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Problems Associated with Trenchless Techniques for Sewer Construction in Hong Kong Wilson Mok

- 1 - January 2001

Problems Associated with Trenchless Techniquesfor Sewer Construction in Hong Kong

Wilson W.S. Mok BASc BA CEng CPEng RPE (Civil & Geotechnical)MICE MIMM MIEAust MHKIE MASCE FGS

Atkins China Limited

In the last ten years, the adoption of trenchless techniques for sewer construction in Hong Kong has become popular,with the length of the pipelines being increased from only a few hundred metres to more than five to six kilometers in acontract. This paper highlights the problems observed from such techniques on various contracts and the solutions orprecautionary measures provided. The design engineers may make reference thereto so that in future similar contractscan be implemented in a more effective and smooth manner.

Keywords : Pipejacking, Tunnel, Settlement, Obstructions, Safety

Introduction

In order to execute the sewer construction using thetrenchless techniques, it is essential to carry out site

investigation works at suitable locations along thealignment of the pipelines in the design stage, and theresults are made available for inspection by each tendererwho will, based on the design requirements such as thesize, depth and length of each section of the pipelines, theground conditions, and site constraints, etc, prepare atechnical report detailing how the works can be carried outif his tender is successful later on.

In general, three types of techniques can be classified,namely, open mode, close mode and mixed mode. Theopen mode, by means of compressed air hand tunnellingor free air hand tunnelling, is applicable when the

pipelines are required to pass through artificialobstructions like left-in sheetpiles, disused piles and oldseawalls. The tunnel constructed is only a temporarylining and the pipeline needs to be pushed thereinafterwards with the gap between the two features beingfilled up with grouting materials. The close mode is toemploy a mechanically operated pipejacking machine,equipped with cutting bits or discs on its face forexcavation, using slurry to balance the earth pressure andgroundwater ahead of the machine. The mixed modeuses a pipejacking machine with a bulkhead in its face toresist the earth pressure and the excavation is undertakenby a mechanically operated boom or backhoe mounted

thereon, and with an access to the face of the machine forremoval of obstructions manually under a compressed airenvironment.

Problems

No matter which type of technique is selected, differentproblems would arise in different stages of the works.They can be broadly summarized as follows:

(i) Application of excavation permit;(ii) Temporary traffic arrangements;(iii) Locations of jacking and receiving shafts;

(iv) Temporary works for shaft construction;(v) Non-compliance of excavation permit conditions(vi) Checking of level and line of pipeline;(vii) Ground settlement / subsidence and heaving;(viii) Settlement monitoring;

(ix) Pipejacking / tunnelling being stopped byobstructions;

(x) Condition of completed pipeline;(xi) Layout of pipeline;

(xii) Environmental issues; and(xiii) Safety.

Application of Excavation Permit

Whenever excavation works are to be allowed in road andfootway, an excavation permit needs to be obtained fromthe Highways Department (HyD). For Governmentcontracts, the client offices are responsible for applyingthe excavation permits for the contractors through theUtility Management System (UMS), in which therespective application is digitized by uploading in acomputer network linked with HyD. When the draft plan

stage is passed, a formal application form, duly enteredand stamped by the client office, is submitted to HyD andusually the excavation permit is granted in about 3 weekstime. However, sometimes the application is rejected inthe draft plan stage due to conflict with other parties’works. This would result in the necessity in thecoordination in agreeing the programme between allconcerned parties before HyD is to consider theapplication. If the coordination fails, HyD will not issue theexcavation permit to the latter applicant until the formerapplicant has completed his work. Consequently, thework would be delayed due to no excavation permit, thusattracting a contractual claim.

For work sites located in busy carriageways, the issue of a“Seek Traffic Advice” by the Transport Department (TD) isnecessary before the application can be considered byHyD. Hence, an excavation permit would be held up if itsproposed temporary traffic arrangement (TTA) scheme isrejected by the authorities.

Temporary Traffic Arrangements

Prior to commencing the excavation work, it is necessaryto implement a temporary traffic arrangement scheme,consented by the authorities, to cone off the works area.

The contractor is to design such a scheme based on thecontract requirements which summarize the trafficconstraints imposed by TD when the design of sewer, inparticular the proposed locations of manholes, iscirculated for his comment in the design stage. The




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problem is that, in reality, the location of some manholesmight have to be shifted to suit the site conditions such asthe conflict of existing utilities which diversion would takea time more than that allowed in the contract, or the as-built foundations of DSD’s box culverts or nullahs or HyD’sstructures (i.e. flyovers, footbridges and subways) beingdifferent from those of the design obstructing the seweralignment. As a result, the traffic impact needs to be re-assessed before the temporary traffic arrangementscheme for the revised manhole location can be acceptedby the authorities. It is not uncommon that the temporaryshaft is to be constructed in more than one stage, withone portion being completed, decked over and resumed totraffic before commencement of another portion, or undernight work. This would no doubt affect the progress ofwork and would also incur additional cost.

Figure 1 - Implementation of TTA Scheme for Shaft Construction

Location of Jacking and Receiving Shafts

To enable the carrying out of trenchless techniques, a pairof shafts is required between each section of the pipelines.They are the jacking shaft and the receiving shaft, whichare also used for construction of manholes after theinstallation of pipeline. A jacking shaft must be largeenough to accommodate the pipejacking machine or thehandshield in the case of tunnelling, the thrust wall at therear, the launching eye in the front, and the auxiliaryequipment. Its size is dependent on the type, diameterand length of the machine and could vary from 5m x 5m to7.5m x 9m. The size of the receiving shaft could, however,be smaller correspondingly.

The locations and spacings of manholes are fixed by theengineer, in consideration of the alignment of the pipelines,the directions of incoming side connection sewers and themaximum length of the sewer for future maintenance.The length of a jacked pipeline could be from less than50m to more than 250m whereas construction of a tunneldrive is limited to a short length due to its high risk, slowprogress and expensive cost.

There is usually a contract requirement that the locationsof manholes are to be ascertained on site after theexisting utilities and services at the respective locations

have been identified by trial trenches. Thereafter, it is thecontractor’s responsibility to determine which locations areto be used as jacking and receiving shafts. His selectionis generally governed by the size of the works areaavailable for placing the plant and construction materials

which is in turn determined by whether the proposedtemporary traffic arrangement scheme for coning it off isapproved by the authorities. There have been cases that,due to heavy traffic, the contractor is only allowed tooccupy a minimum works area for constructing the shaft,in which it can only be used as a transition shaft betweenthe jacking and receiving shafts entailing the pipelinebeing jacked through.

