1 Development of Sewerage

Rehabilitation

Geoffrey F. Read MSc, CEng, FICE, FIStructE, FClWEM, FIHT,

MILE, FconsE, Fcmi, MAE

1.1 Introduction Sewerage is usually defined as a network or system of sewers and associated works designed for the collection of waste water or foul sewage, conveying it via pipes, conduits and ancillary works, discharging it at a treatment works or other place of disposal prior to returning it to the environment in appropriate condition.

Discharge to natural water courses of foul sewage or of any sewage containing putrescible matter cannot be permitted unless either the quantity is so small that dilution prevents nuisance or the sewage is treated in such a manner that the 'purified' effluent is not offensive or deleterious when so discharged. The quantity of water in rivers in Great Britain is generally not sufficiently high to give the degree of dilution (i.e. not less than 500 times the dry weather flow) that is considered necessary for the permissible discharge of crude sewage and therefore the foul sewage from all British inland towns and many coastal towns has to be treated at a disposal works.

As well as domestic waste water the network takes the used water of business and industry and accommodates part or all of the surface water, namely, the flow of storm water from roofs and paved surfaces of all types.

1.2 Sewerage systems

Towns are sewered according to three basic systems:

1. The combined system in which the foul and surface water sewage are discharged into one system of sewers which lead to the sewage treatment works or point of outfall, this being the format existing in the older towns and cities.

2. The separate system in which the foul sewage is carried by an individual system of sewers to the sewage treatment works or point of outfall while surface water is carried away by a number of local systems each discharging at various points into natural watercourses or discharging into soakaways - the system generally being found in the newer towns and cities.

3. The partially separate system in which the greatest part of the surface water is dealt with by the surface water sewers while the surface water from the backyards and back part of roofs is passed to the foul sewers.

2 Sewers

All these systems have their specific uses. The separate system is considered the best for most purposes, whereas combined drainage is desirable where the runoff from road or other hard surfaces is so foul that it requires some form of treatment before passing to a water course. The partially separate system is sometimes applied where the local water consumption is not sufficiently high to keep the foul sewers clean.

Because of the time required to design, finance and construct such facilities the engineer is obliged to estimate future population and industrial growth. The type of industry present or anticipated is particularly important owing to the wide range of water required by various industries - up to 2 27 000 litres of water may be required to produce a ton of steel and up to 4 55 000 litres to produce a ton of paper.

To maintain the health and general well-being of the community the efficient disposal of all types of refuse including sewage is clearly of paramount importance. In Volume I we examined in some detail the earlier forms of sewerage and sewage disposal and traced the development of public health engineering from early times to the present day. Works of sanitation are known to have existed in ancient times and historians have shown that drainage systems were used in Roman cities over 2000 years ago and by other civilisations at a much earlier period in history.

Civilisation has been described as the art of living in towns. Some people, even as long as 5000 years ago, were skilled in that art - others as recently as 500 years ago neglected it and suffered the consequences.

The first civilisations grew in Egypt, Crete, Iraq, northern India and China and they all learned at some stage to provide good water supplies and some sort of drainage system. The need to have convenient access to water has always been one of the most powerful influences on human life and settlement patterns. Everyone needs water for personal health and hygiene - indeed for life itself. As water circulates in the hydrological cycle (Fig. 1.1) with its own momentum, everyone has, in principle, the same equitable right to share in its abundance and its scarcity. The development of the ability to carry water enabled early

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Development of sewerage rehabilitation 3

people to extend their hunting range and gave them the possibility of greater mobility generally, but for permanent settlement and the cultivation of crops, etc., ready access to water or a frequent and reliable rainfall is essential.

Sewers were commonplace in the Indus valley (now western Pakistan) around 2500 BC. In Mohenjo-Daro, one of the largest towns of that early civilisation, every house had a latrine, many had bathrooms and there was a large public bath.

The location of settlements was usually conditioned by the availability of a water supply. However, as a community grew it would need to supplement its water supply and for public health to organise the safe and efficient disposal of its waste products - especially if occurring in congested or confined conditions. The Greeks instinctively equated hygiene with health and, like some other ancient civilisations, had very clear ideas on how to achieve it (Figs 1.2 and 1.3). On the island of Crete, for instance, people knew how to build drainage systems long before the time we think of as 'ancient Greece' and the Minoan palace of Knossos even had clay drainage pipes taking away human waste - the pipes being apparently tapered to increase the velocity and provide a self-cleansing f l o w - all some 3600 years ago.

