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74
hydraulically through lines extending from the top unit
(sometimes called a rodless drill), which is fixed to the top of
the casing. Alternatively, a drill rod may be used to transmit
torque from the top unit to the bit. The bit has a central hol-
low orifice connected to a flexible line extending back up to
the top unit. During drilling, a vacuum pump or air lift is used
to draw the drilling fluid with the cuttings upward to a clean-
ing plant, from where it is circulated back into the hole. The
unit shown in Figure 56 was used to drill 3.35-m-diameter
rock sockets in Franciscan Formation sandstone and ser-
pentinite. Some manufacturers are now producing reverse
circulation units that can be installed on a conventional rotary
hydraulic drilling rig to provide similar capability, at a
smaller diameter. It is likely that these units will become
more common in North America for rock-socket drilling
FIGURE 55 Downhole hammer tool for drilling in hard rock. (D. Poland, Anderson Drilling, personal communication,
Aug. 2, 2005). Reverse circulation drilling can also be car-
A technique used for drilling large rock sockets at the ried out with any type of rotary drill rig equipped with a hol-
RichmondSan Rafael Bridge (Byles 2004) is reverse circu- low Kelly bar (drill stem) that allows circulation of the
lation drilling with a "pile top" rig. The unit consists of two drilling fluid from the cutting surface up through the bar.
main components. A top unit (Figure 56) is fixed to the top
of a steel casing. The "bottom hole assembly" (Figure 57) is
a drill bit lowered to the bottom of the hole through a casing, FIELD LOAD TESTING
submerged in water or other drilling fluid. The bit is operated
The most direct method to determine the performance of full-
scale rock-socketed drilled shafts is through field load test-
ing. Clearly there have been advances in engineers' ability to
predict rock-socket behavior. However, there will always be
sources of uncertainty in the applicability of analysis meth-
ods, in the rock mass properties used in the analysis, and with
respect to the unknown effects of construction. Load testing
provides direct measurement of load displacement response
for the particular conditions of the test foundation, and can
also provide data against which analytical models can be
evaluated and calibrated.
Objectives
Field load testing may be conducted with different objectives
and this should determine the scope of testing, type of tests,
and instrumentation. A partial listing of valid reasons for
transportation agencies to undertake load testing of rock-
socketed shafts includes:
· Confirm design assumptions,
· Evaluate rock resistance properties,
· Evaluate construction methods,
· Reduce foundation costs, and
· Research aimed at evaluating or improving design
methods.
More than one of these objectives can sometimes be
achieved. For example, load tests conducted primarily for
FIGURE 56 Toredo T40-4 pile top unit being placed over
confirmation of design assumptions (proof test) for a partic-
casing for reverse circulation drilling, RichmondSan Rafael ular project can be useful to researchers by contributing ad-
Bridge (California) (Byles 2004). ditional data for evaluating empirical correlations proposed
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FIGURE 57 Shrouded bottom hole assembly lifted for placement through
the top unit (Byles 2004).
for design. Load tests carried to ultimate capacity of the shaft as the basis for design of productions shafts. Items such as
are especially valuable not only to the agency conducting the construction method (casing, slurry, dry), type of drilling
test or for the specific bridge project, but to the entire deep fluid, cleanout techniques, and others may have influenced
foundation engineering community. the behavior of the test shaft. If possible, the construction
methods anticipated for production shafts should be used to
The costs of conducting field load tests should be offset construct test shafts.
by its benefits. The most obvious costs include the dollar
amount of contracts for conducting testing. Other costs that
are not always as obvious include construction delays, delays Axial Load Testing
in design schedule, and DOT person hours involved in the
testing. Direct cost benefits may be possible if the testing Conventional Axial Load Testing
leads to more economical designs. This requires testing prior
to or during the design phase. Numerous case histories in the Until the early 1990s the most common procedure for con-
literature show that load testing almost always leads to ducting a static axial compression load test on a deep founda-
savings. Lower factors of safety and higher resistance factors tion followed the ASTM Standard Method D1143, referred to
are allowed by AASHTO for deep foundation design when herein as a conventional axial load test. Several load applica-
a load test has been conducted. tion methods are possible, but the most common involves
using either (1) a hydraulic jack acting against a reaction
Other benefits may not be so obvious or may occur over beam that is anchored against uplift by piles or (2) a loading
time. Construction of the test shaft provides the DOT and platform over the pile top on which dead load is placed. Six
all subsequent bidders with valuable information on con- states indicated that they have conducted conventional axial
structability that can result in more competitive bids. Refine- load tests on rock-socketed shafts. Conduct and interpretation
ment in design methods resulting from information gained by of axial compression and uplift load tests specifically for drilled
load testing has economic benefits on future projects. shafts is discussed in detail by Hirany and Kulhawy (1988).
