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Ground works

The Building Regulations cover aspects of ground works in part C

Remediation

If you are building on a brownfield site or if the ground contains alien plant species then remediation may be necessary.

Ground Movement

The Building Regulations have a section in the Approved Documents part A – Structure, covering land which may be prone to movement.

Subbase

WRAP have an overview of Subbase and hydraulically bound materials (HBM)  which covers energy and material savings with the following:

  • Treating existing soil to make HBM subbase/ballast
  • Recycled/secondary aggregates to make HBM subbase
  • Recycled/secondary aggregates as unbound subbase
  • Geogrids/geotextiles to reduce thickness.

Top soil and contouring

There needs to be a certain amount of planning as to where to store topsoil while the building work is in progress. It is important to carefully keep topsoil from mixing with subsoil. If there is a surplus of subsoil then it may be possible to ‘lose’ it around the site by slightly raising ground levels rather than paying the hefty price of taking it away by skip or lorry. On sloping sites you may be able to use cut and fill, especially if a basement is proposed but remember that you cannot place foundations on disturbed soil.

In some cases geotextiles may be useful for maintaining separation between different layers of ground material while still allowing drainage. For instance they can prevent top soil getting mixed in with an underlying drainage layer of gravel or hardcore. In the case of Segal construction they are often used to separate a top layer of gravel or crusher run from the subsoil beneath (under the building).

If flooding is a potential problem then raising the level of the land may be an option (and in fact one which builders are increasingly carrying out, especially if land remediation is necessary at the same time). The Environment Agency have an online map called Risk of Flooding from Rivers and Sea

Trees and tree roots

Local authorities can issue Tree Preservation Orders. TPOs can prohibit the:

  1. cutting down
  2. uprooting
  3. topping
  4. lopping
  5. wilful damage, or
  6. wilful destruction
of trees. The cutting of roots can also be considered damaging to a tree.

The Planning Portal has advice on tree protection and also covers hedges. Also the online guidance Trees and development – A guide to best practice.  There are considerable Penalties for non-compliance with tree preservation orders

see also:

Information regarding which trees are subject to TPOs and which trees are to be retained on a building plot as a condition of a planning approval can be found through the local planning authority.

Retaining walls

If it is necessary to do serious contouring of the site there are four main green approaches to retaining walls

Gabions

Gabions are basically boxes made of steel wire or mesh and filled with stones. They are then stacked on top of each other and lashed together with wire to hold back the ground behind. The stones are usually hand placed and can be quite decorative. It is even possible to get gabions which have compartments so that you can have a superior facing stone to the front and cheaper stuff behind. The metal is usually heavily galvanised and sometimes coated in PVC as well. The work is fairly unskilled. You can also add soil in with the stone and then use the ledges for planting. Betafence Gabion Solutions have some good pictures of gabions as retaining walls

Reclaimed sleepers

If reclaimed railway sleepers are available then it is possible to anchor the bottoms in a strip of reinforced concrete and bolt the tops to a top rail of sleepers. A structural engineer will advise on bolt types and size and on what angle to set the sleepers at. They are also suitable for low retaining walls such as for vegetable beds by simply laying them flat on top of each other with some overlap at the joints.

Crib systems

These are interlocking timber frameworks which are filled with soil and planted. They are made of treated timber and checks should be made that the timber is woodmarked. They can be more decorative than the other types of retaining wall once the plants have put some growth on.

Dry stone walls

Ciria do an excellent free study on dry stone retaining walls including repair of existing ones. You may have to register (free)

Soil nailing

As building land gets scarcer, more radical techniques are necessary to utilize difficult plots, especially steeply sloping ones. In some cases soil nailing may be appropriate.

Lidgett Lane

The image above shows soil nailing carried out on a very steep plot. Along with this, the land had been supported by an in-situ cast concrete retaining wall which provides the rear wall to a garage area. Also, gabions were used to help support the neighbour’s garden at the top of the slope.

retaining wall4

Gledhow retaining wall

Carbon footprint

The embodied energy of ground works can be very high, especially where concrete and oil based geotextiles are involved. There is a useful study by WRAP  which compares traditional and low energy approaches, giving examples. Although these tend to be on a much larger scale than most self builders might require, the principles are still the same. Some of the examples such as Crib Wall, Ash Vale, Aldershot are directly applicable. It also has some useful drawing details and costings.

