7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
1/20
1
Load-Displacement Behavior of Underreamed
Footings Bearing in Jointed Clays
Kenneth E. Tand, P.E., M.ASCE, C. Vipulanandan, Ph.D., P.E., M.ASCE,
And Michael W. ONeill, Ph.D., P.E.
ABSTRACT: Underreamed footings, sometimes referred to as belled shafts or
belled piers, are frequently used in areas where geological conditions are favorable fortheir installation. Underreamed footings are sized to provide resistance to compression,
uplift and lateral loads. In this study, the load-displacement behavior of two full size
underreamed footings load tested to failure in compression and three in uplift wereinvestigated. The measured compression loads ranged from 750 to 1090 kips (3340 to
4850 kN), and the uplift loads ranged from 109 to 220 kips (485 to 980 kN).
Due to the relatively high permeability of the slickensided and fissured clays,
four of the load tests were performed under partially drained conditions. FE analysis
using effective stress parameters allowing for the dissipation of pore pressures betweenloading intervals was required to predict the measured load-displacement behavior. The
at-rest earth pressure coefficient, slickensides and fissures in the clays, and permeabilityof the layers were important parameters to model the behavior of the footings.
Introduction
The bottom of drilled shafts are sometimes enlarged to increase their bearing
capacity to optimize foundation costs by reducing the quantity of concrete andconstruction labor required (ONeill, 1985). These belled shafts are also referred to as
underreamed footings or belled piers. This type of foundation system is commonly
used along the Texas Gulf Coast where favorable subsoil conditions exist for theirinstallation.
In order to better understand the load-displacement behavior of drilled shafts instiff clay, four load tests had been performed on drilled shafts, one underreamed, at the
State Highway 225/Loop 610 interchange in Houston, Texas, (ONeill, et. al., 1972).
These drilled shafts were tested to failure and the skin friction and end bearingdeveloped in the different soil layers were analyzed. Further studies of underreamed
footings performed by ONeill showed that underreamed footings were advantageous
because they could be economically installed below the active depth of the expansiveclays typical of the Texas Gulf Coast area, and that underreamed footings provided
adequate resistance for compression, uplift, and lateral loads.
______________________________________________________________________1Principal Engineer, Kenneth E. Tand & Associates; 2817 Aldine Bender, Houston,Texas 77032; Ph: (281) 590-1711; Fax (281-590-1430); Email: [email protected]
2Chairman and Professor; Department of Civil & Environmental Engineering;
University of Houston; Houston, Texas 77204; Ph. (713) 743-4278; Fax (713) 743-4260; Email: [email protected]
3Cullen Distinguished Professor, Department of Civil & Environmental Engineering,
University of Houston; Houston, Texas 77204 (deceased)
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
2/20
2
In another study, an underreamed footing was load tested to failure in
compression at the University of Houston test facility (ONeill, et. al., 1985). The baseof the footing was instrumented with earth pressure cells to determine the pressure
distribution under the footing, and it was observed that a portion of the load was being
carried in suction at the roof (top) of the underream. Later, four more underreamed
footings were load tested in uplift at the same location (Yazdanbod, et. al., 1987), andpiezometers were installed to measure the suction that developed at the base of the
footings.
A recent study investigated the load-displacement behavior of the underreamedfooting load tested to failure in compression at the U of H test facility (Tand and
ONeill, 2003). Finite element analysis was performed to investigate the soil-structure
interaction.
This study included analysis of the two compression load tests, and three uplift
load tests referenced above. The overall objectives were to investigate the effects of the
following factors on the load-displacement behavior of underreamed footings in
compression and uplift:
Suction Fissured and slickensided clay layers. Soil strength parameters and permeability. In situ stress conditions (KO).
Site Geology
The two test sites were located east of downtown Houston, Texas, and they wereabout 4.5 miles apart. Both sites were located on a Pleistocene age deposit known
locally as the Beaumont formation (Mahar and ONeill, 1983). The Beaumont
formation is about 26 feet (8 m) deep at both sites, and it is underlain by an older
Pleistocene deposit known as the Montgomery formation. The subsoils were depositedin shallow coastal river channels and flood plains during the Peorian interglacial stage.
Both formations can be categorized as primarily clay with occasional interbedded seams
and layers of sand and silt. The clays and cohesive silts were overconsolidated to deepdepths due to desiccation that occurred when the water table was lowered during the
Second Wisconsin Ice Age (Bernard, 1962).
After alluvial deposition of the clays, large vertical fissures were formed by
shrinkage due to desiccation. Soil from the surface was washed down into the cracksduring periods of heavy rainfall. The soft sediments in the cracks were then
compressed when the clays swelled leaving locked-in horizontal stress (Al-Layla,
1970). This process was repeated throughout Pleistocene to recent geologic times, and
K0 values of 3.0 and greater have been measured at the U of H test site (ONeill, 2000).The process of desiccation and subsequent rewetting caused cyclic shearing
displacements in the clay mass that produced polished failure planes referred to as
slickensides. The slickensides are widely variable in size and orientation, althoughthere is a slight preference towards inclination at about 45 degrees to the horizontal
(Mahar and ONeill, 1983). The slickensides and fissures are flaws within the clay
mass, and such deposits are often referred to as jointed clays. The system ofslickensides and fissures that occurred during recent drying cycles produced a varying
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
3/20
3
degree of desiccation and overconsolidation within the clay mass (ONeill, et. al.,
1995). The state of stress is a function of the distance from the joints.As shown in Fig. 1, desiccation has produced an assemblage of small to large
oddly shaped clusters that lie within the slickensides and fissures (macro structure), and
the soil within the assemblage is somewhat, but not perfectly, intact (micro structure).