Load test results provide the most benefit when they are Axial load tests may be conducted for the purpose of con-
accompanied by high-quality subsurface characterization. firming the design load for a specific project, in which case
Knowledge of site stratigraphy, soil and rock mass properties, it is typical to load the shaft to twice the anticipated design
site variability, and groundwater conditions are essential for load to prove the shaft can support the load with an accept-
correct interpretation of load test results. The ability to apply able settlement (a proof load test). This type of test is nor-
load test results to other locations is enhanced when subsur- mally conducted under the construction contract and does not
face conditions can be compared on the basis of reliable data. yield a measured ultimate capacity, unless the shaft fails, in
which case the design must be adjusted. Proof tested shafts
Construction factors and their potential effects on shaft normally are not instrumented except to measure load and
behavior should be considered when using load test results displacement at the head of the shaft. When the objective of
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testing is to gain information on behavior of the shaft in terms The following case illustrates effective use of conven-
of load transfer, the shaft should be instrumented to deter- tional axial load test on rock sockets. Zhan and Yin (2000)
mine the distribution of axial load as a function of depth and describe axial load tests on two shafts for the purpose of con-
as a function of axial deformation. firming design allowable side and base resistance values in
moderately weathered volcanic rock for a Hong Kong tran-
Common types of instrumentation for measuring axial sit project. The proposed design end bearing stress (7.5 MPa)
load and deformation at specific points along the length of exceeded the value allowed by the Hong Kong Building
the shaft include sister bars and telltales. A sister bar is a sec- Code (5 MPa). One of the objectives of load testing was,
tion of reinforcing steel, typically 1.2 m in length, with a therefore, to demonstrate that a higher base resistance could
strain gage attached in the center. Either vibrating wire or be used. The project involved 1,000 drilled shafts; therefore,
electrical resistance-type gages can be used. The sister bar is proving the higher proposed values offered considerable
tied to steel of the reinforcing cage and its lead wires are potential cost savings.
routed to the surface, where they are monitored by a com-
puter-controlled data acquisition unit. The gage signals are Figure 58 shows the load test arrangement, consisting of
converted to strain, which is assumed to be equal to the a loading platform for placement of dead load. Figure 59
strain in the concrete and can be used to estimate load using shows details of one of the instrumented shafts. Strain gages
the appropriate elastic modulus and section properties of the were provided at 17 different levels, including 4 levels of
shaft. A telltale is a metal rod installed within a hollow tube gages in the rock socket. Two telltales were installed, one at
embedded in the shaft. The bottom end of the rod is fixed at the base of the socket and one at the top of the socket. Shafts
a predetermined depth in the shaft and is the only point on the were excavated through overburden soils using temporary
rod in physical contact with the shaft. By measuring vertical casing to the top of rock. When weathered rock was encoun-
deformation of the upper end of the telltale during loading, tered, a 1.35-m-diameter reverse circulation drill (RCD) was
deformation of the shaft is determined for the depth at which used to advance to the bearing rock, followed by a 1.05-m-
the telltale is fixed. By measuring the relative displacement diameter RCD to form the rock socket. For the shaft shown
between two successive rods and distance between their in Figure 59, the socket was 2 m in length. A permanent,
bottom ends, the average strain in the shaft between the two bitumen-coated casing (to reduce side resistance in the over-
telltales can be determined. Further information on these and burden materials) was placed to the top of the socket. The
other types of instrumentation is given by Hirany and bottom was cleaned by airlift and concrete placed by tremie
Kulhawy (1988) and O'Neill and Reese (1999). (wet pour).
FIGURE 58 Axial load testing setup (Zhan and Yin 2000).
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FIGURE 60 Unit side and base resistance versus axial load
(Zhan and Yin 2000).
Testing Systems" was undertaken to evaluate these and other
methods for deep foundations and to recommend interim
procedures for their use and interpretation. A draft final
report by Paikowsky et al. (2004b) describes these methods
in detail. The role of each of these tests for rock-socketed
shafts is described here.
FIGURE 59 Details of instrumented rock-socketed shaft (Zhan
and Yin 2000).
Osterberg Load Cell
Figure 60 shows the results in terms of mobilized unit side The O-cell is a hydraulically operated jacking device that can
and base resistances versus load applied at the head of the be embedded in a drilled shaft by attachment to the reinforc-
shaft. Unit side resistance reached a value of 2.63 MPa, well ing cage (Figure 61). After concrete placement and curing, a
exceeding the proposed design allowable value of 0.75 MPa. load test is conducted by expanding the cell against the por-
Zhan and Yin noted that this value agrees well with Eq. 30 in tions of the shaft above and below it (Osterberg 1995). The
chapter three. Load transfer to the base was mobilized im- load is applied through hydraulic piston-type jacks acting
mediately upon loading, indicating excellent base conditions, against the top and bottom cylindrical plates of the cell. The
and reached a value exceeding 10 MPa. In the other shaft (not
shown) a unit base resistance of 20.8 MPa was reached with
no sign of approaching failure.
The case presented by Zhan and Yin demonstrates how a
set of well-instrumented conventional axial load tests can be
used to (1) achieve cost savings on a project with a large
number of shafts, (2) confirm design allowable values of
socket resistance, (3) demonstrate suitability of the construc-
tion method, and (4) provide data against which design meth-
ods can be evaluated.
Conventional axial load testing has largely been replaced
by methods that are easier to set up and conduct, require less
equipment and space, are safer, less time consuming, and
usually less expensive, especially in rock. These methods
include the O-cell, Statnamic (STN), and dynamic impact FIGURE 61 O-cell at bottom of reinforcing cage ready for
load tests. NCHRP Project 21-08, entitled "Innovative Load placement in a drilled shaft. (O'Neill and Reese 1999).
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maximum test load is limited to the ultimate capacity of hydraulic lines
either the section of shaft below the cell, the section above placement channel
the cell, or the capacity of the cell.
