Part Forty Six – Over-thinking things – Part Two

When going through the list of things that we hadn’t used consultants, specialists, etc for, I realised I’d forgotten about a few.  Well, it’s fair to say that I hadn’t really forgotten about them, but had forced common sense (an increasingly rare commodity) to dominate.

I’ll give just one example, but I’m sure others will have many more (especially when it comes to ecology, archaeology and wildlife!).   I had a rough idea of the ground we would be building on, as we HAD to have a hydrogeological survey for the borehole (no easy way out of that one, if you want a quote from driller).  The survey wasn’t expensive, and I could have obtained almost all I needed from the BGS website, plus a look at a lot of the local borehole drilling records, that shown the type of sub-strata and depth.

So, I knew that we were on the exposed top surface of blue gault clay, and that it was at least 3m, perhaps closer to 4m, above the water table.  I also knew that the gault was highly compressed here, so was almost a semi-hard mudstone.  The first thing I was asked by the foundation design structural engineer was the bearing strength of the sub-soil and whether or not the clay was subject to movement with varying water content (heave).

Now, many self-builders today will immediately pick up the phone, or more likely search the web, for a soil testing service, in order to get this information, for fear that if they don’t their house will fall down or subside.  Having lived in a Victorian built house, that just had stepped brick foundations around a foot deep, on to clay, and was still fine over 100 years later, I wonder quite why some have this reaction.

Back before we had all this highly technical (and expensive) kit for measuring soil bearing capacity, building inspectors used to do it, with a lump hammer and a bit of 2″ x 2″, marked with depth marks.  The subsoil would be exposed, the bit of 2″ x 2″, with a squared off end, would be placed on the subsoil and wacked hard with a lump hammer.  If it couldn’t easily be driven in more than about 4″, without really going at it, then the bearing capacity was reckoned to be “good enough”.  I did such a test.  The bit of 2″ x 2″ went in about an inch, and would go in no further, no matter how hard I whacked it.

In fact, this test is pretty good, 4″ penetration is generally “good enough” and is around 100kN/m², and it’s fairly unusual for any domestic foundation to need much more that this.  A slab foundation will usually need a lot less, as the load is spread over a greater area.  Also worth remembering that, as a rough rule of thumb, vertical bearing loads translate out at around 45 deg down and out from the point of load application, so a foundation that’s 1m² at the point of contact with the soil, is applying a load over about 9m² 1m down.

So, despite all the complexities of the big retaining wall, the borehole for water etc, we didn’t spend any money on specialist soil testing.  The wall structural engineer was “old school”, and knew damned well that the design was massively over-engineered, making any testing pointless.  The slab foundation even more so.  Our slab bearing area is around 85m², and the soil is over 100kN/m² bearing capacity.   Under the slab is a 200mm thick layer of compressed crushed stone, that increases the effective soil bearing area to just over 101m².  With a soil bearing capacity of over 100kN/m², the ground under this really simple foundation system could support a mass of well over 10,000 kg/m², or a total house mass of over 1000 tonnes.

In reality, a typical two storey house weighs around 3 tonnes/m², perhaps as much as 4 tonnes/m² for a very heavy stone built house, but more like 2 tonnes/m² for a timber structure like ours (including the foundation mass).  If we’re pessimistic, and say that our house has a foundation loading of around 3 tonnes/m², over the 85m² foundation, then the soil beneath “sees” a loading of around 2.5 tonnes/m² as a worse case (the reduction being due to the load spreading effect of the 200mm stone layer, increasing the effective bearing area).

Now, converting this back to units of force, it’s easy enough to work out what the actual soil loading is.  2.5 tonnes is a force of just under 25 kN/m², or under 25% of the soil bearing capacity.  This shows pretty clearly that anything that passes the old “2 x 2” test is more than good enough, with the sole exception of clays that are subject to heave with changes in moisture level (some are, like London Clay, some aren’t, like our hard Gault mudstone).  Even then there are easy (and relatively cheap) solutions to building on soils subject to heave – use screw piles.  Cheap and easy to install (a mini-digger with torque head can usually do the job).  The bearing capacity can be read out directly from the amount of toque needed to install them, so you know when you’re deep enough.  And, best of all, they can be installed very quickly and are instantly able to bear their full working load, unlike concrete, that takes time to cure.

The problem we have is that no one trusts tried and tested “old school” methods any more, and what’s worse, in this more litigious age, very few people are prepared to just use their judgement, they would rather you, the client, forked out a few thousand for another test or survey.

99% of this is obvious, but how much does it add to the cost of a new build?  For a one-off house, the total cost of all these surveys, tests etc, plus the cost of all the fees to the local authority, licence fees, building control fees etc, etc, can run into the tens of thousands of pounds, very easily, making the whole project non-viable if you’re looking to build a house for less than an equivalent house in the area would cost.

It’s no wonder houses are so damned expensive.  What with the price of land being high, and then there being dozens of people with their hand out for a slice of your cash, even before you’ve started any work, it’s likely that the land value and fees end up being the biggest part of the whole build cost, with actually building the house the cheap bit……………….

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