Project: Statics of drystone walls

Static calculations, implementation in drystone masonry

As dry stone masons, we are interested in the impact of the engineering considerations on the execution of the masonry.

Retaining walls supporting a road or a building must be designed by an engineer. The calculated trapezoidal cross-section must be filled by the dry stone mason in consistent stone quality. In practice, it has been proven that in addition to the team building the wall, another team pre-sorts and prepares the stone material. The effort required to build such a retaining wall is therefore much greater than with traditional dry stone masonry.

Inclination of masonry courses

In general, for both construction methods (traditional and designed by the engineer), the wall courses and bearing surfaces should always be inclined towards the retained material. Usually, the wall courses and bearing surfaces have a right angle to the tilt of the face of the drystone wall.

Traditional dry stone masonry, static active cross section

In traditional dry masonry, the stones of the outer face shell are clearly distinguished from the inner backing masonry. There is also no trapezoidal cross-section  (Fig A below). The load-bearing capacity of this type of masonry cannot be calculated by the engineer. Nevertheless, these walls have proven their worth in many places for centuries. 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 installed at the bottom and that their length then decreases towards the top  ( Fig C below). If this rule is not observed, the result will be an irregular cross-section with reduced load-bearing capacity ( Fig B below).

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Trapez

Place of greatest stress in the cross section

The explanations in the part "Design" show that the retaining walls made of dry masonry are particularly under stress in the lower third. 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 soil is not strong 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. Zudem gilt die Regel, dass die resultierende Kraft die Fusslinie des Trapezquerschnittes im mittleren Drittel schneiden soll.

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 binder blocks can prevent failure of the dry masonry in the base area. In this regard, see also SIA Merkblatt 2053, paragraph 6.3.1.

Einsinken
Kippen
Drawing5

Long trough stones, place and number

Due to the greater load on the masonry in the lower third of the retaining wall, more long through stones must be placed than further up in the masonry.  (cf. Merkblatt SIA 2053, Absatz 6.3.1)

Lower third of wall height:

Through stone placement must start in the foundation course. Min. 1 through stone per linear metre of wall, 3 through stones per m2 of visible surface. Vertical through stone spacing (centre-to-centre spacing) should correspond to the average stone thickness/height of the through stones. The through stones must be offset in each course.