May/June 2022 - 17

contribution, i.e., the proportion of the load reaching the
socket base at the displacement corresponding to maximum
skin resistance.
The most common method for estimating the tip
contribution is based on the elastic approach (Pells and
Turner, 1979) that assumes the concrete and rock materials
respond as elastic isotropic solids, and that the bonds along
the rock-concrete boundary are not broken. Charts and
figures have been developed by the authors for the
determination of load distribution in a rock socket.
Accordingly, the proportion of the load taken on the base of
the socket is a function of the geometry (the ratio of socket
length to diameter) and the ratio of socket stiffness to rock
mass stiffness.
Experimental data published in literature suggest that the
elastic approach is somewhat conservative, and that the
proportion of the load supported by the tip is larger than what
the elastic approach predicts.
Shaft Subject to Lateral Load - When designing rocksocketed
shafts subject to lateral loading, the analysis is
typically performed by representing geomaterials with
springs of different stiffness that simulate the loaddeformation
behavior of the respective layers. This method is
referred to as p-y analysis, which provide the soil resistance p
as a nonlinear function of the pile deflection y. LPile (Ensoft) is
the most used program for analysis of laterally loaded pile.
Several p-y soil and rock models are available in the literature
and implemented in LPile to simulate the behavior of soils
and rock.
Site Geologic Conditions
The general geology of the REM layout consists of a till deposit
overlying the bedrock at varying depths (2 to 17 m [6.5 to
55.7 ft]). The overburden soil overlying the till is mostly
granular fill. The surface of the bedrock is altered and
fractured for depths varying from 1 to 3 m (3.2 to 9.8 ft). Three
rock types are encountered along the 67 km (41.6 mi)
alignment: limestone, dolomite and shale.
Experimental Program
One full-scale, bidirectional Osterberg Cell (O-cell) static load
test was performed in each of the rock formations encountered
along the alignment. Moreover, two fully instrumented lateral
load tests were performed in critical areas.
Prior to carrying out the static load tests, intensive soil and
rock investigation was performed at the load test locations,
which included SPT (standard penetration test) borings, CPT
(cone penetration testing), vane shear in cohesive soil,
dilatometer and pressuremeter tests. Rock core samples were
inspected and logged by experienced geologists and a detailed
record of the rock structural descriptions, including
condition, orientation, spacing, weathering and alteration of
di scont inui t i es , was car r i ed out for rock mass
Axial Load Tests
The bidirectional static O-cell load tests on full-scale
nonproduction test shafts were performed by LoadTest, in
accordance with ASTM D1143 Loading Procedure A - Quick
Test. The tests were performed using one 13.8 MN (3,102 kips)
bidirectional embedded jack (O-cell) to load the base area of
the shaft against the side resistance of the socket above the
base. Four levels of two sister bar vibrating wire strain gages
DEEP FOUNDATIONS * MAY/JUNE 2022 * 17
characterization including the determination of the
Geological Strength Index (GSI). The GSI, along with the
pressuremeter results, were used to accurately estimate the
rock mass stiffness (modulus).
The sacrificial load test shafts were constructed using a
1,300 mm (51 in) diameter permanent steel casing. The
overburden was excavated with an auger mounted on the
drilling equipment. Once the shafts were sealed into sound
rock, an 1,180 mm (70.8 in) diameter auger drilled the rock
surface at the bottom of the steel casing. A cleaning bucket
was used to clean and flatten the surface. Then the bottom of
the rock socket was cleaned with an airlift multiple times
until the tolerance requirements were met. The concrete was
then pumped through a 127 mm (5 in) outside diameter
tremie line into the base of the shafts. Upon completion of
successful drilled shaft construction, concrete integrity was
verified using crosshole sonic logging (CSL).
Figure 1 shows the placement of the casing and
reinforcing cage.
Figure 1: Test pile construction:
placement of reinforced cage
May/June 2022

Table of Contents for the Digital Edition of May/June 2022
