High potential for groundwater pollution
The collector "Heimlinger Tal" transports combined sewage from the village of Losheim into South-West direction to the waste water treatment plant of Merzig. TV-inspections performed in 1998 revealed leakages at almost all joint connections of the 3.3 km long section, mainly constructed with asbestos cement pipes ranging from diameter 400mm to 1300mm. Since the pipes themselves were still in excellent condition, rehabilitation would not have been necessary under structural aspects.
However, due to being located within an important water protection area of the Merzig municipality, an hydro-geological risk-assessment was initiated for the collector. The task of said assessment was to define the threat potential of groundwater pollution connected with the found leakages. Concerns existed because of the collector’s course through a water protection area type WSZ I, which means that groundwater is extracted through wells for drinking purposes. Another aim of the study was to determine the actual degree of rehabilitation need.
And the survey confirmed the initial concerns: Because of the low cleaning abilities of the upper soil-layers, contaminations were found in the groundwater after long rain-periods. The threat potential of groundwater pollution connected with the leaking collector was therefore given a high classification.
To improve the groundwater protection, the EVS decided to initiate an extensive rehab concept, which should not only include the collector’s sealing, but also the optimization of the existing storm water-treatment and –release constructions located within the mentioned water protection zone
Through some changes of the existing release- and retention-constructions, the inflow point of the collector’s emergency–release could be re-located from the water protection zone type WSZ I (where the water wells are located) to the water protection zone type WSZ II. An old sewer which had been out of use for years was re-activated for this purpose. That concept led to cost savings and likewise to reduced intervention into the upper soil layers within this area.
The second concept stage planned the realisation of a soil-filter with retention abilities to further treat the arising storm water. This measure should not only improve the ground water protection but also the "Heimlinger"-creek’s water quality. However, the main project focus was still the rehabilitation of the approx. 3.3 km long collector between manholes number 1 and number 60 (Figure 1).
Rehabilitation under special conditions
Being faced with a very special hydro-geological situation, a close communication with the public authorities became necessary from the initial planning phase on. The applicable ATV-DVWK work-sheet number A 142, "Waste water collectors and pipes within drinking water extraction areas" required an increased safety by application of a double-layer lining system. Excavations had to be kept at an absolutely minimum. Therefore, but also because of the expected costs and impact on nature and landscape, open trench methods were excluded as solution. However, spot-wise and very limited digging was considered acceptable for the rehabilitation works.
Choice of the rehabilitation method
Considering the given surrounding conditions and the technical requirements, Trolining’s GIPP
ipe) method was chosen as the most appropriate one to tackle the project. The combination of an outer Preliner and an additional inner HDPE liner with studs (also called "Troliner") provided a double-layer solution as required by the project specification. The fact that only few excavations were necessary to remove some of the manhole covers complied with the authority’s guidelines in a quite satisfying way.
Another essential aspect for the method’s approval was the complete avoidance of water threatening construction materials during the rehabilitation works within the water protection area. A total of 1.1km dia. 400mm, 1.5km dia. 600mm to 900mm, 0.6km dia. 1300mm and 170m dia. 2000mm were relined with the GIPP method.
Trolining’s GIPP method is based on various HDPE
thylene) liner components which can be combined in a wide range of alternative configurations. The heart of the system is the Troliner with its V-shaped embedment studs on its outer side. The annular void created by these studs is filled with TROLINING-Injektor®, a specially engineered injection grout. This high-strength grout fixes the Troliner permanently in place and provides the load-bearing shell of the system.
TROLINING liners are available for pipes ranging from 250mm to 2000mm (10" to 80") I.D.. Segmental lining sheets are offered for rehabilitation of lines greater than 2000mm (80") I.D. and for elbows and corners in man-sized lines. The rehab system improves the host pipe’s flow characteristics and provides exceeding mechanical and chemical resistance.
Every liner is tailor made according to the exact measurements taken on the job site. That procedure guarantees the systems "Close-fit" characteristics, thus limiting the cross sectional reduction to an absolutely minimum. Prior to the liner insertion, the pipe section to be rehabilitated is plugged-off at both ends, cleaned and visually inspected. A cable winch is then used to unroll each HDPE liner from its transport drum and draw it through the line into position. After insertion, all HDPE liners are welded together at their both ends. The resulting liner –"sandwich" is then inflated with a defined pressure through passages in the temporary plugs closing the ends of the liner section.
When the required inflation pressure is attained, the annular void is filled with TROLINING injection grout fed from the down stream end. The grout’s extremely low viscosity ensures 100% grouting of the annular void, even for long distances. Additional grout characteristics are a defined material expansion and an exceeding short term strength. The latter is decisive to keep the interruption time at an acceptable low level.
The contractor Trolining, chosen to implement the rehabilitation works, has many years of experience and owns certificates and approvals according to the applicable environmental, safety and quality regulations. Trolining’s GIPP-System itself is approved by the DIBt (German Institute for Construction Technology) and regulated by instruction documents of the DWA (German Association for Water, Wastewater and Waste) and the RSV (Association for Pipe Rehabilitation). Another essential aspect with regard to the installation within a water protection zone was the method’s approval for potable water applications (according DVGW work sheet W 270).
Due to the partially difficult terrain within the "Heimlinger"-valley, not all of the manholes along the collector were directly accessible. To keep disturbance of the environment as low as possible, site-planning considered construction of access paths just to those manholes that were absolutely necessary for the purpose of liner-installation or where the collector changed direction within the manhole chamber.
