Projects
Culvert/Bridge install, Stanwick, Case Study
We had already been asked to do the groundwork for a new build on this site, but the existing bridge was not suitable for bringing multiple lorries across, to remove hundreds of tonnes of spoil from site and bring all the materials in etc. Therefore a new bridge in the form of a culvert was proposed and designed, and we were instructed to install this.
Objective
Our objective was to demolish the existing concrete bridge with its steel supports, install the culvert sections and then form a road on top including kerbs and tarmac etc.
About the project
We decided to carry out these works in the middle of the summer, as the flow of the stream would be at its lowest, as opposed to the wetter months, when the stream was significantly faster-flowing. We started by demolishing the existing bridge, taking care not to pollute the stream with debris from the old structure. The culvert sections could have been laid on consolidated type 1 MOT, but due to the stream being live, and the volume of water being considerable even during the drier months, we decided to lay a concrete slab instead.
By laying a concrete slab, we could envelop the shuttering in plastic and divert the stream around. However we were struggling with room, as the concrete culvert sections took up more than the width of the stream, there was not much room to divert the stream around. Therefore we installed a small catchpit ‘upstream’, and from here we ‘over-pumped’ the work area to reduce the flow of the stream.
With the flow significantly reduced, we were able to excavate for and form shuttering for a concrete pad, for the culvert sections to sit on. We put a double-layer of thick dpm plastic within the shuttering, and formed a concrete slab 200mm thick, with 2 layers of reinforcing mesh to the middle. The concrete was allowed to cure for two weeks before the concrete culverts sections were scheduled to be delivered.
We looked into different ways of lifting the concrete culvert sections into place. We originally planned to have a 13-tonne ‘track machine’ on site, this would lift the sections off the lorry and onto the concrete pad. But due to trees on either side of the site entrance, some of which had protection orders on, we had to look at bringing a crane onto site. The size of crane required meant that it would have to sit in the road, and this in turn would close half of the road off for many hours. This also had implications for pedestrians as they would be passing on the pavement underneath the crane and through a live site, in effect. We spoke to Northants Highways and it was decided that we would use 2-way traffic lights to control the traffic. And that the pedestrians would be diverted into a temporary pedestrian walkway around the crane as it sat in the road.
We instructed A-Lift Crane Hire from Northampton to assist us with these works. (A few other local crane companies told us this job couldn’t be done!) We had to co-ordinate the crane hire, the delivery of the culvert sections, and the traffic lights, and all other deliveries, all to happen on the same day. The crane lift was particularly difficult due a combination of various overhead BT cables and also the many trees that surrounded the front of the site, it was very tight as the 7-tonne culvert sections were lowered down amongst all these overhead obstacles! We decided on a strong sand and cement mix to bed the culvert sections onto, on top of the pre-formed concrete. There were only 3 culvert sections to go onto the concrete base, but at 7 tonnes each, we didn’t want to be lifting them on and off our screed too many times, so we spent a lot of time getting the screed exactly right, on top of the concrete slab.
The culvert sections interlocked where they met, and we had to seal them at this point as well, with bitumen and with thick flexible bitumen strips as well.
Looking down the culvert sections either side, we laid a double sand-bag wall 450mm thick, to retain the fill between the long edges of the culvert sections. We used 6F2 for the fill, which we compacted in layers with a heavy duty trench wacker, as the road above was to go on top of all this construction.
Before we could form the road on top of the new culvert, we had to lay services across to the proposed new build. These included electric, BT & Water. With these in, we brought type 1 in to form the sub-base of the new road. We laid kerbs either side to retain the tarmac and then laid 150mm of 20mm base course tarmac, to enable heavy traffic across the culvert/bridge for the duration of the build. The tarmac will be topped later on at the very end of the build, with a 6mm top coat.
Our last job was to lay oak sleepers at each end of the culvert, purely for cosmetic reasons. We finished these works just in time, as the wetter weather set in towards the end of September and the flow increased significantly. It was a pleasure to carry out this very exciting and challenging job for our customer, and from here we were ready to make a start on the groundwork for the new build. Case study coming soon for this.