The soil sampler was very weak at the threaded section and was breaking very easily with very little force applied to it. To fix this, I went increased the diameter for the circular base and increase the dimensions of the base on which the threads rest on. A fillet was also added to the bottom of the base. This serves as a smooth transition from the round base to the square extrusion, making it less susceptible to fracturing in the union between those two sections. 

Section analysis of the soil vapor sampler 

Section analysis of the soil vapor sampler