May/June 2022 - 18

were attached diametrically opposed to the reinforcing cage.
The bottom two levels were in the rock socket area below the
steel casing. A photograph of the frame with the O-cell
assembly is provided in Figure 2.
To maximize the mobilized end bearing at a lower cost, the
concept known as the " Chicago Method, " which consists of
utilizing a smaller base area to maximize the unit base
pressure, was used. Since the bottom O-cell plate is smaller
than the upper one and much smaller than the shaft diameter,
a larger unit end bearing pressure is mobilized.
* Length of pile below bottom of O-cell plate: 100 mm (3.9 in)
A summary of tested drilled shafts dimensions are as
* O-cell top plate diameter:
* O-cell bottom plate diameter:
Lateral Load Test
* Socket length:
880 m (2,887 ft)
720 mm (28.3 in)
3.1 m (101.7 ft)
LoadTest also performed lateral load tests in accordance with
ASTM D3966, Standard Test Methods for Deep Foundations
Under Lateral Load. The drilled shafts tested previously for
axial load (O-cell) were used as reaction piles and were
located at about 6 m (19.6 ft) from the laterally tested shafts. In
each tested shaft, two inclinometer casings were attached to
the reinforcing cage and positioned at 90° to the direction of
loading (See Figure 3). In each of the two casings, 7
inclinometers were installed to monitor the variation of
deflection response with depth.
follows:
Figure 2: O-cell assembly
Figure 4: O-cell for lateral load test
Selected Load Test Results and Analysis
Axial Load Test - As mentioned earlier, one of the load test
objectives was to determine the load distribution in the shaft
socket, i.e., the proportion of the total axial load reaching the
base of the shaft.
At the end of the load test, the downward and upward loads
were about 20.5 MN (4,608 kips). The upper shaft
displacements (above the O-cell) were 2.0 (0.08 in) (limestone),
3.6 (0.14 in) (dolomite) and 4.8 mm (0.1 in) (shale). The skin
friction in all cases was at or near its ultimate value. The
mobilized unit skin friction was 1.87 MPa (271 psi) for dolomite,
1.80 MPa (261 psi) for limestone and 1.6 MPa (232 psi) for shale.
The displacements of the bottom plate (below the shaft tip)
Figure 3: Rebar cage with inclinometers for the laterally tested shafts
The hydraulic jack used to apply the load consisted of a
330 mm (12.9 in) diameter O-cell calibrated to 4,996 kN
(1,123 kips) (See Figure 4).
A W12x79 beam section connected the loading assembly
to the reaction pile (See Figure 5). The loading was applied in
10 increments in accordance with the ASTM Standard.
18 * DEEP FOUNDATIONS * MAY/JUNE 2022
were in the range between 9.6 mm (0.37 in) (dolomite) and
14.2 mm (0.55 in) (limestone). At these displacements, the
mobilized end bearing was only a small portion of the ultimate
end resistance.
Plotting on the same graph, the mobilized skin friction and
tip resistance against the displacement shows that approximately
9 MN (2,023 kips) for dolomite (See Figure 6), 5 MN
(1,124 kips) for limestone and 14 MN (3,147 kips) for shale
were mobilized in end bearing at the vertical displacement at
which maximum (near ultimate) skin friction was mobilized.
May/June 2022

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