12 Structural Foundation Tips, a Horror Story and a Good Method

Choose the right Expert

Groundworks and foundations are potential financial black holes. They are often misunderstood and mistakes are seldom cheap or delay free. It is as much about knowing what not to do as how best to obtain the support you must have to carry off a successful project. There are a surprising number of so called experts who are ill equipped to lead technical projects with an equally surprising lack of understanding of the subject you are taking to them. This too often results in people leading projects for industry on the same basis as they would approach smaller or domestic works and that inevitably leads to delays, misunderstandings and unnecessary avoidable frustrations. This article helps to clarify various roles and milestones which will stabilise and smooth the path to a successful conclusion.

What do Engineers do?

The best foundation you can have for strength is a strong rock, like granite. However it is unlikely to be the right shape and rock is not easy to shape so what is the best sort of ground to have? Geotechnical engineering is a very big subject and often confused with structural and civil engineering. The answer to the question is ground with sufficient strength to be able to take the weight you wish to impose on it. The geotechnical engineer will be able to tell you what you have got by taking samples, measuring the compressive strength and analysing the materials sampled.

Structural engineering is simply about structures. Civil engineering has become associated with altering the shape of the landscape but its origins quite simply emanate from the civilian end of military engineering.

I divide it up into two categories for the purposes of building and construction. Above ground is everything which is factory prepared and brought to site to be erected and below groundworks, which include all the services works, preparations, slab and foundations works, comprise the onsite preparations.

The structural engineers need metallurgists and the civil engineers need geotechnical engineers. This latter part is something which vanishes into the mists of time until you want some work doing which involves the floor or beneath it so when the information is published at the very outset of the project it usually disappears into the groundworks designs and never re-emerges. Make sure you always keep a copy of it. If you want to develop the site in future, it is the first thing the engineers will ask you for.

Before anything can be constructed on land (or below water) you have to establish exactly what you are building on and whether or not it will support the weight of the proposed structure.

I am amazed at how many experts there are in this field who are prepared to take massive risks and place amazing confidence in the properties of concrete even after 60 years of use. It's a bit like assuming Blondini's tight rope would be as good today as it was the day it was put up and then deciding to go for an afternoon stroll across it in ski boots.

Ground moves, it's as much alive as you or me, there are hundreds of small tremors and vibrations at work all the time and this has an effect on everything we do or make. To just assume it will be safe to erect a mezzanine floor on a concrete slab is an unsound decision which could be costly. The same applies to shelving and racking especially if you intend to erect it on a mezzanine floor. If you load it to full capacity without knowing what you are doing you could be applying the equivalent of a stiletto heel load on balsa wood and we all know what stiletto heels do to lino and timber floors!

I was shocked after structural engineers had stated that a former canal side engineering works should have the floor strengthened that the directors of the company overruled the structural engineers and refused to pay for the works to be carried out. They went on to build cantilever racks designed to take 20 tonnes a bay onto a cracked floor stating that they had done the same at other locations and never had any problems. It's like driving south on the north bound carriage way at 2.30 in the morning in thick fog and saying it is safe just because you can't see anyone else and you are driving slowly!

Why does Pisa lean?

When we build load bearing structures these connect inevitably to the ground and that is where, with the aid of gravity, the full force is finally taken. You can make most things stable and perfectly able to accept very heavy and extreme loads. Steel will float if it is designed to do so and sink and re-surface again. Air will support metal. Groundworks are every bit as important and there are many ways of finding out what is going on in the ground. The leaning tower of Pisa is an amazing and famous example of problem ground. There was much argument and discussion regarding the work to save the tower from falling over completely and in particular how much of the "lean" to preserve in the reconstruction works. The answer was they reduced the lean by approximately 1.5 degrees to just short of 4 degrees and it leans because the ground is very weak and unstable. It was constructed over a substantial period of time which probably saved it in a period when little was understood about foundations. There were a lot of corrective building works which in fact make the tower curved as well as leaning. This is fine as a tourist attraction but you certainly do not want it in your factory!

