Ground strain comprises two components, being normal strain and shear strain, which can be interrelated using Mohr’s Circle. The magnitudes of the normal strain and shear strain components are, therefore, dependant on the orientation in which they are measured. The maximum normal strains, referred to as the principal strains, are those in the direction where the corresponding shear strain is zero.
Normal strains along ground monitoring lines can be measured using 2D and 3D techniques, by taking the change in horizontal distance between two marks on the ground and dividing by the original horizontal distance between them. This provides the magnitude of normal strain along the orientation of the monitoring line and, therefore, this strain may not necessarily be the maximum (i.e. principal) normal strain. It should then be noted, that observed strains are dependent on the method of measurement, including the orientation of the monitoring lines, the spacing of the survey marks and survey tolerance.