Years later, water supplies and sewers were common at the time of the Roman civilisation - the famous sewer of ancient Rome, the Cloaca Maxima ( 'Cloacina' was the goddess of sewers), was built about 588 BC to drain the valleys between the Esquiline, the Viminal and the Quirinal to prevent disease and to carry away the surplus water to the River Tiber. It was originally a natural water course but had to be artificially diverted where building made this necessary and in the sixth century Br the Romans also arched o v e r - a similar treatment to that given to the water courses in the centres of our industrial towns during the early nineteenth century. The Romans' fondness for baths and fountains is well known and they certainly appreciated the importance of a proper sewerage system, epitomised by the shrine to Venus Cloacina, although the River Tiber was ill-treated - nearly all the city's waste water and sewage were eventually allowed to flow into it including the flow from the Cloacina Maxima.

Roman engineers introduced their techniques throughout their vast empire; even today, at such ancient bathhouses as that at Bath, may be seen the lead pipes for bringing water in and the drains to take away the used water.

Notwithstanding the efforts of the Romans during their occupation of this country there was really no subsequent effort to provide proper sani tat ion- as we know it today - in the United Kingdom and indeed in other modern countries until the early nineteenth century.

In the garrison towns and frontier outposts of northwest Europe the disposal of human waste was largely in keeping with the 23rd chapter of Deuteronomy which described withdrawal outside the camp. But in the rapidly growing cities of the high Middle Ages, the first attempts at organised waste removal were appearing. Cesspools were introduced in place of the privy but throughout the Renaissance the general practice was to dump all waste in the town's gutters hopefully to be flushed through the primitive 'surface water drains' next time it rained.

Not surprisingly, these crude sanitary arrangements contributed to the spread of disease. John Snow, a nineteenth century English physician, compiled a list of outbreaks of cholera, which he believed had moved westwards from India reaching London and Paris in 1849. He traced a London occurrence to a public well, known as the Broad Street Pump, in Golden Square, which he determined was being contaminated by nearby privy vaults. This was a noteworthy achievement, especially since it predated by several years the discovery of the role of bacteria in disease transmissions.

4 Sewers

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Figure 1.2 (a) Remains of second/third century AD toilets at KOS. (b) KOS Culvert in tersect ion- part of drainage from Thermopylae Roman baths between second century BC and first century AD.

Development of sewerage rehabilitation 5

Figure 1.3 Clay water pipes c. first century BC at KOS. Note: Lime deposits inside pipes and moulded pipe joints,

The sewerage systems as we now understand them have developed from experience gained virtually only over the last 200 years - sewerage has in fact only really developed since the advent of a piped water supply and the consequential need for an effective means of removal of the greater quantity of waste water.

Modern foul water sewerage is descended from the permitted discharge of domestic waste into the underground sewers laid for the purpose of draining rainwater from the s t ree ts , etc. H o w this c a m e a b o u t is i l l u s t r a t ed in the r e p o r t o f the L o n d o n C o u n t y C o u n c i l ,

in w h i c h it is m e n t i o n e d that :

Afew years before the passing of this Act [i.e. the MichaeIAngelo Taylor's Act, 1817], an invention had been introduced which was to have a very important effect on sanitation. This was the water closet, which offered facilities, before unknown, for the entire removal of sewage. At first the watercloset was made to discharge not into the sewers, but into a cesspool, the ancient receptacle for offensive household refuse, the contents of which were removed from time to time. The large addition thus caused to the contents of the cesspools, however, made it necessary to introduce overflow drains running from them into the street sewers; and other reasons also gave inducement to discharge the sewage, with the aid of a sufficient water supply, direct from the water closet to the street sewers. Originally, the discharge of offensive matter into the sewers was a penal offence, and so continued up to about the year 1815. Afterwards it became permissive to drain houses into sewers, and in 1947 it was made compulsory.

These sewers were originally banked-up open watercourses, intended solely for the purpose of carrying off the surface drainage.

1.3 General principles of sewerage

Modern sewers, although they are usually constructed underground, are designed for the purpose of hydraulics, as if they were open channels. Although they are usually round

6 Sewers

pipes or culverts, the crown of the pipe or the arch of the culvert may be considered as being there only to prevent the ground above from falling in. Except in the comparatively rare conditions of surcharge, the crown of the pipes is not designed with the intention of confining the flow of sewage, this being done by the invert and sides.

Sewers are considered in practice to be discharging the maximum capacity for which they are designed when flowing just full - usually for the greater part of the time they are flowing only partly full.

1.4 Public utilities

A sewerage system is one of a number of vital public utilities upon which the modem community is so dependent. The exact value to the public of any particular service is extremely difficult to ascertain. It is not possible to say precisely the degree of expenditure that can be justified in overcoming nuisance and inconvenience - expenditure is largely regulated by practice, current opinion and government policy backed up more recently by environmental considerations generally originating from policies determined by the EC.

