Nowadays, the deformation monitoring of dam work has done mainly by conventional geodetic methods, these advantages are high accuracy but which only allows to monitor the movement of dam at the top. Thanks to developing techniques which are able to monitor the settlement of layer soils while parts of structures are building. This paper presents the magnetic disk using for monitoring movement of embankment at HuongDien hydroelectric plant, Hue province.

1. The principle of operation of magnetic disk method

The magnetic disk, a kind of matter technical tool for monitoring the subsidence with layers, is firstly used to observe at a number of constructions such as hydroelectric dams, air platforms, roads, etc. These days, this instrument allows to measure and record data continuously, which is put into special positions to determine the subsidence in a continuous way (Ha, N.V (2013), Khanh. T (2010)). The magnetic disk surveying method bases on electromagnetic field between a top reading and a round magnet ring which is fixed at stable soil, where the height of observation marks, as follows: At the observation mark Pi (i=1,2,...,n) putting n magnetic disks with surface of disk is in the horizon, this surface creates a magnetic, when a wire fence is put on the disk, an electric current is created and detected by alarm accessories (alarm light, alarm ring). These signals help surveyors to determine when the wire fence lies on the surface of the disk. In the next step, the rule is rolled to measure a differential height between Pi and a reference datum (A) or another mark (O) which is fixed in a directional tunnel before (Fig. 1) (Ha, N.V (2013), Khanh. T (2010)).

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