The cyclic movements resulted in residual strength parameters along the slickensidesand fissures. The micro and macro structure affects the strength, deformation and
permeability properties of the soil mass. The clays are spatially inhomogeneous, and
exhibit some anisotropic properties due to their stress history.
Soil Samples
Fig. 1 - Conceptual Sketch of Jointed Clay Mass
The jointing structure and loading pattern greatly affects whether macro ormicro behavior will govern overall behavior of the foundation. The macro strength of
jointed clay will have small influence on the bearing strength of clay under a large matwhere the soil is somewhat confined, but the macro strength will have great influence
on the uplift resistance of an underreamed footing with limited embedment.
Compression loading on a footing will tend to squeeze the joints closer and reduce the
mass permeability, but uplift loading will tend to open the joints significantly increasingthe mass permeability. There can be significant differences in the pore pressure
gradient between the clay at the edge of a fissure that has been opened by uplift loading,
and in the center of the clay assemblage. Thus, selection of a constitutive model forjointed clays and soil parameters for that model present a significant challenge to the
geotechnical engineer.The subsoils properties at the U H test site have been documented in the
literature (Mahar and ONeill, 1983, and ONeill, et. al., 1995). A summary of thedatabase can be found on the web at www.uh.edu/nges. Shown in Fig. 2 is a subsoil
profile with a brief summary of key soil parameters. It should be noted that the
database was constructed from a very large number of laboratory tests on smalldiameter samples, and in situ tests.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
4/20
4
Depth
ft. m
0 0ft m
2
2
4
4
6
6
8
8
10
10
12
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
StratumW
-%-OCR
Fig. 2 - Generalized Subsoil Profile at U H Test Site
Symbol Key
w - moisture content
LL - liquid limit
Ip - plasticity index
- unit dry densitySu - undrained shear strength
c' - effective stress cohesion
' - internal angle of resistance
qc - cone tip bearingE - soil modulus
KO - at rest earth pressure coefficient
The engineering properties of the subsoils have also been extensively studied at
the state highway interchange (ONeill and Reese, 1972 and Woodward-Clyde, 1982).The site geology is very similar to the U H site, but the water table was at a depth of 15
feet (4.6 m) in 1969. When comparing the subsoil stratigraphy in the two different
Su c' qc ELL Ip - pcf- -ksf- -ksf- ' -ksf- ksf- KO(kN/m3) (kPa) (kPa) (MPa) (MPa)I
20 55 40 115 1.5 .2 25 15 240 3 10(18) (72) (9.6) (0.7) (11.5)
II
_W.T.
III 23 35 20 112 2.0 - - 100 280 2.5 8(17.5) (96) (4.8) (13.4)
IV 28 70 45 110 2.5 .6 20 45 350 2.0 6(17.3) (120) (28.7) (2.2) (16.8)
Stratum I Fill: Mixture of shell fragments and sand with clay binder
II Stiff to very stiff clay
III Very stiff sandy clay, with sand seams and calcareous layers
IV Stiff to very stiff clay
V - Stiff silty clay with silt lenses
VI - Very stiff sandy clay
V
VI
18 24 10 118 2.0 - - 50 480 1.5 5
(18.5) (100) (2.4) (23)
15 30 18 128 4.2 .8 30 70 600 1.2 4
(20) (200) (37) (3.4) (29)
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
5/20
5
engineering reports for this site, it should be noted that the site grade was raised about 6
feet (1.8 m) with fill after the load tests were performed in 1969. Cone penetration testsperformed at a later date (ONeill, et. al., 1985) clearly showed that the footing was
bearing in stratum V, but that the strata was 20 feet (6.1 m) below grade at this site.
Finite Element ModelingThe complex subsoil profile and geologic stress history, as well as the variable
stress paths that occur during loading, make conventional methods of analysis
impractical. Hence, the finite element method was selected to investigate the subsoilparameters affecting the load-displacement behavior of the underreamed footings.
The commercially available computer finite element based code PLAXIS (2D
version 8/2002) was used. The following features within the code were used:
Axisymmetric was used due to the symmetry of the geometry and loading. Triangular elements with 15 nodes were used which provided fourth order
displacement field.
Interface elements were used to model differential movement between theconcrete and soil.
Varying KO values were input to model the horizontal locked in stressesoccurring in the different soil layers.
Volumetric contraction was used to model tension stresses induced at thesurface due to shrinkage of the expansive clays.
The tension cut off feature was used so that no effective stress tensionpoints occurred.
A cavitation cut off pressure of 2.1 ksf (100 kPa) was used to limit suctionpressures.