Unfortunately as so much of the infrastructure is not visible to the general public a lack of appreciation of its importance is commonplace until such time as it requires attention - the latter varying from unplanned emergency works through to planned maintenance and renewal; all of which have an impact on the day-to-day life of the community which normally reacts at any early stage notwithstanding the longer-term benefits.

1.5 Outline design

Foul sewerage design is based on the number and density of buildings, the number of families per building, the size of the family and the varying habits of the population in regard to the use of water.

Sewers should be laid at gradients that will produce velocities sufficient to prevent permanent deposit of solids. A velocity of 0.75 m/s occurring sufficiently frequently is usually enough to sustain self-cleansing conditions and avoid long-term deposition of solids.

For the adequate design of surface water sewers, the engineer must have knowledge of the topography, together with details of the intensities of rainfall of the particular district. There is clearly an economic level to the intensity of storm that can reasonably be catered for and in practice such sewers are usually designed for the worst storms likely to occur every year or two years.

In view of the size of the flows involved, a surface water sewerage system is usually designed if possible as a gravity system, pumping being kept to a minimum. For a foul sewerage scheme, it is often necessary to compare the alternative engineering considerations and economies of gravity sewers and a pumping scheme. Nevertheless in a fiat district pumping or a vacuum system may be the only practical solution. The structural design of buried pipelines was developed in America in the early twentieth century and has been in use in the UK for many years. The importance of structural design became even more apparent with the development of flexible joints for all types of pipe and with the increasing use of plastics.

Development of sewerage rehabilitation 7

1.6 Development of public health engineering As mentioned earlier, in Volume I we traced in some detail the development of public health engineering from as long as 5000 years ago through to the present day.

It was emphasised that public health in Britain and in fact elsewhere in Europe did not automatically improve with the passage of time. In the Roman period, for instance, going to the toilet was usually a clean, comfortable and relaxing experience. Hundreds of years later, in the Middle Ages, toilets were often quite dangerous and disgusting places to enter.

The Industrial Revolution was really the turning point in the development of public health in Britain. It was within this period of rapid change and invention that civilisation in this country entered a new era and a multiplicity of factories and industrial plants rapidly developed in the townships, especially those having abundant supplies of river water, coal and other raw materials such as existed in Manchester.

The population rapidly began to leave the countryside and the agricultural way of life, attracted by the relatively higher wages offered by the new industries and towns began to develop at an alarming rate as they entered the nineteenth century.

Previously the country had been primarily agricultural in character but with the massive population movement to the quickly developing towns and cities, the Victorians had to take some rapid action to combat the health problems that quickly developed. A scenario of masses of people living immediately adjacent to the factories where they worked using water and producing waste of all types meant both a separate supply of potable water and means of disposing of it when used before it became a health hazard was of paramount importance.

Water is often described as the foundation of life but in an industrialised community it quickly becomes a health hazard in itself unless properly treated.

Until the middle of the nineteenth century most British towns obtained their water from convenient wells, springs and rivers, which quickly became polluted from the overloaded piecemeal surface water drainage system which in the main only collected filthy water from the rapidly developing areas to discharge it into the nearest water course where it soon again became drinking water! At that time the link between polluted water and disease did not appear to have been taken seriously. The early mains did not supply water constantly. As the factories grew there was an ever-increasing demand for water power as a result of the installation of water wheels and the damming of water courses to provide the necessary operating head for machinery, in addition to the needs for processing water. This retention of flow and the pollution resulting from the many industrial processes reduced the supply for domestic purposes although we should always remember that the Victorians did not wash or bath to any extent. Nevertheless as a result of severe health problems - cholera killed many thousands of people in the first epidemic of 1831-2 - they were forced to quickly come to terms with the s i tua t ion- and Britain's early combined sewers were born from a necessity to eliminate the breeding grounds of urban disease. For example, Manchester- the birthplace of the Industrial Revolution - saw its population increase from 5000 in 1750 to 1 42 000 in 1842 and by 1867 it had reached 3 63 000 and some 7 00 000 by the end of the Victorian period.

In fact Manchester experienced the most rapid growth of any city in this country and is described in Manchester through the Ages by David Rhodes. By 1840, Manchester had been transformed from an eighteenth century provincial market town to the foremost manufacturing and technological centre in England. It became the centre of the manufacturing

8 Sewers

industry for cotton, in addition to machinery of all types, machine tools as well as coal mining, bridge building, construction of railway locomotives, gas works, chemical plant, etc. The public health problems of town housing, already bad, soon become acute and the results were justifiably described as both foul and fatal. The life expectancy of mechanics, labourers and their families was only some 17 years in Manchester compared with a figure of 38 years in a rural community such as Rutland.

