Environmental Engineering Reference
In-Depth Information
TABLE 3.31
Values of Vertical Modulus of Subgrade Reaction (
k
t
) for Plates on Sand
a
Relative Density of Sand
Loose
b
Medium
b
Dense
b
Dry or moist sand, limiting values for
k
t
20-60
60-300
300-1000
Dry or moist sand, proposed values for
k
t
40
130
500
Submerged sand, proposed values for
k
t
25
80
300
a
From Terzaghi, K.,
Geotechnique,
4, 1955.
b
k
t
in ton/ft
3
for square plates, 1
1 ft, or beams 1 ft wide (1 ton/ft
3
0.032 kg/cm
3
32 ton/m
3
).
TABLE 3.32
Values of Vertical Modulus of Subgrade Reaction (
k
t
) for Plates on Preconsolidated Clay
a
Consistency of Clay
Stiff
b
Very stiff
b
Hard
b
Values of
U
c
(ton/ft
2
)
1-2
2-4
4
Ranges for
k
t
, square plates
50-100
100-200
200
Proposed values, square plates
75
150
300
a
From Terzaghi, K.,
Geotechnique,
4, 1955.
b
k
t
in ton/ft
3
for square plates 1
1 ft, or long strips 1 ft wide (1 ton/ft
3
1.15 lb/in.
3
).
less constant as long as the consistency remains constant. The test is not applicable to
weaker clays because of the effect of consolidation.
Applications
The modulus of vertical subgrade reaction is used to determine shears and bending
moments resulting from deflections of pavements, mat foundations, and continuous foot-
ings by employing the concept of a beam or plate on an elastic subgrade.
The modulus of horizontal subgrade reaction is determined by a pile-load test. It is used
for the evaluation of shears and bending moments in flexible retaining structures and lat-
erally loaded piles, employing the concept of a beam or plate on an elastic subgrade.
Full-Scale Load Tests
Footings
The test is performed in a manner similar to the plate-load test except that a poured con-
crete footing is used rather than a plate. Tests performed on various sizes and at various
depths provide much useful information on the compressibility of sand subgrades, and
offer the most reliable procedures for measuring settlements in these materials if repre-
sentative sites are tested.
Pile-Load Tests
Vertical pile-load tests are performed to obtain measures of
E
and shaft friction in large-
diameter piles, or end-bearing capacity and shaft friction on slender piles. A load test
setup is illustrated in
Figure 4.36.
Lateral load tests are performed on piles to obtain data on the horizontal modulus of
subgrade reaction
K
sh
.