Pressure transducers are used to monitor hydraulic jack dial gages
pressures and converted to load. Linear vibrating wire dis-
placement transducers (LVWDTs) between the two plates
measure total expansion of the cell and telltales are installed
to measure vertical movements at the top and bottom of the
test sections. The downward movement of the bottom plate Test shaft
is obtained by subtracting the upward movement of the top
test section from the total extension of the O-cell as deter- SOIL
mined by the LVWDTs. Telltale deformations are monitored
with digital gages mounted on a reference beam. All of the
instrumentation is electronic and readings are collected by a
data acquisition unit.
The O-cell testing method provides some important ad-
vantages. There is no structural loading system at the ground
surface. Load can be applied at or very close to the base of a
socket for measurement of base resistance. In conventional
top load testing, most or all of the side resistance must be mo- ROCK
bilized before there is significant load transfer to the base.
Some of the cited disadvantages are that the O-cell is sacri-
O-cell
ficial and requires prior installation, so it is not useful for test-
ing existing foundations. Using a single O-cell, it is possible
to mobilize the ultimate capacity of one portion of the shaft
only, so that other sections of the shaft are not loaded to their
ultimate capacity. FIGURE 62 Shaft and O-cell test setup (adapted from
Gunnink and Kiehne 2002).
According to DiMillio (1998), the majority of load tests
on drilled shafts are now being done with the O-cell. This is
supported by results of this study, in which 17 of 32 states re- tain or increase load without continuous upward deflection
sponding to the survey reported using the O-cell for axial of the top of the shaft, whereas the average base displacement
load testing of rock-socketed shafts. Of these, 13 stated that did not change. From these tests, it is not possible to deter-
ultimate side resistance was determined and 7 reported that mine ultimate base resistance values. The base load dis-
the ultimate base resistance was determined. Five states in- placement curves show an interesting difference. For Shaft
dicated the test was used for proof load testing, in which de- No. 1, the downward base movement is small (around 1 mm)
sign values of shaft resistance were verified. These responses up to the maximum test load, suggesting a very stiff base and
show that the O-cell has become a widely used method for good contact between the concrete and underlying rock.
axial load testing of rock sockets. However, the curve for Shaft No. 3 shows downward move-
ment approaching 10 mm upon application of the load, fol-
A set of O-cell tests reported by Gunnink and Kiehne lowed by a flattening of the curve. This behavior suggests the
(2002) serves to illustrate the type of information that is ob- presence of a compressible layer between the concrete and
tained from a typical test in which a single O-cell is installed underlying rock, possibly the result of inadequate cleanout of
at the base of a rock socket. Figure 62 shows the test setup the hole before pouring concrete. Both shafts were poured
for three test shafts socketed into Burlington limestone. As under dry conditions and both were cleaned using the same
shown, the shafts extended through soil before being sock- method, reported as "rapidly spinning the auger bit after the
eted into limestone. All shafts were 0.46 m in diameter and addition of water and then lifting out the rock cuttings."
socket lengths ranged from 3.45 m to 3.85 m. Depth of soil
was approximately 4 m. Figure 63 shows test results for two Gunnink and Kiehne (2002) reported that it is common
of the shafts (Shaft Nos. 1 and 3), respectively. Each graph practice to design drilled shafts founded in sound Burlington
shows two curves, one of the O-cell load versus average mea- limestone for base resistance only, using a presumptive al-
sured uplift of the upper portion of the shaft, and the other of lowable unit base resistance of 1.9 MPa. Side resistance is of-
the O-cell load versus downward displacement of the base of ten neglected for design. Even the lowest observed base re-
the cell. Both figures are typical of failure of the shaft in up- sistance measured by the O-cell tests yielded an allowable
lift. At the maximum test load, it was not possible to main- unit base resistance of 5 MPa, assuming a factor of safety
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79
30 30
25 25
upward displacement
Average displacement (mm)
of shaft above O-cell upward displ of shaft above O-cell
20
Average Displacement (mm)
20
15 15
10 10
5 5
0 0
-5 -5
downward displacement
of O-cell base plate
-10 -10
downward displ of O-cell base plate
-15 -15
-20 -20
0 500 1000 1500 2000 2500 3000 3500 4000 0 500 1000 1500 2000 2500 3000 3500 4000
Load (kN) Load (kN)
FIGURE 63 Results of single O-cell load tests: (left) Shaft No. 1; (right) Shaft No. 3 (Gunnink and Kiehne 2002).
of 3. The tests shown in Figure 63 yield ultimate unit side testing results make it possible to achieve more economical
resistances of 2.34 and 2.28 MPa, respectively. These tests designs. The O-cell tests also identify construction deficien-
illustrate a typical outcome when field load testing is con- cies, such as inadequate base cleanout (Figure 63 left).
ducted; that is, measured unit side and base resistances
exceed presumptive values, sometimes significantly. Load The tests reported by Gunnink and Kiehne also illustrate
a limitation of testing with a single O-cell at the bottom of
the socket. The values of ultimate unit side resistance re-
ported by the authors are based on the assumption that all
of the load was resisted by the rock socket, neglecting any
contribution of the overlying soil. It is not known how sig-
nificant the error is for this case, but testing with multiple
O-cells makes it possible to isolate the section of shaft in rock
for evaluation of average side resistance (however, multiple
O-cells increase the cost of load testing). For example, if
a second O-cell is located at the top of the rock socket, a test
conducted with that cell can be used to determine the com-
bined side resistance of all layers above the rock. An innova-
tive approach based on this concept is illustrated in the testing
sequence shown in Figure 64. The figure and description are
from O'Neill et al. (1997) based on tests conducted by
LOADTEST, Inc., for the Alabama DOT. Arrangement of the
O-cells and the 4-step testing sequence depicted in the figure
made it possible to measure ultimate base resistance, side re-
sistance of the socket (in both directions), and side resistance
of the cased portion of the shaft above the socket. It is noted
that this arrangement made it possible to measure a total
foundation resistance of 80 MN, compared with approxi-
mately 11 MN for the largest standard surface jacks. Instal-
lation of multiple O-cells makes it necessary to provide a
tremie bypass line to facilitate placement of concrete below
and around the upper cells.
