Weathering Influence on Properties of Siltstones from Istria, Croatia

Table 3

Descriptions, typical characteristics, ranges of field-estimated uniaxial compressive strengths, and point load strength index values for siltstones of different weathering grades in the Istria Peninsula.

Term	Symbol	Description and typical characteristics [15]	Grade [15, 40]	Description and typical characteristics of siltstone in the study area [30]	Uniaxial compressive strength, (MPa); min.–mean–max. [30, 41]	Point load strength index, (MPa) mean
Fresh (unweathered)	FR	No visible sign of rock material weathering. There may be slight discoloration on major discontinuity surfaces.	I; R4	Grey, fresh, and intact siltstone. No signs of weathering. Slight discoloration along cracks. Water impermeable. Breaking of sample requires more than one blow of geological hammer. Original rock structure is visible.	46–55–65	2.2
Slightly weathered	SW	Discoloration indicates weathering of rock material and discontinuity surfaces. All the rock material may be discolored by weathering and may be somewhat weaker than its fresh condition.	II; R3	Bright grey siltstone with slightly weathered joint surfaces. Water impermeable. Sample cannot be cut with a knife but it can be broken in blocks with one blow of geological hammer. Original rock structure is visible.	27–33–38	—
Moderately weathered	MW	Less than half of the rock material is decomposed and/or disintegrated to a soil. Fresh or discolored rock is present either as a discontinuous framework or as corestones.	III; R2	Grey to brown siltstone with brown joint surfaces and low water permeability of discontinuities. Sample can be cut with a knife with difficulty and some blocks can be easily broken with light blows of geological hammer. Original rock structure is visible.	20–22–25	0.9
Highly weathered	HW	More than half of the rock material is decomposed and/or disintegrated to a soil. Fresh or discolored rock is present either as a discontinuous framework or as corestones.	IV; R1	Brown siltstone with frequent microcracks and brown weathered surface. High water permeability of discontinuities. Sample disintegrates with light blow of geological hammer and can be cut with a knife. Original rock structure is visible.	<20; NA	NA
Completely weathered	CW	All rock material is decomposed and/or disintegrated to soil. The original mass structure remains largely intact.	V; R0	Light brown silty-clayey soil mixed with remains of siltstone. Pieces can be broken by hand. Original rock structure is visible.	NA
Residual soil	RS	Rock is completely altered to a soil with the original fabric destroyed.	VI; R0	Brown siltstone coarse debris material mixed with clayey, sandy silt. Pieces can be broken by hand.	NA

Schmidt hammer results on rocks with a uniaxial compressive strength below 20 MPa are likely to yield highly ambiguous results.
NA not applicable.