During the past few years a great number of experimental model and numerical analysis results on the uplift resistance of anchor plate embedded in homo geneous cohesion less soil has been reported by many researchers. A review of related literature shows that not much research has been done to analyze the performance of anchor plates in layered soils a problem, which is often encountered by the professional engineers in the field.
Although some experiments, beginning from Stewart (1985)
that studied the behavior of anchor plate embedded in a saturated clay layer
overlain by a compacted sand deposit, the experimental consisted of laboratory
model tests on circular anchor plate of 50 mm diameter and 5 mm thick as shown
in Fig. 1. From this research it was found that the cohesion
less soil overlay significantly increased the ultimate uplift capacity of the
anchor plate compared with its value when embedded in clay alone that Fig.
2 shows them. The increase in uplift capacity is due to two main factors.
The first is the additional over burden pressure, which converts the original
shallow anchor plate into a deep anchor plate; the second is the mobilization
of the frictional loading of the overlays. He showed that large displacements
were needed to mobilize the frictional loading of the overlying cohesion less
Sutherland (1988) pointed out that in practice little
real benefit to uplift capacity could be a anchor plate embedded in cohesive
soil since a large displacement is needed to mobilize the shear strength of
the overlays. It was suggested that achieved by placing cohesion less soil over
if a sand overburden were to be used, a more sensible solution would be to place
the anchor plate on the surface of the cohesive soil layer and then place cohesion
less soil on top.
Manjunath (1998) suggested a theory for define of vertical
uplift capacity of a shallow horizontal strip anchor plate in two layered frictional
- cohesive soil. The effect of surcharge has also been considered. The theory
has been developed by using theory of characteristics coupled with log spiral
failure surface having different foci for different layers shown in Fig.
3. Uplift capacity factors, separately for cohesion (Fc), surcharge
(Fq) and unit weight (Fγ) have been presented as
a function of embedment ratio, friction angle of each layer and ratio of top
layer to the total embedment depth.
||Experimental investigations layered soil system used by Stewart
||Load-displacement curves of anchor plates in layered soil
system by Stewart (1985)
Hence using these factors for the selection values of friction angle of bottom
layer (φb), top layer (φt) and embedment ratio
the vertical uplift capacity of shallow horizontal strip and circular anchor
plates can be defined in two layered soils with surcharge for any value of Df/D
ratio. The net average ultimate uplift capacity (U ↓ (u- net) has been
written in the form:
Uunet = (dt ct
+ db cb) Fc + qFq + 0.5 (B
drγr + dbγb) Fγ)
And the average ultimate uplift capacity (Uu) is as:
Uu = Uunet + (Drγr
Niroumand and Kassim (2010a, b)
reported the behavior of an irregular anchor plate buried in a two layered frictional-cohesive
||Faliure mechanism in two layered soils by Manjunath
(1998). where, γot: Initial length of radial line of
log-spiral failure surface in the top layer, γob: Initial
length of radial line of log-spiral failure surface in the bottom layer,
γt: Final length of radial line of log-spiral failure surface
in the top layer, γb: Final length of radial line of log-spiral
failure surface in the bottom layer, βt: angle between the
final radial line () of log-spiral failure surface in the top layer and
the horizontal at the interface
The testing program consisted of two 159 and 297 mm long irregular anchor plates
buried in soft clay overlain by loose sandy soil as shown in Fig.
4. The uplift tests were carried out on an irregular anchor plate embedded
at a depth D in a combination of layers of clay-sand. The testing program consisted
of an irregular anchor plates buried in soft clay overlain by loose sandy soil
as shown in Fig. 5 (a, b). The thickness
of each layer was increased to a certain proportion of the anchor long and it
was increased from 1 to 4 and 1 to 7 times the long irregular anchor plates.
It was reported that for upper layer thickness ratio of less than one and for
a given ratio, D/B there was no difference between the uplifting an anchor plate
from a clay-loose sand bed.
For a given D/B ratio and the upper layer embedment ratio of 1 to 4 a clay-loose
sand bed in bigger irregular anchor plate gives a greater uplift than upper
layer embedment ratio of 1 to 7 a clay-loose sand bed in smaller irregular anchor
As a conclusion, this study shows that the last experimental works have been
done regarding to performance of the anchor plate in layered frictional-cohesive
soils. Inevitably such a wide range of parameters will contribute to conflicting
conclusions for the ultimate uplift load of the anchor plates. These researches
have been done, using different regular /irregular anchor plates and soil parameters.
Unfortunately, the results obtained from the laboratory tests are typically
a specific problem and are difficult to extend and develop to field problems,
due to the different materials or the geometric parameters used in the field
scale. It is observed that the ultimate uplifting capacity is dependent on the
relative strength of the two layers, the depth of embedment ratio and the upper
layer thickness ratio.