Interpretation of O-cell tests in rock sockets is typically
based on the assumption that total applied load at the ultimate
condition is distributed uniformly over the shaft/rock side
interface, and used to calculate an average unit side resis-
tance by
Qoc
FIGURE 64 Test setup and loading sequence with two O-cells fs = (148)
(O'Neill et al. 1997). BD
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80
where added to obtain the equivalent top load for a downward dis-
fs = average unit side resistance (stress), placement of 10 mm and plotted on a load-displacement
QOC = O-cell test load, curve as shown in Figure 65b. This procedure is used to
B = shaft diameter, and obtain points on the load-displacement curve up to a displace-
D = socket length. ment corresponding to the least of the two values (side or
base displacement) at the maximum test load. In Figure 65a,
The degree to which this average unit side resistance is valid this corresponds to side displacement. Total resistance cor-
for design of rock sockets loaded at the head depends on the responding to further displacements is approximated as fol-
degree to which side load transfer under O-cell test condi- lows. For the section of shaft loaded to higher displacement,
tions is similar to conditions under head loading. Detailed the actual measured load can be determined for each value of
knowledge of site stratigraphy is needed to interpret side load displacement up to the maximum test load (in Figure 65a this
transfer. is the base resistance curve). The resistance provided by the
other section must be estimated by extrapolating its curve
O-cell test results typically are used to construct an equiv- beyond the maximum test load. In Figure 65a, the side resis-
alent top-loaded settlement curve, as illustrated in Figure 65. tance curve is extrapolated. The resulting equivalent top-
At equivalent values of displacement both components of loaded settlement curve shown in Figure 65b is therefore
load are added. For example, in Figure 65a, the displacement based on direct measurements up to a certain point, and par-
for both points labeled "4" is 10 mm. The measured upward tially on extrapolated estimates beyond that point.
and downward loads determined for this displacement are
According to Paikowsky et al. (2004b), most state DOT
80
geotechnical engineers using O-cell testing tend to accept
12
the measurements as indicative of drilled shaft performance
60
11
under conventional top-down loading. O-cell test results are
40
10
applied in design by construction of an equivalent top-load
9
side load-deformation
curve is measured
8 settlement curve, as illustrated earlier, or by using the
6 7
Movement (mm)
20
3 4
5 measured unit side and base resistances as design nominal
2
1 side resistance curve values. However, some researchers (O'Neill et al. 1997;
0 is extrapolated
1 2
3
Paikowsky et al. 2004b) have pointed out differences be-
-20
4
5
6
tween O-cell test conditions and top loading conditions that
7
8 may require interpretation. The most significant difference is
-40 9
measured base load- 10 that compressional loading at the head of a shaft causes com-
deformation curve
-60
11
pression in the concrete, outward radial strain (Poisson's
12
effect), and a load transfer distribution in which axial load in
-80
the shaft decreases with depth. Loading from an embedded
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
O-cell also produces compression in the concrete, but a load
O-Cell Load (MN)
transfer distribution in which axial load in the shaft decreases
(a)
upward from a maximum at the O-cell to zero at the head of
the shaft. It is possible that different load transfer distribu-
30
tions could result in different distributions of side resistance
with depth and, depending on subsurface conditions, differ-
25
ent total side resistance of a rock socket.
Equivalent Top Load (MN)
12
10 11
9
20 8
6
7 In shallow rock sockets under bottom-up (O-cell) loading
5 base resistance measured,
conditions, a potential failure mode is by formation of a con-
15 side resistance extrapolated ical wedge-type failure surface ("cone breakout"). This type
4
3
of failure mode would not yield results equivalent to a shaft
10 2 loaded in compression from the top. A construction detail
noted by Crapps and Schmertmann (2002) that could poten-
5 1 tially influence load test results is the change in shaft diame-
ter that might exist at the top of a rock socket. A common
0 practice is to use temporary casing to the top of rock, fol-
0 10 20 30 40 50 60 70 80
lowed by a change in the tooling and a decrease in the diam-
Downward Displacement (mm) eter of the rock socket relative to the diameter of the shaft
(b) above the socket. Top-down compression loading produces
FIGURE 65 Construction of equivalent top-loaded settlement
perimeter bearing stress at the diameter change as illustrated
curve from O-cell test results (a) O-cell measured load- in Figure 66, whereas loading from an O-cell at the bottom
displacement; (b) equivalent top-load settlement results. of the socket would lift the shaft from the bearing surface.
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81
compression
SOIL
change in diameter at
soil/rock interface
ROCK
FIGURE 67 Comparison of load-displacement curves; O-cell
versus FEM (Paikowsky et al. 2004b).