Consolidation was used to model the dissipation of pore pressures.The constitutive model chosen was the hardening soil model (Schanz et. al.,
1999) because it includes the following soil parameters to model real soil behavior:
Mohr-Coulomb failure criteria (c and ) Secant modulus for zero volumetric strains Hyperbolic relationship to model reduction in stiffness due to strain Constrained (odometer) modulus for volumetric strains Both secant and constrained modulus are stress dependent Unloading/reloading modulus Coefficient of permeability to model dissipation of pore pressures Effective stress parameters were input to model undrained and partially
drained conditions. The computer code automatically computes the pore
pressures based on the stiffness of the soil skeleton.
To model the load-displacement behavior of the footings, a detailed iterative
procedure was used whereby soil parameters were changed until good agreement
between measured versus FE predicted load-displacement behavior was obtained for allthe footings tested in compression and uplift at the U H test site. FE analysis was
performed on a desktop PC computer.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
6/20
6
Compression Load Test on F-1
Underreamed footing F-1 was installed at the U H site, and it was bearing at a
depth of 8 feet (2.4 m) in stiff clay. The 2.5 foot (0.8 m) diameter shaft wasunderreamed to a base diameter of 7.9 feet (2.4 m) without the use of drilling slurry.
The as-built details of the footing load test are shown in Fig. 3.
Fig. 3 Footing F-1Load Test Configuration
The reaction system consisted of two underreamed footings placed 9.5 feet (2.9
m) on each side of the test footing (center-to-center spacing). Their diameter was the
same as that of the test footing, but the bearing depth was increased to 18 feet (5.5 m)below grade. A pertinent observation was that groundwater seepage was large enough
that the reaction footings were installed using the slurry displacement method. Thus,
the permeability of the clay mass is much higher than that of intact clays.
Earth pressure cells were installed at the base of the footing to measure the stress
distribution. A 2.5 foot (0.8 m) diameter cardboard casing (sonatube) was set inside theshaft to act as a form to prevent shear from being transferred between the concrete shaft
and clay. However, the concrete rose up 0.75 feet (23 cm) into the annulus between the
cardboard casing and the soil forming a small collar around the shaft at the top of theunderream when concreting the footing.
The load was applied to the top of the shaft in 100 kips (445 kN) increments and
held for sixty minutes to a load of 500 kips (2224 kN) with a hydraulic jack.
Thereafter, the load was applied in 50 kips (222 kN) increments and held for thirty
minutes so that the failure load could be defined clearly. The footing had to beunloaded at the end of the 700 kips (3115 kN) load cycle to insert shims due to the large
displacement. Plunging failure occurred at a load of 750 kips (3334 kN) during thereload cycle. The footing was excavated after completion of the load test, and there was
no indications of structural failure.
Settlement was measured using dial gages attached to an independent reference
beam, and ground heave was measured using optical surveying methods. The earthpressure cells were read at the start and end of each load interval.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
7/20
7
Analysis of F-1 Load Test
During the initial FE study (Tand and ONeill, 2003), no attempt was made to
model the increasing displacement that occurred during each holding period due to theincreased base load resulting from dissipation of suction on the roof of the underream
because it was deduced that most of the suction had dissipated by the end of each
holding period. FE modeling was achieved by placing a small void directly above theunderream during the calculation phase to break any suction.
The initial calculations used soil parameters c', ' (CIU Bar tests), E' and KO
obtained from the U H site database. FE analysis was performed using 1967 triangular
elements, and undrained conditions were assumed. Detailed parameter studies were
performed to investigate the effects of various soil properties on the load-displacementbehavior. The FE computed load-displacement curves are compared to the field loading
test results in Fig. 4a.
(a) Database/Optimized Parameters (b) Effects of KO and E
Fig. 4 Load Displacement for Compression Footing F-1
FE calculations were made using KO = 1 for each layer to evaluate the influenceof the in situ horizontal stresses that occurred due to the geologic weathering process.
As shown in Fig. 4b, there was limited influence when the load is less than one-third
ultimate, but it was significant thereafter. Displacements are significantly increased,and the ultimate bearing capacity was reduced by about 25 percent. Increasing the soil
modulus of each layer by 50 percent indicated that displacements were reduced, but the
ultimate bearing capacity was unchanged. Simple 3D FE analysis indicates that the
influence of uplift from the reaction footings on the test footing was less than 5 percentwhen the loads were less than 50 percent of failure, and less than 10 percent at failure.
Additional FE analyses were performed to determine whether the load-
displacement behavior of F-1 during the loading intervals could be modeled if porepressures were allowed to build up and dissipate on the roof of the underream, and in
the clay mass below the footing during the holding intervals. The boundary conditions
and finite element mesh are shown in Fig. 5. The FE calculations were performed using
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
8/20
8
2322 triangular elements. Consolidation calculations were made after each load interval
allowing the pore water pressures to partially dissipate.
Fig. 5 - Boundary Conditions and FE Mesh
As shown in Fig. 6a, the load-displacement behavior of the F-1 footing load test
is compared to the FE predictions. The optimized values of c', ', E' and KO are
compared with the soil parameters in the database in Table 1.
(a) Predicted/MeasuredBehavior (b) Suction above the Underream
Fig. 6 Modeling Field Loading Path for Footing F-1
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
9/20
9
Table 1 - Summary of Soil Parameters at U H Test Site
U H Data Base* Optimized Parameters (FEM)
Soil
Layer
Depth
- ft
(m) KO
E
-ksf-(MPa)
c'
-ksf-(kPa)
'
KO
E**
-ksf-(MPa)
c'
-ksf-(kPa)
Kperm.