The speed of this massive population increase and the buildings that went with it resulted in many kilometres of sewer and water main being constructed over a relatively short period with the result that although much of it is still in use today parts of it have reached the end of their working life over a relatively short period. A similar pattern emerges in regard to the massive networks of private culverts. During the Industrial Revolution, extensive culverting of minor rivers and streams, often spanned by early buildings, became commonplace, and due to a lack of maintenance work their condition today is generally much worse than the adjacent public sewerage networks constructed at about the same time. The situation is reflected over many of our northern industrial towns and cities which grew up during the Industrial Revolution. Against this background one can appreciate why so many sewer collapses have materialised over a relatively short period. Where towns and cities have developed more gradually as in the south, the infrastructure has followed a similar pattern and is also likely to wear out over a longer period although other factors may come into play.

In Volume I we researched the development of the national sewerage networks using Manches ter - which possesses the oldest extensive sewerage network in the coun t ry - as a typical example. It was reported that initially the two main forums of construction were the 'U'-shaped brick sewer with stone flag tops and the egg-shaped butt-jointed clayware pipe s ewer s - both still operational under the heart of Manchester today.

Clayware pipes had begun to appear again in this country about the middle of the nineteenth c e n t u r y - such pipes having been known to the Romans but the technique apparently disappeared with their civilisation. From about 1850 the standard Manchester sewer was constructed using butt-jointed ovoid clayware pipes which had been introduced by John Francis, the then Surveyor to Manchester's Paving and Soughing Committee ( ' s o u g h ' - an underground channel) and they were extensively used until about 1880. Apparently, the brick 'U'-shaped sewer type was still favoured rather than clayware pipes when sewers of a larger capacity were required. Generally construction was by tunnelling - round shafts (blind eyes) usually 900 mm internal diameter of double skin b r i ckwork- were sunk 10.8 metres apart and the ground between then excavated to the exact form of the invert without the use of timber. The cost of this particular pipe was claimed to be less than the equivalent brick sewer and construction time was halved. Its additional advantage lay in its shape and the resultant hydraulic benefits obtained. Its main disadvantage, although apparently this was not realised at the time, was the fact that each section of pipe was butt jointed to the next, clay puddle being used to seal it in some instances but more commonly left open so that the sewers could also s e r v e - it was h o p e d - as land drains. This type of construction with its 'joint' failure over years of continuous use is one contributory factor to today's legacy of sewer dereliction in Manchester and elsewhere.

Circular clayware pipes were introduced around 1880 and represented a considerable improvement but although these were spigot and socketed, the sockets were initially only manufactured over the lower half of the pipe. It was again apparently intended that the sewers would also act as land drains.

The dual function which these early sewers were apparently intended to provide is a

Development of sewerage rehabilitation 9

recurring feature of all the early sewer types including brick sewers and suggests that the brickwork was actually laid dry so that any ground water would infiltrate.

The use of clayware pipes continued to increase in the latter part of the nineteenth century with brick construction predominating for the larger conduits. This latter form of labour intensive construction continued into the early part of the twentieth century until the introduction of concrete pipes and in due course pre-cast concrete bolted segments, although for many years the latter technique involved an inner lining of engineering brickwork in cement m o r t a r - a practice which has now virtually ceased in view of the concrete quality now obtained.

1.7 Sewer r e h a b i l i t a t i o n

It is obvious that all forms of engineering construction deteriorate with time and require consequent maintenance and repair. Without doubt, rehabilitation in some form or other has taken place since the first appearance of sewers, namely, in Roman and Greek times or even earlier, utilising the particular techniques then available.

Life cycle management in relation to sewerage networks is of paramount importance - whether they are the responsibility of the water companies or private/industrial commercial sewers - if the maximum cost effective life span is to be obtained. Sewers are hidden from public view and consequently there has always been a tendency to neglect t h e m - out of sight, out of mind!

If repairs and renovations are not carried out at the proper time the useful service life is shortened with the risk of a dangerous collapse situation developing, as shown in Figure 1.4.

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In sewerage generally, the actual conduit represents some 20% of the initial cost depending on the size of the pipe - excavation, temporary support, backfilling and reinstatement (both temporary and permanent) account for the r e m a i n d e r - so that the largest part of the infrastructure asset is 'the existing hole in the ground'. Figure 1.5, which comes from the former Institution of Public Health Engineers' Report on 'Trenchless Pipelaying', illustrates graphically the relative costs of the individual components involved in constructing a sewer.

