Civil Engineering Reference
In-Depth Information
predict settlement behavior will often produce highly inaccurate results, with the inaccuracy
increasing as the rock quality designation (RQD) decreases. Because of this limitation, field
tests are preferable, with some options for determining the allowable bearing pressure of
heavily loaded foundations on rock as follows:
1.
Foundation load test:
If the proposed foundation were to consist of piles or
piers founded on rock, then test piles or piers could be constructed and load tested. The
seismic behavior of the pile or pier could be determined by subjecting them to alternating
loads. Performing load tests would be the most accurate method of determining the load-
settlement behavior of a deep foundation element supported on rock (i.e., see ASTM D
1143, “Standard Test Method for Piles Under Static Axial Compressive Load”). Unfortu-
nately, such a test would be expensive and time-consuming and would normally only be
justified for essential facilities or very high foundation loads.
2.
Plate loading method:
Similar to a standard plate load test, the rock could be
subjected to a plate loading test in order to determine the load-deformation behavior of
the rock. If the rock should contain layers or seams of clay, the plate loading method may
significantly underestimate the amount of rock deformation. For further details, see ASTM
D 4394, “Standard Test Method for Determining the In Situ Modulus of Deformation of
Rock Mass Using the Rigid Plate Loading Method” and ASTM D 4395, “Standard Test
Method for Determining the In Situ Modulus of Deformation of Rock Mass Using the
Flexible Plate Loading Method.”
3.
Rock mass strength:
The allowable bearing pressure
q
all
could be based on the
rock mass strength. The approach would be to determine the unconfined compressive
strength
q
c
and then apply a factor or safety, or
q
all
q
c
/
F
. The goal is to take into account
the effect of both intact material behavior and the behavior of discontinuities and weak
layers contained within the specimen block. The test is described in ASTM D 4555, “Stan-
dard Test Method for Determining Deformability and Strength of Weak Rock by an In
Situ Uniaxial Compressive Test.” The procedure is to perform in situ compression tests
on specimens of rock that are large enough so that the rock mass unconfined compressive
strength
q
c
is obtained. Similar to the load tests described above, this approach is expensive
and time-consuming.
Two other approaches for determining the settlement of heavily loaded foundations on
rock are the elastic method and the finite element method. Unfortunately, both of these
methods require that the modulus of elasticity
E
and Poisson's ratio µ be determined for
the rock mass. A major limitation of these two methods is that the value of
E
is often
obtained from an unconfined compression test on a small rock specimen (i.e., ASTM
D 3148, “Standard Test Method for Elastic Moduli of Intact Rock Core Specimens in
Uniaxial Compression”). Using this approach will undoubtedly overestimate the value of
E
, with the overestimation increasing as the RQD decreases. Likewise, this approach will
overestimate the value of
E
for rock that contains layers or lenses of clay. The methods
will have much greater accuracy if the value of
E
is obtained from in situ tests on large
rock specimens (i.e., ASTM D 4555). For large-scale in situ tests on rock, the value of
E
is obtained from the vertical stress versus vertical strain curve and is often defined as the
tangent modulus at 50% of the maximum strength.
When using high bearing pressures for rock, it is essential that the footing or deep foun-
dation excavation be inspected and cleaned of all loose debris so that the foundation bears
directly on intact rock. Even a thin layer of disturbed and loose material left at the bottom
of the foundation excavation can lead to settlement that is greatly in excess of the calculated
static and seismic values. For heavily loaded foundations on rock, a one-third increase in
the rock bearing pressure is usually recommended for earthquake loading provided the rock
will not be weakened or fractured apart during the earthquake.
