Credits: NASA/Goddard/University of Arizona
The image above is part of a close-up acquired by NASA's OSIRIS-REx spacecraft of Bennu's surface.
101955 Bennu is a carbonaceous asteroid that is currently accompanied by the OSIRIS-REx spacecraft that will take samples from it to return to Earth.
Its diameter is 490 m. and it has a density of about 1.19 g/cm³ and therefore the predicted macroporosity is about 40 %, suggesting its interior has a rubble pile structure.
Measurements from the MASCOT lander, part of the Hayabusa2 mission, have indicated that boulders on the C-type asteroid 162173 Ryugu have high porosities and low tensile strengths in the few hundred kPa range.
Bennu is a B-type asteroid, falling into the wider C-group, and spectroscopy suggests major surface constituents of anhydrous silicates, hydrated clay minerals, organic polymers, magnetite, and sulfides.
Here's what I think at the moment:
Because it can be expected that the hydrated clay minerals and the organic polymers are better preserved below the surface, protected from radiation of the Sun, the subsurface of Bennu is likely to be more cohesive.
With Bennu having a surface gravity of only 6 micro-g, I'm guessing that it might be easy for an astronaut to dig deep below the surface with first the brittle rocks and grains and then the more cohesive material.
Taking an average bulk density of 2000 kg/m³ for CI and CM meteorites, a one cubic meter piece of rock would "weigh" 12 grams so lifting up and removing rocks and grains should be no problem.
But to cleave larger boulders (with tensile strengths about 2 x higher than brick), to cut into the cohesive grain material, and to operate within the dug shaft one will need the right equipment.