If, therefore, a new pipe or the equivalent can be inserted inside the old sewer or it can be renovated without further excavations, theoretically a saving of up to 80% of the traditional cost can be made in addition to the savings on social or indirect costs which might have arisen.

Consequently in recent years sewer renovation has been increasingly advocated as a means of coming to terms with the immense problem of dealing with an often outdated

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Development of sewerage rehabilitation 11

Table 1.2 Implied asset life of critical sewers based on rate of renovation and replacement over the past

11 years. The following table includes data from the report for 2000-1 and shows that two water companies

have now joined the '1000 year club' and now expect their critical sewers to last over a thousand years at

the present rate of renewal

Company Critical sewers Total length Total length Implied asset life

- total length replaced/renovated renovated/replaced based on activity

(km) in 00/01 (km) 90-01 (11 years) since 1990

(km)

Anglia 8191 9 150 601

Dwr Cymru 4321 2 158 301

United Utilities* 10 674 106 481 244

Northumbrian 5982 6 306 215

Severn Trent 7471 2 451 182

South West 1815 1 51 391

Southern 6460 1 43 1653

Thames 18 936 11 479 435

Wessex 2841 20 119 263

Yorkshire 6846 3 75 1004

Total 73 537 161 2313 350

*Formerly North West Water

Source: OFWAT: Financial performance and expenditure of the water companies in England and Wales.

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sewerage network now faced by the water industry generally. This principle of maximising the potential of the existing 'hole in the ground' is further endorsed in Water Research Council's (WRC) Sewerage Rehabilitation Manual. A main factor favouring renovation of a sewer against its complete replacement is reduced initial cost thereby permitting more widespread use of limited financial resources. Sewer renovation should also considerably reduce the surface disruption (particularly if the alternative is sewer replacement in open cut) and hence reduce the 'social' costs associated with the work. While these latter costs can be considerable, representing perhaps three or even ten times the capital value of the

12 Sewers

scheme, other factors need to be fully assessed when considering renovation work. It should be remembered that renovation is only one of the options to be considered in any sewerage improvement programme. Nevertheless it should always be appreciated that renovation in itself will still involve considerable resources in time, manpower and funding. The potentially hazardous nature of some renovation works and the long-term implications of renovated sewers should never be overlooked. Notwithstanding the saving in social costs the author is conscious of the need to fully assess other long-term factors when considering the option of renovation in comparison with replacement. For instance, the difficulty in obtaining the design conditions for an old dilapidated brick sewer, probably with external cavitations, so as to ensure the designed lifespan materialises is a significant factor. A synopsis is given of the problems in Volume I, which have arisen during actual renovation schemes, leading to a prognosis of the way in which some of the problems may be overcome.

Rehabilitation has in fact been defined in the WRC's Sewerage Rehabilitation Manual as covering all aspects of upgrading the performance of existing sewerage systems and includes repair, renovation and renewal. Renovation is not a panacea merely an important option worthy of consideration at the o u t s e t - in fact, when the manual was launched the WRc suggested that it was not a case of renovation or renewal but rather 'has renovation been considered as an option?' The manual has provided a great deal of detail for the design of renovation schemes and is an excellent common sense treatise on the subject for which WRc are to be congratulated- although it does not offer any fundamentally dramatic new concepts it was intended to provide a framework for decision making which is consistent and technically justifiable, while still allowing the necessary flexibility for civil engineers in this field to apply personal judgement in particular situations.

The main theme of the new a p p r o a c h - which has been described as the new age in sewer technology - is to help engineers to maintain 'the hole in the ground' so as to prevent costly excavations and often high social costs, to optimise hydraulic performance and maximise the use of renovation.

At this stage one might hope that perhaps Volume IV of the manual might follow in due course so as to include coverage of conventional and new methods of reconstruction that may well materialise during the next few years. Rather than a scheme being 'problem orientated' the new strategy is a review of the complete drainage area followed by a fundamental reassessment of the network as a whole from which it is assumed the problems will emerge.

Overall the policy suggested concentrates pre-emptive rehabilitation on the critical sewers which generally make up about 20% - although in larger UK towns and cities the figure is much higher of the national network leaving the problems in the others to be dealt with via reactive response. Of the 20% it is hoped that 5 % will enjoy a failure-free situation with the likelihood of failure in the remaining 15% being significantly reduced.

Sewer renovation has to date appeared in different parts of the world for various but completely different reasons. In North America, for instance, the prime justification has been to control infiltration while in hotter Middle East countries the need has been related to serious corrosion difficulties in relation to concrete pipes. In the UK structural deterioration, flooding and pollution are the main problems.