Setting out

soil heaped in corner, hrdstanding laid and profile boards in
soil heaped in corner, hardstanding laid and profile boards in

Setting out is one of the more difficult parts of the building process but it is extremely important to get it right. If you get it right everything else can follow on smoothly. Get it wrong and major problems will occur such as –

  • the house gets built in the wrong place and needs taking down and rebuilding (this happens! especially if it contravenes planning stipulations)
  • you get the levels wrong. This can really hurt if there is not much fall available to the drains. Sewage will not go uphill! It can also badly upset the relationship between parts of the building which are at different levels and particularly if you are adding an extension where the adjoining floors need to be at the same level. The plans you are working from will probably only mention the invert levels of drains and sewers and not say much or anything definite about how high the house is with respect to anything else such as neighbouring houses or roads.
  • things go out of square so that what were meant to be right angles go adrift (now, interestingly, the Georgians and Victorians were not too fussy about this as long as it was all nearly square. This is why kitchen units often don’t quite fit perfectly into the corner of the kitchen with older houses). If you are adding an extension or modifying an older house you may well find you are having to do a bit of fancy footwork to keep things lined up.
  • you get the levels of the foundation tops slightly wrong and you end up having to trim all the below-ground concrete blocks to reach to the correct level. (There is a precise distance between the top of the concrete footings and the level of the damp proof course and this needs to be got right. For instance it might be 2 full blocks and 3 courses of brickwork).

There are two parts to setting out

  • horizontally, using profile boards     see more......
    This tells the JCB where to dig the foundation trenches. The procedure for setting out horizontally is usually done by knocking wooden pegs into the ground to mark out the positions of the foundation trenches. A nail is knocked into the top of each peg to get the exact position.
    The starting points for the first pegs are determined from the drawings which will show dimensions from certain set features such as a boundary fence, the middle of an existing road or the corners of existing buildings etc. Remember that these dimensions will be horizontal ones so if slopes are involved then you will need to take account of them using a level. You can then work out the positions of the other walls from the drawings.Right angles can be formed using 3/4/5 triangles and squareness can also be checked by making sure diagonals are the same between both sets of corners. Given that the pegs will get dug up when the digger gets started, the next step is to put profile boards in place about a metre away from the pegs and sawing a small notch into the board to mark where the line goes. This can be done by tying a nylon line to the peg at the opposite end, stretching it along the line of the proposed wall, lining it up over the nail and then marking where it is on the board.


  • vertically, using levels     see more......
    This establishes how deep to dig. Very often there is no precise information on the approved drawings as to the actual vertical position of a building!  However there may well be a reference to the invert level of a nearby manhole. (The invert level is the level of the bottom of the pipe at that point). This gives you a small amount of leeway regarding levels which can help with getting the slope of drains to work correctly and also it may help with adjusting the surrounding ground features so that the DPC is always at least 150mm above ground level. The gradients for drains can be found here in the Building Regulations, Approved Document H.
    First you need to establish a datum. This is something on the building site which is not going to move or get moved (maybe a solid gate post or stake). Mark a point on it that is preferably visible from all over the site and this point is then given the notional level of 100m. (so that you are not going to get into negative numbers which can get confusing). Next, hire a dumpy level and calibrated staff.  You can then knock wooden pegs into the ground to establish levels at key places such as corners of the building, invert levels etc. These pegs should be close enough to the key points that a levelling board can be used for the local measurements to be made.As the digging of the foundations proceeds you can check the level of the trench bottom to make sure it is correct. (There is a precise distance between the top of the concrete footings and the level of the damp proof course and this needs to be got right. For instance it might be 2 full blocks and 3 courses of brickwork).


Bear in mind that a digger needs to be able to manoeuvre around the site without damaging the profile boards and leveling pegs so a bit of thought needs to go into the planning of how the digger will get around.

Foundations

The drawings you have submitted to Building Control will state what foundations are required and this will depend on the loading due to the superstructure and the ground conditions.

The Approved Documents part A, (England) give the design loads and minimum widths for concrete foundations on various types of ground. See Section 2E

Your engineer may well have requested one or two trial holes to be dug beforehand to establish what the ground is like and their calculations will be based on this. If there are trees close by then this will affect the depth of foundations needed. There is an on line calculator which indicates the minimum depths where trees are concerned. The Building Regulations – Approved Document – A covers basic concrete foundation design and gives sets of tables for how soil conditions influence the foundation dimensions. As the digging of the foundations proceeds you may find that the ground conditions vary from the assumptions based on the trial holes. The building inspector may ask for changes to be made.

