D7.4. Report on the development of mechanistic models able to simulate the temporal deterioration of drainage assets

This report is aimed at academic researchers, water utility engineers and R&D engineers involved in the design, construction and maintenance of wastewater sewer and stormwater drainage pipes who need to (i) understand the impact of joint defects on pipe performance, and (ii) to provide evidence on the key parameters regarding displaced pipe joints that impact on the amount of exfiltration/infiltration experienced.

In-pipe joint defects are one of the most common defects found in sewer and stormwater drainage systems. They are known to cause both infiltration and exfiltration; both of these processes can have a significant impact on the performance of such systems. There is strong evidence that exfiltration of wastewater can contaminate local groundwater. This has an environmental impact but can have public health implications if water supply pipes located close to the failing sewer are also leaking. If the water supply pipes are managed inappropriately in terms of their pressure management so that transients (with +/- ve pressures) are allowed to be created, then there is a significant risk that pathogens within the wastewater could enter the public water supply. Infiltration also causes significant issues in that in areas with groundwater higher or similar levels to the wastewater collection sewer or stormwater sewer levels then a significant proportion of the flows carried by these sewers is simply infiltrated groundwater. This results in excess flows to the end of system wastewater treatment plants, resulting in poor quality treatment and high energy use, or more frequent spills from combined sewer and storm sewer overflows due to the hydraulic overloading caused by the infiltration of groundwater. Both issues cause significant environmental harm, either through the more frequent spills, or the enhanced energy use (and carbon emissions) from the large volumes treated at end of system wastewater treatment plants.

Given the significant impacts caused by displaced joints and the widespread prevalence of displaced joints it was decided to focus on the underlying mechanisms associated with joint displacement and infiltration and exfiltration. Two complementary series of experiments were carried out at IKT, in Germany and the University of Sheffield in the UK. These experiments required the joint development of new measurement techniques, visits and the sharing of data. The results of these experiments provided the data to start to identify and quantify the potential joint mechanisms that control exfiltration from jointed pipes in soil

The results indicated that the pipe material (and relative stiffness) of the joints and pipes were important. In stiff pipe materials such as concrete and vitrified clay the exfiltration rates increased as the angle of relative pipe articulation increased. In PVC pipes after a small angular articulation the exfiltration rate did not rise. Detailed in-pipe measurements using a Digital Image Correlation system indicated that the pipe walls had deformed, and this is an explanation as to why the rise in exfiltration rate essentially stopped even though the pipes were articulated to a larger angle.

Exfiltration was only observed when the joint seals were damaged. In the initial tests in which undamaged joint seals were used, negligible amounts of exfiltration were observed.

In tests with a concrete pipe buried in soil exfiltration rates were larger in size than the exfiltration rates observed in the tests at IKT, in which the pipes that were not buried. This shows that the presence of the pipe bedding material does not appear to impact on the exfiltration rate and that the exfiltration rate is controlled by the condition of the joint and not the surrounding material.

Further tests were carried out in which the buried pipe was loaded with a cyclic load that mimicked a heavy goods vehicle. Even with cycles that represented over 238,000 cycles no appreciable increase in exfiltration rate was observed.

In summary, pipe material is significant, it is important to measure the internal displacement within the pipe as the external angular displacements do not correlate well with the joint displacement especially if the pipe material has a relatively low stiffness. (e.g. PVC). In pipes with stiff pipe wall material such as vitrified clay and concrete exfiltration rates rose with the size of the relative angular articulation between the pipes. Exfiltration rates were also related to the size of joint seal damage. Exfiltration rates were related to the level of water in the IKT pipe and the USFD upstream manhole. More analysis is needed to assess the reason for the differences. Comparing pipes in the air and those buried in high porosity bedding material, the rate of observed exfiltration does not appear to be restricted by the bedding material.