In comparison the main advantage of on-line sewer replacement methods are that reliance on the uncertain structural properties of existing sewers is not required and opportunities may be taken during the planning of reconstruction work to rationalise the system and generally to improve access to the network as a whole.

Development of sewerage rehabilitation 13

Cost effectiveness remains the primary design criterion and if a renovation scheme, taking into account all the safety and quality control requirements cannot be shown to be cost effective, the WRc Manual strategy would recommend discarding the renovation options. If the engineer decides that for a variety of possible reasons the old sewer is not in fact suitable for renovation then replacement should be considered. This could be replacement on-line which, although having to deal with the existing flow during the works, has an advantage so far as the connections of laterals is concerned. There may nevertheless be a case for off-line replacement perhaps retaining the old sewer in a minor capacity of making it no longer part of the primary network and thus carrying less flow.

Although it is clearly possible to replace both on- and off-line by construction in open cut this is no longer normally acceptable in more densely trafficked urban areas and construction operations normally proceed via the use of trenchless technology.

If the engineer is able to prove that the social and environmental costs which are likely to occur if open cut is used added to the construction costs of the smaller conduit, are likely to be less than the equivalent costs of the alternative larger size tunnel construction, then in such an unlikely event work would proceed via open cut.

The most common method where complete on-line replacement is necessary is via the traditional 1520 mm diameter precast reinforced concrete (usually shield driven), bolted sequential construction generally with smooth invert on the line of the existing sewer (Fig. 1.6, 1.7). Although generally oversized from a hydraulic consideration, tunnels of this size permit the existing structure to be encompassed within the face excavation and enable connections to be readily located and satisfactorily made without excavation from the surface.

6 segments/ring

Figure 1.6 Typical 5 and 6 segment bolted tunnel lining units with smooth invert.

14 Sewers

Figure 1.7 Existing brick sewer being replaced by bolted segmental tunnel.

Where the existing sewer wanders from the tunnel face (which is not usual - Fig. 1.8), it is a relatively easy matter to remove side segments and head out to pick up any isolated connections. On complet ion, full man access is possible for future maintenance, which

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Figure 1.8 Existing sewer deviating from line of new tunnel.

Development of sewerage rehabilitation 15

should then become more cost effective. In addition, this arrangement does away with the need for manholes at junctions with minor connecting sewers.

In considering the economics of the situation it should not be forgotten that a completely reconstructed sewer has a longer life than a renovated one, especially since connections are more readily made and more effectively protected from further collapse.

It is considered that when the effective lifespan of a renovated sewer has been reached, there will normally be no alternative to replacement by a new sewer, but time will tell.

Taking into account all the factors which influence the selection of renewal methods, such as mode of sewer construction and surrounding ground, depth below ground level and the location of the sewer relative to other aspects of the environment, it is clearly not possible to arrive at a 'standard' solution applicable to all situations; the choice for each particular sewer length must be taken on its merit.

However, sufficient experience has been gained from renewal work (particularly in Manchester) for guidelines to be suggested for assessing the feasibility of renovation methods. Renovation methods within the criteria laid down in the manual are acceptable for the larger man entry and man accessible sewer but the condition and size of the earlier and smaller sewers (less than 900 mm diameter after lining) make renovation by manual labour completely unacceptable.

The prospects are better in these sewers for the use of renovation techniques relying on remote installation arrangements (e.g. Insituform, sliplining and impact moleing). For sub- man entry sewers, however, these techniques do not offer any convenient means of dealing with voids external to the sewer and this is considered a serious disadvantage when dealing with the early brick sewers and the open jointed clayware pipelines of this era which the Victorians also designed to act as land drains.

Inevitably, the risks associated with all aspects of sewer renovation are greater than those involved in replacement.

However, there are many situations where renovation is the obvious answer but notwithstanding the excellent work carried out by WRc as a result of which a 50-year design life can be anticipated, there is nevertheless a certain reluctance apparent in the use of these 'new' materials so far as sewerage is concerned, perhaps not unrelated to the problems that have arisen in regard to other 'new' techniques such as system buildings, particularly in high rise flats, high alumina cement. Nevertheless, renovation is now an option which must be carefully examined as part of the overall engineering feasibility investigation undertaken in connection with sewerage rehabilitation.

It is the degree of control that can be exercised over the execution of the renovation work that is of paramount importance to the longevity and success of the system. It must be fully understood that size limitations of the existing sewer considerably affect the quality of workmanship and supervision that can be achieved.

