sleeves or grouting tech-niques are used to make a watertight joint usually working fromwithin the pipe.

In cases where excavation is unavoidable for reconnectionspecial 'keyhole' techniques have been evolved which require aminimum of spoil to be removed so that special pipe-fitting toolscan be used from ground level.

40.11 Costs

The total cost of constructing any buried pipeline is made up ofthe costs incurred by its promoter and the social costs borne bythe community at large. The social costs can arise in a variety ofways. Examples are traffic delays, additional wear on unsuitablediversion roads, business losses, etc. which are not directcharges to the promoter. There is increasing concern in somecritical urban situations that the social cost may be greater thanthe initial construction cost of the pipeline, particularly wheretrenched construction is used.44 Thus, concern has given addedimpetus to the development of trenchless techniques which mayoffer significant benefits in congested areas by reducing disrup-tion.

40.11.1 Project cost appraisal

The prudent promoter of a new pipeline will consider its totallifetime costs which simply stated are:

Initial construction + Running + Maintenance + Replacementcost cost cost cost

It is likely that several different routes and types of constructionappear feasible and the promoter will undertake cost/benefitstudies to ascertain which combination offers the best advantagewithin given financial limits. The studies will require a numberof estimates to be made of future events. Consequently, theresults are unlikely to be accurate in absolute terms but willoffer a relative assessment of the choices available. A furtherfactor to be considered in cost/benefit analysis is the possibilityof maximizing benefit for a small increase in construction cost.An example of this is the enlargement of ducts and tunnels topermit them to be used by two or more separate services.

The degree to which social costs are considered in such studieswill depend on legal requirements and the nature of the pro-moter. The law gives some third parties entitlement to recom-pense for financial damage suffered as a result of the works. Inother cases, for instance, where diverted traffic uses more fuel,the promoter is unlikely to have to foot the bill and will notconsider such costs in the feasibility study unless politicalrequirements or public policy dictate otherwise. Table 40.8

Table 40.8 Typical direct and indirect costs associated with sewer construction in trench

Engineering costsPlanning and designSite investigationsConstructionSupervisionService diversionsReinstatement and

maintenance

Social costsOther service

replacementsTraffic diversions

and delaysLoss of businessEnvironmental

Dislocation of supply

Direct costs

YesYesYesYesYesInitial

reinstatement

For example, gas mains,water mains, etc.

Traffic lights,signs

CompensationInclude in

permanent worksTemporary works

Indirect costs

NoNoNoNoNoIncreased long-

term maintenance(damage due totrenching)

No

Cost to road users of delay and extra fuel, additionalpolicing, wear and tear on alternative routesAny costs not met by compensationYes

Consumer inconvenience

indicates typical direct and indirect costs which may arise in theconstruction of a sewer in trench.

40.11.2 Construction costs

The estimation of costs for civil engineering construction is askilled procedure. If prices are required to be within a reason-able order of accuracy then it is necessary to build them up fromfirst principles. The variation of conditions for design andconstruction will tend to make the use of historical overall unitcosts unreliable except on the simplest jobs.

Ground conditions have a major influence on both the designand the risks arising during construction and in any type ofconstruction the cost of a proper site investigation should beregarded as money well spent. The report should consider theproblems likely to arise in both the permanent and temporaryconditions and the report should be made available to thetenderers.

The promoter must decide at an early stage what the contrac-tual basis will be for the design and the construction of theworks. The contracts with the designer and the constructor mustplace the obligations and responsibilities for the risks in an

equitable manner. The promoter must recognize that by bearingsome part of the potential risks himself, e.g. unexpected groundconditions, then the prices tendered are likely to be morerealistic.

Decisions made during the design phase may result in somemethods of construction being precluded and the effect of thismust be assessed. Also different construction methods impresstheir own individual characteristics on the serviceability and lifeof the completed works which must be recognized by thedesigner.

The total construction cost of a pipeline is made up of:

(1) Labour cost.(2) Plant cost.(3) Materials cost.(4) Constructor's on-costs including temporary works.(5) Constructor's profit.(6) Design cost.(7) Promoter's supervision cost.

In most cases, the cost of the pipe and other permanentmaterials is not affected significantly by the depth of construc-

Effective pipe diameter (mm)

Note: Based on 600-m longsewer including manholes

Figure 40.20 Comparison of direct engineering costs of varioustunnelling methods in good ground

Microtunnellingrange

Man-accessibletunnelling

(Note effect of lining downlarge bore to give hydraulicefficiency in some methods)

Keypipe jacktrenchtimber headingtunnellingmini tunnellingremote control bentonite shield

Cost

per m

etre

as

a pe

rcen

tage

of15

00 m

m d

iamet

er tu

nnel

cost

tion. For a given diameter of pipe constructed in trench the costof labour and plant increases as a power function of the depth.The cost of pipelines laid by non-disruptive methods is largelyunaffected by depth.

The comparison of costs for different types of constructionmust be made on the basis of the same ground and siteconditions. Where conditions change, then the relativities andranking of the various methods are also likely to change. Someconstruction methods can cope readily with unexpected groundconditions, whereas others may need to be supplemented withexpensive secondary operations such as extensive grouting orground freezing. The potential cost of such risks must beassessed at the feasibility stage of a project.

The total job size will also have an influence on the unit costsof construction especially when using methods with a highinitial set-up cost. Figure 40.20 illustrates the relative costs forinstalling 600-m long sewers by different methods in goodground.

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