Project: Statics of drystone walls
Static calculations, consequences for the drystone mason
As dry stone masons, we want to know what effect the engineer's considerations have on the craft of drystone masonry.
Traditional or engineered building technique? Lessons for the traditional way of building drystone walls.
As drystone masons we build our walls most of the time in the traditional way (Fig A ). It is only when we have to build retaining walls supporting a road or a building that the wall dimensions must be calculated by an engineer. In the traditional building technique the outer face of the drystone masonry is clearly distinguished from the backfilling. There is also no trapezoidal cross-section (Fig A ). In the engineered building technique a trapezoidal cross section is calculated which must be filled by the dry stone mason in consistent stone quality (Fig B) (In practice, it has been proven that in addition to the team building the wall, another team presorts and prepares the stone material. The effort required to build such a retaining wall is therefore much greater than with traditional dry stone masonry).
While in the engineered building technique, the bearing capacity of the masonry can be calculated precisely, this is not the case in the traditional building technique. Still the traditional building technique does its duty without any problems in many places. The load-bearing behaviour of such walls can be understood as follows:
The traditional dry stone wall forms a kind of trapezoidal cross-section in the area of the long trough stones, if the backing stones are carefully placed in this area. Care must be taken to ensure that the longest trough stones are placed at the bottom and that their length then decreases towards the top ( Fig C). If this rule is not observed, the result will be an irregular cross-section with reduced load-bearing capacity ( Fig B).
Inclination of masonry courses
As explained in the section "Friction" and also in the section "Earthquake", for all construction methods (traditional and designed by the engineer), the stones should always be inclined towards the retained material. Usually, the wall courses and bearing surfaces have a right angle to the batter of the face of the drystone wall.
Foundation stones and long through stones
The explanations in the section "Design" show that the retaining walls made of dry masonry are particularly under stress in the lower third of their height. In this area, the following points must be taken into account during construction:
If the resulting force cuts the footing line in the foundation area very far out, there is a danger that foundation stones will be pressed into the soil. The foundation stones sink in towards the outside and a bulge is formed (Fig B below). This damage is often observed when the foundation stones are too small or the bearing capacity of the soil is not high enough.
As a countermeasure, sufficiently large foundation stones are to be installed (as binders and if possible protruding). In the lower third of the wall height, more long through stones are to be installed than in the remaining wall height (Fig C below). See also the SIA Merkblatt 2053, paragraph 6.3.1. In addition, the rule applies that the resulting force should intersect the foot line of the trapezoidal cross-section in the middle third.
The resulting force acting far out against the face of the wall can cause very narrow and high stones installed as runners to tilt. Here, too, undesirable movements and deformations of the masonry can result (Fig E below). This effect can be prevented if (especially in the lower third of the wall height) sufficiently long through stones are placed (Fig F below).
The same applies if a failure from tilting and sliding occurs (cf. section "Dimensioning"). Here, too, the installation of sufficiently long through stones can prevent failure of the dry masonry in the base area. In this regard, see also SIA Merkblatt 2053, paragraph 6.3.1.