There are those who might assume that assessing whether a masonry wall can bear an additional load is a straightforward exercise: one simply compares the anticipated load in kilonewtons to the bearing capacity of the underlying ground, using values obtained from laboratory core tests or relying on commonly accepted figures for materials such as Upper Globigerina Limestone, where this constitutes the founding base. Then, it is a question of computing a basic stress calculation. Yet the reality may be far more complex. When a masonry wall is expected to carry vertical loads—particularly as part of a shared or party structure—its true load-bearing capacity is determined by a constellation of interrelated factors.
First and foremost, the masonry itself must also be properly assessed. Strength values of stone and mortar vary, and results from compressive testing on isolated stone blocks—especially when very high—should be interpreted cautiously. Conservative estimates should be used, especially so when detailed information about mortar quality and construction technique is unavailable. In design terms, therefore, what matters is not the theoretical strength of an individual stone block, but the actual composite performance of the wall.
In particular, one would also need to see whether the wall is laterally restrained since a slender wall with no return walls or slab anchorage lacks the stiffness to resist lateral forces, even if vertical loading is within acceptable bounds. Meanwhile, foundation movement, though sometimes perceived as minor, can have serious effects in masonry structures. Calculated settlements in the range of 6 to 9 millimetres may be enough to induce cracks, particularly where the wall is not well tied or where the floor structure depends on continuous support.
The design must also consider future excavation nearby. Even if excavation is carried out at the legally required offset, its effects may still propagate through fractured or weak rock and compromise the wall’s stability.
Last, but not least, it must be kept in mind that the quality of the rock, often Upper Globigerina Limestone, varies by location and depth. Its mechanical strength near the surface is frequently reduced due to weathering, and the portion of the rock that directly supports the wall may be far weaker than deeper strata. Thus, one has to look at the condition of the underlying rock and the characteristics of the contact between wall and rock. Meanwhile, if the wall’s footprint on the rock is narrow—say, 19 to 23 centimetres—the vertical stress becomes concentrated before the load spreads downward through what we call a “bulb of pressure.” This makes the condition of the uppermost rock layer even more critical because if that layer is soft or fractured, settlement or cracking is likely to occur, even if the ‘deeper’ rock is strong. Equally important is the nature of the interface itself. In buildings constructed in the past 50 years, there is often a thin layer of plain concrete between the wall and the rock. This is sometimes mistaken for a foundation, but if it contains no steel reinforcement and lacks substantial thickness or bearing area, it does not function structurally as a strip foundation would. In such a scenario, it should be treated as part of the wall mass, not as a load-spreading base.
The ultimate question, therefore, is whether a load bearing wall can safely and reliably carry the intended loads without unacceptable movement or distress. That judgment does not depend solely on the strength of an individual stone block. It requires a careful assessment of the wall’s actual geometry, the materials used, the presence or absence of lateral restraint, the condition of the uppermost rock layer apart, of course, from the rock’s bearing capacity as well as the potential for ground disturbance in the surrounding area.
One last word of advice – while the use of lightweight block systems in proposals for additional storeys may, to the logical observer, appear advantageous, such solutions must still be approached with caution. Their structural performance should still be grounded in verified material certification, reliable manufacturing data, and the application of appropriate construction detailing.