-ft/day-(m/day)
Ia0-3
(0-0.9)~3.0
230
(11)
0.2
(10)25 1.0
50
(2)
0.2
(10)15 Drained
Ib3-5
(0.9-1.5)~3.0
230
(11)
0.2
(10)25 2.0
100
(5)
0.2
(10)15
50.0
(15)
IIa5-7
(1.5-2.1)~3.0
230
(11)
0.2
(10)25 2.0
400
(19)
0.4
(19)20
1.0
(0.3)
IIb7-9
(2.1-2.7)~2.5
270
(13)
0.2
(10)25 3.0
500
(24)
0.7
(34)25
1 x 10-5
(.3 x 10-5
)
III 9-14(2.7-4.3) ~2.0 300(14) *** *** 2.5 600(29) 1.4(67) 30 1 x 10
-4
(.3 x 10-4
)
IV14-26
(4.3-7.9)~1.5
400
(19)0 25 2.0
500
(24)
0.7
(34)20
1 x 10-5
(.3 x 10-5
)
V
26-50
(7.9-
15.2)
~1.2
710
(34)
0.8
(38) 30 1.5
600
(29)
0.8
(38) 30
1 x 10-4
(.3 x 10-4
)
The following comments pertain to the items marked with asterisks in Table 1.
* c' and ' are from CIU Bar tests. See web database for CKOU Bar and
residual soil parameters.
** E is reported as 2 x the E modulus for comparison purposes only. TheE modulus is the secant modulus in the hardening soil model, and themobilized E is a function of the minor principal stress (3) and power
function (m).
*** Layer 3 is very stiff to hard sandy clay with sand seams and numerouscalcareous nodules. The clay appears to be lightly cemented with
calcium carbonate. This layer was not addressed in the web database.
FE analysis captured the general trend of the loading/holding intervals during
the field loading test. The initial displacements predicted by FE were slightly greater
than measured because the hyperbolic soil model does not consider small strain
stiffness or anisotropy. The permeability of the clays above the underreams, whichcontrols buildup and dissipation of suction, are physically changing during the load testdue to opening of fissures and slickensides and this cannot be modeled.
Analysis indicated that decreasing the permeability of each layer by a factor of
10 had little effect on the load-displacement behavior indicating that the permeabilityused for analysis is resulting in undrained failure of the bearing clays. However,
increasing permeability by a factor of 10 increased the stiffness of the load-
ref
50ref
50
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
10/20
10
displacement behavior (Fig. 6a), and increased the bearing capacity by about 15 percent
due to partially drained conditions.The earth pressure cell measurements indicated that about 50 percent of the load
was carried above the base of the footing at about 50 percent of the ultimate bearing
capacity. Since a void had been cast around the footing shaft to break side shear, most
of this load would have been carried in suction on the roof of the underream. Maximumsuction inferred from the measurements was about 4 ksf (192 kPa), which is about 2
times greater than the maximum suction measured during the subsequent uplift load
tests (F-2/3/4) at the U H test site. The most plausible explanation for the high suctioninferred from the earth pressure cells is that the sand layer used to bed the earth pressure
cells was more compressible than the stiff clay surrounding the cells, and that a
disproportionate amount of the bearing stress was being carried by the stiff clay aroundthe cells (arching effect) until the sand was densified.
The earth pressure cell measurements did indicate that the base load increased
during the holding periods thus revealing that suction forces were building up on theroof of the underream when load was applied, and that suction was dissipating with
time. For example, the earth pressure cells indicated an increased base load of 14 kips(62 kN) during the 60 minute holding period at 400 kips (1780 kN). Thus, they infer
that 0.3 ksf (15 kPa) of suction on the roof of the underream dissipated during theholding period, which transferred load to the base.
FE predictions showing buildup and dissipation of suction pressures versus
displacement of the underream are shown on Fig. 6b. The predicted gap above theunderream started to form during the initial 100 kip (445 kN) loading interval. At the
400 kip (1780 kN) load, FE analysis predicts that 0.35 ksf (17 kPa) of suction
developed, and that it completely dissipated during the one hour holding period. TheFE predicted dissipation of suction correlates well with the 0.3 ksf (15 kP a) suction
inferred from the earth pressure cells. It should be noted that the buildup anddissipation of suction is a dynamic process because suction is a function of
displacement, and displacement is a function of base load which is also a function of
suction. FE analysis can model this complex process.FE analysis indicated that suction continued to build up on the roof of the
underream after the 400 kip (1,780 kN) load was applied, but it did not entirely
dissipate thereafter. At the failure load, analysis predicts that about 0.6 ksf (29 kPa) of
suction pressure was still present on the roof of the underream. Thus, the base load atfailure was about 25 kips (110 kN) less than the load applied to the top of the footing
(~3% less). Analysis of the earth pressure cells indicated that the base load was about
715 kips (3,180 kN) at failure which correlates well with the FE prediction of 725 kips(3,230 kN).
The roof of the underream was situated in a weather clay crust where thepermeability was high enough that most of the suction dissipated during the loading
intervals. This is the prime reason that good agreement was obtained when bothneglecting suction on the underream (Fig. 4a) and when including suction (Fig. 6a).
