ENGINEER'S NOTEBOOK
A direct
r�ading
pocket shear
vane
STANLEY SEROTA. M. ASCE
Managlnr Director
ANIL JANGLE. A.M. ASCE
Senior Soll,test Ena!neer
Foundation Enalneerlng Ltd
London, England
Polnler
The
use of the in-situ vane device for
determining the undrained shear
strength or saturated clay has been
known since 1918. but it was not unlit
ufccr
the
development work o{ the
1940's that it was generally accepted
u. 11.n alternative to trinial tests on
undi'ilurbed samples from boreholes
and pits.
In 19S6, Scrota
devised
a
pocket-size vane instrument for the
11ole purp05e of assessing the shear
11trcn1th of cohesive soils in those cir
cum.'ltanc:es which ari5e so often dur
lr>g con111ruction work when a quick
�heck reading is cs!1cn1ial. This early
1ns1rument had two unfortunate char
acteristics: (I) The right-angled cor
nei:s of the vane tended to rouna off.
This "'as e.isily cured by giving them a
rudiu� in the first ptucc.
(2) The
in
!llrumcnt readings were in angular
measure, whic.:h had
10
be convenet.l
Doc plate
/
Dial
I 7� ..
Body
19mmvane
Fig, 1. Principe! components •.
into shear strengths Cram II c:aiihnnion
graph.
E?couragcd hy the di:;c:overy th:u
I!'@ mslrumcnt was giving more con
s1!1c�1 n:1111hs th:in the lahora1ory
lra�1ul. tcs1, a comprehensive in
vc.,11ga11np nnd dcvelopmenl p1'ognm
was un"'imukcn 10 determine hnw ac
�uralcly the vane would h.: rated and
lo improve its scope and precision as
both a field and lahoratory tool.
Special :lllcntion wai; riven 10: the
lorquc-dcffcltion characteristic."S; the ef,
f�t of ralc of ;ipplicatian to torque·
rcproducihilil� of results: optimu�
numher :ind i;1zes of vanes to give the
mosl useful working ran,e (Jr a 11inglc
torsion head: calibrating each vane
against rcmolded clays of accurately
known srrengrh: testing performance
in undi�lurhed s:imples: elimina1ing
the need for a calibration chart. The
pholo and Fig. I illustrate the in,
strumeni developed a� a result <1f 1h1s
program. The inner end of a steel he·
lical spring is held in a central spanJI.:
carrying the v11ne rod: the ou1er end is
attached ICI the aluminum casing. In
operation. the vane Is pushed into 1he
soil and the torsion head
ill
rotated al
constant speed. The torque is regis,
lcred by movement of a "maximum··
pointer from a factory-set zero, and
was shown to he virtually propor
tional to the applied shear stress for
each vane size. Every deviation from
regularity was round to he auributable
to human error in the corresponding
triaxial tc:.�t. The hc11t rate or applica•
tion
of the torque was
shown to be
one
revolution per minute-not
criti
cal
and ea$ilv paced by the second
hand
or
a watch (see Fig. 2).
Repraducihilitv test showed that the
ins1rument will iive a deviation from
average of less than one percent in
two readings
out
or three: in practice
the average of three readings is accept·
able, unlc.'iS Ofle
of
them shows a sig
nificant deviation.
Vanes
or
four differenl diameters
( O.S, 0. 7S. 1.3 and I.SS in.) were
tested and it was found 1ha1 two were
sufficient. The 1.3-in.-diam vane cov,
cred the strensth range or 200 to SOO
lh/:.q ft {normally consolidated cla)-s)
and overlapped well the range of the
0, 7 5-in.-diam vane. which proved to
be accurate up to 3.000 lb/sq ft in
remolded clays. There were Indications
that Che limit might be higher for un
disturbed clays.
For empirical calibration of the
vanes. clay was compacted dynami
cally in a C.B.R. mold and three l'h
!n.-diam sampling tubes were jacked
anto the
day.
Vane tests were made in
the clay betwc;en the tubes; then the
samples
were
extracted and tesled in
triaxial compression. A very large
Reprinted from Civll Enrfn1terln1-ASCE January 19'72
