Engineering & Environmental Geology

Rock Mechanics & Hydrogeology

RQD, Mohr–Coulomb joint strength, slope stability and Darcy flow — the geomechanics and groundwater of applied geology.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Engineering geology evaluates rock and soil as materials. Rock-mass quality is graded by RQD (sum of intact core pieces ≥ 10 cm over the run length). Discontinuity strength follows the Mohr–Coulomb law

τ = c + σ_{n} tan ϕ

; slope stability is the ratio of resisting to driving forces (factor of safety). Groundwater flows by Darcy's law

Q = K i A

, with transmissivity

T = K b

describing an aquifer's capacity to transmit water.

The Core Formula Matrix

RQD:

\frac{\sum ( intact pieces \geq 10 c m )}{run length} \times 100

.

Mohr–Coulomb:

τ = c + σ_{n} tan ϕ

.

Factor of safety:

F S = \frac{resisting}{driving}

.

Darcy / transmissivity:

Q = K i A

T = K b

Joint failure envelope: c=30 kPa, φ=30°.

The ‘IIT Traps’

⚠
RQD uses ≥10 cm pieces only. Shorter fragments don't count.
⚠
φ is the friction angle, not the dip. Use $tan ϕ$ for strength.
⚠
Darcy velocity ≠ pore velocity. Seepage velocity $= K i / n$ (divide by porosity).

📚 Standard references

Engineering Geology — F.G. Bell
Foundations of Engineering Geology — Tony Waltham

PART 2

Progressive 3-Tier Question Suite

Q1MEDIUM2 Marks · NAT

A rock joint has

c = 30

kPa and

ϕ = 3 0^{\circ}

. Under a normal stress of

100

kPa, its shear strength is _____ kPa.

c=30 kPa, φ=30°.

kPa

Q2BASIC1 Mark · NAT

In a

150

cm core run, intact pieces ≥10 cm total

120

cm. The RQD is _____ %.

Q3HARD2 Marks · NAT

An aquifer has

K = 5 \times 1 0^{- 5}

m/s and saturated thickness

20

m. Its transmissivity is _____ ×10⁻³ m²/s.