However, if the footing was situated below the water table and the rate of loading was
high, suction effects may be included in the design.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
11/20
11
Two cone penetration tests (CPT) next to the test footing detected a 5 foot (1.5
m) thick layer of very stiff to hard sandy clay about 1.5 feet (0.5 m) below the footing.FE parameter studies were performed to evaluate the effects of subsoil layering:
The thin layer of stiff clay (IIb) directly below the footing was replaced withthe underlying layer of hard sandy clay (III). Qu is computed as 833 kips
(3710 kN) at a displacement of 0.4 feet (S/B = 5%) indicating a 10 percentincrease in Qu.
All the clays below the footing were replaced with the layer of stiff clay(IIb). Qu is computed as 638 kips (2840 kN), indicating a 15 percentreduction in Qu.
All the clays below the footing were replaced with the layer of very stiff tohard sandy clay (III). Qu is computed as 910 kips (4050 kN), indicating an
increase of 20 percent in Qu.
Parameter studies thus indicate that subsoil layering had moderate influence onthe ultimate bearing capacity at this site. However, layering had only minor influence
on the initial slope of the load-displacement curves when loads were less than one-thirdultimate. The prime reason is that there were not large differences in the soil modulusbetween the clay layers.
FE analysis was performed to evaluate the effect of slickensides and fissures on
underreamed footings loaded in compression. As shown in Fig. 7, a random pattern of
slickensides and fissures within the soil mass was modeled using zero thicknessinterface elements. A stiff clay profile [cohesion (c') = 0.75 ksf (36 kPa) and internal
friction angle () = 20] with a constant soil modulus [E = 250 ksf (12 mPa)] and KO
(2.0) was chosen. Residual soil parameters [c' = 0.2 ksf (10 kPa) and ' = 12] were
input for the interface elements. A 6 feet (1.8 m) diameter underreamed footing bearingat depths of 8 feet (2.4 m) and 13 feet (4.0 m) was used. Undrained conditions were
assumed, and suction was allowed to build up on the roof of the underream.
Fig. 7 Use of Interface Element to Model Jointed Clay
ref
50
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
12/20
12
As shown in Fig. 8, FE analysis predicted that the effects of jointing on the load-
displacement behavior was minor at loads less than one third of failure. Displacementswere increased thereafter, and the ultimate bearing capacity was reduced by about 6
percent for the shallow footing, and 2 percent for the deeper footing. Also shown in the
figures, there was about a 25 percent reduction in the ultimate bearing capacity when
reducing KO from 2.0 to 1.0 for the jointed clays.Yielding was taking place along numerous slickenside surfaces for the shallow
footing. However, it was only taking place along isolated slickensides in the deeper
footings due to higher confining pressures. FE analysis predicts that a cavity started toform above the roof of the underream for both footings when the initial load was
applied. The maximum suction pressure of 2.1 ksf (100 kPa) developed on the roof of
the underream after a top displacement of 0.6 inches (1.6 cm) for the deep footing, but 3inches (7.6 cm) of displacement was required to develop maximum suction for the
shallow footing. Thus, the base load for both footings was reduced about 53 kips (235
kN) by suction (~ 8%).
(a) Footings at 8 feet (2.4 m) (b) Footings at 13 feet (4.0 m)
Fig. 8 Effects of Jointed Clays for Footings in Compression
The 725 kip (3230 kN) base load at failure computed by FE analysis indicated a
net ultimate bearing pressure of 14.7 ksf (705 kPa). The equivalent undrained shearstrength of the jointed clay mass was computed to be about 1.9 ksf (91 kP a) assuming a
bearing capacity factor Nc = 7.7 (Skempton, 1951) for D/B (depth/diameter) = 1. For
comparison, the average undrained shear strength of the bearing clays below the footing(ONeill, 1983) was about 2.4 ksf (115 kPa). However, the equivalent shear strengthdoes not consider the effects of layering of the clays above or below the bearing
elevation. FE analysis can best be used to analyze geotechnical problems involving
multiple layers, and complex in situ stress conditions.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
13/20
13
Uplift Load Tests on F-2, F-3, and F-4
The load tests on footings F-2, F-3 and F-4 were performed as a series at the U
H test site to study the load-displacement behavior of underreamed footings tested inuplift (ONeill, et. al., 1985). Underreamed footing F-2 had a base diameter of 7.5 ft.
(2.3 m), and it was bearing at a depth of 8 ft. (2.4 m) in stiff clay. Footings F-3 and F-4
had base diameters of 6 ft. (1.8 m), and they were bearing at a depth of 10 ft. (3.0 m) instiff clay.
The reaction system consisted of two W 33 x 220 beams bearing on two timber
mats at the surface. The distance from the center of footing to edge of mat was 11 ft.
(3.4 m). The as built plan of the load test setup for footings F-3/4 is shown in Fig. 9.
Fig. 9 - F-3/4 Load Test Configuration
Two vibrating wire type piezometers were placed near the center of each footing
at the base to measure pore pressures. A pertinent observation is that relief wells had to
be installed to lower the water table so that the piezometers could be placed in the dry.Thus, the permeability of the clay mass is much higher than that of intact clays.
