Keeping the cornerstone secure

When talking about sign foundations it is generally understood to mean buried concrete blocks. Other more innovative options exist, such as large steel anchors and Kongcrete for smaller applications, but there is as yet very few solution for the fixing of very large signs into the ground.

Sign foundations will vary according to the mix of a number of different issues, namely windloading, weight, ground conditions, location features and services.

Windloading

This is the starting place for any external foundation design. Small signs with insignificant windloads and overturning moments—leverage exerted on the foundation by the wind hitting the sign—will require only small concrete foundation blocks. But larger signs will need concrete foundations designed to counter the overturning moments. This may consist of a single block or just one block per post. Generally the length, breadth and depth can be varied within certain limits to suit local conditions. For example, a cramped site may require a deeper block where length and breadth are restricted.

However, care should be taken in this instance where a smaller ‘footprint’ can dramatically increase the ground pressure exerted by the overall foundation. Where shallow foundations are needed, more concrete reinforcement will probably be required. Here care over sign positioning on the foundation—weight distribution—together with possible frost penetration issues may need considering.

Weight

Generally, a centralised sign weight will help stabilise the foundation and aid resistance to the overturning moment. However, where weight is applied asymmetrically to a foundation—for example towards an edge or corner—care must be taken that the weight does not stress the concrete to the point where additional high windloads will cause cracking and failure. Asymmetrical loading can also cause higher ground pressure at certain points leading to possible subsidence.

Ground conditions

The nature of the ground, particularly at the base of any excavated pit ready for concrete, may affect the overall dimensions of a concrete foundation. Sandy or loose land, ground with larger roots, or earth with holes due to buried debris will offer less resistance to the loads applied. Compacted ground will offer good resistance and withstand greater ground pressures. Ideally, ground conditions should be assessed prior to calculating the foundation dimensions, but realistically this is often not possible. However, unless signs are very large, the difference between foundation dimensions for loose or compact ground will be small. Therefore, if calculations are based on a worst-case scenario, it will generally suit most conditions and little ‘overdesign’ will occur.

Location features

Where sign foundations are located on reasonably level ground in an open area, the actual location will only be relevant for initial windload calculations. However, if a sign is located near features such as earthen banks, marsh, open water, building foundations, woodland or areas containing buried debris and chemicals, a site specific assessment may be needed.

Each of these features could provide issues which need taking into account. For instance, a foundation located near or on a bank may cause unequal resistance to windloads and weight; this in turn could cause slippage or cracking. If a sign is situated near existing foundations, care must be taken that the additional pressures exerted on the ground by the sign foundation are acceptable alongside those already imposed by any existing foundations. If in doubt, check.

Services

Prior to any mechanical digging, always undertake checks for underground services. A small hand-dug hole is usually the only realistic method to finally ascertain if services are present. If concrete foundations are cast over or near services, consent from the client and/or appropriate bodies must be sought as increased pressure from weight or windload could crack pipes and distort cable runs. Again, check first, it could save a very costly insurance claim.

In conclusion, foundations well be many and varied according to the sign design, consequent windloads and site conditions. There are no ‘one size fits all’ solutions and care should be taken to assess the basics for each individual sign foundation.