Earthworks - Geotechnical Investigation and Design Tables - page 139

•

The blasting term as used here refers to the difficulty level and can include rock
breakers, or expanding grouts.

12.4 Excavation characteristics
•

The excavatability characteristics based on rock hardness and strength.

•

The above is combined with its bulk properties (seismic velocity) and joint spacing.

Table 12.4 Excavation characteristics (Bell, 1992).

Rock hardness Unconfined compressive Seismic wave Spacing of Excavation

description strength (MPa) velocity (m/s) joints (mm) characteristics

Very soft 1.7-3.0 450-1200 50 Easy ripping

<

Soft 3.0-10 1200-1500 50-300 Hard ripping

Hard 10-20 1500-1850 300-1000 Very hard ripping

Very hard 20-70 1850-2150 1000-3000 Extremely hard

Ripping or blasting

Extremely hard 70 2150 3000 Blasting > > >

•

Table below combines both factors of strength and fractures into one assessment.

12.5 Excavatability assessment
•

The excavatability data shown are extracted from charts. It is therefore approxi-
mate values only.

•

Higher strengths combined with closer discontinuity spacing shifts the excavata-
bility rating.

Table 12.5 Excavatability assessment (Franklin et al. 1971 with updates from Walton and Wong, 1993).

Parameter Easy digging Marginal digging Blast to loosen Blast to fracture

without blasting

Strength, I s (50) (MPa) 0.1 0.3 0.3 0.3 < < > >

Discontinuity spacing (m)

<

0.02

<

0.2 0.2 to 0.6

>

0.6

RQD (%) 10% 90% 90% 90%

< < > >

•

Blast to loosen can be equated to using a rock breaker.

•

Ripping involves using a tine attached to the rear of the bulldozer.

12.6 Diggability index
•

The rock weathering term is another term incorporated in this table as well as the
type of equipment (backhoe or excavator).

•

This table classifies the diggability only. The following table provides the
implication for the type of equipment.