Darley Abbey Culvert Replacement Scheme - Questions answered by the City Council
Darley Abbey Culvert Replacement Scheme
Setting the scene
The culvert in question is the historic route of the Nut Brook which rises as a spring in farmland to the north of Nut Wood, adjacent to South Avenue. It runs south along the boundary of the wood and South Avenue properties until it reaches the River Derwent. Instead of flowing into the river, where it would be above the weir, and thus affected by the rising river levels, it was long ago diverted into a brick lined culvert which follows the river, flowing down what is now the canoe club access, Old Lane and Darley Street. The culvert flows underneath what is now the British Legion Club where there is an old gear chamber, and then continues south across Darley Park to an outlet where it returns to an open channel and ultimately rejoins the River Derwent. This culvert, being of a brick construction, is several hundred years old and is in keeping with the history of Darley Abbey. Due to the nature of its construction the culvert does not follow a precise line and may indeed have followed the line of the river as was.
We know the route in general terms as we have Close Circuit Television (CCTV) surveys of large sections of the culvert.
During the mid-1970's the Allestree Trunk Sewer was constructed, flowing down the west side of the river and following the route of the Nut Brook culvert from the southern extent of South Avenue. Because of the different methods of construction, and indeed material, the horizontal alignment (route) of the two pipes crosses at numerous points. Unfortunately, it would appear that the vertical alignment (or level) also converges, such that the two pipes were trying to occupy the same geographical space. It would further appear that, in order to make the line of the sewer more conducive to achieving maximum capacity, the older brick culvert was "adjusted" to fit. Several of our CCTV surveys terminate in the brick culvert changing to a pair of small parallel pipes surrounded in concrete.
To replace the culvert between the upper end of Darley Street and the Royal British Legion Club.
The design scheme as proposed is a trenchless (thrust bored) dig with open excavations at the top and bottom, merely to tie-in with existing. The work will require three surface excavations in order to permit the directional dig. This is currently out to tender.
As part of this proposal, three trial holes were dug during the week commencing 19th November 2012, these were to ascertain the exact line and level of the STW sewer, the status of the culvert (if possible), the likely ground conditions and the status of the ground in relation to the local archaeology.
Following this work, it seems likely that the original proposal may not have been the best choice and that a better solution may be to directional drill the majority of the distance apart from the last connection at each end.
We can't do anything to change the plan until the tenders are returned, but we can then talk to a couple of the best to find out what effect this would have on the overall cost and construction programme.
We are sensitive to the concerns of residents and will as best we can to make the whole scheme as beneficial to all as possible.
The Implications for NOT doing the work
The existing culvert is leaking and will ultimately have a detrimental impact on nearby properties.
There exists a flood risk to Darley Street from the highway drainage in Old Lane.
There is no viable outfall above Darley Street as the presence of the weirs means that the standing water levels in the river are too high to allow the culvert a free outfall. A drowned outfall is not viable as there would be no way of preventing the river in flood from flooding Darley Street.
If the culvert WERE to be terminated above Darley Street, it would leave a dry outfall in Darley Park and a dry culvert upstream. This would be likely to be a haven for all kinds of vermin and would require backfilling/sealing.
Questions and responses received to date
The following questions were asked by the residents during the trial hole exercise and illustrate the local depth of feeling:-
1. Exactly what are the planned works to replace the Culvert?
(A) The replacement works will involve the laying of a new culvert parallel with the Severn Trent Sewer, to run from a manhole beneath the car parking bays in Old Lane to an existing manhole immediately outside the British Legion Club. This will mean that any residual elements of the brick culvert beneath Darley Street will be isolated from the brook itself, but they will remain under the road. The culvert itself will be routed through a new pipe, laying alongside (but not interfering with) the Allestree Trunk sewer. Our trial holes have served to show
(a) that the proposed line has no obvious complications; and
(b) that there is no recorded archaeology which we need to avoid.
2. Why does it need replacing?
(A) As indicated above, the culvert is the historic route of a flowing brook, it is fed from a spring on nearby farmland. Naturally the normal (or base) flow will depend upon the pressure of ground water, and will be greater following periods of wet weather. In addition the lower section of Old Lane, between Haslam's Lane and Darley Street, flows into the culvert resulting in possibly high rates of flow from highway drainage in extreme conditions. The limitations placed on flows by restricting the culvert to very small pipes has resulted in a degree of siltation as large debris cannot be carried forward. This poses a significant risk of blockage which could lead to extensive flooding.
3. How will it be replaced, particularly in such close proximity very old houses with no foundations?
(A) The three surface pits will be opened with, the larger drive pits located at the upper and lower ends of Darley Street and a smaller reception chamber located in the middle. The open pits will be lined to protect against collapse and settlement. There is a need to construct some shallow highway carrier drainage, but much of this can be accommodated into the proposed construction pits. The thrust bore dig itself is almost free of vibration and should not affect the existing properties.
4. What impact might this have on the current water table levels and hence the impact on subsidence (see point 9)?
(A) Essentially, replacing the culvert through this area will have no detrimental effect on ground water levels or subsidence. The removal of water leaking from the damaged culvert will reduce the volumes of water moving through the soil and will reduce the likelihood of soil erosion and thus the risk of future settlement. The natural water table, driven by the standing river level will, however, remain as is.
5. Who is doing the works and are they sufficiently skilled and knowledgeable in doing such works. Having seen the contractors in operation last week I would challenge that, e.g. attempting to scoop out water with the shovel on a digger and pour the water down the drain? Not only will this block a drain you can never drain a hole below the water table by such a method.
