What grout should we use with the magnet extensometer? Should the grout be weaker, stronger or the same strength as the surrounding ground ?

In soft ground applications, you might use a ratio of 6.6 water : 1 cement : 0.4 bentonite. This provides a 28 day compressive strength of about 4 psi. You should also consider using telescoping joints. These will accommodate settlement that would otherwise break the access pipe. They will also improve compliance to the surrounding ground.

In a hard clay, you might use a ratio (by weight) of 2.5 water : 1 cement : 0.3 sodium bentonite. This provides a compressive strength of about 100 psi and a modulus of about 10K psi. The strength and modulus of the grout are controlled mainly by the ratio of water to cement.