Lateral support monitoring

[Patgeotech]

We undertake regular monitoring/survey of targets during installation of lateral support (as well as for a certain time period after completion to check that there is not movement, although we all know that with any form of excavation, some form of movement will occur). It has become practice that when monitoring readings of say 5 to 10 mm are recorded, many geotech chaps ignore these results and relate the result to thermal differences and instrument error. What I wanted to know from others out there, what would you accept as thermal or instrument error on say a 200mm thick, shotcreted, mesh reinforced wall? Is 10mm acceptable?

 

[ishvaaag]

By measuring controlled similar specimens in alike environment one can get the idea of what is actually happening. 10 mm looks too much for a start except that some restriction and curve pushes out some bulge, or as volume change for say one day, just on temperature; if water is involved and seasonally that would be one target of your interest, so anyone working continuosly in these things should establish a method in which even relative movement can reveal the true pattern of movement happening. Help of someone good at tridimensional geometry would be paramount.

One can choose an initial point (to remain more or less stable) as reference and then refer everything happening to it; it would be surmised that point doesn't move even if it does, and at any time t all coordinates would be referred to that purportedly fixed poing. Later analysis of the evolution of the shapes will help in deciding what movement to adjudicate to that point itself, and so the evolution of the shape.

 

[bridgebuster]

10 mm could be OK. I'm in the same boat on one of my projects. We designed a replacement for a dry rubble retaining wall - the existing wall is about 300'(90M) long with varying exposed height from 3' (1M) to about 30'(9M). The owner - a city agency - has been monitoring it for the past four years. They installed prisms on the wall and perform 3D survey on a monthly basis; it started out with daily measurements and gradually went to monthly measurements.

Typically, we would see a 3 to 6 mm change; occasionally a few locations might move on the order of 10 to 12 mm. Same thing - the geotechs didn't seem worried. Movements were always attributed to thermal changes. In our case water wasn't an issue becuase these types of walls drain through the open joints.

I agree with Ishvaaag's approach. Another method is to place sensors on the wall. This gives continuous real-time monitoring.