The Flat Dilatometer Test (DMT) in Soil Investigations - Marchetti DMT

NOTE: Possible problems with light rigs
Possible problems with light rigs (such as many SPT
rigs) are:
– Light rigs have typically a pushing capacity of only
2 tons, hence refusal is found very soon (often at
1-2 m depth).
– Often there is no collar near ground surface (i.e. no
ground surface side-guidance of the rods).
– There is a hinge-type connection in the rods just
below the pushing head, which permits excessive
freedom and oscillations of the rods inside the hole.
– The distance between the pushing head of the rig
and the bottom of the hole is several meters, hence
the free/buckling length of the rods is high. In some
cases the loaded rods have been observed to
assume a "Z" shape.
– Oscillations of the rods may cause wrong results.
In case of short penetration in hard layers it was
occasionally observed that the "Z" shape of the
rods suddenly reverted to the opposite side. This is
one of the few cases in which the DMT readings
may be instrumentally incorrect: oscillations of the
rods cause tilting of the blade, and the membrane is
moved without control close to/far from the soil.
NOTE: Pushing vs driving
Various researchers (US DOT 1992, Schmertmann
1988) have observed that "hammer-driving alters the
DMT results and decreases the accuracy of
correlations", i.e. the insertion method does affect the
test results, and static penetration should be
preferred.
According to ASTM (1986), in soils sensitive to
impact and vibrations, such as very loose sand or
very sensitive clays, dynamic insertion methods can
significantly change the test results compared to
those obtained using a static push. In general,
structurally sensitive soils will appear conservatively
more compressible when tested using dynamic
insertion methods. In such cases the engineer may
need to check such dynamic effects and, possibly,
calibrate and adjust test interpretation accordingly.
US DOT (1992) recommends that, if the driving
technique is used, as a minimum 2 soundings be
performed side by side, one by pushing and one by
driving. This would give a site/soil specific
correlation, which would allow to get back to the
parameters obtained from correlations based on the
pushing insertion (with added imprecision, however).
According to Eurocode 7 (1997), driving should be
avoided except when advancing the blade through
stiff or strongly cemented layers which cannot be
penetrated by static push.
3.2 PUSH RODS
While in principle any kind of rod can be used, most
commonly CPT rods or drill rig rods are employed.
A friction reducer is sometimes used. However the
consequent reduction in rod friction is moderate,
because of the multi-lobate shape of the cavity
produced in the penetrated soil by the blade-rod
system.
If used, the friction reducer should be located at
least 200 mm above the center of the membrane
(Eurocode 7 1997).
NOTE: Use of stronger rods
Many heavy penetrometer trucks performing DMT
are today also equipped with rods much stronger than
the common 36 mm CPT rods. Such stronger rods
are typically 44 to 50 mm in diameter, 1 m length,
same steel as CPT rods (yield strength > 1000 MPa).
A very suitable and convenient type of rod is the
commercially available 44 mm rod used for pushing
15 cm 2 cones.
The stronger rods have been introduced since the
rods are "the weakest element in the chain" when
working with heavy trucks and the current high
strength DMT blades, able to withstand a working
load of approximately 250 kN.
The stronger rods have several advantages:
– Capability of penetrating through cemented
layers/obstacles.
– Better lateral stability against buckling in the first
few meters in soft soils or when the rods are
pushed inside an empty borehole.
– Possibility of using completely the push capacity of
the truck.
– Reduced risk of deviation from the verticality in
deep tests.
– Drastically reduced risk of loosing the rods.
Obvious drawbacks are the initial cost and the
heavier weight. Also, their use may not be convenient
in OC clay sites because of the increased skin friction.
3.3 ROD ADAPTORS
The DMT blade is connected to the push rods by a
lower adaptor (Fig. 8).
The most common adaptor has its top connectable
to CPT rods, its bottom connectable to the DMT
blade (ending cylindrical male M27x3mm).
If rods other than CPT rods are used, specific
adaptors need to be prepared (see Fig. 8).
An upper slotted adaptor is also needed to allow
lateral exit of cable, otherwise pinched by the pushing
head.
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