Key Issue 2 - Drainage/Hardstandings Flashcards
You state that the drainage to the rear of site was in poor condition, why was this?
The drains were blocked with cracking and fractures noted. There was also no separation to separate the trade effluent prior to it entering the public foul water.
You discounted permeable surfacing due to the cost implications, but are you aware of the benefits permeable surfacing can provide?
Yes, I understand that with permeable surfacing the water penetrates through the pourous top coat through the sub base, so attenuation would not be required and less drainage would be required to the rear of site. The levels of site don’t have to be tailored in a way to get the surface water to drain to gulley.
SuDS, what are they?
Sustainable Drainage Systems. They are designed to managed stormwater locally to essentially mimic natural drainage. They aim to reduce surface water flooding, SuDS achieve this by lowering flow rate and increasing water storage capacity on site. The water is held in an attenuation tank and passes through an outlet when the water reaches a certain level.
You mention about submitting the drainage design to the LPA to discharge a pre commencement condition, did you do anything else?
I requested that the engineer submitted a pre development enquiry to Yorkshire Water to get their initial thoughts on the design, upon confirmation this was deemed acceptable, I sent onto the LPA for approval, who also consulted with YW during their consultation process.
This makes the engineer sound incompetent, were they qualified to undertake the job that you requested.
The engineer was put forward by the client, due to their cheap quote. I did due diligence and looked at their website and requested previous work, all of which looked acceptable and in line with what was expected.
You suggest that the drainage was faulty, did your initial drainage survey not pick this up?
This area was out of our site boundary and as such was not surveyed as part of our initial drainage survey. This only became an apparent issue when water was backing up within our new foul system.
With the addition of more surface water to the front of site, did this need attenuating.
It was not feasible to attenuate this water, as it would either need pumping back to the attenuation tank at the rear of site. The attenuation could not be positioned at the front of site as this area of concrete was to be overlayed. To compromise the engineer altered the falls of the area where the building was to be demolished to fall to the rear of site so this would not be discharged directly into the front aco channel. This was accepted by the LPA and YW.
Under which Parliamentary Act are trade effluent consents issued?
Trade effluent consents are issued under the Water Industry Act 1991. This legislation regulates the discharge of trade effluent into public sewers, ensuring that discharges do not harm the sewerage system or the environment.
What do CBR values represent?
California Bearing Ratio (CBR) values represent a measure of the soil’s strength and load-bearing capacity. The CBR test evaluates how much load a soil can support compared to a standard crushed rock material. High CBR Values: Indicate a stronger soil with good load-bearing capacity, suitable for supporting heavier loads. Low CBR Values: Suggest weaker soil conditions, potentially requiring additional treatment or reinforcement. Add lime to clayey soils to reduce plasticity. Add portland cement to weaker soils to make them more solid.
What prompted you to commission a geotechnical investigation for the project, and what specific objectives did you aim to achieve?
- Assess soil properties for the wash bay foundations and new hardstanding’s.
- Evaluate bearing capacity.
- Detect any contamination that would be an additional cost on site.
From which body is this consent obtained, and what are the necessary steps?
Submit an application to the local water company stating the nature of consent, volume and discharge point. The water company will assess to ensure it does not adversely effect the sewerage system. The water company will issue trade effluent consent which will give conditions such as limits on pollutants.
How did you specify the requirements for the ground investigations?
– Clearly defined the area to investigate, which was the rear of site where new hardstanding’s were to be installed and new drainage.
- Outlined the objectives, soil strength, drainage potential and contaminants.
What are the typical CBR values one would expect?
High-Quality Subgrade Soil: Typically ranges from 10% to 20%. This range indicates strong, well-compacted soil suitable for most pavement applications.
Good Quality Granular Materials: Can exceed 20%, representing high strength and load-bearing capacity.
Poor Quality or Undisturbed Soil: May be less than 5%, indicating weaker soil that might require additional treatment or stabilisation.
How did you assess the costs associated with both options?
- Initial cost analysis, cost per square meter of material and installation process. Permeable was more expensive due to a deeper sub base which increased material cost and installation.
- Maintenance costs, permeable requires ongoing maintenance such as cleaning to maintain permeability.
- Lifecycle cost, generally SMA offers longer term durability and requires less frequent repairs or replacement.
The choice of Stone Mastic Asphalt (SMA) was based on a comprehensive cost assessment, which considered initial costs, installation and maintenance expenses, lifecycle costs, and operational benefits. SMA was selected for its durability, lower maintenance needs, and overall cost-effectiveness given the project’s specific requirements and constraints.
