Question $2$: Rock mass classification and rock failure criterion

$2\mathrm{.}1.$ A rock mass is transected by $3$ intersecting joint sets. $J_1$ has $30cm$ joint mean spacing, is continuous, with moderately rough unweathered surfaces, no infill and is dry. $J_2$ has a spacing of $20cm,$ is planar and discontinuous, with signs of water and is moderately rough with some iron staining. $J_3$ is undulating and continuous, spaced $20-30$ cm apart, relatively smooth, no infill and dry, but with iron staining. Intact rock uniaxial compressive strength is $80$MPa, and overburden density is $2720k\frac{g}{m^3}.$

$2\mathrm{.}1\mathrm{.}1.$ Calculate the vertical virgin stress at a depth of $1500m$ below surface. Comment on the ratio of rock strength to stress and likely consequences thereof.

$2\mathrm{.}1\mathrm{.}2.$ Calculate the number of joints per unit volume of the rock mass. $(2)$

$2\mathrm{.}1\mathrm{.}3.$ Estimate the $RQD$ of the rock mass

$2\mathrm{.}1\mathrm{.}4.$ List the six variables in Barton's Q System. Determine their values and calculate the Q$-$rating of the rock mass using the attached tables $($ANNEXTURE $1).$

$2\mathrm{.}1\mathrm{.}5.$ The Geomechanics Classification of the rock mass is $69,$ indicating Good Rock. Compare this to your Q$-$rating above and provide possible reasons for any difference between the ratings.

$2\mathrm{.}1\mathrm{.}6.$ What adjustments are required to the basic RMR value to obtain an overall rock mass description?

$2\mathrm{.}2.$ Identify the criterion utilized for evaluating the strength of in$-$situ rock intersected by numerous closely spaced joint sets with various angles. Discuss the limitations associated with this criterion $(5)$

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