Settlement was measured using dial gages attached to an independent reference beam,
and ground heave by optical surveying methods. The piezometers were read at the startand end of each load interval when the equipment was functioning properly.
Footing F-2 [7.5 ft. (2.3 m) dia.] was load tested in steps of 20 kips (89 kN).
Failure occurred at 109 kips (485 kN) at an upward displacement of about 2.2 inches(5.6 cm). Each load was held for 5 minutes, and measurements were taken at the start
and end of each load cycle.
Footing F-3 [6.0 ft. (1.8 m) dia.] was load tested in steps of 25 kips (111 kN).
Failure occurred at 220 kips (979 kN) at an upward displacement of about 1.8 inches(4.6 cm). Each load was held for 5 minutes, and measurements were taken at the start
and end of each load cycle.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
14/20
14
Footing F-4 [6.0 ft. (1.8 m) dia.] was load tested over a 10 day period at load
steps of 40 kips (178 kN). Failure occurred at 190 kips (846 kN) at a deflection ofabout 0.8 inches (2.0 cm). Each load was held for a 2-day period, and settlement was
monitored periodically.
After completion of the initial loading cycle for each test, additional loading
cycles were then applied. The failure load increased during the additional load cyclesdue to build up of suction pressures that occurred with increasing displacement.
Analysis of F-2, F-3 and F-4 Load Tests
Footings F-3 and F-4 were both bearing at a depth of 10 feet (3 m) below grade,and both had underream diameters of 6 feet (1.8 m). However, F-3 was load tested to
failure in about 1 hour and F-4 was load tested to failure in about 10 days. The ultimate
capacity of F-3 was 220 kips (980 kN) versus 190 kips (845 kN) for F-4. The subsoilconditions indicated by the CPT tests indicate that reasonably uniform conditions
existed, and thus most of the differences are due to short term versus sustained loading.
This difference suggests that about 1.2 ksf (57 kPa) of suction built up on the base of F-
3. This is a simplified assumption because some of the additional 30 kips (134 kN) offorce could have been due to the difference between the partially undrained and drained
strength of the clays.
Both F-2 and F-3 were load tested in about 1 hour, but F-2 carried only half theuplift load of F-3 although the diameter was 25 percent greater. The reasons are due to
2 feet less embedment for F-2, and the fact that the roof of the underream was situated
in the weathered clay crust.
About 0.2 ksf (10 kPa) of suction was measured at an upward displacement ofabout 0.5 inch (1.3 cm) during the first loading cycle in F-3. The maximum measured
suction of 1.9 ksf (90 kPa) occurred during the second loading cycle for F-3 at an
upward displacement of about 7 inches (18 cm). As predicted by Boyles law, suctionis a function of displacement (volumetric expansion).
FE analysis was performed to determine whether the effects of suction on theuplift capacity could be modeled. The boundary conditions and finite element mesh for
typical analysis is shown in Fig. 5. The optimized soil parameters in Table 1 were the
input. The FE calculations were performed using 2214 triangular elements for F-2, and2404 elements for F-3 and F-4. Consolidation calculations were made after each load
interval allowing the pore pressures to dissipate.
The load-displacement behavior for footings F-2, F-3 and F-4 are compared in
Fig. 10a. The measured versus predicted load-settlement behavior for footings F-2, F-3,and F-4 are shown in Figs. 10b, 10c, and 10d, respectively. For comparison purposes,
undrained and drained loading are shown.As shown on Fig. 10a, the load-displacement behavior of footings F-3 (short
term test) and F-4 (sustained test) were very similar up to a load of 190 kips (845 kN).Thereafter, the load-settlement curve of footing F-3 was stiffer, and the ultimate uplift
capacity was about 15 percent greater.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
15/20
15
(a) Footings F-2/3/4 (b) Footing F-2
(c) Footing F-3 (d) Footing F-4
Fig. 10 Load-Displacement Behavior for Uplift Tests
As shown in Fig. 10b, the load-displacement behavior of footing F-2 (short term
test) is very similar to FE predictions for drained, not undrained, conditions. The primereason is that the footing is situated in the weathered clay crust where the permeability
is so high that dissipation of pore pressures approaches drained conditions.
As shown in Fig. 10c and 10d, the load-displacement behavior of footings F-3and F-4 is closer to FE predictions for undrained loading up to a load of 160 kips (720
kN). Thereafter, the load-displacement behavior was softer due to dissipation ofsuction. The relationship between undrained and drained conditions is complex for
jointed clays. It was concluded (ONeill, 1986) that air passages formed during theuplift tests, probably due to opening of the slickensides and fissures, which allowed
suction to dissipate rapidly. However, undrained conditions were probably present in
the intact clumps of clay between the slickensides and fissures. For FE modeling, thepermeability of layers IIb and III were input as 1.0 ft./day (0.3 m/day), and 0.5 ft./day
(0.2 m/day), respectively.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
16/20
16
Strength parameters very close to residual properties [c' = 0.2 ksf (10 kPa) and '
= 15] were required to model the weathered clay crust. KO =1.0 was used to computethe in situ stresses for this layer because the tests had been run in the summer when
shrinkage had reduced the horizontal stress. The permeability and the loading rate areimportant parameters because they control dissipation of pore pressures.
