Testing of gravity sewers

According to Scottish Water, in Scotland gravity sewers must be inspected and tested in two stages: as the work precedes and after backfilling sewer line. The checks requested in Sewers for Scotland (Scottish Water, 2018) follow the advice on testing requirements of BS EN 1610:2015 “Construction and Testing of Drains and Sewers”. Actually, the testing requirements for sewers for the UK are also based on the same British Standard, and consequently, the checking procedures are identical (Water UK, 2018).

The tests done after laying the pipes and before backfilling or concreting are done to ensure the correct assembly of joins and detecting possible workmanship and site handling damage.

Test performed after backfilling may guarantee that the sewers have not been compromised during the completion of the works and meet all the standards. Also, the testing process entails the contractor to clean the sewers before the handling.

Scottish Water in Sewers for Scotland V.4 (2018) requests the successful completion of the following tests/inspections for gravity sewers (foul and storm lines):

• Test as the work precedes:
• Air or water test (clauses 4.7.4. and 4.7.5. respectively). 
• Tests after backfilling: 
• Air test of the sewer line (between manholes) (Cause 4.7.4.) 
• Visual inspection: CCTV survey (Clause 4.7.6) 
• Infiltration (Clause 4.7.7) 
• Water-tightness (Clause 4.7.8) 

As follows, all the tests on the previous list are briefly described highlighting some aspects that help me to understand better these practices. I have not entered into detail, for further information is recommended to consult the official codes and guidance (references).

Tests as the work precedes

Air test for gravity sewers

The industry prefers the air test than the water test because it is less time demanding, more straight forward and avoids additional safety precautions needed when using the higher test pressures. Both of them serve the same purpose, and the acceptance of the sewers entails pass only one of them.

The test consists of blocking all the inlets of the sewer line tested and check if the pipes assembled could maintain a pressure of 100mm head of water pumped into the system for 5 minutes. The test is successful if the air pressure remains above 75 mm head of water after these 5 minutes.
 

There are two reasons why the test is done before backfilling: first, to be able to solve any deficiencies on time and second, because the test could be not valid due to possible water infiltration after backfilling.

A sewer is only accepted if the sewer passes the test. If not, the contractor should find and correct the issue and repeat the test. However, failure to pass this test is not conclusive; the test could give a fake fail. It is essential to ensure that the testing process is carried out correctly. Some reasons for a false test failure could be the following ones:

• Damaged equipment.
• Installation of the test equipment. For example leaks on the connections, valves not closed correctly or Inflatable Stoppers against the inside of the pipe not wholly sealing the pipe. 
• Insufficient time for stabilisation: Temperature and weather has a substantial effect on the test and stabilisation. In conditions of hot weather, due to the expansion of the air when heated, more stabilisation time could be required. On the other hand, testing below 1 Celsius degree could result in a significant loss of pressure during the test and the consequent test failure. In this case, more stabilisation time could be needed as well. 

Sewers shall be tested regularly throughout the installation before any concreting or backfilling. Sewers for adoption (Water UK, 2018) recommends every three jointed segments (clause E7.3.2), but it could be done every 3 or 5 pipes. In general, it is recommended to test all the line before constructing the manholes in the extremes.

Lastly, I would like to highlight that the test is more severe for short and small pipelines than pipelines long with large diameters. The air test requests the same pressure requirements (pressure above 75mm head of water after 5 min) regardless of the diameter and length of the pipeline. As a consequence, in a short run of small diameter pipes that contains small air volume, minor loses of air will drop the pressure quickly, and then, it is easy to fail the test. It is recommended to do the test every 4 or 5 pipes for small pipes diameters (>150mm) to increase the volume of air tested and avoid the over-severity of this test. 

Water test for gravity sewers

The water test has the same purpose and philosophy than the air test: testing the water-tightness capacity of the sewer by measuring the loses of fluid in a pressured set of pipes.

In this case, water is the fluid used instead of air to give pressure. The pipes should be filled with water until the pipeline contains no less than 1 m head of water over the soffit of the highest point. The pressure in the pipeline’s lowest point always must be lower than 5m head of water. In such cases the pressure is more than 5m head in the bottom of the line, the pipeline should be tested on several stages.

The water test takes long to stabilise before proceeding. The pipeline should be filled with water over an hour to allow the absorption of water by the pipe material. Once the system is wet, the water level should be restored. 

The test consists of measure the water needed to maintain the water level (and the pressure) above 1 m head of water in the top of the pipeline. During the test, the water level is restored every 5 minutes, writing down the litres of water added. If the loses in 30 min do not excess 0.5 litres/m3, the pipeline passes the test. 

This test is more tedious and time-consuming than the air test but takes into account the difference of diameter and length of the pipeline setting different parameter. Also, the leaks are easy to detect.

Tests after backfilling

Air test of the sewer line

After backfilling, the pipeline should be Air tested again from manhole to manhole. This last air test is to ensure that the pipeline has not been compromised during the backfilling works, especially when compacting.

Visual inspection

The visual inspection usually includes the use of a CCTV survey and a light-line inspection that use light rings to measure deformations. All the net should be completely clean to carry out the visual inspection. Usually, the net is cleaned with water pressure and dewatered before the inspection.

The CCTV survey film any infiltration on the system, so it supports the infiltration and Water-tightness verification.

Infiltration

The pipelines and manholes should be accepted as satisfactory if the infiltration, including the infiltration into manholes, does not exceed 0,2 litres/m2 in 30 min, where m2 refers to the total internal surface area of the pipeline and manholes.

To perform this test, all inlets of the system shall be effectively blocked and any residual flow should be deemed to be infiltration.

Water-tightness

No discernible water flow penetration into the manholes and chambers should be detected. This could be checked when conducting the visual inspection and infiltration tests.

References:

• Scottish Water, Sewers for Scotland – A technical specification for the design and construction of sewerage infrastructure, Version 4.0 – October 2018
• Water UK, Sewers for Adoption – A Design and Construction Guide for Developers, Eighth edition – August 2018. 
• BS EN 1610:2015, Construction and testing of drains and sewers, September 2015