12 Tips to help you understand foundations

1) What weight will the ground take? Core samples must be collected from key areas for any proposed load bearing application whether green, brown or developed sites. If you don't have the results of this sampling you are guessing.

2) Pick the correct professionals, they are faster and right. These samples are then analysed and a ground bearing capacity assigned to them and some sums, that could look like the ones below,may be done relative to the location and proposed work. If you don't understand them you probably require professional help.

Moment causing rotation

= load x lever arm = [(q - qo) x B] x [½B]

Moment resisting rotation

= shear strength x length of arc x lever arm= [s] x [p.B] x [B]

At failure these are equal:

(q - qo ) x B x ½B = s x p.B x B

Net pressure (q - qo ) at failure = 2 p x shear strength of the soil

This is an upper-bound solution.

3) Decide what weights and loads your process imposes. This is just the beginning. It now goes to the structural or consulting engineer who uses the data to construct a suitable specification for the base building materials. You are never going to see these. Before this can happen they need to know what weights and loads are to be placed on them and how. Its all a bit like a computer, there are several levels between you and the plug in the wall and what you can do on the key board. It is this that is so frequently missed and simply not understood.

4) You need calculations. Once the soil conditions are understood a technical design can proceed. There are pages of calculations and scenarios required to do this. When you take over someone else's property or building you assume the floor won't collapse beneath you and thanks to building regulations it probably won't.

5) Research as much as you can from existing archived information. In order to start a building project from the ground up, the first thing you need is a floor that will take the weight. To do this you will need to trace back the history of the build to the original core samples and calculations, not the drawings of the slab and foundations but the two prior stages. The architect won't have them but may know where you can get a copy. There should be a set lodged at the local authority if your building is less than 10 or 12 years old but if you can't find them its back to item 1 again, core samples.

6) Understand good ground conditions. There are many ways to construct foundations. Before you can commence you need to support and stabilise the ground into which they are going. Generally speaking you will need more than 50 kNm2 ground bearing pressure and most fabricators cover themselves by stating minimally 150 kNm2. The fact is that without this information building regulations won't allow you to proceed or worse still will allow you to proceed and then condemn the building.

7) Organise good control procedures If a floor in its current condition won't take a load you can open up the ground and strengthen it. This is a good way of doing things because you can see what you are dealing with. However disturbing the ground can be fraught and if it is brown ground or very soft ground you might as well bury your wallet in it because the costs will just spiral.

8) Stabilisation options. With the right type of ground you can stabilise it by adding man made resins. This bonds the soil material together and self seals itself so damp proof membranes are less of a problem. As it is the engineer's equivalent of keyhole surgery there is little or no mess and it is very quick. It is cheaper than piling and the ground can be used straightaway once the process is installed.

9) Get to grips with the basic site. With good ground or new works it is always going to be cheaper to conventionally construct foundations. If the ground is poor, restoring the strength is going to be expensive. It is always dirty and takes a minimum of 15 days before you can really use it.

10) What you should be concerned about. Foundations can, and do, cost as much as the intended build on them. They can be a bottomless pit for money and beware of anyone who does or says any of the following:

a) Oh planning won't be a problem - you cannot say that until it is awarded.

b) I don't think building regulations will apply - building regulations always apply.

c) I have already done the building regulation drawings for you - why would you do that? They are supposed to be submitted after the works are complete, as constructed.

d) Well first we will go for planning approval, that will only take 4 weeks - in your dreams!

e) Your building project can be done in less than 6 months - highly unlikely.

f) What are percolation tests? - these test what happens to surface water and are an indication of the drainage requirements and also are used to check they are working properly, for example French drains. The local authorities will expect to see drawings and the results of these tests and might make you dig the drainage up if you can't prove them.

g) Oh the builder will sort the foundations out and you won't need planning for this - this is a cavalier approach, almost guaranteed to cost you extra money. If the foundations have not been done the builder is not going to be in a position to provide the calculations required by the local authority.

h) I know this man who can do that cheaply for you - in this case I would find out who he is and go and see some works he has done cheaply for other people.

i) Doesn't know anything about your business - or want to - an indication that he is working to a hidden agenda.

j) Doesn't quote, advise or know about any of the content of this article - deeply suspicious - avoid.

