In the coastal zone, wood species used for vertical constructions during the Late Middle Ages ranges from alder to ash, with occasionally oak. Oak as a local building material was long available in the sandy parts of Groningen, but became scarce during the 16th century, requiring an increase in import of oak timber (Groenendijk & Van Rijn 1996). In the clay region there has always been a shortage of oak. Alder (Alnus) was the most popular wood species for foundation purposes, as this tree was abundant in both clay and peat regions and in the stream valleys of the sand regions. Trunks of Alnus are well capable of carrying vertical loads and are resistant to rot when applied below the ground water level. Willow (Salix), in spite of being a typical wetland species and easily available, is not suitable for timber, as it is very soft, flexible and susceptible to rot. Following Wiselius (1994), the mechanical strength index is 26 and the durability grade is V, which indeed means wood classifications unfavourable for construction purposes. In domestic building therefore willow may be used for wattle work, but it is unknown in over-ground structures (Casparie et al. 1995). The use of willow for foundation purposes under the medieval Steentil seems rather anomalous and suggests that immediate availability was more important than structural demands at the time.
Yet, willow has been used in vertical structures, even in the Cistercian monastery at Aduard itself. The second abbey church of the monastery rested completely on pile grids, the number of wooden piles being estimated as 12,500. They measure between 10 and 35 cm in diameter and with lengths not exceeding 2.40 m. Despite the lack of wood determinations in these older excavations, a 1976 test pit in the monastery area produced pile grids only consisting of willow and birch (Praamstra & Boersma 1977, 187). So we may conclude that willow must have been known in monastic building engineering, including foundations in a wet and weak subsoil.
There is little doubt that monks, or building contractors with ample experience under supervision of the Cistercian monastery, constructed the first Steentil bridge at a distance of 1.5 km from Aduard. They chose untreated willow and were not concerned about the bark left. Though we are ignorant about the lengths of the piles, the willow pile sampled may have exceeded the average length of 2.40 m as encountered in the 1976 trench in Aduard. However, we think that the builders had no intention to reach the firm Pleistocene subsoil, as its top lies more than 10 m below Ordnance Level here. Was it the heavy clay, allowing to found on the basis of the sticky property of clay (kleef in Dutch) that encouraged them to do so? This might well be the case, compared with the shallow foundation depths found in the monastery itself. Whatever the consideration of the constructors might have been, the application has been successful, as is clearly proven by the perfect condition of the wood after 700 years (fig. 5). Though the wood had become very weak, the trunk itself still retained its original shape. The flattening of the upper ends suggests that the piles were meant to carry a weight, probably spread over transverse beams. A possible transverse beam was encountered in 2010 as a stray find, but mistakenly remained undocumented. On the other hand, part of the pile grid was covered directly with masonry of large medieval bricks and it may be that the piles were kept upright only by means of a wooden frame around the edges. Both foundation constructions, with and without intermediate transverse beams, occur with medieval buildings in the weak Groningen soil.
Figure 5 Poles of the wooden foundation at the bottom of the Aduarderdiep next to the present eastern land abutment; the upper ends are flattened and the sapwood of the trunk is still present (photo: H.A. Groenendijk).
Under the bridge, a collection of stray moulded bricks was found. It looked like a pavement but its distribution and composition were random. Therefore, this ‘pavement’ is likely the result of the demolition of the first and the second stone bridge. So, it is imaginable that the original foundation consisted of vertical piles with joisted beams, the latter being removed when the three-arched bridge was demolished, to be replaced by the 1722 design. On the other hand, the observation of masonry directly placed on the vertical piles points to joisting by means of a wooden frame around the pile’s edges. Bricks were produced at the Aduard monastery and were already widely used during the end of the 13th century.