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Concrete Strip Foundations

 

Excavation for the Foundations

 


Work Included in this Section

Foundation Excavation and Leanmix Concrete backfill to Softspots. The work in this section is covered in the Technical Guidance Documents (TGD) Part C.


What You Will Need to Do


Ensure the Foundations to be excavated are set out accurately and double checked.


Ensure your Engineer will be available to examine and certify the foundation excavation prior to any concrete being poured.


Ensure any Instructions issued by your Engineer, such as digging out softspots etc. are followed exactly.


Ensure that before excavating, care is taken to identify and clearly mark any underground pipes, drains or cables and if it is necessary to operate machinery beneath overhead power lines the ESB is notified to ensure that the lines are disconnected or other precautions taken.


Ensure the foundation excavation is taken down to a firm base and that all soft spots and rubble are removed and backfilled with leanmix concrete prior to pouring foundation concrete.


Ensure that if the foundations become water logged a sump is formed and all water is pumped out away from the foundations.


Ensure that if it is not intended to use the excavated materials at a later date for siteworks they should be transported to a licensed tip.

 

What to Look Out For


Poor ground conditions, as problems can be expected with loose sand and loose silty clayey sand, soft sandy silty clays, peat, fill or made up ground.


Excessively wide trenches can cause considerable cost overrun due to the amount of additional concrete required to fill them. If the ground is inclined to give way you should provide shuttering to the sides of the concrete to minimize waste.


If you decide to use the excavated soil later, and to avoid double handling and the associated extra costs, ensure that the excavated soil is placed in spoil heaps where it will not interfere with the construction work.

 


What You Will Need to Provide

 

Provide leveling and setting out materials, such as timber pegs, lime, builder’s line, nails, cement, sand, etc.


Lean mix concrete, if required, to make up levels and fill excavated softspots


Pumping Equipment if necessary.

 

Watching the Pennies


Ensure that the digger driver is using the right size bucket on the digger to match the specified width of the foundations. If the bucket is too wide, the trench in turn will be too wide and this will lead to extra costs for concrete waste or unnecessary formwork.

 

Ensure that the trenches are not over excavated below the formation level as any over excavation will have to be made up in leanmix concrete.


If possible the excavated materials should be used at a later date in the landscaping siteworks. This will save on expensive transport and tipping charges.

 

To avoid double handling and the associated extra costs, ensure that the excavated soil, if being kept on site for re use later, is placed in spoil heaps where it will not interfere with the construction work.

 

Concrete to the Foundations


Work Included in this Section


Concrete and Reinforcement to Foundations.


Concrete used in foundations for housing should be a minimum strength classification of C12/15.


Lean mix concrete used for filling to softspots and blinding under foundations should be a minimum strength classification of C8/10


Some foundations depending on ground conditions may require stronger concrete mixes than those mentioned above and may also require steel reinforcement. It is essential that the Engineer details the concrete strengths and where and how he wants the steel placed for such foundations.


Foundations should be provided for all block and brick walls, chimneys, columns & load bearing stud partitions.


Minimum Foundation Dimensions


Depth:
The minimum depth of foundation excavation below finished ground level should be not less than 600 mm


Width:
The minimum width of foundation should not be less than three times the thickness of the wall it is supporting.


Thickness:
The minimum concrete thickness of foundation should not be less than 300mm


These are minimum dimensions for standard foundations. Foundations for chimneys and other large structures may need to be more substantial. Your engineer should be at hand to make these decisions.

 

What You Will Need to Do


Appoint a Concreting Subcontractor if work is not already being carried out by the previously appointed Groundworks Contractor.


Ensure the concrete foundations to be poured are set out accurately and have been double checked.


Ensure your Engineer will be available to examine and certify the foundation concrete being poured.


Ensure any Instructions issued by your Engineer, such as increasing or altering reinforcement are followed exactly.


Ensure the bottom of the foundation trenches are dry, level and compacted before the concrete is poured. Any ground water should be allowed to discharge to a sump and be pumped away from the foundation area.


Ensure the top of the concrete foundation is level when it is poured. If it is necessary to have steps in the foundation ensure they are arranged to suit the coursing of blockwork in the rising walls.

 

What to Look Out For

 

Weather conditions: Concrete work should never be carried out in freezing conditions as the concrete may not set properly.


Excessively wide trenches can cause considerable cost overrun due to the amount of additional concrete required to fill them
.

 
What You Will Need to Provide

 

Provide leveling and setting out materials, such as timber pegs, lime, builder’s line, nails, cement, sand, etc


Lean mix concrete, if required, to make up levels and fill excavated softspots.

 

Foundation concrete.


Mesh or Bar reinforcement complete with tying wire and spacers, if required.

 

Timber for formwork, if required.

 

Watching the Pennies

 

If the trenches are wider than required due to having been over dug, or trench sides collapsing, use formwork to save wasting concrete unnecessarily.

 


Detailed Costs

 

Get detailed costs for this section of your project here


Rough Guides

 

1.00m3 of concrete in a 1200 mm wide x 400 mm thick concrete strip foundation will cover a length of 2.08m (No allowance for waste or over pour allowed)

 

1.00m3 of concrete in a 900 mm wide x 300 mm thick concrete strip foundation will cover a length of 3.70m (No allowance for waste or over pour allowed)

 

1.00m3 of concrete in a 750 mm wide x 300 mm thick concrete strip foundation will cover a length of 4.44m (No allowance for waste or over pour allowed)

 

1.00m3 of concrete in a 600 mm wide x 300 mm thick concrete strip foundation will cover a length of 5.55m (No allowance for waste or over pour allowed)


Other Types of Foundations
 

Raft or Pile Foundations

 

Your site conditions may require other foundation types such as raft foundations, pile foundations etc. As these foundations are a specialist form of construction it is essential that they be designed and supervised by professional engineers.

 

 

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