Guide to working on Public Sewers affected by Building Works Introduction Severn Trent Water Ltd requires the work to be done in order to protect the public sewer and minimise the risk of us having to break out the floor of the proposed building so that we can excavate down on the sewer, in the foreseeable future. Works to be carried out to public sewers which will be located under the new building may include the following: Removal of existing sewer connections Provision of replacement connections Replacement of sewer pipes. Removal of existing manholes Construction of replacement manholes This leaflet gives guidance on how the above works are to be carried out. Health and Safety Severn Trent Water is responsible for protecting public sewers, the health and safety of anyone who works on public sewers and any person in proximity to the works. We satisfy our duty of care by giving approval to builders/contractors only after confirming they have the necessary experience and competence. It is illegal to interfere with a public sewer unless our consent has been given. Method Statements and Risk Assessments. Prior to giving approval to builders/contractors allowing them to work on our public sewers, we require them to send us Method Statements and Risk Assessments. These should demonstrate that the work to be carried out will be done safely and that the builder/contractor carrying out the work has the necessary competence and experience. Severn Trent must be satisfied that risks are being properly dealt with. In addition to the normal risks associated with construction such as manual handling, use of plant and equipment, general site safety, etc. the particular risks affecting work on sewers includes: Working in deep excavations, trenches deeper than 1.5 metres must be supported Working in confined spaces, these are manholes and trenches where a workman can not immediately stand up and be in open air. Only workman with confined spaces training and certificates can work in confined spaces
Physical contact with sewage can cause infection and illness. Appropriate hygiene measures should be taken to avoid risk to health, protective clothing is essential. Dealing with flows in sewer pipes, risk of flooding must be eliminated The method statement should outline the procedure for the work, e.g. how flow is to be dealt with, how trenches are to be supported, how the sewer pipe is to be cut, how the connection is to be made, etc and must cover all activities from the start of the work until the sewer is back in operation. The method statement must include details of how risks identified above are to be handled. Specification All work carried out to public sewers must comply with the provisions of Sewers for Adoption 6th edition a design and construction guide for developers and Severn Trent Water addenda. Copies of the guide can be obtained from WRc, Frankland Road, Blagrove, Swindon, Wiltshire, SN5 8YF. Tel 01793 865138/865069. However the following guidance is given for commonly required work Permitted pipe materials Clayware Pipes as per Sewers for Adoption Guidelines Concrete Pipes as per Sewers for Adoption Guidelines Solid wall plastic pipes should comply with the relevant provisions of BS4660 and BS EN 1401-1 Structured Wall Plastic Pipes as per Sewers for Adoption Guidelines and subject to the following: Structured wall plastic pipes approved for use in Severn Trent Water are as the table below. For limitations on their use please consult Asset Protection office. Polysewer Polypipe Building Products Quantum Marley Ultrarib Uponor Ultrarib Wavin Removal of existing sewer connections, manholes, rodding eyes and replacement of pipes The existing pipe on either side of the junction, manhole or section of pipe to be replaced is to be neatly cut out using a suitable pipe cutter or disc saw. New pipes are to be inserted into the gap. The new pipes are to be connected to the existing pipe using secured banded couplers to form flexible joints. The banded couplers are easier to fit if the new pipe is of the same material as the section of pipe removed. Arrangement of banded couplers is shown in diagram 1 at the rear of this document. The new pipe must be laid on a gravel bed and trench filled in as shown in diagram 2.
Please note that if the depth of the trench is more than 1.5 metres, YOU MUST support the sides of the trench in order to avoid collapse and risk of death to persons within the trench. Making a new connections to the public sewer You must make the connection using a pre formed junction. Saddle type connections are not permitted, unless the existing pipe diameter is more than 300 mm and the diameter of the connection pipe does not exceed 100 mm. A section of the existing sewer is to be neatly cut out using a suitable pipe cutter or disc saw and the cut out section removed. The junction is to be inserted and connected to the existing pipe using secured banded couplers to form flexible joints. The banded couplers are easier to fit if the new pipe is of the same material as the section of pipe removed. The new junction pipe must be laid on a gravel bed and trench filled in as shown in the diagram 2. Connection to existing manhole The new connection should be set so that the top of the connection pipe is at the same level as the top of the main pipe within the manhole. A hole should be made through the existing manhole side wall by cutting out a core using appropriate tools. The connection stub pipe should then be inserted through the hole and concreted in place. Manhole benching should be formed to direct new flows into the main channel. Please see manhole detail diagram 3 or 4 for materials required in benching work. Preformed half channels may be used. Construction of new manholes The type of manhole construction is dependent on size and depth of sewer as shown in diagram s 3 and 4. Manholes constructed in plastic used for normal domestic drains are not permitted.