If the replacement is to be off-line there is still usually a case to be made for the use of a bolted (man access) tunnel which facilitates the reconnection of laterals. If the old sewer is to remain as a rider sewer dealing with the localised flow only, this criterion is no longer significant and the provision of a smaller diameter pipeline - still constructed via trenchless t e chno logy - may be the answer.

Just as it is a case of 'horses for courses' in sewerage rehabilitation generally, the same situation applies where replacement is under design consideration. East situation should be treated individually and a solution determined without predisposition towards any one technique.

When selecting appropriate rehabilitation methods (whether renovation or replacement)

16 Sewers

assessment of the adequacy of the existing sewer is reqlaired. First, its hydraulic carrying capacity for present and future needs (some renovation methods may significantly reduce sewer cross-sectional areas) must be considered. Second, to assess the condition of the existing sewer structure in the short term for carrying out inspection and investigation as well as possible renovation work from within it and its long-term viability as part of the sewerage system.

1.8 Strategy The problem of sewer dereliction is immense. It is perhaps the greatest problem facing the water companies since the design and construction of the sewerage interceptors and treatment works, so far as Manchester is concerned, some 90 years ago. On the national scale sewer dereliction is one aspect of underground service dereliction (mainly sewers and water mains) which may be compared with the housing problem of the mid-twentieth century, which was overcome by the dramatic slum clearance programme of the post-war years. It is extremely unlikely, unless some major catastrophe occurs, that the underground dereliction problem will ever attract a similar level of public attention and resources and the national drive which was harnessed during the slum clearance era. Surface development and improvements, because the results are visible to all, are more likely to be favoured compared with underground infrastructure.

It should always be remembered that Britain's sewerage network has, in practice, been an important factor in ensuring the health and prosperity of the public for a considerable period, although some of it has now become a liability as a result of the lack of maintenance.

It is worthwhile noting in this context that one other public service industry, nemely, British Gas, because of the different characteristics of natural and town gas had little alternative but to embark on an extensive programme of renovation and/or replacement of its underground network within a relatively short period. Some 47 000 km of new mains were laid in ten years and in this case 'age' was not the major factor responsible.

Although the water industry can take heed from this achievement there are two important differences. First, the costs for replacing relatively small diameter shallow gas mains are a different order of magnitude to those for replacing larger and deeper sewers. Second, British Gas, following the revitalisation of the industry with the exploitation of natural gas, has been able to finance such a replacement programme. Traditionally the water industry has in the past made little allowance for replacement costs in setting its charges to the consumer. Although attempts are now being made to rectify this situation it is recognised that this could not be fully achieved within the present system without apparently increasing charges to unacceptable levels. Thus the problem is one of enormous magnitude tainted with the realisation that resources for its eradication are extremely limited and likely to remain so over the foreseeable future.

In the past, the policy dictated by the availability of finance has allowed events to determine the priorities for action, i.e. only nominal resources were allocated for investigations and maintenance, the incidents being dealt with as they occurred. Such works were essentially of an emergency nature in costs and manpower resources and are not easily predicted or effectively managed. The results of such an approach are that the extent of the problem is never fully recognised. For the civil engineer the basic objection to this thinking is that it contravenes the principles of planned and efficient use of resources. The continuation of such an approach led to the major crisis situation in Manchester reported in Volume I, which could have even resulted in a complete breakdown of the sewerage system.

Development of sewerage rehabilitation 17

It was interesting to note in early 1998 that the House of Commons' Environment Sub- committee included in their recommendations that water companies should have a renewal programme in place by 2002 to ensure that sewers are replaced as quickly as they deteriorate. To comply with this long overdue sewer replacement proposal water companies would have to considerably increase their level of investment. They currently replace some 1% of faulty sewers per year against a background of 8% to 10% of the network being in very poor condition. Since 1991 only around 1200 km of the total network (3 20 000 km) has been replaced.

In a paper on 'The Development, Renovation and Reconstruction of Manchester's Sewerage System', which I presented to the Manchester Literary and Philosophical Society in 1982 while holding the appointment of City Engineer, Manchester, I warned 'that a losing battle is being fought and dereliction is taking place much more rapidly than renewal'.

Sewers are hidden from public view and consequently there has also been a tendency to neglect t h e m - in fact, some have never been repaired since construction.

Engineering works do not last forever - even Victorian sewers - and regular maintenance works and ultimate replacement are of paramount importance if collapses, with the serious associated public health and injury risks, are to be avoided.

It is generally accepted that particularly the older parts of the British sewerage network are in poor structural condition, their hydraulic performance is often inadequate and they are having to carry much greater external live loads than those envisaged at the time of construction. Consequently the need for substantial Civil engineering works on a regular basis is unquestionable in order to deal with sewers having serious structural defects and where the lack of capacity is causing river or stream pollution - often with associated public health risks!