Figure 2 - View of a Typical Jacking Shaft

The identification of existing utilities and services at shaftlocations is the contractor’s obligation as stipulated in theGeneral Specification for Civil Engineering Works and is atime consuming and complicated activity. Sincetrenchless techniques are mainly adopted in urban areas,it would be impracticable to look for a shaft completelyfree of existing utilities and services. It is only a questionhow many utilities exist at the respective location.

With reference to the records of utilities and servicessupplied by the utility undertakers and the authorities, thecontractor will decide the sizes and extent of the trialtrenches after the location of a manhole has beenpreliminarily confirmed by the engineer. Before trialtrenches are carried out, the contractor is required to useunderground services detection equipment to investigatethe locations and depths of underground utilities andservices, and the excavation is conducted with extremecare to avoid causing damage. It is often that theexposed utilities and services are different from those

indicated in the record drawings in terms of location, size,extent and depth. Occasionally, uncharted features areidentified necessitating a considerable time to find out theownership.

The performance of the underground utilities detectionequipment depends on its brand and model, theexperience of the equipment operator, the spacing, depthand configuration of existing utilities and services, as wellas their materials.

For utilities / services made of metal, they could begenerally detected in an accurate manner and vice versa

for those made of other materials. For different types ofcables at different depths within the same detection zone,the signals induced therein would be interfered with eachother, possibly mixing up the result. A better result wouldbe obtained when the utilities are widely spaced without




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overlapping with each other. Hence, the data provided bythe equipment should be carefully interpretated and usedin conjunction with the as-built records of utilities andservices. It has been found that the equipment isineffective to detect concrete pipes and asbestos cementwatermains. Utilities surrounded with concrete might alsoaffect its accuracy.

After completion of the identification, the contractor isrequired to plot up details of all the existing utilities andservices with respect to the shaft location on drawings andto assess whether any of them would need to be divertedin order to give sufficient room for the pipejacking / tunneling work and the subsequent manhole construction.It is the obligation of the utility undertakers under theRoads (Works, Use and Compensation) Regulation toalter their features at their own cost should they be in theway of Government’s works. The lowering of the top slab,relocation of the access, desilting and ventilation openings,and offsetting the pipeline centre in the shaft, asappropriate, might be the alternatives to account for theutilities and services such as high voltage cables andlarge diameter watermains, which diversion is extremelydifficult in terms of the time frame of the contract or publicconvenience. Sometimes, it is even infeasible to divertthe utilities and services due to their congestion over thearea. Under such circumstances, a new location of themanhole has to be looked at by the engineer, with trialtrench excavation being required again. In some cases,several locations need to be compared prior to making thefinal decision. It is possible that the final location of ashaft which consists of less utilities and services is moveddistantly from its original location to avoid the timely utilitydiversions. As a result, additional time and cost would beincurred and the shaft construction delayed.

Figure 3 - Relocation of Openings in Manhole to Suit HighVoltage Cables

For utility diversions, it is the contractor’s responsibilityunder the contract to agree the programme with the utilityundertakers, to closely monitor the progress and to notifythe engineer of any slippage. In the event of a slippage,there is not too much action that both the engineer andthe contractor can do but to keep on chasing. Tominimize the occurrence of such a situation, the contractorwill normally carry out all the civil works necessary for theutility diversion, at the discretion of the engineer so thatthe cost associated therewith can be reimbursed.

Temporary Works for Shaft Construction

After the location of a manhole has been finalized, thecontractor is to design the temporary works for the shaftsuitable for carrying out the trenchless techniques and themanhole construction. A drill hole is sunk at the shaftlocation to determine the ground and groundwaterconditions. The contractor will select the type oftemporary works with reference to the site investigationresults and site constraints, if any. The temporary worksdesign, after being certified by the Independent CheckingEngineer as being satisfactory, is forwarded to theengineer for consent prior to commencement of work.

The temporary works generally appear in rectangularshape and sheetpiles are commonly adopted due to fastoperation and cheaper cost. Sometimes, it is requisite tomodify their shape and size to suit the existing utilities andservices. When boulders are encountered, preboring bymeans of Odex method using the down-to-hole hammer isused to remove obstructions. If the boulder zone is toothick, the sheetpiles still might not be able to be driven tothe required depth due to the rock fragments falling intothe prebored holes. When this situation arises, theaffected sheetpiles are to be extracted and the holesprebored again. Alternately, steel pipe piles can be usedfor penetrating through boulders, with their hollow sectionbeing filled with sand or grouting materials to increase therigidity. The cost of using this method is about twice asthat of the sheetpiling work, and the time required iscomparatively longer. Among the two types of piles, pipepiles are widely used at locations where headroom isconstrained.

Upon completion of excavation, the shaft is to bemaintained in a dry condition to allow the execution of thepipejacking / tunnelling work and the manholeconstruction. Groundwater would ingress into the shaftthrough the gaps of the temporary works. The situation ismore critical at locations where steel laggings are installedbeneath the existing utilities being required to betemporarily supported inside the shaft, and wherelaunching and receiving eyes are placed. Dewateringinside the shaft is not an effective measure as it woulddraw down groundwater table causing ground settlementin the surrounding soils and in turn affecting the stability ofthe utilities and services underneath. Watertightness can,however, be achieved by providing a grout curtain alongthe perimeter of the sheetpiling cofferdam. This curtain,with a thickness of 450 to 600mm, is constructed beforethe excavation and is effected by firstly using bentonitecement grout to fill up the voids of soil particles andsecondly applying chemical grout to increase the strengthand to decrease the permeability. Outside the launchingand receiving eyes, the thickness of the grout plug is to beincreased to 3 to 4m to prevent the ground being punchedby the pipejacking machine or the tunnelling shield whenleaving and entering the shaft. The grout mix is adjustedto suit the ground conditions, and grout pipes are left inthe ground so that re-grouting can be carried out if theinitial grout is proved to be ineffective. In porous groundor near the seafront, it is possible to have a highconsumption of grout due to it being migrated. The grout




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pressure must be properly controlled to avoid blowing outthe ground or encroaching into ductings of utilities.

