Part Three – The Devil Is In The Detail (Episode One)

This third entry was published originally  on the 8th April 2013 and received 2,149 views on the old forum

Having received planning approval, the next step was to get the pre-commencement planning conditions discharged. We had four of these, one needing details of surface water management to be approved, one asking for details of a scheme to keep mud off the road during construction, one that required external materials to be approved prior to commencement and one for approval of a landscaping plan before we start.

I addressed the surface water run off and landscaping condition together. Having looked at the cost of getting a professional landscape design (upwards of £800) I opted to do it myself. I kept the detail to a minimum, on the basis that being overly specific might well cause problems later if we changed our minds about plant species etc. I did detail how surface water from the house and garage roof would be fed to soakaways, and that the drive would be surfaced with permeable, SuDS*** compliant, pavers. A fairly simple site plan, showing the layout of the drive, soakaways etc, together with some details of key planting, was drawn and accompanied with a photo of the scale model to illustrate how the landscaping might look.

Dealing with the “mud on the road” was just a matter of providing an assurance that contractor’s vehicles would be inspected before leaving the site and their tyres/tracks cleaned if necessary.

The final condition was to get approval of the roof and external cladding material. I’d included photos of these in the Design and Access Statement, so assumed that the planning officer would now need to see samples, rather than just photos. I duly drove around the country gathering bits of wood, roofing etc, and then called the planning officer to ask when and where he’d like to inspect them. This resulted in an interesting Catch 22 moment. I was told that, because of health and safety issues, I would not be allowed to bring the samples to the council offices. Instead the planning office suggested a site visit. I pointed out that the site was overgrown with 4ft high brambles and surrounded with security fencing, and that I couldn’t lawfully start work on it to make it accessible until I had the pre-commencement conditions cleared, one of which was the approval of external materials. The planning officer then suggested that I take photos of the materials and that he would accept them instead. I then pointed out that I had already done this, and that they were in the D&A Statement that formed part of our application. The response was for me to just reference the D&A Statement in the application for discharge of conditions. I had to wonder why the approval of materials had been added as a pre-commencement planning condition, particularly as obtaining approval turned out to be just reminding the planners of what I’d put in the original application.

The up side of this was that I submitted my form (and fee) for discharge of these conditions on the Monday afternoon and received a letter telling me they had been discharged the following Thursday morning. Quite a remarkably quick turnaround, and far faster than I’d assumed.

Having finally got the paper work out of the way, I could make a start on getting some of the details sorted out. We knew that services would be a problem (more on that in the next episode) but the first priority was getting the design of the big retaining wall we needed right. I knew this was going to be costly and potentially problematic, as it is around 2.5m high in one corner and around 40m long in total. To add to the potential difficulty, the wall needed to be as thin as possible, particularly in the north east corner, in order to get the house as far back in the plot as possible, leaving room for the drive at the front. Having looked at various options, I first settled on this being a plain hollow block reinforced concrete wall, rendered to make it look reasonable. I duly went to a couple of structural engineering firms and asked for quotes to design the wall, explaining the constraints. The first came back with a quote for £1500 (inc VAT) just to do the design work, which seemed massively OTT. The second was a fair bit better, at £360 (inc VAT), so we went with them. This turned out to not be a great idea, as when I got quotes for constructing the wall they came in at £25k to £32k, far more than I’d expected. There was also a problem pointed out by one of the ground works companies, in that the width of the wall footings was such that the house wouldn’t fit where it was supposed to be.

Having wasted the money on the SE’s design, I decided that I’d look at all available options and design the wall myself, doing my own structural calcs. With hindsight I should have had the confidence to do this at the start. I looked at several different systems, from the big, dry laid, block systems, like Porcupine, Redi-Rock, Allan Block etc, through dry laid, but reinforced concrete filled systems, like Stepoc, to gabions and timber crib.

The cheapest were gabions, but when I’d originally looked at using them, the advice I’d had was that they would take up too much space, plus they don’t look that great, IMHO. Timber crib is somewhat more expensive than gabions, but takes up as much, or even more, space, but does look more attractive.

Although I’ve done a fair bit of structural engineering work in the past, it has all been associated with aircraft structures, not civil engineering. A quick crash refresher course on retaining structures, Rankine etc revealed that the engineering design for retaining walls is pretty straightforward, so I cobbled together a spreadsheet to allow the calculations to be done quickly and repeatedly. The idea was that I could try lots of different designs by just sticking some numbers into a spreadsheet and seeing if they stacked up.

The first discovery was that vertical gravity walls, using gabions or blocks, are a bit of a disaster structurally. For them to be structurally sound at anything over about a metre high they need to be ridiculously deep (front to back). My 2.5m high wall would need to be 2m deep at the base if I used vertically stacked gabions, just to get an acceptable safety factor against overturning. This was no better than the reinforced concrete block wall.

However, by creating a batter angle (effectively leaning the wall back a few degrees), the resistance to overturning increases massively, and the overturning moment from the backfill and soil behind actually decreases, allowing the wall to be made a fair bit thinner. In practice this batter angle can be achieved without actually building on a slope, by just stepping back rows of gabions to form a shallow terrace. The depth of the gabions at the top can also be reduced, so reducing the overall footprint of the wall.

After several iterations of different gabion designs I ended up with something that would fit in the available space, be significantly cheaper and be OK for the less visible wall section behind the house. We didn’t want gabions for the more visible wall section behind the garden, though, so opted to use timber crib (the Permacrib system from Phi). The same basic design spreadsheet worked OK for this wall system, so after a few hours work I ended up saving money and getting a reasonable compromise design for this wall.

I also learned during this design process that gabion wall design can be very sensitive to things like rock fill density and backfill material choices. For example, my ground works chap suggested that backfill with shingle, rather than crushed concrete, would be much quicker, as it wouldn’t need to be compacted in layers. A quick check with the spreadsheet revealed that doing this would result in an increased risk of the wall falling over, just because the internal friction angle for pea shingle was so much lower as to put excessive horizontal force on the rear of the wall.

***SuDS – Sustainable Urban Drainage System, see:


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