Table 11-1. Excavation Protection.
Soil Type
Identification
Problems and Mechanism
Preventative Measures
Overconsolidated
(1) Stiff plastic clays,
(1) One dimensional
(1) Rapid collection of sur-
clays
natural water content
heave, maximum
face water, or grading
near plastic limit
heave at excava-
around excavation
(2) See figure 3-14 for
tion center
(2) Deep pressure relief to
swelling potential
(2) Swelling dependent
minimize rebound
on plasticity
(3) Place 4" - 6" working mat
(3) Usually fractured
of lean concrete immedi-
and fissured. Ex-
ately after exposing sub-
cavation opens
grade (mat may be placed
these, causing
over underseepage and
softening and
pressure relief systems
strength loss
placed in sand blanket)
(4) If sloped walls, use as-
phalt sealer on vertical
walls, burlap with rubber
sheeting or other mem-
brane on flatter slopes
Chemically
(1) Lab testing
(1) Swelling, slaking
(1) Protect from wetting and
inert, unce-
(2) Soil and geologic maps
and strength loss
drying by limiting area
mented clay-
(3) Local experience
if water
open at subgrade
stone or shale
infiltration
(2) Concrete working mat
(2) Rebound if over-
(3) Paving, impervious mate-
consolidated
rials to avoid water in-
(generally the
filtration; place sealing
case)
coats immediately after
(3) Cracking if dry
exposure
or evaporation
(4) Reduce evaporation and
allowed
drying to prevent
swelling on resaturation
(Continued)
(Sheet 1 of 3)
11-6
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