Tunnels and Structural Elements

Mechanised tunnel boring technique is generally employed to realise deep tunnels within rock mass having good geo-mechanical characteristics. Nevertheless, tunnels often cross rock portions where the rock mass presents either poor mechanical parameters or particularly high coverage which may lead to develop large convergences. The development of large convergences after liner placement is generally not compatible with tunnel liner’s resistance and designers have to take particular care of these areas.

The range of variation of these parameters is quite limited, consequently designers cannot easily adjust them to fit all conditions. Furthermore, if increasing overcut area leads to positive effects controlling large convergences, the gap has to be quickly filled to ensure ring’s segments stability. When a proper adjustment cannot be found, complex technical solutions might be required, leading to slower excavation advance or other un-economical side-effects.

The most desirable solution is a layer of collapsible material between rock mass and liner being stiff enough to ensure liner stability but collapsible at load levels inferior to liner mechanical resistance. Other authors investigated possible solutions using different materials, mostly based on mortar / aggregate combination.

Expanded Clay Aggregate (ECA®) is generally employed for civil engineering purposes where light weight solutions are required such as in road/railway embankments and in light weight concrete slabs. Expanded Clay Aggregate (ECA®) is a granular material having a large void ratio, a relatively low peak resistance and is available in different granulometries. In civil applications, Expanded Clay Aggregate (ECA®) has proven good mechanical characteristics and the ability to be “designed” and combined with other construction materials in order to obtain the desired behaviour.

In areas, prone to mining subsidence or where ground conditions contain natural sub formation voids, drainage, culvert or tunnels and there is a danger of collapse, expensive techniques such as load transfer are often considered. However, using Expanded Clay Aggregate (ECA®) can eliminate these costs and considerably lighten the load to provide similar benefits to those when used in weak soil areas.

During the construction of tunnels, unexpected voids and small cavities around the concrete may collapse on the structure. Lightweight filling with the use of loose or cement bound expanded clay can be archived with very low costs of handling thanks to the pumping opportunity. To protect the tunnel from falling rocks, Expanded Clay Aggregate (ECA®) reduces the impact.

Apart from tunnel, land base closer to mining activities easily forms voids, drainage or culvert. Similar like, tunnels, Expanded Clay Aggregate (ECA®) can be used to prevent  danger of collapse in a very cost effective way.