150 Ground improvement by deep vibratory methods
combination of the densification effect of vibro compaction with the reinforcement effect of stone columns. In these instances, the degree of compaction can be measured in the sand layers by the methods described in
Section 3.4. Improvement of the cohesive soils is then achieved in the first
place by a sufficiently high area replacement value a c and the friction angle
φ c of the stone column material.
The stiffness of the improved soil is controlled primarily by the shear
strength of the stone column material. It is therefore recommended to verify, for each major site, the friction angle φ c of the stone backfill chosen in
the design with suitable laboratory tests, if insufficient experience with the
material exists. In exceptional cases, and if the size of the project merits
the expenditure, the shearing resistance of the stone column alone or of
stone column and tributary soil (i.e., the unit cell) can also be measured
directly on-site by full-scale shear tests (see Figure 2.15). Barksdale and
Bachus (1983) recommended these shear tests to be performed as double
ring tests as described in the Jourdan Road Terminal Test Embankment
report (Parsons-Brinkerhoff et al., 1980).
Suitability of the stone backfill material in terms of hardness and abrasion resistance may be controlled by relevant testing methods such as the
Los Angeles Test following ASTM C131 designations or similar regulations
such as the European standards EN 1097–2 and EN 13450.
When verification of the performance of single or strip foundations on
column groups is needed, full-scale loading tests are recommended with
actual footing dimensions because of the complex interaction between
stone columns, footing size, and stiffness. Alternatively, it may be possible
to achieve sufficient confidence in some cases by performing tests with a
footing on at least three columns.
For verification of the settlement performance of large column groups
or infinite column grids, zone tests are necessary for modeling this loading
scenario realistically. Load tests on single columns with the load only being
applied directly on the column itself do not reflect actual stress conditions
in these situations.
In cases where the soft soil to be improved is close to foundation level,
say up to five times the unit cell diameter d e , load tests on single columns
are, however, indicative for the performance of an infinite column grid
with the test footing size being equal to the unit cell area A
d
=
⋅ ⋅
0
2
.
.
25
e
π
From the load-settlement curve of this unit cell loading test, an equivalent
Young’s modulus E* can be defined as the ratio of the uniform load q
multiplied by an equivalent column length l* and divided by the measured
settlement s m . The uniform load q on the infinite grid causes the settlement
of the infinite grid s g with the actual column length l and the equivalent
modulus E* as follows:
q l
s
E
q l
s
⋅ =
=
⋅
*
*
m
g
(4.47)
combination of the densification effect of vibro compaction with the reinforcement effect of stone columns. In these instances, the degree of compaction can be measured in the sand layers by the methods described in
Section 3.4. Improvement of the cohesive soils is then achieved in the first
place by a sufficiently high area replacement value a c and the friction angle
φ c of the stone column material.
The stiffness of the improved soil is controlled primarily by the shear
strength of the stone column material. It is therefore recommended to verify, for each major site, the friction angle φ c of the stone backfill chosen in
the design with suitable laboratory tests, if insufficient experience with the
material exists. In exceptional cases, and if the size of the project merits
the expenditure, the shearing resistance of the stone column alone or of
stone column and tributary soil (i.e., the unit cell) can also be measured
directly on-site by full-scale shear tests (see Figure 2.15). Barksdale and
Bachus (1983) recommended these shear tests to be performed as double
ring tests as described in the Jourdan Road Terminal Test Embankment
report (Parsons-Brinkerhoff et al., 1980).
Suitability of the stone backfill material in terms of hardness and abrasion resistance may be controlled by relevant testing methods such as the
Los Angeles Test following ASTM C131 designations or similar regulations
such as the European standards EN 1097–2 and EN 13450.
When verification of the performance of single or strip foundations on
column groups is needed, full-scale loading tests are recommended with
actual footing dimensions because of the complex interaction between
stone columns, footing size, and stiffness. Alternatively, it may be possible
to achieve sufficient confidence in some cases by performing tests with a
footing on at least three columns.
For verification of the settlement performance of large column groups
or infinite column grids, zone tests are necessary for modeling this loading
scenario realistically. Load tests on single columns with the load only being
applied directly on the column itself do not reflect actual stress conditions
in these situations.
In cases where the soft soil to be improved is close to foundation level,
say up to five times the unit cell diameter d e , load tests on single columns
are, however, indicative for the performance of an infinite column grid
with the test footing size being equal to the unit cell area A
d
=
⋅ ⋅
0
2
.
.
25
e
π
From the load-settlement curve of this unit cell loading test, an equivalent
Young’s modulus E* can be defined as the ratio of the uniform load q
multiplied by an equivalent column length l* and divided by the measured
settlement s m . The uniform load q on the infinite grid causes the settlement
of the infinite grid s g with the actual column length l and the equivalent
modulus E* as follows:
q l
s
E
q l
s
⋅ =
=
⋅
*
*
m
g
(4.47)