Onsite with Sam at London Colney Social Club
dbstructural has been appointed to provide civil and structural engineering services for the redevelopment of London Colney Social Club in St Albans. The site is being redeveloped into 8 residential properties, a new social club and additional parking.
Our Civil Engineering Technician Sam Weavers has been working on the civils and recently had the opportunity to spend 2 days onsite. Thank you to our client Oak Developments and their groundworker MouldingPBS for facilitating Sam’s 2 day visit. Sam recounts his experience as follows:
London Colney – site experience by Sam Weavers
During my time on site at London Colney, I saw a range of things including surface water and foul water drainage, construction of beam and block floors and CBR testing.
The site is proposing 8 new residential properties with a social club, there’s also a road down the center of the site with additional parking.
Surface Water
We have proposed a surface water strategy as follows:
The houses are gathered using rainwater downpipes from the roof areas, gravity fed. The road is collected using an array of gullies and is discharged to a soakaway in the parking area. Each row of plots has its own dedicated soakaway to deal with the roof discharge.
While on site the first thing I saw was the excavation and setting out of the drainage behind plots 4-8. This was set out using a laser levelling station which could identify the staff at any point on site, beeping when held at the desired level, enabling the groundworker to easily identify the correct level at any point on site.
Once the level and position had been successfully been set out, the plastic manhole bottom is placed in the excavation, as is the other junctions and manholes until all elements of the run have been set out. Once the setting out is complete, the pipe can be laid and joined into the junctions and manholes, this is done using grease and just pushing each component together.
Each junction and section of pipe has a watertight seal as standard, so any need for further adherence is not needed. Once all the pipe is connected, Invert levels will be checked, and every run will be checked using a string line from collar to collar. This is to ensure that the run is straight and consistent and isn’t bulging up or sagging in any areas.
Once everything has been verified, the runs will be backfilled using small shingle. Small shingle is used to ensure the pipe is thoroughly supported and able to get underneath the pipe in the trench, and on top of the shingle topsoil will be sued to complete the backfill. The manholes will be built up using precast sections, as they come in two parts, the base with preformed pipe insertions, and the chamber walls which come in a large variety of depths, I believe the one used on site was around 250mm. The manholes are then built up and backfilled also.
The final part of this section of surface water drainage was the soak away crates. This, again, begins with excavation and setting out to the required dimensions, in this case it was 4m by 5.5m by 0.4m deep. This was excavated down to the designed depth and verified using the same levelling device as previously mentioned, smoothed off and ready for crates.
Beneath the crates a terram sheet is placed down to ensure no soil gets into the crates and clogging it up. This sheet is also placed over the top before it is backfilled. Once the sheet is down the crates are placed in and clipped together, then the sheet is placed over and backfilled with sand to secure the crates, then finished with topsoil. This concludes the surface water drainage for plots 4-8.
Foul Water
The foul water drainage for plots 4-8 follows a very similar process as the surface water drainage. It begins with setting out and excavation of the trenches, placing all manhole base and junctions in the correct positions and verifying levels.
When setting out the first shallow 225 dia. manhole, we noticed that the pipe from the internal soil vent pipe would be really tight against the lintel. This was checked by setting out the rest bend and the pipe out of the plot to the manhole, ensuring there was space with the right fall. While this solution worked, I learnt that the first chamber on a run needs to be checked against the lintel to make sure there is enough cover to get the pipe out.
After this was confirmed as okay, the rest of the foul manholes were set out and excavated, then connected all of the pipework and checking falls once again. The backfill compared to surface water is different for foul water manholes. These are backfilled and surrounded with a concrete type mix, offering support for the manhole and stopping any groundwater from getting in or foul discharge getting out. It also ensured the pipework stays in place.
For this project the foul water was connected into the existing chamber on site. Upon inspection the two existing manholes were in poor condition. As a result these old brick chambers will need to be rebuilt.
The connection of the existing chamber provided another learning point. The invert level of the existing chamber we were connecting with was 70mm higher than expected, making the fall to it very tight. It is crucial to check before any drainage goes in that the outfall is obtainable, and in this case it was but on the limit of working.
CBR Testing
While on site the CBR testing was completed alongside the drainage being installed. This process involved excavating to construction depth for the road. The excavator was placed in said excavation (to use as a platform for the CBR kit to operate against), and the CBR testing was carried out. This process involved a plate being placed on the floor and varying amounts of pressure applied to the ground so that compression values could be taken to provide a CBR value. This test was done five times at multiple points across the site.
In conclusion
This has been a fantastic project to work on alongside our Senior Civil Engineers and client Oak Developments. It has been a great learning experience and I look forward to seeing the site progress.
