Milburngate Weir Emergency Repairs
The Council recently commissioned a dive survey of the weir in Durham City the passes below Milburngate Gate upstream of the new passport office below Milburngate Bridge. The survey identified a large area where the weir has scoured and been undermined to the extent where we have serious concerns about the stability of the structure.
Whilst ownership of the weir is not known it was considered that we needed to arrange repairs before the winter as there was a significant risk the weir could be breached in high flows.
The work is programmed to start on 21st August and take 7 weeks to complete with a programmed completion date of 7th October 2017.
Milburngate Weir Refurbishment
Method Statements and Management System
Introduction and General Synopsis
A scheme is required to repair scour damage that has developed over the life of the weir that has caused significant undermining and weakened the weir to the extent were failure is foreseeable.
Work will comprise filling voids below or within the weir with mass concrete and making good damage to the Downstream scour apron. A Section of weir wall approx. 35m in length has been identified towards the centre of the river which requires emergency repair work undertaken, using a combination of steel sheet piles and mass concrete.
To facilitate filling of the voids with concrete, water that flows through the weir will be stemmed by using steel piles driven into the river bed, approximately 1m below bed level and approximately 200mm Upstream of the existing weir wall. The void created between the piles and wall and within the existing weir will then be filled with mass concrete specifically designed to be placed in running water and prevent segregation and loss of cementitious materials into the watercourse. Washout will further be controlled by placing Visqueen to line the voids where possible. Once flow through the weir has been stemmed with mass concrete fill then repairs will be made to the surface of the scour apron Downstream of the main wall within the same area, using stone pitching to match the existing surface finish or through the use of concrete filled Soluform bags or mattresses pinned to the underlying substrate and grouted into position to fill any remaining voids/bedding joints.
There appears to be stone from the existing apron in the bed adjacent to the
Downstream face and this will be recovered and utilised wherever possible.
To further protect the river and adjacent environment, silt curtains will be located in the river Downstream of the working areas.
- To provide a safe working platform a temporary mobile pontoon will be constructed on the Upstream side of the weir with “spud legs” at each corner to improve stability and used as a working platform to drive the sheet piles using a 26T 360º tracked excavator, placed upon the pontoon.
- When not required for work the pontoon will be securely moored to anchor points to be fixed in or to the rear of the river training wall below Milburngate Bridge at the site compound. Weather will be monitored and the contractor will register with the EA’s floodline facility. In the event of forecast storm flows the pontoon will be dismantled and stored clear of the watercourse.
- The site compound will be located on the East bank below Milburngate Bridge and secured with suitable security fencing. Security shall be provided out of hours to secure the compound and stored materials.
Method Statement for use of pontoon
See separate RAMS for installation and use of pontoon.
Method Statement for constructing sheet piled wall
Steel sheet piles shall be stored in the site compound and only the quantity required in a single shift to be loaded onto the pontoon each day. Piles shall be located and driven into the river bed using a 26T 360º tracked excavator secured to the deck of the pontoon and driven using a pile hammer.
Method Statement for placing concrete behind piles and into the weir wall voids.
Concrete to be Toproc UW specified to minimise risk to environment in the event of a spillage as concrete limits bleeding.
Concrete to be batched off site and delivered in concrete trucks then pumped approximately 35m over the river to steel pipes fixed to the top of the existing concrete weir with brackets at 1.5m centre to centre clear of the water course. Each bracket to be secured to the top of the weir with two 16mm diameter expansion anchors fixed into the existing concrete. The concrete will be pumped across the top of the weir to where it is required and shall be discharged directly into the voids via a 6m length of flexible pipe and controlled by the tracked excavator. Washout of concrete from below the weir should be controlled through the use of steel sheet piles driven into the river bed upstream of the weir and though the use of Visqueen sheeting placed at the back of the voids.
Silt curtains shall be located Downstream of the weir to trap sediment in the event of a spillage or leaching out of the weir during placement.
Method Statement for repairing holes to Downstream scour apron
Where possible scour holes on the Downstream side of the apron will be filled to a suitable level with clean stone or mass concrete using the tracked excavator located on the pontoon. Once filled to a suitable level the surface of the apron shall be repaired using existing pitching stone recovered from the river adjacent to the weir or through the use of concrete filled Soluform bags pinned into the substrate. The Soluform bags will be delivered to site pre‐filled and be loaded onto the pontoon on pallets to be transported to where required. Any remaining voids shall be filled using Rockbond grout to form a solid repair.
Ecological management and mitigation
The site has been surveyed by DCC’s Ecology Section taking into account the proposed work. There have been no issues identified that will adversely affect the environment with respect to wild life or flora and fauna. I understand that the issues have also been discussed with the fisheries team within the Environment Agency and they have raised no concerns.
|Hazard||Debris entering watercourse off pontoon Risk Medium|
|Mitigation||No materials to be stored on the pontoon beyond those required in any given work shift. As work is to be carried out by qualified divers and lifting equipment will be located in the river on the pontoon then it is anticipated that any large debris that falls into the river will be recovered within the same working day.|
|Hazard||Contamination from concrete entering watercourse. Risk High|
|Mitigation||Concrete mixed off site and pumped to required areas in specially designed piped delivery system fixed to the weir crest. Specialist Toproc UW underwater concrete to be used to eliminate potential for damage to the environment in the event of a spillage.|
|Hazard||Loss of pontoon and plant in flood event. Risk High|
|Mitigation||The site will register with Floodline Warnings Direct and weather forecasts and river flows will be monitored daily. No works will take place if flooding or high flows are anticipated. If high flows become apparent only after a shift has begun, all works will cease and materials, equipment and staff moved to high ground. If flood events are predicted then all plant will be removed from the pontoon and the pontoon will be dismantled and removed from the river and stored in a suitable location.|
Hazard Suspended solids leaching into the watercourse. Risk High
Mitigation Use of Visqueen sheeting to all voids in the weir wall and the use of Soluform bags to contain concrete whilst repairing the Downstream Spillway.
Site and equipment maintenance plan
All plant is maintained in a serviceable condition to the manufactures recommendations and will be located clear of the watercourse. Additionally, Murlac silt curtains will be placed downstream of the works as a final defence against contamination of the river.
Fuelling will be undertaken, where necessary, in a designated bunded area away from the watercourse within the site compound. Records of all maintenance to plant will be maintained by the supplier.
Durham County Council operates a formal complaints procedure with all communication directed through the Highways Action Line, HAL. Resultant actions and records of related communication are also referenced to specific complaints on the computer based system.
Copies of all permits, health and safety information, risk assessments and method statements will be maintained in the site file compiled by the Contract Manager and kept by the site supervisor. In addition, copies of all additional records required by the permit will be kept in the site file.
Reviewing the management system
The duration of site work will be limited and it is unlikely that there will be opportunity to audit the procedures. Checks shall be carried out to check that the information noted in “Records” is available in the site file.
Making the management system available on site
All site staff will be made aware of the Environmental Management System during the site induction and a copy of the management system will be maintained in the site file and shall be available to all site staff.