Environmental Engineering Reference
In-Depth Information
Load Cells
Purpose
Load cells are used to measure loads on earth or rock anchors, in the various elements of
tunnel linings, in struts in braced cuts, at the bottom of piles (total pile load minus tip load
equals shaft friction), and at the top of piles in pile load tests.
Device
Cell function is often based on the electric transducer principle. The cells are mounted
within variously shaped containers. Various capacities are available.
Calibrated hydraulic jacks are used for the application of load to rock bolts, tieback
anchors, piles, drilled shafts, and cross-lot struts.
Tell Tales
Purpose
Tell tales are installed in piles at various depths for strain measurements during load test-
ing, from which shaft friction and end bearing are calculated.
Device and Installation
A small-diameter rod or pipe is fixed at the bottom of the concrete or steel pipe. It is
extended up through the pile top and encased in a slightly larger pipe to permit free move-
ment. A dial gage at the top reads deflections. A tell-tale set in the pile bottom provides a
measure of both base movement and shaft shortening when compared with the total pile
deflection measured at the top by separate instruments.
A group of rods is usually installed at the same depth in large-diameter piles, and often
at various depths in long piles. They are distributed as a function of the soil profile to
measure shaft friction for the various soil types.
Analysis
The average load
P
s
from shaft friction, carried by a segment of the pile
∆
L
from the meas-
urement of two tell tales, is expressed as
P
s
a
s
E
p
((
R
1
−
R
2
)/
∆
L
)
(4.1)
Pressure chamber
Bypass orifice
Return line
Filling tube
Gloetzl cell
Pressure in material
Filter
Precision ground
orifice
Pressure diaphragm
Oil-filled space
Pressure line
Pressure
pad sensor
FIGURE 4.30
Schematic of the hydraulic pressure cell. (From Hartman, B. E.,
Terrametrics
, Wheatridge, Colorado, 1966. With
permission.)