FE analysis was performed to evaluate the effect of slickensides and fissuresusing a random pattern of slickensides and fissures modeled using interface elements(Fig. 8a). Shown in Fig. 11a is the load-displacement response for a 6 foot (1.8 m)
diameter footing bearing at a depth of 8 feet (2.4 m), and on Fig. 11b for a footing
bearing at a depth of 13 ft. (4.0 m). FE analysis indicated that there was about a 10percent reduction in the uplift capacity due to the presence of slickensides and fissures
above the underreams for both embedment depths. Also, there was a 10 to 20 percent
reduction in the uplift capacity when KO was decreased from 2.0 to 1.0.
(a) Footings at 8 ft. (2.4 m)(b) Footings at 13 ft. (4.0 m)
Fig. 11 Effects of Jointed Clays on Footings in Uplift
Compression Test on F-5
Underreamed footing F-5 was installed at a test site for the Texas Department of
Transportation at the interchange of Highway 225 and Loop 610 . What is referred to
as shaft S-2 (ONeill et. al., 1972) had been underreamed to a diameter of 7.5 feet (2.4m). The load was applied in 50 kip (222 kN) load steps up to 500 kips (2225 kN) in 2.5
minute intervals, and thereafter the steps were increased to 200 kips (890 kN). Failure
occurred at a load of 1080 kips (4800 kN) at a displacement of 2.8 inches (7.1 cm).Settlement was measured using dial gages attached to an independent reference
beam, and they were read at the start and end of each load interval. The shaft wasinstrumented with 7 levels of Mustran cells (strain gages) to measure the load
distribution with depth, and 4 levels of telltails had been installed to measure
compression of the footing shaft.
The reaction system consisted of two underreamed footings placed 10 feet (3.0m) on each side of the test footing (center-to-center spacing). Their diameter was the
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
17/20
17
same as that of the test footing, but the bearing depth was increased to 10 feet (3.0 m)
below the test footing. The as built details of the load test set up are typical to thoseshown in Fig. 3, except for the embedment depths.
Analysis of F-5 Load Test
The strain gages indicated that 190 kips (845 kN) was carried in side frictionalong the shaft. Suction occurring above the roof of the underream was not measured,
and thus the compression loads reaching the base during the load test can only be
estimated using FE analysis.
For initial FE calculations soil parameters c', ' (CIU Bar tests), E' and KO
obtained from the site data base were used. The measured value of 2.4 ksf (105 kPa)side shear from the strain gages was input for the maximum value in the interface
elements along the shaft to reduce the unknowns. Analysis was performed using 2288
triangular elements, and undrained conditions were assumed. In Fig. 12a the load-displacement behavior of F-5 is compared to the FE predictions. The optimized values
of c', ', E', and KO with values reported in the database are summarized in Table 2.
(a) Load-Displacement Behavior (b) Incremented Displacements
Fig. 12 Load-Displacement for Footing F-5 in Compression
It is surmised that undrained conditions existed during this test due to the rapid
rate of loading (~30 minute duration). The fact that the underream was bearing in clayabout 8 feet (2.4 m) below the water table suggests that suction close to the maximum
theoretical value of 2.1 ksf (100 kPa) probably developed. FE analysis predicts that the
maximum suction pressure of 2.1 ksf (100 kPa) build up on the roof of the underreamafter a load of 150 kips (670 kN) had been applied, and that it was present thereafter.
Thus, the base load at failure was reduced about 80 kips (356 kN) due to suction (~9%less).
The CPT test closest to F-5 showed that the strata of stiff silty clay with silt
layers was between 20 and 26 feet (6.1 and 7.9 m) below the 1969 grade. As shown inFig. 12b, FE analysis indicated that this layer is somewhat confined by the stiffer clay
layers above and below, and the silty clay is being squeezed out laterally.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
18/20
18
The 810 kip (3604 kN) base load at failure computed by FE analysis indicates an
ultimate bearing pressure of 18.3 ksf (876 kPa). The equivalent undrained shearstrength of the clay mass was computed to be about 2.1 ksf (100 kPa) assuming a
bearing capacity factor Nc = 8.8 (Skempton, 1951) for D/B (depth/diameter) = 3. For
comparison, the average undrained shear strength of the bearing clays below the footing
(ONeill, 1972) was about 2.0 ksf (96 kPa). However, the equivalent shear strengthdoes not consider effects of layering of the clays above or below the bearing elevation.
Table 2 - Summary of Soil Parameters
SH 225 and Loop 610 Interchange
Site Data Base Optimized Parameters (FEM)
Soil Layer
Depth
- ft
(m) KO
E
-ksf-
(MPa)
c'
-ksf-
(kPa)
'KO
E
-ksf-
(MPa)
c'
-ksf-
(kPa)
Ia 0 5(0-1.5)
2.5 375(18) 0.4(19) 23 1.0 50 .2(10) 15
Ib5 10
(1.5-3)1.7
375
(18)
0.4
(19) 23 3.0 400.5
(24) 20
II10 22
(3-6.7)1.3
375
(18)
0.4
(19) 23 1.5 400.6
(29) 20
III*22 26
(6.7-7.9)1.3
35
(2)0.3
(14)30 1.5 200
.5
(24) 30
IVa*26 30
(7.9-9.1)
1.3330
(16)0.5
(24)
30 1.2 6001.5
(72) 25
IVb*30 60
(9.1-18)1.1
360
(17)0.8
(37)30 1.0 600
2.0
(95) 25
The following comments pertain to the items marked with asterisks in Table 2.