The immediate construction cost of these works is high - probably over s million per year - but in addition there are indirect or social costs arising out of the consequential impact on the communi ty- perhaps varying between three and ten times the civil engineering costs in the most critical places. These social costs have in general to be met by the public at large or the highway authority without recourse to the water company. Clearly at some stage the industry will have to move towards a fully evaluated cost-benefit approach to take into account such social costs along with the anticipated benefits.

1.9 Environmental impact of sewer collapses

It is clear that serious environmental and public health consequences result from the ageing sewer network.

The personal injury risks for the public travelling over a worn-out network is also considerable and would doubtless result in greater priority being given to rectification if it were a structure above ground such as a bridge rather than an underground problem - typically out of sight, out of mind! In addition the immediate consequences of a sewer collapse may include the contamination of drinking water supply, flooding of habitable properties with crude sewage or other equally obnoxious and unacceptable environmental conditions. The results of such problems - particularly where a number of collapses occur concurrently - has an impact on the normal business and commercial life of a town or city which, in turn, can lead to a loss of trading confidence in its future well-being.

The cavity associated with a sewer collapse is usually onion shaped with an undercut format at the top adjacent to the road surface. These often spectacular collapses highlight the immense problem of sewer dereliction which now has to be faced against a background

18 Sewers

Figure 1.9 Bolted segmental tunnel replacing 'LI' shaped brick sewer with flag top.

of underfunding. The potential risk of vehicles falling into these large cavities is horrifying to consider particularly when one is immediately aware of the potential risk to life and limb. To enable the general public to readily appreciate the void size the author introduced a comparison factor which has now been adopted worldwide, namely, the DDB factor. That is to say the number of double decker buses which could be engulfed in the particular cavity - hopefully each not carrying the maximum of 92 passengers. The largest cavity in Manchester to date has been 4DDB which necessitated excavating some 21 m to the 3.66 m diameter trunk sewer.

1.10 Cost of sewer collapses

A typical sewer collapse account for a city centre location might be made up as follows:

Direct costs to water company

1. Repair and reinstatement of damaged sewer 2. Business loss claims under Public Health Act legislation

45 000 1 77 000

2 22 000

Indirect cost to general public, etc 1. Losses to bus or tram operator 2. Traffic disruption 3. Damage to other utilities plant 4. Emergency services 5. Flooding

10 500 82 815 90 000

2 000 70 000

s 55 315

Development o f sewerage rehabilitation 19

Therefore, the likely cost to a water company and the public of a major sewer collapse is s 77 315.

Based on the recent Manchester scenario of six major collapses per year in the central area the overall cost (direct and indirect) just for localised repair could be:

s 315 x 6 = s 863 890

This assessment of course ignores the potential ever-present personal injury risk in such an emergency situation.

(The above figures are based on estimates produced for a socio-economic cost-benefit analysis for a large industrial city in the north of E n g l a n d - not Manches te r - for which the author was responsible.)

1.11 Funding When sewerage was a local authority responsibility, it had to compete at budget time with other more sociably attractive services such as housing, social services, education, etc., and did not attract adequate funding. Privatisation of the industry was intended to overcome this and enable adequate finance to be made available. However, this has not obtained, the socially attractive services have been replaced in practice by shareholders who wish the maximum return on their investment and European legislation related to fiver and coastal pollution. Clearly the first water regulator (OFWAT) who was appointed in 1989 following the privatisation of the water and sewerage companies has to ensure that the sewerage network is obtaining adequate funding within the overall budgets and that a comprehensive planned maintenance strategy is introduced forthwith.

Over the last few years a new organisation has developed viz. Campaign for the Renewal of Older Sewerage Systems (CROSS) and Figs 11.1 and 11.2 which they have published highlight the implied asset life of critical sewers based on the rate of renovation and replacement over the last 8 and 11 years which confirm the seriousness of the present situation.

Bibliography 1. The Federation of Civil Engineering Contractors, 'Public Health and the Engineers'. 2. Read, Brian Healthy - A Study of Urban Hygiene, Blackie, 1970. 3. House of Commons' Environment, Transport and Regional Affairs Committee. 'Sewage Treatment and

Disposal', Stationery Office. 4. Proceedings of ISTT Conference 'No-Dig 90', Rotterdam. 5. Water Research Centre. 'Sewerage Rehabilitation Manual', fourth edition. 6. Read, Geoffrey F. 'Sewer Dereliction and Renovation- an Industrial City's View'. Repair of Sewerage

Systems Conference, Institution of Civil Engineers, London, 1981. 7. BS 8005 Parts 1 and 2.