11) Legs - Finally if you are going to have anything with legs or pillars involved you have to know how much weight it will put on the floor and be sure the floor (slab) can take it. You may be asked to mathematically prove it. If it fails load tests you will need a block of concrete or construction to support it in the ground. I have seen this crop up as an expensive afterthought on dozens of occasions - don't get caught out.

12) The difference between planning and building regulations

If you did the building regulations drawings they would be wrong. The only reason for doing them prior to planning consent is ignorance or increasing fees to the unsuspecting client. Either way your money is down the drain. Building regulations are the main area of flexibility, whereas planning is much more restrictive. Why?... because the engineers run that part so the build may have to vary, but it still has to comply with regulations because planning governs shape and building regulations deals with method and content or put another way planning is about 'what' and the building regulations are about 'how'.

My customer took 5 months to get planning consent with a book of conditions that his professional adviser would really struggle to deal with. If he thought he should do building regulations drawings first he is unlikely to understand or take seriously the consent terms.

Horror story tip

If you think none of this happens I have a customer who was told by his "professional" adviser (who had full professional memberships and accreditation with internationally respected Royal Charter Institutes) that he was going to get planning approval in 2 weeks prior to the government's re-organisation of the local authorities in 2009. He couldn't make certain changes because he had already done the building regulation drawings so the work had to be built the way he had drawn it. This is what actually happened:

• There were serious issues with the highways department that our "professional" knew nothing about.

• He had no idea about groundworks and hadn't even had a ground survey done.

• He obviously had no idea of how to approach a planning application.

• He seriously underestimated the timescales.

If he did know all this then the implications are even worse because he knew what he told my customer was wrong. I would like to say this is rare but it is far from rare, it is common place to tell the customer what they want to hear to get their work. NEVER ever get building regulations drawings done prior to planning approval because at that stage you don't have:

• Permission to build

• Technical designs

• Planning conditions

• Ground or environmental survey information

The epilogue - How to do it right - A Good Method

8 months later his project had still not concluded. It should have taken 12 weeks to plan, 8 weeks to build and 4 weeks to fit out. Unfortunately 6 months was too slow for my customer and then when someone told him it could be done in two weeks it was time for us to walk away. This is very common. This is what should have been done.

• The newly re-organised authority should have been contacted straightaway. They would have had better facilities anyway. Getting snarled up in the changeovers with the demised departments guaranteed this project was going to fail to meet expectation from the outset.

• Consult the highways department where parking or road access is an issue.

• Consult the client with a suggested range of solutions.

• I would then have taken a set of discussion plans (really cheap rough information) and tested the local authority for other lurking problems.

• Don't do the ground survey until there is a strong chance of obtaining planning consent. The survey usually is the lesser of the two costs but you may find the likely terms of consent exceed your budget expectations in which case the survey money is wasted in the event you abort.

• Work backwards from the finished article/design to ensure your ground designs are right. If you think design work is expensive and time consuming wait until you try demolition!

• Only when all this is agreed should there be a move to a planning application. Why? Because if you want a door or window moving or changing, it is a new application and/or associated cost. It could even delay or abort the project so always keep the exit route of least cost open

• Nail down contractors, suppliers and work schedules with fixed price contracts.

• Check for hidden costs and delays. Obviously you need to know where these are likely to occur but that is one very good reason to implement regulations prior to work beginning

• Once the work is completed to the planning terms, then and only then, are the final "as built", building regulation drawings done. Up to this point you will be using manufacturer's drawings derived from the planning consent drawings which is why you never do the building regulation drawings otherwise the cost of revisions would be enormous. One of my projects had 31 drawing revisions prior to the building regulation drawings. Revisions are normal, they are there to save costs and not to incur them.

• You can't change the procedures. The planners make sure you don't try because many people depend upon them. Their job is to help everyone safely use the system and benefit from its supporting role. This attitude is what gets applications through the quickest.

The moral of all this is if it sounds too good to be true it probably isn't true.