Figure 4 - Failure of Grout Plug in Receiving Shaft

The as-constructed temporary works are to be inspectedby the Independent Checking Engineer and aConstruction Check Certificate issued stating that thetemporary works have been constructed in accordancewith the certified design, before commencement of thenext stage of work inside the shaft is allowed.

After manhole construction in the shaft is complete,extraction of sheetpiles in the proximity of sensitive utilitiessuch as gas main and watermain should be prohibited toavoid causing stability problem.

Non-compliance of Excavation Permit Conditions

There are conditions attached to the excavation permitsissued by the Highways Department (HyD), for which thepermittee (the contractor) is to abide by during the courseof works. These conditions, apart from outlining thegeneral obligations, also cover the requirements ontemporary traffic arrangement and control, vehicular andpedestrian facilities, unattended sites, noise control, sitecleanliness, and backfilling and reinstatement, as well asmaintenance and defects liability. Staff of HyD will carryout inspections on the work sites on a regular interval andprepare checklists if any of the excavation permitconditions has not been complied with. The checklists willthen be forwarded to the permittee for follow-up action as

soon as possible. By the end of each month, HyD willsummarize all the non-compliance cases for each worksite and request the consultant supervising the respectivecontract to provide a written explanation on the causes ofsuch cases and to advise the remedial measures thathave been taken to avoid recurrence of similar situations.If the figure of non-compliance cases for a particularcontract is persistently high, the consultant will be requiredto attend the Road Opening Coordination Committee(ROCC) meeting held by HyD for further explanation.

In view of the work sites requiring excavation permitsbeing located at either carriageways or footways,

argument between HyD and the contractor always ariseson the standard of defining non-compliance, particularlywhen a HyD construction site is in the vicinity forcomparison.

A typical example is that whenever works are to be carriedout at a location, an excavation permit must be availableand a temporary traffic arrangement scheme implemented,with lighting, signing and guarding being provided. Sincethe form for applying excavation permit only specifies theapproximate length, width and depth of excavation, thecontractor will assume that the TTA coned-off area can beused as a works area for placing plant and constructionmaterials necessary for execution of the works. HyD,however, has a different view that such a works areashould not be used unless it is also indicated in theapplication.

Figure 5 - Typical Setup of Slurry Pipejacking Equipment inWorks Area

Another example is the maintenance of traffic cones. Forwork sites located at carriageways, traffic cones are usedto enclose the works area. From time to time, some ofthese cones are temporarily shifted to allow parking of

lorries for loading and unloading, but they have not beenimmediately reverted to their original spacing after suchan activity. There have also been occasions that thetraffic cones are removed by outsiders for parking purpose,or by the adjacent shops for illegal loading and unloading,and this situation is particularly critical outside the normalworking hours and during public holidays, and in the worksites where one activity has been completed but await thecommencement of the next activity such as utilitydiversion. All the above cases would cause a situationthat the traffic cones have not been placed in accordancewith the Road Traffic (Traffic Control) Regulations. Sitetidiness is another issue which has caused HyD’s concern,

in which the contractor is always found to have notremoved disused construction materials and excavatedspoils off the site at the end of a day, in particular thosebuilding debris and rubbish dumped by others during non-working hours.

In the above context, although it is true that, under theexcavation permit conditions, the contractor is fullyresponsible for maintaining his work site in a tidy, properand secure manner, some of the non-compliance casesmight be beyond his control as it would be extremelydifficult, if not impossible, to provide guarding in each worksite at all times. Practically speaking, there is no way toprevent occurrence of such cases. What the contractorcan do is to continuously educate his workers andsupervisory staff, to properly fence off his work site afterwork, to strengthen the site patrols and to allocate moreresources for cleaning up the site and maintaining the




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TTA scheme, so that the non-compliance cases can bereduced to a minimum. On the other hand, although it isthe contractor’s responsibility to abide by the excavationpermit conditions, the engineer’s staff should morallyadvise the contractor immediately of any irregularitiesbeing observed on site so as to rectify the situation assoon as possible.

Figure 6 - Condition of a Works Area Coned Off by TTA Scheme

Checking of Level and Line of Pipeline

The checking of the level and line of a pipeline is carriedout by means of a laser device mounted on the sheetpilesat the rear of the jacking shaft with reference to the targetplate installed in the pipejacking machine. The machineoperator will, based on the readings shown in thecomputer monitor, adjust the steering cylinders so that thealignment of the pipeline can be reverted to the designedprofile soonest. The laser device might have to be

temporarily shifted to maintain the laser beam being incontact with the target plate due to excessive deviation inalignment of the pipeline during the course of pipejacking.It is vital to ensure that this device is prevented frommovement accidentally during the course of works and tocarry out daily checking on its position by surveyingmethod. Otherwise, misalignment might arise turning thepipeline outside the receiving shaft even though themovement of the laser device is very small. It is also vitalto clean the laser device and the target plate installed inthe pipejacking machine on a regular basis so that theirfunctionality will not be affected by dust.

As a double assurance, the level and line of a pipelineshould also be checked using a gyro theodolite at suitablelength of the pipeline when the pipejacking is in progress.

Figure 7 - Laser Device Used for Pipejacking / Tunnelling Works

Ground Settlement / Subsidence and Heaving

Shaft Construction and driving of pipejacking / tunnellingare the two major operations in trenchless techniques. Itis therefore necessary to properly control groundsettlement to avoid damaging the adjacent roads,footways, utilities, services and structures.

Ground settlement would occur in the following situations:

1) For shaft construction, sheetpiles or pipe piles or acombination of both are commonly adopted astemporary works. Sheetpiling would inducevibration tending to consolidate the soils in thevicinity. The pipe piles, installed in the ground byusing either the rotary drilling method or thepercussive drilling method, would also generateground movement but with a comparatively smallmagnitude.

The extent of ground settlement caused by the

above two construction methods is very limited,generally less than 1m from the edge of the piles.The situation can be improved if sandy materials,with the aid of water, are constantly used to fill upthe gaps during the piling operation.

2) For removing underground boulder obstructions,Odex drilling method is used, in which compressedair is constantly supplied to the down-to-holehammer to enable the breaking of boulders into fineparticles, thus causing caving-in of the surroundingsoils above, particularly near the ground surface.Even though sandy materials are filled in the drill

hole continuously, settlement would still exist as therate of blowing out is fasten than the rate of infilling.In rectification of the situation, casing should beequipped with the hammer down to rockhead andair pressure properly controlled to avoid causingexcessive voids in the adjacent ground.