* The effective stress parameters for layers III and IV were not reported in
the site database. These parameters were assumed from data at the U ofH site for similar subsoil conditions.
** E is reported as 2 x the E modulus for comparison purposes only. The
E modulus is the secant modulus in the hardening soil model, and the
mobilized E is a function of the minor principal stress (
3) and powerfunction (m).
Conclusions
The load-displacement behavior of two footings load tested in compression andthree in uplift was investigated. The complex subsoil profile and geologic stress history
of the Beaumont clays required numerical analysis to capture all the important features.
ref
50
ref
50ref
50
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
19/20
19
Very good agreement was obtained between predicted load-displacement behavior and
the field load tests using FE analyses. The following conclusions are advanced fromthis study:
The slickensides and fissures within the clays were modeled usingrandomly distributed interface elements. There was about a 2 to 6 percent
reduction in bearing capacity for footings loaded in compression, but itwas about 10 percent for footings loaded in uplift.
Strength parameters for the weathered clay crust close to residual [c' = 0.2ksf (10 kPa) and ' = 15] were required to model the load-displacement
behavior for footings in uplift.
A cavity was formed above the roof (top) of the underreamed footingsduring the compression load tests which caused a build up of suction. Theultimate bearing capacity was increased by less than 5 percent for the
shallow footing F-1, and it was increased by about 10 percent for the deep
underreamed footing F-5.
Suction built up on the base of the underreamed footings during the upliftload tests. The uplift capacity of footing F-4 tested under sustained
loading was about 15 percent less than the capacity of footing F-3 testedunder short term loading. The magnitude of suction developed was a
function of the displacement, loading rate, and permeability of the clays.
KO was an important parameter in the modeling of the footings. The soilmodulus had a substantial effect on displacements, but it did not effect the
ultimate bearing capacity.
7/28/2019 Underreamed FoUnderreamed Footings in Jointed Clayotings in Jointed Clay
20/20
Appendix - References
Al-Layla, M. T. H. (1970) Study of Certain Geotechnical Properties of Beaumont
Clay,Ph. D. Thesis Texas A & M University.
Bernard, H. A., LeBlanc, R. J., and Major, C. F., Recent and Pleistocene Geology of
Southeast Texas, Geology of the Gulf Coast and Central Texas and Guide Book ofExcursions, Houston Geological Society, 1962, pp. 175-224.
Kim, M. H., ONeill, M. W., and Kramer, S. L. (1999). Performance of Drilled Shafts
with Isolation Tubes in an Expansive Soil Environment, Geotechnical Testing
Journal, ASTM
Mahar, L. J., and M. W. ONeill, Geotechnical Characterization of Desiccated Clay,
Journal of Geotechnical Engineering, ASCE, January, 1983, pp. 56-71.
ONeill, M. W., and S. A. Sheikh, Geotechnical Behavior of Underreams in
Pleistocene Clay,Drilled Piers and Caissons, C. N. Baker, Jr., Editor, ASCE, May,
1985, pp. 57-75.
ONeill, M. W., and Reese, L. C., Behavior of Bored Piles in Beaumont Clay,
Journal of the Soil Mechanics and Foundations Division, ASCE, Vol. 98, No. SM2,
Feb. 1972, pp 195 213.
ONeill, M. W., and Sheikh, S. A. (1985). Geotechnical Behavior of Underreams in
Pleistocene Clay,Drilled Piers and Caissons II, ed. by C. N. Baker, Jr., ASCE,May, pp 57 75.
ONeill, M. W., and Yoon, G. (1995). Some Engineering Properties of Over-
consolidated Pleistocene Soil of the Texas Gulf Coast, Transportation Research
Record No. 1479, Transportation Research Board, Washington, DC, pp 81 88.
Schanz, T., Vermeer, P. A., and Bonnier, P. G. (1999). Formulation and Verification
of the Hardening Soil Model,Beyond 2000 in Computational Geotechnics,
Balkema, Rotterdam, pp 281 290.
Sheikh, S. A., and ONeill, M. W. (1988). Structural Behavior of 45-Degree
Underream Footings, Transportation Research Record 1119. Transportation
Research Board, Washington, DC, May pp 83 90.
Skempton, A. W., The Bearing Capacity of Clays,Proceedings, The Building
Research Congress (U.K.), Division I, 1951.
Tand, K. E., and ONeill, M. W. (2003), Comparison of Computed vs. Measured
Load/Settlement Response of a Footing Bearing on Stiff to Very Stiff Clay, PlaxisBulletin No. 14, pp 10-13.
Woodward-Clyde Consultants (1982), Study to Investigate the Performance of Skin
Friction on the Performance of Drilled Shafts in Cohesive Soils, Technical Report
GL-82-1, U. S. Army Engineer Waterways Experiment Station.
Yazdanbod, A., Sheikh, S. A., and ONeill, M. W. (1987). Uplift of Shallow
Underreams in Jointed Clay,Foundations for Transmission Line Towers, ASCEGeotechnical Special Publication No. 8, ed. by J-L Briaud, April, pp 110 127.
Top Related