87
OF ENGINEERING FORMCTLÆ.
Mining. (Lefroy’s 'Handbook of Field Service.’)
In the demolition of walls the line of least
resistance L = half the thickness, and a is a
coefficient depending on the structure.
The charge in lbs. = a X L3. __
In a wall without counterforts, where the in-
terval between the charge is 2L, a = 0'15.
In a wall with counterforts the charge to be
placed in the centre of each counterfort at the
junction with the wall, a = 0’2.
Where the charge is placed under a foundation,
having equal support on both, sides, « = 0'4.
If the foundation rests on woodwork, a = 0’5
to 0 • 6. .
If the charge is placed'in the centre of a cir-
cular or polygonal mass of masonry, a — 0 1.
A leather bag containing 50 or 60 lbs. of pow-
der, hung or propped up, will demolish almost any
gate or barrier.
For ordinary mines in average soil, the charge
L3
in lbs. = — ■
Bulk of Rock EARTtfwoKK, &c., when placed
in Embankment, original Excavation being
ASSUMED AT 1*00.
Rock, large blocks .. about
Medium, miselected__________
Metal.......................
Chalk ___________________
Sand, and gravel ..........
Clay and earth after subsidence
„ „ before „
__________________________________
1 50
1-25 to 1 30
1-20
130
107
I-08
1-20