3) Upon completion of the shaft construction, groundsettlement would continue as a result of thedeformation of temporary works and the dewateringrequired to keep the shaft dry. The effect of groundsettlement in the former case might be significant ifthe temporary struts are widely spaced, whereas

watertighness would be a problem for those shaftswith inadequate penetration depth of temporaryworks due to irregular rockhead above theformation level, or varied ground conditions limitingthe penetration. The provision of windows in thetemporary works, necessary for accommodation ofexisting utilities and services in the shaft, also givesrise to the ingress of groundwater through the gapsbetween the steel laggings below such features.

The ground settlement due to the deformation oftemporary works could be controlled if thicksections of piles are used and the spacing of struts

is reduced. Alternately, grout curtain can beformed outside the perimeter of the piles toincrease the rigidly of the surrounding soil. Thiswould also help to minimize the necessity ofdewatering in the shaft.




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4) During the course of pipejacking works, overcut isto be required in order to allow the pipeline to passthrough the ground from the jacking shaft to thereceiving shaft. This phenomenon would onlycause insignificant ground settlement in normalground. In soft ground, the overcut situation wouldbecome critical if the slurry injected at the head ofthe pipejacking machine is too thin to preventexcessive materials falling into the cutting wheel,thus attracting ground settlement. Anotherphenomenon is that, in mixed ground consisting ofsoil and boulders, the ground settlement could beserious as the pipejacking machine tends to movetowards a direction with soft materials resulting inthe migration of more soil thereinto due to the rateof cutting through boulders being much slower thanthat of soil. The effect of ground settlement underthis situation depends on the location and sizes ofthe boulders in front of the cutting wheel, depth ofthe overburden above the pipeline and density ofsoil. If boulders are located at the lower half of thecutting wheel or where a boulder zone isencountered, the overlying soil would be disturbedby the vibration of the pipejacking machine when itis in contact with the hard materials forcing the soilto fall into the voids of the boulders, eventuallycausing ground settlement.

To minimize the effect, the cutting wheel pressureshould be properly controlled in the range of 90– 100 bar.

Over-excavation may occur at lower pressures ifthe cutting wheel is not always maintainingpressure against the face. If high cutting wheelrotation speeds are being used and cutting wheelpressure is 80 bar or less, the water flow should beregulated to the lowest permissible level in order toprevent over-excavation resulting from too muchmaterials being flushed out from the excavatedface creating voids.

For slurry pipejacking work, vibration would beproduced by the desander used for separating theslurry from the spoils for recycling use, resulting inlocal ground settlement if the filling materials belowthe road / footway, especially flexible type, are notproperly compacted. In this respect, rubber padsmay be used for placing below the desander toabsorb vibration. This would also eliminate theresonance of the glass windows of the nearbybuildings induced thereby causing breakage.

5) When the pipeline is being jacked in the ground,groundwater and soil would migrate into the shaftthrough the gap between the pipeline and thelaunching eye. Ingress of groundwater isunavoidable but a rubber seal can be installed inthe eye to prevent escape of slurry and soilmigration which would cause ground settlementdue to loss of fines. If the work is carried out inclayey materials, the installation of double rubberseal may be necessary for retaining the fines.

Figure 8 - Condition of a Free Air Hand Tunnel

Figure 9 - Filling Up Voids between Temporary Linings and Soil

6) For hand tunnelling works, the size of overcut inorder to allow the advancement of the shield headis difficult to be controlled. If the voids between thetemporary linings and the soil cannot be filled up bygrouting materials rapidly, they would be closed upwithin a short time causing the ground above tosettle. Dewatering inside the tunnel would causedrawdown of the groundwater table and in turnattract ground settlement. This situation can becompensated by carrying out horizontal groutingwork along the pipeline alignment in advance, or bycompressed air pressure, or a combination of both,

depending on the ground conditions, depth of thetunnel and depth of groundwater table.

Ground heaving is a product of excessive pressureinjected into the ground during the course of the work.This situation would occur if the slurry pressure is greaterthan the overburden pressure above the pipeline or theground is too loose with high porosity. To overcome theproblem, the machine operator’s experience and reactionin that instance are very important that he should closelymonitor the volume of slurry actually injected into theground ahead of the pipejacking machine and thepressure used and make the necessary adjustment of its

mix by increasing the viscosity and of its applied pressure.

Having stated the above, sufficient overburden needs tobe provided above the pipeline for slurry pipejackingworks so that the ground will not be blown up by the slurry




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pressure injected to the face of the machine. By virtue ofthe physically required pressure for maintaining theworkability of slurry in the charging pipe to avoid blockage,it might not be unreasonable to assume a 6m thick soil asbeing a minimium thickness for such overburden. Thisdepth should also be a safe margin to get rid of theconflict with utilities and services in the ground, whichgenerally exists in a depth range of 1.5m to 3.5m.Similarly, the pressure applied to a compressed air tunnelshould be checked against the overburden to ensure asafe working.

Settlement Monitoring

Both the pipejacking and hand tunnelling works wouldinduce ground settlement. Its magnitude can beestimated after the site investigation and laboratory dataat the respective locations have been gathered. Theacceptability of this magnitude depends on whether thestability of the adjacent roads, footways, utilities, servicesand structures will be affected, in which an assessmentneeds to be carried out. If the value is excessive, theground has to be treated by grouting in advance of theworks. Therefore, it is necessary to monitor theperformance of such works by installing markers atsuitable locations so that the actual values can bemeasured and compared with that obtained fromtheoretical calculations, and to set out the triggered andaction values for which measures are to be taken, whichinclude increase of the monitoring frequency, stopping ofwork for investigation of the cause(s) of the settlementand remedial measures before the works are resumed.The same approach should also apply to the estimatedsettlement being within the tolerated value.