FIGURE 66 Perimeter bearing stress at diameter change
under top loading.
equivalent top-load settlement curve derived from an O-cell
Paikowsky et al. (2004b) reviewed the available data that load test may underpredict side resistance for higher dis-
might allow direct comparisons between O-cell and conven- placements; that is, the O-cell derived curve is conservative.
tional top-down loading tests on drilled shafts. Three sets of Further FEM analyses reported by Paikowsky et al. (2004b)
load tests reported in the literature and involving rock sockets suggest that the differences between loading from the bot-
were reviewed. However, in two of the cases the test sequence tom (O-cell) and loading in compression from the top are the
involved conventional top-down compression loading (Phase result of differing normal stress conditions at the interface,
1) followed by O-cell testing from the bottom up (Phase 2). and that these differences become more significant with in-
Mobilization of side resistance in Phase 1 is believed to have creasing rock mass modulus and increasing interface friction
caused a loss of bond, thereby influencing results of the O-cell angle.
tests and precluding any direct comparison. The third case in-
volved STN and O-cell tests of shafts in Florida limestone. These numerical analyses suggest that differences in the
Paikowsky et al. stated that several factors, including highly response of rock sockets to O-cell test loading and top-down
variable site conditions and factors related to the tests, pre- compression loading may warrant consideration in some
vented a direct comparison of results. cases. Ideally, side-by-side comparisons on identical test
shafts constructed in the same manner and in rock with sim-
FEM reported by Paikowsky et al. (2004b) suggests that ilar characteristics and properties are needed to assess differ-
differences in rock-socket response between O-cell testing ences in response. However, it is expected that the potential
and top-load testing may be affected by (1) modulus of the differences, if any, will eventually be identified and incorpo-
rock mass, EM, and (2) interface friction angle, i. Paikowsky rated into interpretation methods for O-cell testing. In the
first calibrated the FEM model to provide good agreement meantime, the O-cell test is providing state transportation
with the results of O-cell tests on full-scale rock-socketed agencies with a practical and cost-effective tool for evaluat-
shafts, including a test shaft socketed into shale in Wilsonville, ing the performance of rock sockets and it is expected that
Alabama, and a test shaft in claystone in Denver, Colorado, the O-cell test will continue to be used extensively.
described by Abu-Hejleh et al. (2003). In the FEM, load was
applied similarly to the field O-cell test; that is, loading from Instrumentation such as sister bars with strain gages
the bottom upward. The model was then used to predict be- makes it possible to better determine the load distribution and
havior of the test shafts under a compression load applied at load transfer behavior during an O-cell load test. This infor-
the top and compared with the equivalent top-load settlement mation can then be used to make more refined predictions of
curve determined from O-cell test results. Figure 67 shows a load transfer behavior under head load conditions.
comparison of the top-load versus displacement curves for
the Alabama test, one as calculated from the O-cell test and In summary, some of the advantages of the O-cell for ax-
the other as predicted by FEM analysis. The curves show ial load testing of rock-socketed shafts include:
good agreement at small displacement (<0.1 in. or 2.5 mm);
however, the curve derived from FEM analysis is much · Ability to apply larger loads than any of the available
stiffer at higher displacement. This exercise suggests that the methods (important for rock sockets) and
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82
· With multiple cells or proper instrumentation, it can an overall program leading to increased use and improved de-
isolate socket base and side resistances from resistance sign methods for rock-socketed foundations. The Colorado
of other geomaterial layers. DOT has also used O-cell testing to improve its design
procedures for rock-socketed shafts, as documented by Abu-
Limitations of the O-cell test for use by state DOTs include: Hejleh et al. (2003).
· Shaft to be tested must be predetermined, because it is
not possible to test an existing shaft; Statnamic
· For each installed device, test is limited to failure of one
part of the shaft only; The STN load test was developed in the late 1980s by
· There are possible concerns using test shaft as a pro- Berminghammer Foundation Equipment of Hamilton, On-
duction shaft; tario. Its use in the U.S. transportation industry has been
· Interpretation methods that account for differences in supported by FHWA through sponsorship of load testing
loading mode are not yet fully developed; and programs, as well as tests conducted with an STN device
· There are currently no ASTM or AASHTO standards owned by FHWA for research purposes.
specifically for O-cell load tests.
In this test, load is applied to the top of a deep foundation
Interviews with state DOT engineers for this study show by igniting a high-energy, fast-burning solid fuel within a
that the O-cell test has been an integral tool in advancing the pressure chamber. As the fuel pressure increases, a set of re-
understanding and use of rock-socketed drilled shafts. The action masses is accelerated upward, generating a downward
Kansas DOT (KDOT) experience is representative of several force on the foundation element equal to the product of the
other states. The following is based on an interview with reaction mass and the acceleration. Loading occurs over a
Robert Henthorne, KDOT Chief Geologist. The geology period of approximately 100 to 200 ms, followed by venting
of the western half of Kansas, located in the High Plains of the pressure to control the unloading cycle. Load applied
physiographic province, is dominated by thick sequences of to the foundation is monitored by a load cell and displace-
sedimentary rocks, mostly sandstone, shale, and limestone. ment is monitored with a photovoltaic laser sensor. The con-
Until approximately 1995, virtually all highway bridges were cept is illustrated schematically in Figure 68. STN equipment
founded on shallow foundations or H-piles driven to refusal is available for test loads as high as 30 MN.
on rock. Drilled shafts were not considered a viable alterna-
tive because of uncertainties associated with both design and Processing of the load and displacement time histories is
construction. With encouragement from FHWA, KDOT required to convert the STN measurements into an equivalent,
engineers and geologists initiated a long-term program of static load-displacement curve. The analysis accounts for
training, education, and field load testing to better match dynamic effects that may include damping and inertial
foundation technologies with subsurface conditions. Work- effects. The unloading-point method as reported by Horvath
shops on drilled shaft design, construction, inspection, and et al. (1993) provides a relatively straightforward method for
nondestructive testing (NDT), sponsored by FHWA and the determining static resistance using measurements made at the
International Association of Foundation Drilling (ADSC), top of the shaft during a STN test. Test interpretation is also
were conducted at the invitation of KDOT. KDOT began us- discussed by Brown (1994) and El Naggar and Baldinelli
ing drilled shafts as bridge foundations where appropriate. (2000).
