Had the head of an archaeology department told me he had never excavated a posthole, I would not have been the least surprised. Many academics may have little contact with real soil, and may not even get muddy at work. Apparently it's not part of their job description. So they may never have enjoyed lying on the ground with their arm lost down a deep and narrow posthole, excavating mostly by feel, and struggling to get the loose soil out from the bottom of the hole (an old fashioned ladle is ideal), which may account for the lack of scholarship on the subject.
So, for the sake of all those calluses and rheumatic knees, we have to get to grips with interpreting postholes, and the building and structures they represent. Thinking of postholes as foundations is an important part of the mindset of theoretical structural archaeology, and so it is necessary to consider how this type of foundation works in principle.
When we walk in soft soil, our feet sink in, displacing and compressing the material, to a depth where it is resistant enough to take our weight. Buildings' foundations [or footings!] are usually dug to a level that will support the load from the outset, to avoid the distortion and collapse of the ground represented by a footprint. However, it's important to understand, just as your foot does when you continue to stand in the same spot, foundations will continue to compress the material below them for some time, sinking or ‘settling’ until equilibrium is reached. It is worth pointing out, for completeness, that the larger the area of the bottom of the shoe, the less it will sink in, and a larger area of foundation will put less load on the soil below it.
An important distinction between feet and buildings is that latter are ‘static’, and just sit there, whereas your foot moves, and is therefore ‘dynamic’. Obviously, the ‘dynamic load’ of landing in the ground, has a different effect from ‘the static’ load of just standing still. However, foundations do have to deal with ‘dynamic loads’, since buildings are affected by the wind, and may accumulate many tonnes of snow and ice, in addition to any loading imposed by the activities of the occupants.
In a modern context the design of foundations is governed by a series of agreed-upon formulas and tests that can be performed on the soil. This allows the engineer to calculate the point at which the soil is likely to break, or rapidly distort, when a load is applied to it; a ‘safe bearing pressure’ for a foundation is then back-calculated by applying a generous safety margin, perhaps 300%. An acceptable degree of settlement is also an important consideration in this calculation.
Settlement is a key issue; it is inevitable that the foundation will sink, mostly due to pressure's squeezing out water from between the soil particles, a process that may take years. While it is difficult to be precise, in modern structures 0.05m of settlement would probably be acceptable. However, what is vital is that the foundation sinks evenly; thus consistent underlying ground conditions are most important. This is why it is important for foundations to be at the same depth. Foundation design becomes more complicated if there are variations in the ground across a site, or if the superstructure has elements with different heights and mass. Seasonal variations in ground conditions can also be a significant factor. Changes in the water content of the soil, perhaps a rise in the water table, can completely change the nature of the site, highlighting the importance of drainage.
What any builder should be trying to achieve is a stable base for a structure, so that it will settle evenly and remain level, or ‘true’, throughout the life of the building. Gravity tends to deal harshly with anything that does not abide by the basic rules about staying perpendicular and level.
A 0.3m diameter post has the potential to support the weight of between one and four Ford Fiestas, depending on the soil below it, and allowing for a reasonable degree of settlement. On a site like Orsett, with 850 holes cut into compact gravel, it is an interesting concept to try and visualise four Fiestas balanced on each post, or perhaps two Mondeos with a Fiesta on the top would be easier!
Using experimental constructs, or theoretical values, we can back-calculate the sort of static loads that were regarded as ‘safe’ in prehistoric terms, especially if, as I suggested in the previous article, there is a fairly constant ratio between floor area and the number of posts supporting the roof. Precisely, how this limit is understood, explored, and refined, is central to the development of building technology in the 1st and 2nd millennia BC, and will be discussed in later articles.
The prehistoric builder, regardless of any personal religious convictions or world view, was subject to the same laws dictated by the force that has made a universe in its own image, Gravity. We are justified in assuming that prehistoric builders had developed a system of engineering; the fact that substantial structures were built at all in prehistory proves that the basic principles were understood. The evidence is also inherent in the building plans, and it has been observed that the postholes forming a structure are usually the same depth. This is a key observation when associating postholes with a structure: that depth is the most important factor, and, for example, the postholes representing the ring beam in a roundhouse should be the same depth.
In the previous article we saw how the diameter of the posthole can tell us quite a lot about the post and the tree it came from. However, the fact that trees and the timbers they produce have a taper means that a long horizontal timber will have a thick end and a thin end, and the posts supporting it may reflect this. It is something of a theoretical consideration, but for simplicity of carpentry, the size of the post may be matched to the diameter of the timber it supports.
To enter the world of theoretical structural archaeology and share its view of prehistoric built environment, these are the first keys to the kingdom:
- Posts set in postholes of the same depth will support each horizontal element in a structure.
- The depth of the posthole increases with the height of the post.
Using what you have just learnt, answer the following:
- (i) What structural principle is being broken in A?
- (ii) Suggest an alternative interpretation of the building section in B.