Normally, nail markers are installed in the road surfaceadjacent to excavated shafts at 5m intervals directlyabove the centre line of the drive alignment and at offsetsfrom that line at a distance which depends on thediameter of the pipeline and the depth of overburden.This installation is only good for flexible pavements andcan always provide reasonably accurate readings. Inaddition, ground settlement under this type of pavementcan be easily detected by visual inspections on the cracksdeveloping therein. For pipejacking and tunnelling worksbelow rigid pavements, experience has told that sub-surface markers in the form of a steel rod with a baseplate should be used as the pavement itself can withholdby its rigidity for some time even though the soilunderneath has settled. These markers require the coringof holes, say 150mm in diameter, through the concreteslab, and a PVC tube is inserted to encase the steel rod.Its depth should be limited to not more than 600mm belowthe slab to avoid damaging the utilities and services laid inthe ground. The holes are then filled with sandy materialsto maintain the verticality of the steel rods, and steel capsare provided to prevent ingress of surface runoff whichwould affect the accuracy of the readings. Uponcompletion of the works, the steel rods are to be removedand the holes in the concrete slab properly reinstated.

When settlement is detected on ground surface, itsinfluence zone would be in the range of 45 O to 60 O projected from the face and rear of the pipejacking

machine / handshield in the longitudinal direction and fromits two sides in the transeverse direction. Depending onthe ground conditions, the respective settlement couldtake a few days to complete. Hence, it is essential tocontinue the monitoring for some time after completion ofthe work so that an overall picture on ground settlementcan be obtained before the permanent road reinstatementis executed.

Figure 10 - Apparatus Used for Coring Holes in ConcreteCarriageway

Figure 11 - Installation of Sub-surface Settlement Marker

Figure 12 - Cracks Noted in Carriageway During the course ofPipejacking

Pipejacking / Tunnelling Being Stopped byObstructions

The success of pipeline installation by trenchlesstechniques depends very much on the method selected.In old reclaimed areas where the type and sizes of thefilling materials are susceptible, the earth pressurebalancing pipejacking machine is commonly adopted,whereas the slurry pressure balancing pipejacking




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machine has proved to be effective in virgin ground and innewly reclaimed areas where the filling is under propercontrol. The hand tunnelling method, due to its high costand slow daily production rate, is only considered in areaswhere artificial obstructions exist.

In order to avoid the work being stopped by boulder orartificial obstructions, the contractor needs to determine

which type of method should be used with care, withregard to the ground conditions along the respectivepipeline alignment as revealed by the site investigationworks carried out in the design stage, and other detailsavailable in the working drawings such as locations ofseawalls, locations of left-in sheetpiles, etc. Additionalboreholes are usually sunk by the contractor at selectedlocations before the pipejacking / tunneling machines areordered. This is because up to the present moment, noneof the pipejacking machines in the world can remove largesizes of artificial obstructions.

The configuration of the cutting wheel in a pipejacking

machine plays an important role as to whether the pipelinecan be constructed in a fast and smooth manner. Spoketype cutting wheel with cutting bits equipped in the slurrypipejacking machine has been proved to be efficient in soilwith occasional boulders in the size of less than 25% ofthe diameter of the machine. This arrangement is nogood for penetrating through a boulder zone, and themachine would be stuck resulting therefrom. A rescueshaft has to be sunk to remove the obstructions if trafficpermits. Otherwise, a rescue tunnel is to be constructedfrom the receiving shaft, which would be expensive andtime consuming. The cutting wheel which appears in adome with discs and / or roller cutters is capable of cutting

through rock, but some cutters may have to be replacedduring the course of the pipejacking work due to wearing.Access to the face of the cutting wheel for suchreplacement can only be gained in the machines with adiameter of 1500mm or above, after the ground ahead isfully stabilized by slurry. To minimize the problem, anexperienced contractor would forward samples of rockcores obtained from site investigations to the pipejackingmachine manufacturer for determination of thecompatibility of the cutting discs before the machine isdelivered to Hong Kong. To avoid improper functioningleading to stoppage, those cutting discs repaired in localshops for re-use should be carefully examined prior to

launching the machine into ground.

Figure 13 - Cutting Bits in Pipejacking Machine Worn out byBoulders

Figure 14 - Condition of a Worn Out Cutting Disc after RockExcavation

For earth pressure balancing pipejacking machines,isolated boulders ahead can be handled by the boomcutter or mini backhoe mounted on its face if their sizesare less than 25% of the diameter of the machine. If thesizes or extent of boulders are excessive or where smallartificial obstructions are encountered, the removal can beeffected by pneumatic tools operated by a worker at theface of the machine under a compressed air environment,which access is gained from the airlock chamber therein.This type of machine also cannot deal with boulders orartifical obstructions if their sizes almost occupy the entirearea of the face of the bulkhead. Rescue operations,similar to those of the slurry pipejacking machine, are tobe required in this regard.

The pipejacking and tunnelling works would also bestopped by obstruction of existing utilities crossing thealignment of the pipeline. This situation is very rare as inthe design stage, utility record drawings have beenobtained from the utility undertakers. The design engineerwill, relying on the details contained in these drawings,which generally include the nature, approximate alignmentand extent of the respective features, the site investigationresults and site constraints, choose the alignment of thepipeline and the locations of manholes. Discussions onthe possible conflicts would also be carried out with theutility undertakers prior to making such a decision. Sincethe actual alignment, extent and top and bottom levels ofexisting utilities are not available in the record drawings,trial trenches are required to be executed in theconstruction stage before the actual location of a manholecan be decided. However, if an utility exists at anabnormal depth say 6 to 7m below ground somewherebetween two manholes, the pipeline at the same levelwould be obstructed. This situation cannot be foreseen inthe design stage as it would be impracticable to open andinspect every utility drawpit or chamber due to the timeand resources being involved, particularly that the levelsof some utilities like cables might not be linear along theiralignment. Although close liaison is maintained by theengineer, the contractor and the utility undertakers, theproblem might not be known until the time when the utilityundertakers identify the features revealed by trial trenchesfor the manholes near the “problemed” utility, as theirexcavation might not be allowed at the same time whiletrial trench excavation is being carried out at othermanhole locations, due to traffic constraint. Consequently,




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part of the pipeline in the upstream and downstreamwould have been installed at the time of the incidentrendering adjustment of the level to suit being impossible.

Should the above situation be encountered, a trial trenchneeds to be opened at the conflicting location to fullyexpose the bottom level of the utility. The engineer has todecide whether the design can be amended by usingopen cut method locally with a reduced pipe size, or theutility must go. Nonetheless, the progress of thepipejacking / tunneling works would be interruptedattracting a claim from the contractor.

