Ruby - 139 129 126 123 121
Hoisted "puts" outside of array creation (suggestion from bitpwner).
puts (0..9).map{|i|(i*3...(i+1)*4).map{|j|"/#{'**'*j}\\".rjust(82+j,' ')}}+["%86s"%'|--------|']*11<<'%86s'%'|________|'
Ungolfed ("puts" unhoisted):
# print fill patterns from the intervals
# [0..3, 3..7, 6..11, 9..15, 12..19, 15..23, 18..27, 21..31, 24..35, 27..39]
# centered on columns 81-82
(0..9).each { |i|
(i*3...(i+1)*4).each { |j| # x...y vs x..y-1 saves a char
puts "/#{'**'*j}\\".rjust(82+j,' ')
}
}
# print the stump
puts ("%86s\n" % '|--------|') * 11
puts '%86s' % '|________|'