At my work whenever a new roadway is being built there comes a point where things are ready for the gravel and asphalt or concrete are placed over the top of the material used to build this on top of. That point of construction of a road is called "reaching subgrade" as that is grade (or elevation) below the "finished grade", which is the grade (or elevation) we drive on... the surface of the finished roadway.
When it rains the subgrade can become soft. For the road to last it should be firm before placing gravel etc over it. Before that happens a "proof roll" takes place. Proof of firmness is done by rolling a heavily loaded dump truck over it. Proof roll. The wheels of the truck squash soft subgrade causing it to appear to be moving up and down like water on a pond on a windy day. We walk beside the truck watching for any movement. A person walks on each side since normally both sides cannot be seen from one side... oh, but contrare mon-frair...
Stand off to the side at about 25 feet (8 meters) at about 45 degrees from the rear of the truck and wheels on both sides can be seen but... under the truck is mighty dark. Enter a good strong flashlight to light up underneath of the truck and light up the area the wheel just rolled over.
While 2 young engineers walked inches from the rear wheels of the truck I was off to the side watching front and rear on both sides. Young engineers do not typically realize about 60% of the loads are equally distributed among 8 tires at the rear while only 2 up front carry about 40% of the weight. Much more pounds per square inch, meaning much more "wave action" is seen up front.
Anyway another experienced guy was walking with me and watching what I was watching. He said "it's mighty dark under that truck". I said "not much longer", whipped out my PK PR-1 and the other fellow said "holy *** that's a bright flashlight!"