A common problem with retrofit mezzanine projects?

Structural engineers will be involved from the start and Building Regulations Part A require the concrete slab to be assessed to ensure it can bear the increased loads imposed by the new structure. Checks also include mezzanine floor load capacity to ensure it can safely support both live and dead loads based on the intended usage.

What is too often overlooked, however, is nature of the substrate under the slab. Retrofit mezzanine projects are often treated as interior refurbishment and teams sometimes skip full geotechnical surveys, assuming that the existing slab is stable.

This may turn out to be an expensive omission: unexpected subsurface conditions are a frequent cause of project delays and of performance issues in the finished structure.

Poor subsoil conditions such as soft soil, compressible silts or made-up fills contribute to a high risk of settlement beneath new support loads. This can lead to industrial floors sinking by centimetres causing sloping floors and buckled slabs which in turn leads to

unsafe operating conditions for fork-lifts and other machinery. The extent of sinking can be dramatic – as much as 150 mm of movement is not uncommon.

Calculating the risk of subsoil problems in mezzanine projects

A comprehensive ground investigation report is always advised which should identify the type of subsoil and its condition. Red flags include the presence of reactive clay soil, low bearing capacity, excessive moisture or silty, fine-grained compressible soils that may not provide uniform support.

This assessment is then used to understand whether the existing slab will be sufficient to support the new structure. It’s particularly important to consider the point loading of new structural steelwork, which will be distributing loads not anticipated in the building’s original design. You also need to verify whether the current slab and ground conditions are suitable for the intended mezzanine floor load capacity.

How to stabilise subsoil under a concrete slab?

Once a potential weakness in the subsoil has been identified, there are a range of options available to rectify the situation. Soil excavation and replacement, the installation of additional piling or the use of geogrid platforms will all stabilise the ground but involve significant disruption and lengthy work programmes.

The preferred solution is likely to be the use of resin injection. In this method, small access holes are drilled in the slab and a two-part resin is pumped into the subsoil. When the two components meet underground a chemical reaction causes them to expand. This expansion compresses and densifies loose soil and the resin cures within minutes to form a rigid durable material providing a stabilised load-bearing structure.

This quick method of dealing with unstable subsoil will provide the additional ground stability to support the increased weight of a retrofitted mezzanine floor. The Mainmark team is well placed to advice ground engineers on the quantity and application of resin required for the specific ground conditions and project parameters.

Solving a subsidence problem

Should the worst happen, and subsidence happens during or after the mezzanine installation, the resin injection underpinning method provides a fast and effective subsidence repair solution that can be carried out with minimal disruption to the operation of the warehouse facility. The resin cures in hours and the whole project can be completed in days.

An additional benefit of this system is that the expansion of the resin provides an uplift that can lift floor slabs that have already dropped. Hosetech chose Mainmark’s Teretek solution for their warehouse building in Ireland, which achieved lifts of up to 300mm across the 2,000m2 concrete slab. This is a prime example of concrete slab lifting, a resin injection method which not only stabilises but restores the original level of the floor quickly and cleanly.

MK Survey needed a rapid solution when they noticed accelerating signs of subsidence in one of their warehouses. Mainmark provided a solution that recognised the urgency of the problem, completing all work required to raise and re-level the structure by 86mm in just seven days.

If your warehousing is showing signs of subsidence, or you’re wanting to check the site is future-proof against it, don’t hesitate to get in contact with us.