31 January, 2009

20. Everything you ever wanted to know about postholes – but were afraid to ask

I was once asked by the head of an archaeological department,“What is the difference between a stake hole and a posthole?” If you don’t know, may I suggest you read my previous article first; this article is for advanced students who have grasped the basic idea: Postholes held posts that supported a vertical load.

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.
The interior of the Iron Age enclosure at Orsett, Essex, with some of the 840 excavated postholes [1]
By contrast, much bad pay, conditions, and weather have been endured in excavating these features, so it is not unknown for ‘f**king postholes’ to be discussed down at the pub by disgruntled muddy archaeological diggers, having spent their day excavating these ubiquitous, and often meaningless, features. However, the thousands of man-hours of excavation and painstaking recording of plans and sections have given us the only record of the prehistoric built environment, even if that was not always understood at the time.

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.

Cartoon By B Kliban [2]
As a general rule we don’t think about foundations; we sit in our houses and the thought of what is supporting it does not cross our minds. Architects and builders worry about that for us. However, as archaeologists, a great deal of what we dig up are foundations; we don’t actually excavate many ‘Iron Age roundhouses’, only their foundations. Luckily, the basic principles of foundation design are fairly simple; the detail is far more complex and mathematical, but thankfully, the people we study did not grasp this, so we don’t really have to either.

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.
It is worth briefly considering what a typical prehistoric post might be capable of supporting on various types of soil. These are very rough figures (so don’t try this at home), but it is interesting to see the range and scale of values for common types of soil.[3] I have only ever used such calculations for working out the maximum safe bearing pressure, in modern terms, for an Iron Age granary, which gave me an upper limit for the capacity of the granary.[1] Remember, what we are discussing is load bearing posts and postholes, and that for much of prehistory, walls were not load bearing and can discounted as part of the structure.
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.[4] 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.

Bersu’s 1940 sketch section of Little Woodbury Roundhouse, showing taller posts in deeper postholes [4].
However, what is equally important is that, in prehistoric structures, the depth of a posthole seems to be related to the height of the post it held; taller elements have deeper postholes. In modern terms, this is not structurally necessary, for although the post may be taller and heavier, it will not necessarily be carrying more load. However, a deeper hole for a taller post makes it more stable, and is sensible and intuitively correct.

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.
Theoretical posthole sections
Without the impression of the timber post, or ‘post pipe’, we cannot be certain of its size. The shape of the posthole may be a result of its being robbed; wood, being a useful resource, will often be recycled. The collapse or demolition of a structure may also distort the shape of the posthole. In reality, at many sites we find only the bottoms of features, a few centimetres deep, and distinguishing between a small pit and a large posthole is not always easy. Thus, in some respects, posthole diameter is a secondary consideration to its depth.

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.
These two principles are the starting point when interpreting prehistoric postholes as structural foundations, and are important in the more fundamental and underlying problem of identifying a structure in the first place.
Exercise 1: Reconstruction of a Neolithic longhouse from Flögeln, North Germany [5]
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.
(Continue on a separate sheet if necessary).
Sources & Further Reading:
[1] G. A. Carter 1998: Excavations at the Orsett ‘Cock’ enclosure, Essex, 1976. East Anglian Archaeology Report No 86.
[2] Cartoon by B. Kliban from: The Biggest Tongue in Tunisia and other drawings. Penguin 1986, ISBN0140072209. The cartoonist B Kliban : http://www.pbase.com/csw62/kliban
[3 ] Using BS 8004. Code of practice for foundations. Based on; Medium dense gravel: 200 to 600 kN/m², Medium dense sand: 100 to 300 kN/m², Very stiff bolder clays & hard clays: 300 to 600 kN/m², Firm clay: 75 to 150 kN/m²
[4] The two phases of the Little Woodbury roundhouse can be separated on the basis of depth: G. Bersu 1940 Excavations at Little Woodbury, Wiltshire. Part 1, The settlement revealed by excavation. Proceedings of the Prehistoric Society, 6, 30-111. This was also noted by Peter Reynolds at Pimperne Down: D. W. Harding, I. M. Blake, and P. J. Reynolds 1993 An Iron Age settlement in Dorsett: Excavation and reconstruction. University of Edinburgh. Department of Archaeology Monograph series No. 1
[5] W. Haio Zimmermann, Die trichtenbecherzeitlichen Häuser von Flögeln-Eekhöltjen im nördlichen Elbe-Weser-Gebiet. In: Rüdiger Kelm (Hg.), Vom Pfostenloch zum Steinzeithaus. Archäologische Forschung und Rekonstruktion jungsteinzeitlicher Haus- und Siedlungsbefunde im nordwestlichen Mitteleuropa. Heide 2000, 111-115. See reconstruction: http://www.pcl-eu.de/virt_ex/detail.php?entry=01


Timothy Reid said...

Hi Geoff
Interesting read though I must admit I have no idea what a cock enclosure is? Should I be embarrassed for myself?

Geoff Carter said...

Hi Tim
The Orsett 'Cock' Enclosure, was named after a local pub, The Cock Inn, I think to avoid confusion with other enclosures nr Orsett, - bit of a hostage to fortune.

You still hear about the Orsett 'cock ' Roundabout, (built over the Iron Age enclosure), on our traffic bulletins (A128/A1013).

Produces some dubious results from search engines!

Anonymous said...

Hi Geoff,

Amazingly interesting site and highly professional!

Dave P
(DIABLO and old relic!)

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