Several bridge sites in western Kansas were designed with
rock-socketed shafts. To address the lack of experience with
these conditions, O-cell testing was incorporated into the larger
bridge projects. In almost every case, the O-cell test results
showed side and base resistances considerably higher than
the values used for preliminary sizing of the shafts, and valuable
experience was gained with construction methods, effective
cleanout strategies, NDT methods, etc. KDOT now has O-cell
test results on rock-socketed shafts from nine projects and has
developed in-house correlations between rock mass properties
and design parameters for commonly encountered geological
formations. Drilled shafts now comprise approximately 70%
to 80% of new bridge foundations, and shaft designs are
more economical because there is a high level of confidence in
capacity predictions, based directly on the load tests.
The approach taken by KDOT illustrates how field load
testing, in this case with the O-cell, can be incorporated into FIGURE 68 Schematic of STN load test (O'Neill et al. 1997).
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Mullins et al. (2002) recently introduced the segmental Derived Static = REF*UP-derived capacity
unloading point method, which uses top and toe measure- REF = 0.96 for rock
ments as well as strain measurements from along the length 0.91 for sand
of the foundation. The segmental unloading point method 0.69 for silt
enables determination of load transfer along various seg- 0.65 for clay.
ments of the foundation, an advantage for rock-socketed
shafts to separate resistance developed in rock from that
developed in portions of the shaft embedded in soil. The analy- Dynamic Impact Testing
sis is automated using software provided by the testing firm
and equivalent static load-displacement graphs are produced A dynamic compression load test can be carried out by drop-
immediately for evaluation. All data are stored for future ping a heavy weight onto the head of the shaft from various
analysis and reference. heights. The shaft is instrumented with strain gages and
accelerometers to measure the force and impact velocity of
During the 1990s, FHWA performed or funded STN and the stress wave generated by the dynamic impact. The mea-
correlation studies with conventional static load tests to surements are correlated to driving resistance to predict load
develop standardized testing procedures and data interpreta- capacity. A review of various available drop weight systems
tion methods (Bermingham et al. 1994). Numerous other and evaluation of the method is given by Paikowsky et al.
studies have further expanded the database of case histories (2004c). A typical drop weight system consists of four
and performance studies. The result is that STN testing is components: (1) a frame or guide for the drop weight, (2) the
now a well-developed technology that is highly suitable for drop weight (ram), (3) a trip mechanism to release the ram, and
use by state DOTs for axial load testing of rock-socketed (4) a striker plate or cushion, as shown in Figure 69. Various
shafts. STN advantages identified by Brown (2000) include: configurations of modular weights can be used to provide
ram weights as high as 265 kN (Hussein et al. 2004) and drop
· Large load capacity, applied at top of shaft; heights are adjustable up to 5 m (Paikowsky et al. 2004c). A
· Can test existing or production shaft; rule of thumb given by Hussein et al. is that a ram weight of
· Economies of scale for multiple tests; 1% to 2% of the expected shaft capacity be available on site.
· Amenable to verification testing on production shafts;
and Drop weight load testing interpretation relies on analysis
· Reaction system not needed. methods similar to those used in standard dynamic pile test-
ing. Strain gage and accelerometer measurements at the top
Disadvantages include: of the pile are used to evaluate characteristics of stress wave
propagation. If sufficient shaft resistance is mobilized, it is
· Capacity high, but still limited (30 MN); possible in theory to relate the stress wave characteristics to
· Rapid loading method, as rate effects can be significant shaft capacity using available PDA (Pile Driving Analyzer)
in some soils (less in rock); technologies. Drop weight testing of drilled shafts has not
· Mobilization costs for reaction weights; and been used extensively on bridge foundations in the United
· Not currently addressed by ASTM or AASHTO stan- States, in part because other available methods (e.g., O-cell
dards. and STN) provide a more direct measurement of static resis-
tance. According to DiMillio (1998), test results on FHWA
Mullins, as reported in Paikowsky et al. (2004b), analyzed projects have not demonstrated sufficiently good agreement
a database of 34 sites at which both STN and static load tests between drop weight and other tests. The drop weight tests
were conducted on deep foundations. The data included load reportedly overpredicted measured capacities.
tests on four drilled shafts in rock at two sites, one site each
in Florida and Taiwan. The objective of the study was to Drop weight testing for rock sockets is suitable for post-
develop recommendations for LRFD resistance factors when construction tests at bridge sites where questions arise during
axial compression capacity is based on STN testing. The au- construction regarding the performance of as-built founda-
thors recommend a resistance factor of 0.74 for all deep foun- tions. This application is illustrated by the case of the Lee
dation types in rock (not specific to drilled shafts) when Roy Selmon Crosstown Expressway, in Tampa, Florida.
