The need for trenchless technology standards

The provision of new water services has not kept pace with urban densification. Spatial constraints make the traditional approach of digging trenches extremely disruptive to the social and business environment. Using trenchless technology (TT) offers many advantages, but a unified approach must be adopted when it comes to local standards. By Alaster Goyns*

Close to 70% of South Africa’s population is now concentrated in urban areas. The rate at which this urban population is increasing, especially in the metropoles, is probably more than double that of the national average. Many of these high-density areas are unserviced or poorly serviced and, for the health of these communities, there is a desperate need for water services.

In addition to this, the densification of established urban areas, through subdividing stands and building townhouse complexes, means that the existing water services are frequently inadequate. What’s more, the condition of many first-generation water services has deteriorated, which now need replacement or rehabilitation. This situation is exacerbated by ageing infrastructure that results in the loss of clean water from the supply lines and soil pollution resulting from leakages from sewers. In many situations, the problem is keeping the clean water in the clean water pipes and the dirty water in the dirty water pipes. Many local authorities and consultants in South Africa are interested in using TT to provide services in the high-density urban areas mentioned above. However, they are reluctant for two main reasons: they need standards on which to base the planning, design and installation; and there are misconceptions about whether this approach will create employment opportunities. The Southern African Society for Trenchless Technology (SASTT) has addressed both issues. The focus of this article is on TT standards.

Function of pipelines

Water pipelines fall into two broad categories,

1 Ongoing condition assessment is essential to ensure the integrity of pipeline assets 2 Pipelines can fail functionally when unable to convey the required volume of water and structurally when unable to carry the imposed loads 3 Open cut trench 4 A deep trench excavation Informal settlements in urban areas tend to be unserviced or poorly serviced in terms of water and sanitation infrastructure

namely freshwater supply pipelines that flow full and operate under pressure, and wastewater disposal pipelines that flow partly full and flow under gravity. Pipelines are considered as assets designed for a specific lifespan; in reality, it is the holes through the soil that are the real assets. Pipelines merely line these holes to ensure that they operate effectively and efficiently. They need to meet the primary requirement of hydraulic performance and the supporting requirements of strength, watertightness and durability.

Pipelines can fail functionally when unable to convey the required volume of water and structurally when unable to carry the imposed loads. A functional failure is usually because the pipeline is not big enough, or there is a blockage. In the former situation, the problem can only be addressed by replacing

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or upsizing the pipeline; in the latter, the problem can usually be rectified by clearing the obstruction.

On the other hand, when there is a structural problem, there is usually permanent damage to a section of the pipeline and this will get progressively worse, resulting in the whole pipeline collapsing if nothing is done. The underlying cause of structural problems can be difficult to determine, as there are several factors that could have a combined effect. There are durability problems with certain pipe materials compromising the pipeline strength and causing infiltration and exfiltration at the deteriorated joints. This means that bedding material can be washed into the pipeline, resulting in the loss of bedding support.

Most of these problems can be rectified with a minimum of surface disruption by using TT. Pipelines that have inadequate capacity can be upsized by using pipe bursting and pipelines that are leaking and losing their strength due to durability issues can be rehabilitated using various lining techniques.

Trenchless standards

SASTT is fully aware of the need to develop TT standards that provide a common base on which to design and install services using these techniques. Although there are international standards that adequately cover the technical and practical aspects of these techniques, the direct adoption of these is not appropriate for South Africa for several reasons: • the format of international standards does not correspond to that of South

African construction standards • these documents are generally too compact as they do not provide sufficient explanation • they are in general not appropriate for the skill levels in the country. In addition, the playing field needs to be levelled so that all who tender on a project price on the same basis.

With these in mind, funding was requested and granted from South Africa’s Water Research Commission in 2001 to establish what standards were needed. This included a survey to ascertain the needs in the country and how to incorporate international developments, with adaptation, where necessary, to suit South African conditions and skills. The output was WRC Report No. KV 133/01 – A Framework for Establishing Appropriate Trenchless Technology Guidelines and Standards in Southern Africa.

At the time, the only TT standards available in South Africa were those related to pipe jacking (concrete pipe) and these were in the format covered by a section of a code of practice for design (SANS 10102 Part II – Rigid pipes), a product standard (SANS 677 for concrete pipe) and a standardised specification for product installation (SANS 1200 LG – Pipe jacking). This combination of standards is well established in the country and is therefore the format that SASTT has adopted in a concerted effort to produce TT standards.

Support services

An additional component for TT standards is the need to cover the support services, such as inspection techniques. The reason that these documents are not being currently produced as SANS standards is that SASTT has the expertise to produce them and has direct access to the technical

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ABOVE Hole through the soil: the real asset BELOW Overflowing sewer due to blockage

content produced by other members of the International Society for Trenchless Technology, of which it is a member. Once these documents have been in use for a few years and any amendments required made to them, their adoption as national standards will be motivated.

To date, the focus of these documents has been producing specifications for the commonly used techniques so that designers can include them in project documentation for contractors to have a schedule of payment items, a bill of quantities to quote on, and details of what work has to be done. The current status of these documents is: • SASTT – TS-TT1 Sliplining of pipelines has been completed • SASTT – TS-TT2 Pipe bursting has been completed • SASTT – TS-TT3 Horizontal directional drilling has been completed • SASTT – TS-TT4 Cured in place pipe lining is currently being finalised • SASTT – TS-TT5 CCTV inspections of pipelines has been drafted. These documents are done in the same format as SANS/SABS 1200 LG – Pipe jacking. There are, however, other documents that need to be produced so that there is a consistent way in which trenchless projects are addressed. Two of these cover condition assessment and liner design.

Traditionally, CCTV has been considered as a means of assessing pipeline condition. However, although effective, this is just a means of gathering visual detail from inside a pipeline, which shows where there are defects, the types of defect, and their extent. This does not necessarily show the severity and underlying causes of these problems.

There are now several other means of gathering data, such as laser and sonar profiling that provide internal dimensions and electro-scanning inspection, which can locate specific pipe defects that are potential leakage points. Condition assessment in a broader sense combines the data gathered from inside the pipeline and that gathered from the external conditions above the pipeline, so that the remaining strength and useful life of the pipeline can be estimated. This provides the designer with the information needed to do a risk analysis and set priorities as to what remedial measures are necessary and when these should be done. A decision can be made about whether to replace or rehabilitate the pipeline.

The principles for designing new pipelines are well documented and understood. As mentioned above, pipelines line the holes through the soil so that they operate effectively and efficiently. So, whether designing a pipeline or a lining through an existing pipeline, the same basic theory is applicable, but the interaction between a pipeline and the surrounding soil and a lining and the host pipeline will be different. In the latter case, the application of these basic principles to the design of various types of liners needs to be covered in a design code.

Concluding comment

When there are no standards, there is the danger of two extreme situations: • An over-conservatism approach, where the technology is not fully understood, generally resulting in more stringent requirements than needed and higher construction costs. However, critical factors may be overlooked, and these could give rise to failures. • The lowest initial cost approach, where the site investigation for a new service or the condition assessment of an existing service is inadequate, or not done at all, resulting in under-specification and a high probability of failure at a later stage. With both scenarios, the cost of rectifying a failure once the service is in use, as with any buried water service, could be several times that of the initial installation itself. The purpose of SASTT producing standards is to provide a uniform means of communication between the various parties involved, comparing alternatives, evaluating the quality of workmanship, and ensuring that the specified requirements are consistently met.

*Alaster Goyns is the owner of Pipes CC. He writes on behalf of SASTT.

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