The pipejacking machine would also be stuck when theground condition suddenly changes from soil to extremelyhard and massive rock, due to the wearing or damage ofthe cutting discs attributing to the excessive jacking loadapplied onto the pipeline while a maximum torque is beingmobilized by the cutting wheel to allow the discs forloosening the rock by the rolling and penetration process,resulting in inadequate or no space for free movement ofthe discs against the rock face, thus leading to gradualexhaustion.

The remedial measures can be in the form of breaking therock ahead of the pipejacking machine by the preboringmethod if occupation of the ground above is allowed. Atemporary shaft is sunk for removing the rock fragments,the inspection of the condition of the cutting discs and thenecessary replacement. If not, a rescue tunnel withhorizontal airlock should be used to recover the machine.

Another stoppage occurs in the slurry pipejackingmachine when the slurry discharging pipe is clogged withmaterials such as spoils (usually clay with high plasticity),metallic pieces or timber fragments. If the cleaning isunsuccessful even by opening the by-pass unit, therespective sections of the pipe have to be dismantled forunblockage. Whereas clogging is found at the inlet valve,high pressure water jet connected to the nearest nozzlecan be used to dilute the spoils pressing them into thedischarging pipe.

Condition of Completed Pipeline

Upon completion of the pipejacking for a pipeline, it isnecessary to inspect its condition and remedial measuresare to be carried out if defects exist. In general, thedefects include spalling or cracking of concrete in pipes,opening up of pipe joints, spalling of PVC internalprotective linings, and irregular profile of pipeline.

Spalling or cracking of concrete in pipes usually occursnear the pipe joints in the sections of a pipeline with a zigzag profile, resulting from encountering non-homogeneous ground during the course of the pipejackingwork. The jacking load acting onto the pipeline wouldbecome eccentric developing unevenly high stress on oneside of the joint. The concrete would crack or spall out ifthe actual deflection at the respective pipe joint exceedsthe allowable value recommended by the pipemanufacturer. The repair work can take place byapplication of epoxy to the damaged area of the pipe afterthe defective concrete has been removed. This workshould be carried out immediately to avoid causing danger,

prior to continuing the pipejacking work if the situation isserious, or else, a steel ring may be used to temporarilywithhold the defective concrete, and the repair is to beconducted after completion of the pipeline. New PVCsheets are then incorporated in the affected area of thepipe body by embedment of the locking keys into theepoxy before it becomes stiff.

Figure 15 - Repair of Damaged Concrete Pipe for Pipejacking Work

Opening up of pipe joints is usually found at locations ofintermediate jacking stations, and occurs after the jackshave been removed but the rear jacking pipe cannot bepushed in contact with the front jacking pipe already fixedin the intermediate jacking station due to the excessivedistortion of pipe joints, formed by the irregular profile inthe pipeline for the reason as stated in the foregoingsection. This would result in the pipeline being failed inthe air test and the infiltration test.

To overcome the problem, epoxy may be used to fill upthe gap, with a steel cage being added if the gap is wide.Remedial measures may not be necessary for otheropened up pipe joints if their widths fall within thetolerance allowed in the pipe.

PVC internal linings are provided in the pipes to preventthe concrete from being attacked by hydrogen suphidegas emitted from sewage. They are placed in the pipebody with locking keys embedded in the concrete whenthe pipes are manufactured, and generally appear from210 o to 359 o with the pipe invert, depending on the designof the sewerage project. One problem is that the pipesmay not be properly placed to allow the lining of each pipealong the pipeline being in the same position. Anotherproblem is that, during the pipejacking work, dirts or waterwould enter into the pipes behind the PVC linings throughthe pipe joints causing the linings to spall out between thelocking keys. This phenomenon is particularly obvious atlocations where slurry pipes have been disconnected forunblocking obstruction or where rails are installed in thepipeline for transportation of spoils by trollies, messing thepipeline with sludge. To overcome the problem, thelinings in the spalled areas are to be cut out so that the airor water entrapped therein can be released, andadditional linings put back and rigidly connected with theadjacent linings by a special made welding equipment. Inaddition, all pipe joints should be protected by the liningsas soon as the pipeline installation is complete so thatoccurrence of similar situations can be avoided.




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Figure 16 - PVC Linings in Pipeline

Irregular profile of a pipeline is a common phenomenonfor pipejacking works. Many contract documents specifythe permissible deviations being 50 to 75mm for line and

35 to 50mm for level. Although there are alwaysprovisions in the contract that the contractor is responsiblefor making good the misalignment to the satisfaction of theengineer if such values are exceeded, the “problemed”sections of pipes in the pipeline cannot be dismantled andreconnected due to the construction method adopted. Itappears that the only solution is to smooth thedepressions at the critical sections by trimming theconcrete surface in the invert of the affected pipes andapplying epoxy or other approved materials thereon asrendering so that ponding and trapping of sewage will notoccur. Adding suitable steel mesh may be required if therendering is thick. From practical point of view, remedial

measures are unnecessary in other sections of thepipeline should the permissible deviations be slightlyexceeded. The contractor should, however, conduct ahydraulic check as to whether the pipeline will still havesufficient capacity in carrying flow after completion ofremedial measures. If the result is unsatisfactory, theconstruction of another smaller diameter pipeline adjacentto the original pipeline would be inevitable, but this wouldinvolve a lot of problems such as space in the shaft, timeand choice of machine.

For hand tunnels, cement mortar is used for rendering theinvert of their whole length so that a smooth and uniform

base is provided before the pipeline is pushed therein byhydraulic jacks.

Layout of Pipeline

Prior to carrying out the pipejacking of a pipeline, thecontractor is necessary to prepare a pipe layout withrespect to the contract requirements such as the positionsof flexible joints from structures, including manholes, andthe locations where intermediate jacking stations are to beplaced. This layout may need to be changed during thepipejacking work, by using more intermediate jackingstations to reduce the face and skin resistance of the

pipeline due to unforeseen ground condition, resulting inthe possibility that the two flexible joints, as stipulated inthe General Specification for Civil Engineering Works,cannot be provided at the correct positions from the outerfaces of manholes.

To account for the above situation and bearing in mindthat the sheetpiles below the pipe cannot be extractedafter work, more short pipes should be allowed at bothends of the pipeline, with their last pipe from the manholebeing extended outside the temporary shaft so thatsuitable flexibility of the pipeline can be maintained.

Environmental Issues

Solid waste, dust, noise, air and water are the majorenvironmental issues relating to pipejacking and tunnelingworks, and would cause significant impact to the public ifthey cannot be properly controlled.