diagram 1 Section of new pipe Existing pipe Coupler Coupler Existing pipe PLAN Existing pipe Coupler New pipe Coupler Existing pipe Existing pipe Coupler New pipe Coupler Existing pipe SECTION
diagram 2 50mm 50mm Backfilling should be compacted in 300mm layers in order to avoid trench settlement surface level Backfill to be clean hardcore minimum size 20mm maximum size 200mm or selected subsoil free from vegetable matter lumps of clay or building rubbish Bedding to be Pea Gravel minimum size 5mm maximum size 20mm 50mm 50mm PIPE BEDDING AND TRENCH BACKFILL
diagram 3 SOLID ENGINEERING BRICKS FOR TOP 2 COURSES WITH 40 KN MORTAR TO BE CORBELLED TO SUIT COVER OR CONCRETE SLAB PLUS 2-4 COURSES OF BRICKWORK. MANHOLE CONSTRUCTION 225 THICK CLASS B ENGINEERING BRICKWORK OR CONCRETE SECTIONS WITH 150MM C20 CONCRETE C20 CONCRETE WITH SULPHATE RESISTING CEMENT SHORT (ROCKER) CUT PIPE STANDARD MANHOLE DEPTH TO TOP OF PIPE 1.5 METRES MAXIMUM CUT PIPE SHORT (ROCKER) PIPE 675 JOINT TO BE AS CLOSE AS POSSIBLE TO FACE OF MANHOLE TO PERMIT SATISFACTORY JOINT AND SUBSEQUENT MOVEMENT 1240 INVERTS TO BE FORMED USING CHANNEL PIECES 150 MIN 150 MIN (MIN THICKNESS 40mm) TOPPING (1 CEMENT:1 SAND:2 SINGLE SIZE COARSE AGGREGATE) TO BE BROUGHT UP TO A DENSE SMOOTH FACE. HIGH STRENGTH CONCRETE 1240 675 DUCTILE IRON COVER BEDDED ON 40KN MORTAR OR RESIN 1220mm x 685mm MULTIPLE 225mm TO BARREL OF PIPE BENCHING SLOPE TO BE 1 IN 10 TO 1 IN 30 ARCH OVER PIPE (BRICK MANHOLE ONLY) OR SEALING STRIPS TO APPROVAL OF THE PCC RING MANUFACTURER USE MORTAR,BITUMEN,MASTIC RESIN BRICKWORK MORTAR TO BE SOLID AND WATERTIGHT WITH PCC SECTIONS M H COVER AND FRAME 40KN MORTAR HAUNCHING TO
diagram 4 40KN MORTAR BED AND HAUNCHING TO M.H. COVER AND FRAME D400 COVER AND FRAME MIN. CLEAR OPENING 600X600mm FRAME TO BE 100mm DEEP. MIN. 2 MAX. 4 COURSES SOLID ENGINEERING BRICKS, PRE CAST CONCRETE MASONRY UNITS OR PRECAST CONCRETE FRAME SEATING RINGS. MORTAR OR APPROVED RESIN COMPOUND TO BE 40 KN. STEP RUNGS 250 250 675mm MAX TO FIRST RUNG MINIMUM CLEAR ACCESS 600X600mm BEHIND STEPS OR LADDER. PRECAST CONCRETE CHAMBER SECTIONS AND COVER SLAB TO BE BEDDED WITH MORTAR, BITUMEN, RESIN MASTIC OR SEALING STRIPS TO THE APPROVAL OF THE PCC RING MANUFACTURER. 1200mm DIA 150mm CONCRETE SURROUND IN UNSTABLE GROUND OR HIGH WATER TABLE CHAMBER HEIGHT (NOT LESS THAN 900mm) HIGH STRENGTH CONCRETE TOPPING TO BS 8204 Pt.2 (1 CEMENT: 1 SAND : 2 SINGLE SIZE COARSE AGGREGATE) BENCHING SLOPE TO BE 1 IN 10 TO 1 IN 30 TO BE BROUGHT UP TO A DENSE SMOOTH FACE (MIN. THICKNESS 40mm) ALL IN SITU CONCRETE TO BE ST4 WITH 20mm AGGREGATE. TO BE CLASS 2 SULPHATE RESISTANT 300 75 MIN DISTANCE BETWEEN TOP OF PIPE AND UNDERSIDE OF PRECAST SECTION TO BE 50mm MIN. INVERTS TO BE FORMED USING CHANNEL PIPES 225mm TO UNDERSIDE OF PIPE SHORT (ROCKER) PIPE JOINT TO BE AS CLOSE AS POSSIBLE TO FACE OF MANHOLE TO PERMIT SATISFACTORY JOINT AND SUBSEQUENT MOVEMENT CUT PIPE PIPE JOINT WITH CHANNEL TO BE LOCATED MIN 100mm INSIDE FACE OF MANHOLE MINIMUM WIDTH OF BENCHING MINIMUM WIDTH OF BENCHING TO BE 500mm TO BE 225mm CUT PIPE SHORT (ROCKER) PIPE STANDARD MANHOLE DEPTH TO TOP OF PIPE MORE THAN 1.5 METRES