Progress Report – New 15mm Hadrian’s Wall range Wall Sections

Jul 2nd, 2010 | By | Category: Rome, Workbench

Builders of the Great Wall of China used horizontal foundations, not contour following like the RomansGarry’s note – Last week I mentioned the differences between construction of the Great Wall of China and the stone walls of the Roman Limes.  This mention refers to how the two sets of engineers handled construction traversing an incline.

The Chinese used a series of deep cutting trenches digging into the hillside, which provided a flat foundation for each length of wall supported.  Because of this, there was little to no erosion or subsidence of their foundation layers in each “step” of the wall progressing up the slope (even after millennia).

Click the photo on the left to see a larger image and examine the stone course layers’ angle.  Additionally, each “step” provided a buttress to the section above it, preventing it from leaning or sliding down the slope – although this was merely an added element of strength because the flat footings took most of the load-stress, allowing the Chinese to build heavier walls on worse slopes, than the Romans could achieve on their frontiers.

Stone courses following the land contour at Hadrian's WallThe Romans however, used uniform foundation trenches that followed the ground’s contours.  This meant the courses of stones also followed the curves of the land, which gave major subsidence and structure-slide risk in the northern British weather systems.

There is evidence of long wall sections on inclines causing down-slope pressure against lower sections and leading to collapse, distortion, and subsidence.

This is easiest imagined as building a line of bricks on an icy sloping pathway – starting at the bottom, the lowest brick may stay in place, but as the line extends up the slope, the weight of the line and the low friction on the slope, causes the whole line to slide down.  This same effect materialises when building a line of stone wall onto a thin layer of sodden earth lying above sloping rock strata.

That so much of Hadrian’s Wall’s lower courses remain as are visible today, may owe more to stone robbing for local construction that reduced the linear weight and pressure, as it does to the Roman tendency to build with concrete and mortar.

The mileforts and turrets were of three different designs, depending on which Roman legion built them.  (Garry notation – Ah ha!! This late-discovered research snippet confirms my surmising in parts one and two of this series, and explains why the style differences exist between the reported gateways and turrets).  Inscriptions of the Second, Sixth, and Twentieth Legions, tell us that they were all involved in the construction of Hadrian’s Wall (Muttering – note the quote below saying no legions were stationed at the wall = too much speculation unearthed?).  All the mileforts and turrets averaged 493 metres (539 yards) apart.

The turrets are reported as measuring 4.27 square metres (46.0 square feet) internally – see last weeks comments about visually apparent size differences.  Construction (of The Wall) was divided into (15) lengths of about 5 miles (8 km) each.  One group from each legion would excavate the foundations and build the mileforts and turrets, and other cohorts followed with the curtain wall’s construction, until it was finished in 128 AD.

Early in its construction, just after reaching the River North Tyne, the width of The Wall was narrowed to 2.5 metres (8.2 ft) or even less (sometimes 1.8 metres – 6 feet), which has been labelled the “Narrow Wall“. However, Broad Wall foundations had already been laid as far as the River Irthing, to the point where the Turf Wall began, demonstrating that construction progressed from east to west.

Many turrets and mileforts were optimistically provided with stub ‘wing walls‘ in preparation for joining to the Broad Wall, offering a handy reference for archaeologists trying to piece together the construction chronology.  (Garry’s note – this is an important observation for the model designing.  It means the turrets and mileforts can be built to a standard design for use on any point of the wall, but that the wall sections themselves, must be built to several different specifications.  See design summary that follows below.)

Within a few years of starting construction, someone decided to add a total of 14 to 17 (sources disagree – possibly due to whether they do, or don’t, include any pre-existing forts / garrison towns) full-sized forts along the length of the southern side of Hadrian’s Wall, including Vercovicium (Housesteads) and Banna (Birdoswald), each holding between 500 and 1,000 auxiliary troops (one source says no legions were posted to the wall, which seems contradictory to other narratives).

The eastern end of the wall was extended further east from Pons Aelius (Newcastle) to Segedunum (Wallsend) on the Tyne estuary. Some of the larger forts along the wall, such as Cilurnum (Chesters) and Vercovicium (Housesteads), were built on top of the footings of mileforts or turrets, showing the change of plan.  An inscription mentioning early governor Aulus Platorius Nepos indicates that the change of plans took place early in the construction period.

Also at some time (still during Hadrian’s reign, i.e. before AD 138) the walling west of the Irthing was rebuilt in sandstone to basically the same dimensions as the limestone sections to the east … but to which of the three known cross-sectional dimensions has not been found during my research to date.  As this was a post commencement revision, for now I’m assuming it was to the Narrow Wall dimensions, during the period when “Get a move on!” orders had been issued.

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