For instance you may come across changes in the soil type, tree roots, water courses, remains of previous constructions such as cellars etc. and this may require a change to the design of the foundations. Small variations such as a change in the firmness of the subsoil may be dealt with quite easily by referring to the regulations mentioned above. Hitting a filled in cellar may involve considerable foundation redesign by the engineer. Occasionally it may be useful to have a survey done using ground-penetrating radar (GPR). This can detect tree roots, cavities and services.

Strip vs. trench fill

Foundations are sometimes loosely referred to as footings but strictly speaking footings refer to the way walls were splayed out at the bottom before concrete was commonly used. You may well come across footings if you are excavating adjacent to walls built in the early part of the last century or before. The two methods in use now (apart from using a concrete raft to build off) are strip foundations and trench fill. Both are covered in the building regulations part A1/2  with minimum sizes for solid concrete foundations in section 2E on page 34. The building inspector will normally want to see the trenches before concrete is poured.

Trench fill uses a lot more concrete which can work out expensive and it accounts for much more embodied energy. It tends to be more expensive than strip but is faster and therefore there is less risk of flooding occuring between digging and pouring the concrete. It also requires more accuracy in digging the trench, otherwise a good deal more concrete can be wasted. Strip foundations usually have steel reinforcing bar in them and that will be detailed on the drawings.

Block foundations  

Block foundations are typically used with Segal timber frame construction as the base for timber posts. With block foundations which might typically be 600 mm square by varying depths (depending on ground conditions) the digger may not be able to form a square hole: it will be longer in one direction because of the swing of the bucket. Some 19 mm. ply cut into 600 mm wide sheets can be used at either end of the short trenches to block off the ends.

Piles.

Under certain circumstances it may be necessary to use piles. This might be because of layers of soft or unreliable strata such as clay or sand or because of previous disturbance of the land due to workings or old basements etc. or to the proximity of tree roots.

Piles are usually concrete cylinders of about 200mm diameter and up to many meters deep, usually driven into the ground by a small tracked machine. They will be spaced a few meters apart and it might take 2 or 3 days to complete the work. A variation on this is screw pile foundations which are steel screws inserted into the ground. These may prove to be a greener option if they can be reused at some time in the future.

Underpinning

Underpinning is usually carried out on existing buildings when foundations are inadequate or there is some kind of ground subsidence. It is specialist work which involves excavating under the existing foundations (usually in short sections at a time), temporarily propping the existing wall and then placing a considerable amount of concrete under it. It can also be carried out when it is necessary for a wall to span an existing drain or sewer.

Dangers and problems with trenches

There are considerable risks around digging foundations, especially if they go deeper than about 1.2 m. Collapsing trenches are a major source of accidents in the building industry. Even with shallow trenches, if a person is working in the bottom of it, say setting out reinforcement, then a sudden collapse of a trench side could be fatal. Trenches over 1.2 m. deep which people have to work in will usually need specialist supervision and  shoring. See the HSE legal requirements on this. And here

Two of the other main risks with foundations are

  • that the trench sides collapse either as the digger is working or afterwards
  • that they get flooded due to rain.This will also probably cause the sides to collapse

To satisfy the building inspector the bottom of the trench needs to be clean and firm to ensure that the concrete goes onto solid ground. If bits of loose earth fall in they will need removing by shovel. The trick is to get the ready mixed concrete laid as soon as possible after excavation (and inspection) and this requires that any steel reinforcing goes in quickly, any special openings such as where drains might need to go through are dealt with and that the concrete is delivered on time.

In case of heavy rain it may be necessary to get hold of a pump at short notice and in the situation where trench sides are loose and tending to collapse it may pay to be able to get hold of some form of disposable shoring. Old sheets of hoarding ply used as shoring can sometimes save a good deal of concrete. This is especially true for block foundations such as those often used with some types of post and beam construction such as Segal.

see also the NHBC Good Craftsmanship guide to Foundations, Substructures and Ground Floors

Levels

The levelling of the bottom of the trench should be checked regularly with a graduated staff or other means to make sure the digger driver is on course.

The level of the top of the concrete when poured can be checked by knocking wooden pegs into the bottom of the trench and trimming their tops to the finished level. Best to err very slightly on the low side with the finished concrete level as it is easier to slightly raise the blocks which sit on the foundations than to trim them.

See ‘Setting out’ which covers the importance of getting the distance right between the top of the footings and the DPC to ensure you get a set number of whole blocks and bricks to fit in correctly. (In the case of post and beam timber systems such as Segal, the top levels of the concrete blocks are approximate, 600 x 600 x 50 concrete paving slabs are laid into the concrete before it cures and the timber posts are trimmed accurately to length later).

