Urban geology: Gabions in the Dutch townscape
Stephen K Donovan (The Netherlands)
Gabions are tools of the engineering geologist, facing elements that are used to stabilize over-steep slopes, such as sea cliffs or railway/roadway cuttings; they also have military applications. The word is derived from the French, gabion, and Italian, gabbione, and originally referred to “A wicker basket, of cylindrical form, usually open at both ends, to be filled with earth, for use in fortification and engineering” (Little et al., 1983, p. 823). A modern gabion used in engineering geology is a cage, box or cylinder, commonly infilled by rocks or concrete, and sometimes sand or soil (https://en.wikipedia.org/wiki/Gabion).
Essentially, gabions provide a stable retaining wall that is semi-permanent. That is, they can be more easily removed, modified or replaced than a permanent structure made in concrete, brick or steel. Although they may be aesthetically unpleasing, gabions provide stability in situations where serious erosion problems may exist, which cannot be controlled by alternatives such as re-vegetation (Freeman and Fischenich, 2000). This is a simplification and studies such as that of Druse (2015) explain something of the complexities.
So, in the low-lying Netherlands, what uses might be and are found for gabions? It is reasonable to suggest that they might be used in canal systems, such as in Amsterdam, where erosion is aggravated by heavy boat traffic (compare with Freeman and Fischenich, 2000). I have looked for this to no avail, mainly because most canals in Amsterdam are walled by brickwork. Further, outside the cities, most canals are not inland waterways as such, but a means of lowering the water table in a region close to sea-level. For example, where I live in Hoofddorp, close to Amsterdam Schiphol International Airport, the surrounding area is actually a drained lake basin and below sea level. In consequence, there are no rivers (which would have to flow uphill to the sea) and the common canals are entirely for control of the water table. So, canal bank erosion by boat is unknown, as are gabions reinforcing the banks.
Some gabions are used for simple walls, which, at best, are bland if not actually ugly (Fig. 1A). The illustrated example is a wall outside a restaurant at Amsterdam Zuid railway station. This is a robust, non-permanent structure, yet it seems to be over-engineered for what is required. All of the structures figured in this article are infilled by cobbles of Mississippian (Lower Carboniferous) limestone, which has no natural outcrop in the Netherlands and therefore must have been imported (Nield, 2014). These may include exotic fossils (see below) or interesting mineral veins, such as the illustrated example (Fig. 1B), which may be sphalerite.
Yet, if treated with imagination in constructing an ‘artificial countryside’, gabions may add to the visual effect and even model geological phenomena. I illustrate two examples that are aesthetically more pleasing to the eye of a geologist and both imitate the appearance of the most basic of sedimentary phenomena, namely bedding. Fig. 2 illustrates gabion pseudo-beds on the southeast side of Hoofddorp station, close to the Novotel (Fig. 2A, right background). (Note for the international traveller: Amsterdam Zuid (Fig. 1) and Hoofddorp (Fig. 2) are only one stop from Schiphol Airport, albeit in opposite directions.)
From a distance, these gabions give a persuasive impression of parallel limestone beds (Fig. 2A and B), although the wire cages become more obvious the closer you get (Fig. 2D). The bridge in the drainage is also distractingly man-made (Fig. 2C). Up close, the limestones are fossiliferous and may one day repay a survey of the fauna. The stick bryozoan (Fig. 2E) and productid brachiopod (Fig. 2F) are typical examples. This feature has been in place for over 15 years, and the limestones have had time to be washed clean and gently etched, exposing any fossils more fully than the newer structure in Fig. 1.
Gabions have played a significant role in remodelling the townscape between the Leiden Centraal railway station and the nearby Leiden University Medical Centre (Fig. 3). Most are rather unimaginative and just walls, although soil and vegetation have been incorporated to produce an appearance (Fig. 3A), still artificial, but less so than, say, Fig. 1. But, adjacent to the railway viaduct, gabions have been integrated with vegetation rather more imaginatively. Rather than being straight and wall-like, they are arranged in a double-layer, which is sinuous and more subtly like a rock exposure (Fig. 3B and C). The two layers of gabions have the appearance of two congruent sedimentary beds, weathered back and having geometries that imperfectly mimic each other, as might be expected in a natural rock exposure.
References
Druse, M. 2015. Numerical verification of geotechnical structure in unfavourable geological conditions – case study. GeoScience Engineering, 61(2): 8-13.
Freeman, G.E. & Fischenich, J.C. 2000. Gabions for streambank erosion control. EMRRP Technical Notes Collection (ERDC TN-EMRRP-SR-22), U.S. Army Engineer Research and Development Center, Vicksburg, MS: 9 pp. www.wes.army.mil/el/emrrp.
Little, W., Fowler, H.W. & Coulson, J. (eds), Onions, C.T. & Friedrichsen, G.W.S. (compilers). 1983. The Shorter Oxford English Dictionary on Historical Principles. Third edition. Volume 1. Book Club Associates, London, xxix+1280 pp.
Nield, Ted. 2014. Underlands: A Journey through Britain’s Lost Landscape. Granta, London, xvii+251 pp.