For solid waste, soil samples are taken at suitable depthsby boreholes at shaft locations for laboratory testing,including chemical analysis, in the initial stage of acontract. Depending on the result, the spoils excavatedfrom the shafts and the jacked / tunnelled pipelines are tobe disposed of at either a public dumping area forreclamation, a marine spoil ground for unsuitablematerials or the Chemical Work Treatment Facility locatedat Tsing Yi for contaminated materials. The problem isthat, during the course of works, the materials disposedfrom the section of sewer between two shafts might not besame as those revealed by boreholes at the same levels,thus leading to them being transported to a wrongdisposal area if they are not instantly identified in thestorage area by the contractor in the event of there beingany change in the type of material.

When shaft construction is carried out, dust will emit, inparticular during rock excavation. It is necessary to adoptsuitable suppression measures to avoid causing nuisanceto the public. Cleaning and watering the site frequentlymay minimize the fugitive dust emissions. In the processof material handling, any material which has the potentialto create dust should be treated with water or sprayedwith wetting agent. Where dusty materials are beingcarried by lorry, suitable fitting side and tail boards shouldbe provided, with a clean tarpaulin being placed on top.Wheel washing facilities should be installed and used byall lorries leaving the site so that earth, mud, debris anddust will not be deposited on public roads. Water in thewheel cleaning facility should be changed at frequentintervals and sediments removed regularly. Whensufficient space is not available for such installation, thewheels of a lorry should be cleaned by high pressurewater jet within an area enclosed by an earth or concretebund which can stop the muddy water flowing onto theadjacent carriageways and footways, prior to leaving thesite.

Noise will be generated by the lifting appliance, excavator,generator, compressor and the like during the pilingoperation for construction of shafts, and the pipejackingand tunnelling works, and would cause nuisance to thepublic if its level is excessive. Under the current EPDregulations, no construction noise permit (CNP) is to berequired for noisy works other than piling works between7a.m. and 7p.m. on a day except Sundays and PublicHolidays. For piling works, a duration of 3 hours in a dayis allowed by EPD, with the time being designated toaccount for the environment at the respective location.




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Any work carried out beyond the legal hours, without avalid CNP, would result in the prosecution by theauthorities, and the bad remark in the contractor’sperformance report for consistent violation of EPDregulations.

To control the situation, the following mitigation measurescan be carried out:

1) Use of acoustic barriers / shields to enclose orpartially enclose noisy activities and to screen thereceivers from direct line of site from constructionactivities. A purpose-built barrier normally canachieve reductions of 5 – 10 decibels;

2) Use of silenced equipment;

3) Carefully planning the construction programmethrough:

- Restriction of times at which concrete

breaking and / or piling is carried out;- Minimization of surface construction and

underground excavation work duringevening and night-time periods;

4) Observation of school hours: noisy operations suchas concrete breaking, excavation plant operation,and / or piling should be avoided near the existingschools during teaching or examination hours;

5) Placing of noisy equipment and the conducting ofactivities as far from sensitivity receivers as ispractical and turning off of idling equipment;

6) Proper plant and equipment maintenance.

Emission of hot and waste air from generators,compressors and other plant always attracts complaintsfrom the residents and pedestrians. Due to most of theworks areas being constrained by traffic and other factors,it may not be possible to place these features in a properlocation to avoid the nuisance. In these circumstances,consideration should be given to increase the height of theair emission pipes and to place their outlets distant fromand at a direction opposite to the residents andpedestrians.

Water pollution or nuisance should be avoided during theshaft construction and the pipejacking or tunneling works.Such phenomenon usually arises when the contaminatedwater is discharged directly or indirectly into any publicsewer, storm drain, channel, stream course or sea, orspills onto the adjacent carriageways or footways,resulting from either the water being pumped out from theshaft or tunnel without being treated or sedimented inadvance, the wet excavated material leaking the effluenton ground surface at the time of being loaded to the lorryfor disposal, oil leakage from the plant and machinery,disused slurry overflowing from the storage tank, muddy /

waste water flowing out during the pipe-piling, preboring,drilling or grouting operations.

All the above acts are a violation of the EPD regulationsleading to the contractor being prosecuted.

To prevent occurrence of the situation, the followingmeasures are considered to be effective:

1) Silt and slurry traps and sedimentation tanksshould be provided at each works location andwaste water (site run-off) should be channelled tothe basins. Regular cleaning of the sediment trapsand tanks, including the adjacent gully system,should be carried out to ensure their efficiency.This would also help to minimize the chance offlooding during heavy rainstorms;

2) Measures to intercept surface runoff onto the workslocation due to rain should be provided so that itwill not flow across the sites;

3) Processing water should be re-used. Prior to itsdischarge, it should be channelled to thesedimentation tanks;

4) Earth or concrete bunds should be constructedaround the areas of working. Any release of mudor slurry within the bunded area should be removedimmediately. All the lorries should be wheelwashed before leaving the site, and provided withfacilities in the load compartment to preventleakage of muddy water onto carriageway duringthe course of transportation.

5) Excavation of open-cut and cover trenches shouldbe avoided during monsoon or rainy seasons ifpossible. Measures should be taken to minimizethe flow of rainwater into trenches. Trenchesshould be dug and backfilled in short sections.Water pumped from the trenches should bechannelled to sedimentation tanks prior to itsdischarge into storm drains.

Safety

Since most of the works are of underground in naturenecessitating the entry into or working in confined spaces,it is essential to maintain a safe working environment forthe workers during the course of works. The riskassociated with each activity must be comprehensivelyassessed by a competent person in advance so thatcorresponding safety measures can be provided based onhis recommendations. It is the contractor’s responsibilityto ensure that any work in a confined space, as defined inthe Factories and Industrial (Confined Spaces)Regulations, complies with the requirements of theRegulations and is in accordance with the Guide to theRegulations published by the Labour Department. It isalso necessary to provide adequate and suitable trainingand tool box talks to all persons involved in the works, toestablish emergency procedures to account for thepossible incidents and to carry out a drill beforecommencement of work. Consideration should also begiven on the design of the pipeline and tunnel such thatsafety problems due to inadequate headroom can beeliminated.

The types of risk for trenchless techniques depend on themethod adopted, which include but not limited to air




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quality, electrical installations, fire prevention and floodingfor pipejacking works; the same and face collapse for freeair hand tunnelling works; and all the above and change ofpressure for compressed air tunnelling works.