As result of the detailed planning, a total of 24 manholes could be abandoned by installing the lining system straight through them without interruption. Hereby the impact on nature and landscape was marginal. Rehab lengths of up to 205m were accomplished to comply with this aim. The system allowed directional changes of up to 5° without wrinkling. After liner-curing, the abandoned manholes were closed and filled-up with none-structural grout. Part of the access paths were preserved for other purposes after completion of the rehab works.
Europe’s largest GIPP liner for the retention pipe dia. 2000mm
The rehabilitation of two parallel pipes dia. 2000mm, each of them approx. 85m long and together providing a total water retention volume of more than 500 cubic meters, represented a special challenge to the contractor. Until that date it was Europe’s longest studded HDPE-liner ever supplied with that diameter by Trolining. A TROLINING Double System with a wall thickness of 45mm was selected to meet the structural requirements. The design calculations were performed by the LGA Nuremberg (part of the TÜV Rhineland group) under application of the FEM (Finite Element Method).
To allow the insertion of the HDPE liners, each of them weighting up to 2.2 tons (Figure 2), the concrete cover plates were removed from the manhole chamber at the down stream end of the retention pipe. A 30-ton telescopic crane was called for this task. Being available anyway, the same crane was used to lift the HDPE liner-coils onto the stands right beside the insertion manhole, which was just reachable through a narrow construction road.
The liner insertion occurred with a 5-ton pulling winch. A diversion roller, fixed right above the insertion manhole, allowed feeding the HDPE-liners into the host pipe one-after-the-other (Figure 3). As required by the design calculations, one Preliner and two studded HDPE-liners had to be inserted. Following the liner insertion, a tailor made sealing plug was introduced from one end into the "liner-sandwich" to apply air-pressure through the plug’s core. This procedure is necessary to unfold the liner over its whole length and give the system a provisional circular shape. Once in circular shape and after some further working steps, inflatable sealing plugs could be inserted from both liner ends to apply the final counter pressure for the grouting process. Thick timbers were used to construct a rigid bracing, which - with a liner of 2m in diameter and an 8m pressure column respectively 800 milibars - had to resist an equivalent horizontal force of approx. 25 tons.
Grouting of the annular space was performed under application of the specific Trolining mixing-technology by using 3 mixing units. First the outer and then the inner annulus were filled. After curing, the supporting water pressure could be released from the liner and the sealing plugs were removed (Figure 4). As a final step, the lining system was cut-back flush to the manhole chamber walls and the space between the HDPE-liners was sealed with a multi-layer PE-weld to guarantee water-tightness (Figure 5).
Due to its exceptional size, the rehabilitation of the retention pipe dia. 2000mm caused massive interest among experts and decision makers of the surrounding municipalities. Two job site presentations were therefore held during the project execution. At the first presentation date, held on 16th May 2008, visitors were given the opportunity to get a picture about the application of state-of-the-art rehabilitation methods for large pipes. The insertion of a pre-welded Preliner was demonstrated "live". Presentations by the responsible project engineers of the customer EVS, the consultant Schweitzer GmbH and the system-applicator Trolining GmbH concentrated on all relevant aspects.
The second job-site demonstration on 12th June 2008 was mainly used to show the finished product. Wooden stairs, especially constructed for this purpose, allowed the visitors entrance from the up-stream end. By walking through the relined retention pipe over its whole length of approx. 85m, participants had the opportunity to convince themselves of the achieved quality (Figures 6 and 7).
Depending on the shaft geometry, HDPE-sheets with a basic thickness ranging from 5 to 8mm were used to line both the existing standard manholes and the shaft chambers of special size. Using the same type of HDPE for the GIPP lining and the manhole rehabilitation allowed to provide 100% tight transitions. Manual PE-extrusion welding was applied to join the manhole sheets and the pipe liners.
Protection of the "Heimlinger Bach", the small creek that runs through this valley, had a major priority due to some fish-ponds fed by its water at the down-stream end. This required setting-up a water-management plan for the time of the rehab works, which should ensure an almost constant discharge volume at the spillway.
At the start point of the rehabilitation section, right beyond the border of Losheim, the design value for the collector’s discharge volume was stated to be 4400 Liters/sec. It was obvious that this huge water quantity could not be bypassed with pumps at reasonable economic conditions.
It was therefore agreed with the approval authority to set-up a bypass system with a discharge volume of Qp = 800 Liters/sec. Two pumps with said capacity were installed, one of them for stand-by reasons. The power supply was provided by a generator. Two parallel pipelines, one of them dia. 250mm (HDPE) and the other dia. 400mm (Steel), were used to transport the combined sewage from the up-stream to the lower end of the first portion of rehab section (Figures 8 and 9).
Precipitation- and emergency-reporting systems were installed to reduce the risk of sudden overflows. Such overflow actually did not occur during the approx. 1 year long project execution. The bypass system which had to be installed for the following rehab sections, located below the intermediate storm water spillway, was much easier. A capacity of Qp = 50 Liters/sec were considered sufficient; the bypass pipeline had a maximum length of 1.3km.
This project confirmed that the chosen GIPP method has special advantages for the application in water protection areas, complying with the requirements of the ATV-DVWK work sheet A 142 and avoiding the use of water threatening materials at the same time.
Furthermore it was possible to cover all pipe dimensions included in this project with just one rehab system. Application of the same type of thermoplastic for both the pipes and the manholes allowed to provide 100% tight transitions from one section to the next one and hereby guarantee a long-lasting and reliable protection of the ground water resources within a sensitive environment.