Maintaining the Drainage Capability
Under Drainage capability there is understood according to DIN 4045 [DIN4045:1985] the "possibility of discharging the flow of water or sewage with natural gradients or by means of artificial lifting (natural or artificial drainage capability)."
Irrespective of whether the rehabilitation is a case of repair, renovation or replacement, the maintenance of the drainage capability (drainage security) during the construction phase requires special attention as it is of decisive importance both from a technical and an economic point of view. Thus, careful investigations must be carried out to find out how this can be technically achieved and what requirements this makes on the process to be used. The influence of heavy rainfalls must be included in these considerations [ATVM143-1:1989].
Depending on the rehabilitation process utilised, the following possibilities can be considered for the quantity of wastewater that must be disposed of, the number of laterals and the cross-sectional size of the current section of the sewer:
1. Interruption of the drainage capability by means of limited backup in the sewer upstream of the section to be rehabilitated
This possibility is used for cleaning and inspection purposes. Its application for rehabilitation measures is only possible for a limited period of time. In all cases, it must be ascertained that the permissible back-up level is not exceeded. The closing elements mentioned in (Abschnitt 188.8.131.52) are applicable here. The region of utilisation of a limited backup can be further extended by the use of a suction vehicle. This pumps out the wastewater, transports it to the next downstream manhole and discharges it there.
2. Maintaining a partial drainage capability by means of measures integrated into the rehabilitation system
A component of a rehabilitation system is the partial drainage capability, which is meant to permit carrying out the work without any additional measures. Examples of this are:
- Utilisation of injection packers with centrally arranged throughflow (Abschnitt 184.108.40.206.1) for ensuring a 25 - 50 % of the discharge quantity as long as no backup occurs.
- Deviating the wastewater via a suction line through the shield tunnelling machine when overriding non-man-accessible sewers (Image 5.5-1) (Image 5.5-19).
- Sectional transfer by creating a partial backup and discharging via channel, pipe or hose. (Image 5.5-1) shows how the sectional length transfer for repair to the invert was carried out around 1921. The actual closing was achieved by heaping up sand between two wooden profile plates or with the use of sandbags [Gürsc21]. The sewage was led off over a wooden channel. Sealing balloons with a throughflow and connected hose are use for this purpose today.
3. Maintaining the drainage capability by means of measures within the section of the sewer to be rehabilitated
In this case the incoming wastewater is acquired by a temporarily laid pipeline within the sewer (Image 5.5-2) or the opened-up gallery (Image 5.5-3) (Image 5.5-18) and, with small quantities and natural gravity, led away without the need for a pump station [Damma85]. A special solution in this respect is shown in (Image 5.5-4) and (Image 5.5-5). The flexible hose utilized for the discharge is laid flat on the invert of the sewer and protected during the rehabilitation process by cover channels [FI-Franc]. The laterals are connected to the discharge line via flexible hoses.
Preconditions for the maintenance of the drainage capability by means of measures within the section of the sewer to be rehabilitated are:
- Man-accessible sewer cross sections or galleries;
- Sufficient space availability for laying the discharge line and for carrying out the rehabilitation measures;
- As accurate as possible estimation of the discharge quantity and corresponding sizing of the discharge line;
- The possibility, in the case of strong sewage flows, of flooding the affected section of the sewer after a short period of preparation. This can become necessary, for instance with combined flow sewers in the case of extraordinary precipitation that exceeds the discharge capability of the discharge line as well as the usable back-up volume.
A variant of this technique that can be used with the short pipe process is known as the "Catheter Method" [Rothe89] (Image 5.5-6). In this method, a flexible hose is pulled into the sewer to be rehabilitated in place of a pipe for the provisional discharge of wastewater. In the region of the starting excavation or manhole, the hose is equipped with an extension piece and valves and these permit a rational insertion of short pipes. Here the hose lies on the invert of the liner.
4. Maintaining the drainage capability by means of measures outside the sewer to be rehabilitated
In this case, the section of the sewer to be rehabilitated is taken completely out of service and an artificial drainage capability comprising pumping or lifting the sewage and discharging it through an above-ground pipeline is created. (Image 5.5-7) shows one such drainage capability installation using pumps with separate discharge lines, and right- and left-hand entering building connections. Ensuring the drainage capability for the building connections in this case includes the following measures [Pause88b] :
- Creating a provisional manhole and installing a solids pump for each building connection. The discharge of the sewage from above is ensured by a corresponding depth of the manhole (Image 5.5-8) (Image 5.5-9) [FI-Broch].
- Installation of always two pumps with similar methods of operation as for the main sewer.
- Creation of a corresponding collector vessel.
- Installing the connection from the collector sewer to the solids lifting installation.
- Installing the (two) provisional pressure pipes as connection of the solids lifting installation with the downstream sewer.
During the operation of the artificial drainage capability, particular attention must be paid to ensuring against operational shutdowns during work pauses. Short-term malfunctions are bridged by the collector vessel [Pause88b].
In order to reduce the installation work, the variant of connecting the laterals directly to the provisional main drainage capability line should be explored in every case.
In the rehabilitation processes that require the cutting-off of the laterals before the start of the work (e.g. the bursting process, overriding and, if necessary, lining of non-man-accessible sewers), the respective excavations are used for installing the solids pumps.
In the place of the provisional pump installations that require electrical power and intensive maintenance, use has been made for some years of the Heber (Lifter) [Karna94].
According to [Meyer70], the lifter is an "apparatus for removal of fluids from open vessels. The suction or angle lifter comprises a bent pipe with which the fluid can be moved via a level higher than that of the fluid level into a lower level. In order to be operational, the fluid must be sucked up. It then flows out as long as the fluid level in the vessel is higher than the outflow level of the lifter, that is, as long as there is a pressure differential between entry opening (air pressure and pressure of the fluid in the container) and outflow opening (air pressure)."
An example of the maintenance of the drainage capability by means of a lifter within the scope of the replacement of a man-accessible masonry sewer 1720/2150 is shown in (Image 5.5-10).
In this application case, the ends of the sewer in front and behind the section of the sewer to be replaced were blocked off and a steel pipe with a diameter of 2 m was sunk besides both of these ends up to below the invert of the sewer. After closing the ends of the steel pipe invert with concrete, both of the steel pipes were connected between the operational ends of the sewer by means of a large steel pipe in order to ensure the discharge of the sewage in these provisional vessels. Both vessels were connected with a lifter piping that ended about 0.5 m above the invert at both ends. A vacuum plant carried out the starting up of the lifter pipe, e.g. the filling with sewage up to the independent flow without provision of energy.
Experience has shown that, for uninterrupted continuous operation, one or more vacuum plants are indispensable at the high point. They must start up automatically when gas has gathered. Depending on the amount of gas generated and the composition of the sewage, it has been found that automatic controls by means of float switches or submerged electrodes have worked well (Image 5.5-11).