Footings and Foundations in Kent

A strip footing puts a shallow band of concrete in the bottom of the trench and builds blockwork off it up to damp-proof course level. Trench fill brings the concrete almost to the top of the trench and does away with most of the below-ground blockwork.

Trench fill uses considerably more concrete. It also takes far less labour and time, gets the site out of an open excavation sooner, and removes the risk of a trench collapsing while somebody is working in it. On deep foundations that trade usually favours trench fill, and on shallow ones it often does not. It is a decision worth making before the concrete is ordered, because the two options need very different volumes.

The building control inspection comes first

Under the Building Regulations, foundation excavations are inspected before they are covered up. That is the point of the exercise: once concrete is in, nobody can see what it went onto.

Give notice to building control, have the trench inspected, and book the concrete around the result rather than ahead of it. An inspector can require the trench to go deeper, and a load batched for the original depth is then the wrong load. This is the single most common reason a foundation delivery has to be moved.

What sets the depth

Foundation depth is set by the ground, not by the concrete. Approved Document A of the Building Regulations covers the ground conditions that matter, and building control confirms the depth for your particular site.

Two factors drive most of it. Frost affects the upper layer of the ground, so a footing has to sit below the zone it reaches. And shrinkable clay swells and contracts with moisture, which is why foundations near mature trees usually go deeper than those on open ground. Where a site investigation exists, its findings govern. Where none exists, building control’s judgement on the open trench is what you work to.

Sulfates and the mix designation

BS 8500 has a family of designated foundation concretes: GEN1 for mass concrete fill in non-aggressive ground, and the FND range, starting at FND2, where the ground carries sulfates that would attack an ordinary mix.

Which one you need is decided by a chemical assessment of the ground, not by guesswork, and the result appears as a design sulfate class. If a ground investigation report exists, the class is in it. If it does not, your approved supplier will ask what is known about the site before batching, because supplying a standard mix into aggressive ground is a failure that shows up years later.

Working out how much concrete a footing needs

Trench volume is length times width times depth, in metres. A trench 20 metres long, 600mm wide and 900mm deep is 20 x 0.6 x 0.9, so 10.8 cubic metres.

Trenches are the pour type where the calculated figure and the real figure diverge most. A machine never digs a perfectly parallel trench, soft spots get dug out and made up in concrete, and a trench that has stood open through rain takes more than a fresh one. Allow a margin, and describe the trench honestly to your approved supplier rather than quoting the drawing at them.

Getting a mixer to a trench

Foundations are poured early in a build, which is usually the point at which a site has the least finished ground to drive on. A full-size mixer is heavy, and it will sink into ground that took a digger without complaint.

The practical questions are always the same: can a loaded lorry get within chute reach of the trench, is the ground firm enough to carry it, and can it turn or reverse out again. Where the answer is no, the pour goes in by pump or by a smaller vehicle making more trips. Kent Readymix asks about ground and access at enquiry stage so that an approved supplier sends something that can actually reach the work.

Placing concrete into a trench

Pump standing water out before the concrete goes in. Concrete discharged into water loses cement from the mix and sets weak exactly where it is carrying most.

Trench fill is normally ordered at a higher consistence class so that it flows along the trench and levels itself rather than needing to be dragged. Say what you are pouring into when you order, because the same strength of concrete can be supplied stiff or flowing and the wrong one turns an easy pour into a hard day. Your quotation, delivery and payment are agreed directly with the approved supplier appointed to your pour.

Getting a foundation pour under way

Send the trench dimensions, the depth building control has agreed, whether you are pouring strip or trench fill, and how a loaded lorry would reach the excavation. Where you hold a ground investigation report, the sulfate class in it decides the designation.

Many of our approved suppliers can deliver next day, and for commercial customers on an agreed contract, delivery within hours is sometimes possible depending on location and availability.