Mechanical Engineering
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# Monitoring Land Subsidence using Extensometer

## Question

Hi I’m undergraduate mechanical engineering student, I’m currently working on a project about monitoring land subsidence. Many source and paper show one of the methods to moniroting land-sub is using rod extensometer. I can understand with the concept, but im having trouble understanding the technical issues.

1. in this pic shows that the anchor is attached to the surrounding soil, but is the anchor outside or inside the borehole? If it’s inside, how is the anchor placed/attached to the soil and take measurement, is the borehole has holes along its length or has a hole where the anchor supposed to placed?

2. if the anchor attached to the surrounding soil, and it moves vertically downward alongside with the soil, how could the head/borehole is not also moves downward? Because if it moves how is the measurement obtained, knowing that measurement is obtained between the anchor at the bottom and the reference point at head/collar of borehole? if the head/borehole also moves downward I think you can’t obtained the measurement, cause you need a steady structure as a reference point

Does anyone has the knowledge and could answer my question and explain it to me? Id be grateful, thanks in advance.

## Recommended Posts

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If you are looking to measure subsidence, it occurs from "underneath", and then - when it gets large enough - becomes evident on the surface.

You note on the diagram the substrate....you are measuring the movement of the bottom when compared to the top. Subsidence occurs from the bottom up, erosion from the top down. (Oh.... and there is - of course - "settlement")

The anchor is slid down through the borehole with the prongs retracted then - at the correct depth, the prongs are extended into the surrounding soil, anchoring the - well - anchor! There is a great image and an expansion of diagrams on this website: https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.geokon.com%2F1950&psig=AOvVaw2PFto479ydbo5ZGyxbpj1j&ust=1645111460891000&source=images&cd=vfe&ved=0CAsQjRxqFwoTCPirp_PDhPYCFQAAAAAdAAAAABAD

This gives a fixed subterranean point from which to gauge any change from the reference at the top of the borehole before there is any sign at the top. The change between the two points is what is being measured.

Hope this helps

Good luck!

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