tested by STN. In addition, a rate effect factor (REF) is rec- The columns supporting an elevated section of roadway are
ommended to account for rate effects when using STN founded on drilled shafts socketed into limestone. During
results by the unloading point method. The REF varies with construction of the superstructure, one of the columns sud-
soil or rock type and recommendations are given here. If the denly underwent more than 3 m (11 ft) of settlement as a
segmental unloading point method is used (requiring strain result of the failure of the drilled shafts. Subsequent investi-
gages), separate REF factors can be applied to each seg- gations determined that the failed shafts were not founded
ment to account for different soil or rock types. This analy- in sound limestone as believed, raising questions about the
sis addresses the disadvantage cited previously regarding rate capacity of all 218 drilled shafts supporting the elevated
effects. roadway. As part of an investigation to determine how many
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FIGURE 69 Schematic of drop weight system (Paikowsky et al. 2004c).
shafts might need remediation, dynamic load tests were con- applicable to shafts that satisfy the criteria for rigid behavior,
ducted on 12 of the shafts supporting existing columns using given as
the pile driving hammer shown in Figure 70. Testing proved
the design capacity of 11 of the 12 shafts tested. This case 2
Ec
also illustrates the need for thorough subsurface investiga- Er 1 (149)
tion when socketing into limestone. In this case, rock eleva- 2D
tions were found to be highly variable. Seismic methods B
used in combination with borings in the post-failure inves-
tigation provided a more detailed geologic model of site in which Ec = modulus of the reinforced-concrete shaft, Er =
conditions. rock mass modulus, D = socket length, and B = socket
diameter.
Interpretation Framework for Static Axial The analysis is applied to two cases: (1) shear socket un-
Load Tests der compression or uplift and (2) complete socket under
compression. The shape of a load-displacement curve from a
Carter and Kulhawy (1988) and Kulhawy and Carter (1992b) load test is modeled in terms of constant slopes (S), which are
proposed a method for interpretation of static axial load tests related mathematically to the model parameters described in
on rock-socketed shafts. The method involves analyzing a chapter three. Consider the load-displacement curve for a
static axial load-displacement curve from a load test accord- shear socket loaded in compression, as shown in Figure 71.
ing to the analytical closed-form solutions presented in chap- Three parameters are required to idealize the geometry of
ter three (Eqs. 6995). The parameters back-calculated from the curve. S1 is the slope of the initial portion, S2 is the
the load test could then be used to evaluate effects of various approximated slope of the full-slip portion of the curve, and
design parameters on the load-displacement behavior of trial Qi is the intercept on the vertical axis (wc = 0) of the line with
designs that differ from that of the test shaft. The method is slope S2. For a rigid shaft, the measured curve parameters
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theoretically are related to the elastic model parameters by
the relationships given here (a):
(a) Shear Socket -- Compression or Uplift
(1 + r )
Er = S1
D
1 S2
tan · tan =
2 S1 - S2
c = ( 2 tan · tan + 1)
Qi
BD
with
= ln 5 (1 - r )
D
(150)
B
in which r = Poisson's ratio of the rock mass, = interface
friction angle, and = interface angle of dilation, and c = in-
terface cohesion.
For a complete socket under compression in which the base
load-displacement is determined (Figure 72), the load-
displacement curve is approximated by S1, S2, Qi, and S3, the
slope of the base load-displacement curve. The curve parame-
ters are related to the elastic model parameters as given in (b),
(b) Complete Socket -- Compression
(1 + r )
Er = ( S1 - S3 )
D
FIGURE 70 Dynamic load testing of shaft-supported column in Eb =
(
1 + b 2 )
S3
Tampa, Florida.
B
1 S - S3
tan · tan = 2
2 S1 - S2
c = ( 2 tan tan + 1)
Qi
BD
in which Eb = modulus of the rock mass beneath the shaft base.
Carter and Kulhawy (1988) applied the technique described
to 25 axial load tests reported in the literature by back-
calculating values of the model parameters Er, Eb, c, and (tan
tan) from load-displacement curves using the equations
given previously. A limitation of the model described earlier
is that the assumption of rigidity may be less acceptable for
shafts in harder rocks where the modulus values for the rock
mass and the shaft material are closer. The reader is advised to
review the original publications for further assumptions and
derivations of the equations.
Lateral Load Testing
FIGURE 71 Interpretation of a side-shear-only test (Carter and A significant number of states indicated in the questionnaire
Kulhawy 1988).
that lateral loading governs the design of rock-socketed
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depths corresponding to approximately the first 10 diameters
of the shaft. Other important points to consider when con-
ducting conventional lateral load tests, as pointed out by
O'Neill and Reese (1999) are summarized as follows.
The test site conditions and test shaft should be selected
and built to match as closely as possible the actual conditions
to which they will be applied. Items such as overburden
stresses acting in the resisting soil and rock layers, ground-
water and surface water conditions, shaft dimensions and re-
inforcing, and construction methods all can have a significant
influence on the lateral load response of a drilled shaft. To
the extent possible, these conditions should be matched by
those of the load test.
Analysis of the load test results will be interpreted using
the analytical methods presented in chapter four. The most
widely used method is the p-y curve method, in which p-y
curves are fit to obtain agreement with the load test mea-
surements. As a minimum, it is therefore necessary to have
reliable measurements of ground line shear load, ground line
FIGURE 72 Interpretation of a complete socket test (Carter and
Kulhawy 1988).
deflection, and rotation (requires two deflection points sepa-
rated by a known vertical distance). To define p-y curves ac-
curately over the length of the shaft requires measurements
of the deflected shape of the shaft, which can be done using
shafts for a significant percentage of projects (Question 25). slope inclinometer measurements. A more accurate method
However, as noted in chapter four, very few lateral load tests to determine p-y curves (or to evaluate any analytical
have been conducted on rock-socketed shafts. Methods for method) is to establish bending moment as a function of
conducting lateral load tests on deep foundations include depth, which can be done by installing a steel tube with
conventional methods, Osterberg load cell, and STN.
closely spaced strain gages along the length of the shaft. This
approach is most appropriate for tests conducted for applied
Conventional Lateral Load Test research; for example, to develop new methods for estab-
lishing p-y curves in rock.