(A) The contractors undertaking the trial holes are a different type of contractor to those who will be chosen to complete the final scheme. We are lucky in Derby to be surrounded by some very experienced local civil engineering contractors They are highly skilled, experienced and professional. They have completed similar schemes to this in a wide variety of sensitive and historic areas.
6. How will the holes be dug without damage to properties? Large diggers are totally unsuitable for use in this area.
(A) Mechanical excavators come in a wide variety of shapes and sizes. The removal of the tarmac will be likely to cause more disruption than the excavation through the soils below. Care will be taken to use methods and plant which minimise vibration and disruption. The sides of the excavation will be shored using an appropriate method and the base of the excavation will be dewatered as necessary to allow the new pipes and granular surrounds to be placed. The removal of trench supports will be timed to coincide with backfilling operations and the trench will remain open for only as long as is necessary. Backfilling operations will ensure the adequate compaction of backfill materials and materials will be chosen that have little or no residual consolidation aspect. This means that settlement of the surface will be controlled at all stages.
7. What are the timescales for doing this? Quicker does not necessarily mean good news particularly given the difficult and confined location.
(A) We propose for this work to commence in mid-February to coincide with the planned closure of Darley Street by National Grid Gas, from that point we have allowed until the end of March to complete the works as necessary and will take as long as is necessary to minimise disruption, while achieving a successful completion.
8. What other works are planned for the village, drains replacement (what/where/why) road resurfacing and when?
(A) With the completion of this work, all of the invasive, deep work will be completed under Darley Street for some time to come. We understand that there will be a full reinstatement/ resurfacing of the road and footways, to return the area to its original state, and this will be jointly financed by Derby City, National Grid Gas and Severn Trent Water as part of their joint obligation to reinstate after works within the carriageway.
9. How will the reinstatement works be done? I am particularly concerned on this point. Any form of extensive excavation work so close to properties can have very serious medium and long term impact on subsidence. I note that a very deep hole was dug only 6 feet from number 11 Darley street and left unsupported overnight. The hole began to "bell out" due to the water table eroding the sides. This cannot be right. I am not an expert but know that the shearing lines from any such hole runs from 45 degrees which would have undermined the property. Reinstatement without changing the water levels is particularly important.
(A) Backfilling and reinstatement are two different questions.
As detailed in question 6 above, the surrounding and backfilling of excavations will take place immediately upon completion of the thrust bores and the manholes which will form the changes in direction. The three chamber excavations will be backfilled by mans of a concrete surround to avoid settlement of the surrounding surface. The hole will be filled as side supports are removed to avoid side collapse.
Reinstatement refers to the replacement of the finished surface and will initially be a temporary surface with a more long term road surface once a surfacing scheme has been identified.
10. In doing these works we would expect the Council to accept responsibility and liability for any structural damages, short and long term. This of course can be mitigated by using the correct contractors and methods, although might not be the cheapest short term approach.
(A) Our contractors for this kind of sensitive work go through a rigorous vetting procedure which heavily weights the quality of their method statements, their skills and experience as well as the price of their submission. We will pick the contractor which offers the best balance of quality and price to ensure overall value for money for residents.
Since we believe that the culvert is already compromised and leaking beneath the road, there is a risk of future subsidence to properties. By completing the work we are reducing the risk of collapse and potential settlement in the future. There will be a period of disruption but we believe that the reduction in the risk of collapse, the removal of a leaking culvert and the reduction in flood risk makes this an important project.
11. The use of heavy diggers and such machinery is unsuitable for our street. During the work our house was vibrating. Items in the bathroom which is at the back of the property were rattling on the shelves. If this effect was felt at the back of the property then the older part of our house will also have been impacted on.
(A) See 6 above
12. The proximity of the holes to our house and the depth of the holes (2.5m) causes a concern. Subsidence caused by this may not be apparent for many years but the implications would be serious for the owner of our house.
(A) See 10 above
13. The proximity of the work to our house means that cosmetic damage to the frontage of our house is a concern. We have recently (Nov 2012) replaced all our windows with wooden sash windows in accordance with the views of the Conservation Office. We will be redecorating and rendering parts of the front of our house soon. Machinery and pieces of debris could all cause damage to the front and side elevations of our house.
(A) Screen protection can be provided to the frontage of nearby properties to prevent damage by flying stone chips. We would expect a considerate contractor to carry out this level of protection as necessary.
14. Who will actually be responsible and accountable for this work? The note below implies that once suitable contractors have been appointed they will perform the work in accordance with best practice and in a way that is sympathetic to the area and residents. Having just had Severn Trent and British Gas in the area for several months we were expected to liaise directly with the contractors. We do not feel this should be the responsibility of local residents and would expect a contact at the council to oversee, be accountable for the work, and be available onsite during the works.
(A) We are your contacts at the council we are responsible for our design work. The contractor will be responsible for the quality of his work and will have suitable insurance to cover this. We will liaise with the contractor on your behalf, but we do ask that any requests for cars to be moved or left clear of the work area are complied with.
15. How will the work impact on older properties, in the shorter and longer term?
(A) Before the work commences we shall arrange for a condition survey of all affected properties in order to create a benchmark of the current status. This will be used to determine any claim of subsidence arising from the works in the short and medium term. Following completion of the works it is proposed to undertake a reconstruction of the road surface.