Bi-planar coil assemblies are disclosed which have DSV’s whose centers are offset by distances D from the origins of x,y,z-coordinate systems, where said origins of x,y,z-coordinate systems are coincident with the geometric centers of the coils. It has\u2014in a holder therefore\u2014a cutting tip for truing engagement with the grindstone. The contact is movable among a first position, a second position, and a third position. A divider is positioned between the first plurality and the second plurality of slack spools. A measuring cell based on a three-chamber construction is used, and upper and lower soil layers with an intermediate soil layer therebetween are alternately subjected to a dynamic repetitive load, and what influence there is on the intermediate soil layer is investigated from the relation between pressure and displacement.