Does water lubrication affect friction differently for rocks and soils? Evidence and open questions

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Static friction coefficient of minerals under different humidity conditions (data from Horn and Deere 1962)_

Mineral	Oven dried	Saturated
Rose quartz	0.13	0.45
Microcline feldspar	0.12	0.77
Calcite	0.14	0.68
Muscovite	0.43	0.23
Chlorite	0.53	0.22

Morphological and friction parameters for two granular soils (DNA-1A and OS) with different roughness, under both dry and wet conditions; the interparticle friction angle is assessed based only on tests carried out at 1 N normal contact force (data from Marzulli et al_ 2021)_

Material	Particle size (for microtest), mm	Hardness, GPa	Roundness	Roughness, nm	φ′bulk low stresses range (°)	φ′bulk medium–higher stresses range (°)	φ′μ (°)
DNA-1A dry	1.0–1.8	0.3	0.6	1476	47.9	40.8	21
DNA-1A wet					45.9		30
OS dry	0.5–0.8	5.8	0.8	204	45.0	36.5	7.6
OS wet					40.8		-

Measured values of φb under different moisture conditions for granite and sandstone (data from Li et al_ 2020)_

Rock type	Moisture condition	φb (°)
Granite	Dry	33.46
	Wet surface	30.59
	Saturated	30.38
Sandstone	Dry	35.30
	Wet surface	32.92
	Saturated	32.62

Shear strength of rock fractures under dry and wet conditions (data from Barton 1973; Alejano et al_ 2012)_

Rock type	φb (°)
	Dry	Wet
Mudstone	32	29
Siltstone	32	30
Limestone	34	31
Sandstone	31	29
Marble	49	42
Shale	29	21
Granite	33	31

Values of residual friction angle of Campolattaro marls for different conditions of humidity and shearing direction (data from ISMES 1982)_

Sample joint	Shearing direction	Number of data	Friction angle by linear regression (°)	Average friction angle (°)
Saturated	Parallel to joint	8	17.7	16.3
Dry	Parallel to joint	9	38.0	37.8
Dry	Perpendicular to joint	8	44.4	44.5