Drains

There is some useful background on drains and sewers on the Directgov web site

The legal requirements for drains are dealt with in detail in the Building Regulations, Approved documents, Part H, which give a very thorough explanation of the requirements, so much so that they can almost be used as a design manual for drain construction.

Essentially there are two types of systems; ‘separate’ and ‘combined’. Newer systems are separate and have two sets of pipes (one for surface rain water and one for foul water). The older type are combined where all the water goes down one drain and into one sewer. The separate systems are better because the rain water is taken directly to a river and doesn’t cause flooding at the sewage works where the foul water is being treated. You need to establish which type of system you have. The local council Mains Drainage department can help with matters such as the type of system in the area and plans of drains and sewers.

Local sewers and drains may be ‘private’ or ‘adopted’ (ie under the control of the local authority). All work involving drains needs notifying to the building inspector

Surface rainwater and SUDS

The Building Regulations part H place emphasis on allowing the surface water to drain into the ground locally where feasible. This is in line with SUDS policy. The full details of the Approved Document covering surface water drainage are available here

SUDS is an approach to dealing with rainwater which is particularly relevant to urban areas where there are increasing amounts of hard impermeable areas which tend to send rainwater straight into the drains and thence to the rivers rather than letting the water soak into the ground and get slowly released. During very heavy rainfall this extra runoff overwhelms the drains and causes flash flooding when sewage overflows into becks and rivers. During periods of drought, tree roots can suffer. The idea of SUDS is to hold back the water locally and release it slowly. There are various ways of doing this

  • Living roofs – which delay the speed of runoff
  • Permeable surfaces such as drives and car parking. Several companies manufacture pervious interlocking systems from fully recycled plastic in a variety of grades depending on bearing requirements. The gravel infill can be from recycled crushed hardcore, slate chippings etc.
  • Soakaways
  • Ponds
  • Wetlands
  • Lawns – which may be allowed to partly flood
  • Attenuation tanks. These are being increasingly required for new build
See also the Susdrain site

With these methods there is sometimes a risk that ground materials might get mixed together over time and this could become a problem. For instance a layer of soil over a layer of hardcore may eventually find its way down into the hardcore and block up the drainage voids. This can be avoided by inserting a layer of geotextile beneath the soil. It is usually a strong non-woven plastic based fabric which allows water to pass through but not soil or sand.

Water saving and SUDS

It has been described as ‘a marriage made in heaven’. Carried out correctly, saving water can work very well with SUDS. SUDS can be implemented using attenuation tanks which hold back storm water and allow it be released slowly. The same tanks can store rain water for the purpose of toilet flushing, washing machines, car washing and garden watering (all uses where wholesome water is not required). There are lots of companies now offering such tanks along with size calculations. See the UK Rainwater Harvesting Association Briefing Paper.

Gutters and rainwater pipes are dealt with in the Approved documents part H3 section 1.

Foul water drainage

Foul water (black water from WCs and sinks,  and grey water from baths, washing machines etc.) is normally discharged into a sewer, septic tank or digester. (but see composting toilets).The Building Regulations part H 1 section 2 have a lot to say on the subject and can pretty much be used as a design guide.

Domestic sewerage pipework and fittings have been largely dominated by uPVC suppliers. PVC has several drawbacks as a material and there has been a recent revival in the use of clay drainage pipes from such companies as Naylor Drainage and Hepworth. The clayware pipes have plastic joints which make them easy to connect and they are more robust than plastic pipes though more expensive. Other drainage products such as inspection chambers are available in HDPE.

Granular backfill for drainage pipes may be available as recycled aggregate.

2 comments to Ground works

  • bobthebuilder

    Hi Julia,
    we are intending to write an article quite soon on the principles of the Segal method.

    Briefly the idea is that it should be:-
    1. Easy for anyone who can use simple tools such as a saw, drill, hammer etc. No wet trades such as plastering or bricklaying.
    2. All materials (such as sheets of ply etc) to be used in the sizes they are supplied (so no cutting or waste is involved except some cutting of posts and timber members) This implies a tartan grid plan to allow for sizes of posts.
    3.Timber posts stand on concrete block foundations rather than strips.

    There are numerous developments have come about over the years since the method was first used. These include high insulation standards and draft proofing.

    Keep an eye on the Segal page on this site for future information.

    Bob.

  • Julia McMaster

    Hello,

    we are shortly planning to build an outhouse and are very interested in the Segal method. There seems to be little information on the internet on construction details, would you be able to tell us more, etc. distance of the upright posts, sheeting material, dampproof course etc? It would be much appreciated.
    With many thanks,
    Julia McMaster

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