The conventional method for conducting a lateral load test is
given in ASTM D3966 and involves pushing or pulling the Boundary conditions must be considered carefully when
head of the test shaft against one or more reaction piles or back-fitting analytical models and then applying the model
shafts. A variety of arrangements for the test shaft and reac- for design. In a lateral load test, the boundary conditions at
tion shaft are possible and these are given in detail in Reese the head of the shaft will normally be free of any rotational
(1984) and Hirany and Kulhawy (1988). One approach is to restraint and have zero applied moment and zero axial load.
use two shafts and apply the load such that both shafts are Service boundary head conditions are likely to include some
tested simultaneously, providing a comparison between two head restraint and possibly axial load and moment. Also, the
shafts. A load cell is used to measure the applied lateral load nonlinear momentEI relationships must be accounted for
and dial gages or displacement transducers attached to a ref- both in the load test and in the analysis.
erence beam can be used to monitor lateral deformation.
Thorough treatment of instrumentation for lateral load tests Four states (California, Massachusetts, New Jersey, and
can be found in Reese (1984) and Hirany and Kulhawy North Carolina) reported the use of conventional lateral load
(1988). tests on rock-socketed shafts. Although lateral load testing is
not as common as axial testing, conventional testing has been
Drilled shafts are often used where the designer wishes to the method of choice for lateral. Other methods have, so far,
take advantage of their large lateral load capacity, especially been used on a limited basis. These include lateral O-cell
that of large-diameter shafts. Analysis often shows that the tests (at least two states, South Carolina and Minnesota) and
geomaterials in the upper part of the ground profile have the lateral STN (Alabama, Florida, Kentucky, North Carolina,
most significant influence on lateral deformations and lateral South Carolina, and Utah). Several states that did not respond
load transfer. A critical part of lateral load testing is to have to the survey are known to have conducted lateral STN (Ohio
detailed knowledge of the site stratigraphy, particularly at the and Virginia).
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87
Lateral Osterberg Load Test allowing the reaction masses to slide on rails, as shown in
Figure 74. The lateral STN test can simulate lateral impact
The O-cell can be embedded in a drilled shaft and oriented loading such as might occur against a bridge pier from a
such that the load is applied in the horizontal direction. The vessel.
method is described by O'Neill et al. (1997) for a case in
which the Minnesota DOT required representative p-y curves The lateral STN test can also be used to derive the static
for a stratum of friable sandstone situated beneath a thick lateral response, but requires appropriate instrumentation
layer of normally consolidated clay. Shafts socketed into the and correct analysis of the test results. In tests described by
sandstone were to support a bridge undergoing ice loading. Brown (2000), the following instrumentation was used:
The test was conducted at a nearby location in which a 26.7
MN O-cell was positioned vertically within a 1.22-m-diameter · Load cell,
socket, as depicted in Figure 73, and used to thrust the two · Displacement transducers,
halves of the socket against the rock. Lateral force and de- · Accelerometers on top of cap or shaft,
flection measurements were used to derive p-y curves. The · Downhole motion sensors,
authors point out that care must be taken in interpreting · Resistance-type strain gages, and
the results, because the stressstrain conditions created by · Megadac Data Acquisition System.
the test are not the same as in a laterally loaded socketed shaft
that is loaded at its head and not split. Figure 75a shows the measured dynamic response of the
shaft in terms of force, acceleration, and lateral displacements
Lateral O-cell testing of rock sockets offers some of the versus time. The curves showing measured lateral displace-
same advantages as for axial O-cell load testing, namely the ment from three measurements are identical and cannot be
elimination of a structural loading system at the ground level. distinguished in the figure. Dynamic response is separated
Also, the test provides the ability to apply lateral loading at pre- into static, inertial, and damping components. A p-y analysis
determined depths, such as within the rock socket. Further re- (using LPILE or FBPIER) is fit to obtain a reasonable match
search is needed to establish guidelines for proper procedures between the measured load-displacement response for each
and to define correct analyses that account for the differences component of force (static, inertial, and damping). Load ver-
in boundary conditions, load transfer, and soil and rock resis- sus displacement curves derived are shown in Figure 75b
tance, compared with a shaft loaded at its head. It is also worth based on analysis of the dynamic response in Figure 75a.
noting that the lateral split socket test may provide a means to
measure the in situ rock mass modulus of deformation (EM). The lateral STN test is reported as to be safe, controlled,
and economical. Its principal advantage lies in the ability to
measure directly the dynamic lateral response and to provide
Lateral Statnamic a derived static response. This test is a valuable tool for the
design of bridge foundations to withstand dynamic lateral
The STN load test has also been adapted for lateral loading.
loading from earthquakes, wind, and vessel impacts. The test
The device is mounted on steel skids supported on the ground
may also be used in place of a conventional static test. Lateral
loads up to 18 MN may be possible.
FIGURE 73 Top view of O-cell arrangement for lateral split
socket test (O'Neill 1997). FIGURE 74 Lateral STN load test (Courtesy: L. Fontaine).
