MR Cooper, BSc, MSc, DIC, MICE, CEng. This paper was first published in GE’s November 1986 edition.
The maintenance, repair and replacement of ageing dry-stone retaining walls is a growing problem for many authorities. This type of wall provides support and protection to hundreds of miles of highway, often on major trunk roads. Jones (1979) has described the construction of these walls, and an account of current engineering problems associated with them was given in New Civil Engineer, 18th August, 1983.The typical construction with horizontally aligned flaggy backfill behind a carefully placed facing is well illustrated by the exposed cross-section of a typical wall shown in Fig. 1. A slight variation employed for downslope walls where a road crosses side-long ground using lateral cut and fill is to use a double facing with flaggy backfill between. In either case the wall will gain much of its stability from being constructed with a backtilt, or batter, as shown in Fig. 2, where the batter is highlighted by the angle between the vertical lamp post and the service pipes on the face of the wall. This batter can play an important part in influencing the eventual failure mode.
In preventive maintenance an essential first step is to observe or deduce any developing failure mechanism and so to identify the cause of distress. This is particularly true for dry-stone walls which can stand large movements before collapsing. The failure modes of rigid retaining walls are well known and form the basis of standard design procedures where the walls are checked for resistance to overturning, sliding and general slope failure. However, for retaining walls which do not act as rigid bodies additional failure modes are possible. In the particular case of dry-stone walls failure is often preceded by the development of a pronounced bulge, and eventually occurs by bursting at the bulge. An understanding of the mechanism involved in this type of failure should be useful in designing an efficient repair.