In order to make certain the safety and health of theworkers who enter and work in a confined space, aPermit-to-Work system should be used, in which acertificate is issued by the contractor listing the findings inthe risk assessment report completed by the competentperson, the results of the gas testing, the nature of work tobe done, the conditions and features of the confinedspace, the period during which workers can remain safetyin the confined space, etc. Only certified workers whohold a relevant certificate after attending an approvedtraining course in connection with confined space areallowed to enter and carry out the work therein. For eachlocation, a Permit-to-Enter system should be enforced toregister the persons entering and leaving the confinedspace.

Prior to each entering into an excavated shaft, a jackedpipeline or a hand tunnel, a suitable testing detector mustbe used to ensure the air at different levels and locationsbeing free of dangerous gases such as carbon monoxide,hydrogen sulphide, methane and other flammable gasesand at the same time sufficient oxygen being present.Fresh air has to be constantly and directly supplied to theworks location through an adequately and effectivelyforced ventilation system in the form of high capacityblowers while work is in progress. Where the atmosphereinside is likely to cause safety or health hazards to theworkers, breathing apparatus should be used.

Electrical installations are another issue which mightcause hazard to the workers. They should be effectivelyearthed and protected to avoid electric shock. Powercables are to be properly fixed and hung up to avoiddamage or leakage which could burn the PVC linings, ifany, in the pipeline causing toxic gas. Adequate lighting isrequired in the excavated shaft and throughout the lengthof the jacked pipeline and hand tunnel to prevent theworkers from being fallen down due to unclear visibility.

Figure 17 - Forced Ventilation, Rails and Electrical Installationsin Pipeline

To account for the possible fire hazard, suitable andadequate fire fighting equipment should be providedadjacent to the shaft, pipeline and tunnel. Neither storageof dangerous good and flammable material nor smoking in

the confined space is to be allowed. To alert the workersin case of fire, fire/emergency alarm has to be installed atthe appropriate location, with the fire/emergency exit routebeing identified by displaying the direction signs.

Flooding or inundation in excavated shafts would occurwhen the perimeter grout curtain is ineffective or thesubmersible pumps are malfunctioning. This situationseldom occurs in jacked pipelines as the work is carriedout in a close mode but the author has experienced acase that the pipejacking machine and a section ofpipeline behind was found inundated with soil and waterafter a holiday, the cause of which was believed to havebeen due to the opening of the valve used fortransportation of spoils by mistake, not known whencompleting the work at the end of a day. For free air handtunnelling works, it is essential to verify the effectivenessof grout ahead of excavation to avoid sudden ingress ofgroundwater and change of ground condition, which couldlead to face collapse endangering the workers. Oncompletion of work each day, shutters should be providedbetween the work face and the handshield, and thecondition of the handshield is to be examined in the nextday before the shutters are removed for continuing thework. Ingress of groundwater in compressed air handtunnels would unlikely occur if suitable pressure is appliedtherein. To achieve this, it is necessary to maintain thefunction of the compressor at all times as the dropping ofpressure would result in the tunnel being completelyflooded. For the same reason as stated above, the workface for the compressed air hand tunnel also needs to besafeguarded by shutters at the end of each day.Horizontal grouting may be necessary ahead ofexcavation if excessive seepage of water or loose workface is noted. Persons who enter into the compressed airenvironment must properly depressurize before leaving.A medical lock is to be provided on the surface near theworks location to allow for emergency events.

Figure 18 - Horizontal Grouting in Compressed Air Hand Tunnel

An effective communication system has to be installedbetween the control room and the jacked pipeline/tunnelat suitable locations, usually in the pipejacking machine

and the handshield and in the intermediate point of apipeline or tunnel via the cable inter phones, so thatinstant contact can be made, any emergency situationnotified immediately, and corresponding response takenbased on the severity of the occurrence.




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Emergency procedures need to be established and listedin the method statement for the respective work, for whichthe workers are to strictly follow. Rescue equipment suchas approved type of breathing apparatus, suitableapparatus for reviving an unconscious worker, vesselscontaining oxygen or air, lifting man-bucket or stretcherand the relevant lifting appliance and gears, safetyharness and ropes, standby water pump, life buoys fittedwith life line, audio alarm installed inside the control room,pipeline and bottom of the shaft for alerting personnel incase of emergency, etc., should be provided. Allemergency contact telephone numbers must be posted atsuitably visible locations so as to enable immediate actionwhen necessary. If practicable, a rescue team consistingof suitably trained persons should be formed. Thedecision to evacuate from underground to surface shouldbe token by the most senior person present at the time.

Notwithstanding the above, the supervisory staff andmanagement level of both the engineer and the contractorshould be proactive and consciously in performing theirduties so as to ensure the safety requirements laid downin the contract documents, the safety plan and the methodstatements being complied with at all times, and takeimmediate actions should irregularities be observed.

Conclusion

The trenchless techniques for sewer construction in HongKong have been proved to be satisfactory with lessconstruction time and less disturbance to the public whencompared with the open trench method. Some of theproblems presented in this paper could have beenavoided or minimized had suitable type of method beenselected based on detailed site investigation results andsite constraints, and proper planning provided in advance.It is hoped that, through the further development andresearch of the techniques, the problems related to thetechnical side can be controlled in a more effectivemanner. It is also essential to include comprehensiverather than general requirements in the contract to fullycover each activity of works, which should be practicaland specific with regard to site constraints, with thecontractor’s responsibility and actions being clearlydefined, so that different views between the engineer andthe contractor on problems, when occurred, can beminimized. The experience, training and attitude of theworking and supervising personnel also have a significanteffect on the success of work.

Wilson W.S. Mok

Wilson Mok graduated from theUniversity of Windsor in Canada andhas over 23 years practical workingexperience in a wide variety ofgeotechnical and civil engineeringprojects in both design office andsite. He is particularly experienced in

dealing with design and construction associated withgeotechnical investigations and instrumentation, deepexcavations, tunnels, ground improvements, settlementanalysis, reclamation, site formation, slope preventivemeasures, foundations and sewerage works. He hasworked as a resident engineer for Fugro, Mouchel, MottMacdonald and Maunsell, and is currently employed byAtkins for the same post on a DSD Contract.

Documents

Problems Associated with Trenchless Techniques for Sew…