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Rockfill Embankment Settlement
Sugarloaf Mountain Bridge Abutment
and Hoover Dam Bypass (US-93)
Scott A. Anderson and Nicholas J. LaFronz
the Abutment 2 fill catches on the side-slope of an adjacent lower
embankment for the Hoover Dam Access Road. The 1,700-ft-long
embankment east of the east abutment is founded on moderately
rugged bedrock terrain.
Published studies of performance of spread footings on embankment are relatively rare. FHWA (1) presented a summary of Washington Department of Transportation studies of bridge spread
footings on compacted fill, including on shot rock. Spread footings
were instrumented and settlement was measured via borehole settlement devices and a SONDEX system at one site. The process of monitoring settlement before placement of superstructure and abutments
was described, as was the use of jacked abutments. The cost of spread
footings was approximately 50% to 65% that of typical pile foundations at the time of the study. The FHWA report (1) generally reported good performance of bridges on spread footings on the basis
of monitoring data, including those founded on shot rock embankment, although the experience cited was primarily focused on smaller
bridges and embankments. The authors indicated that many bridges
tolerated 1 to 3 in. of differential settlement. Most of the measured
settlement was compression of underlying foundation materials
and not the embankments themselves. The authors concluded that
(a) approximately 75% of observed settlement occurred during the
construction period before placing the bridge deck, (b) post–deck settlement was typically less than 0.25 in., and (c) different methods of
estimating settlement varied by approximately 0.4 in., on average.
Subsequent sections of this paper present design considerations
and development, specification and construction, and settlement
monitoring of the Abutment 2 rockfill embankment.
The Hoover Dam Bypass project (US-93) crosses the Colorado River
between Arizona and Nevada below Hoover Dam, 30 mi southeast of
Las Vegas, Nevada. The Arizona approach to the river includes the
seven-span, 900-ft long Sugarloaf Mountain Bridge. The east bridge
abutment footing is supported on a welded-wire–reinforced, special
embankment fill zone within a 130-ft high rockfill embankment. Reinforced zone backfill consists of 4-in. maximum size select granular backfill; the remainder of the embankment consists of 2-ft maximum size
rock. Backfill and rockfill were obtained from project roadway cuts, and
construction and acceptance of the reinforced zone and rockfill embankment were in accordance with FHWA standard specifications and project special provisions. Postconstruction settlement of the embankment was
monitored for 2 years with conventional survey monuments on abutment
wingwalls and the embankment and multipoint borehole extensometers (MPBX) in the roadway shoulders. The monitoring period included
3 months before placement of the closure span and deck. Roughly 1 in.
of settlement beneath the reinforced fill was measured during construction, with an additional 1⁄2 in. of settlement within 2 years after fill
placement. Postconstruction settlement of the wingwalls was approximately 1 in. The rate of settlement decreased through the 2-year period,
and settlement from construction and self-weight is nearly complete.
Use of the MPBX was deemed partially successful in this application.
US-93 is a designated North American Free Trade Agreement route.
Increasing congestion and safety concerns because of existing
switchbacks and the Hoover Dam crossing led to construction of the
Hoover Dam Bypass by the Central Federal Lands Highway Division of the FHWA (CFLHD-FHWA). The bypass project crosses
the Colorado River between Arizona and Nevada downstream of
Hoover Dam, 30 mi southeast of Las Vegas, Nevada. The Arizona
approach of the bypass is 1.7 mi long and includes the seven-span
Sugarloaf Mountain Bridge. The finished grade of the bridge meets
existing ground at the west abutment (Abutment 1) on the northeastern flank of Sugarloaf Mountain. It is a maximum of 160 ft above
existing grade near the midpoint of the structure and is supported on
a 130-ft high rockfill embankment at the east abutment (Abutment 2).
Embankment side-slopes and fore-slope are at 11/2H:1V (steepest),
where H:V is the slope ratio (horizontal to vertical), and the toe of
BRIDGE FOUNDATION DESIGN
CONSIDERATIONS
During design development and design team (Hoover Support Team
or HST) discussions with CFLHD and the Arizona Department of
Transportation (ADOT), support alternatives for Abutment 2 loads,
including both shallow and deep foundations, were evaluated. Deep
foundation alternatives were dismissed because of numerous issues,
including constructability, excessive lateral loading and deflection
of columns, down-drag loading, and column verticality.
Project earthwork estimates indicated that 80% to 90% of the
embankment volume would be derived from roadway cuts in basalt,
yielding mass material with rockfill characteristics. During design,
it appeared that virtually all of the rock and soils from project roadway cuts would be suitable for use as embankment fill. The majority
of the prospective embankment rockfill consisted of basalt, basalt
flow breccia, dacite, rhyolite, and conglomerate.
S. A. Anderson, FHWA, Federal Lands Highway, 12300 West Dakota Avenue,
Suite 210, Lakewood, CO 80228. N. J. LaFronz, HDR Engineering, Inc., 3200
East Camelback Road Suite 350, Phoenix, AZ 85018. Corresponding author:
N. J. LaFronz, Nick.LaFronz@hdrinc.com.
Transportation Research Record: Journal of the Transportation Research Board,
No. 2016, Transportation Research Board of the National Academies, Washington,
D.C., 2007, pp. 3–12.
DOI: 10.3141/2016-01
3
4
Transportation Research Record 2016
For the spread footing on fill option, immediate settlement (compression) of the embankment fill because of self-weight was estimated
to be approximately 1⁄2% to 1% of the fill thickness or a maximum of
approximately 8 to 16 in. for the 130-ft high fill, occurring during
embankment construction. The magnitude of postconstruction differential settlement of the embankment and lateral spreading-related
settlement was deemed insignificant, considering the high shear
strength of rockfill on a bedrock foundation. Compression of the
embankment because of applied abutment loads was estimated to be
approximately 1⁄2 to 1 in. On the basis of AASHTO specifications (2),
estimated tolerable differential settlement for the bridge end span was
approximately 1⁄2 in., based on the span length of 128 ft and a tolerable movement (angular distortion between adjacent footings) of
0.004. This was not expected to be exceeded.
The HST, in consultation with CFLHD and ADOT Bridge and
Materials Groups representatives, selected the spread footing option
based on anticipated performance, constructability, and cost. Initially,
it was decided to modify the selected alternative by founding the
abutment on a low-height, mechanically stabilized earth (MSE) wall
on the crest of the embankment fill to more effectively distribute
footing load, provide a stable upper portion of the embankment
compared with embankment only, and aid in reducing magnitude of
differential settlement. This scenario deviated from ADOT policy
of using columns and deep foundations to support an abutment seat
behind an MSE wall but was deemed preferable to placing the abutment directly on nonstabilized, unreinforced fill. However, geometric design constraints convinced the HST and CFLHD that inclusion
of a reinforced fill zone beneath the bridge abutment footing would
provide equivalent performance expected from the MSE system,
without the added cost and construction issues.
The selected design for Abutment 2 support consisted of a zone
of welded wire-reinforced embankment fill, extending from the bottom of the footing to a depth of 25 ft below the spread footing base,
within the limits of 1:1 lines extending from 5 ft outside the bottom
edges of the footing laterally and longitudinally (Figures 1 and 2).
Backfill for the reinforced zone consisted of 4-in. maximum size
processed fill meeting the requirements for select granular backfill
in the FHWA Standard Specifications FP-96 (3). An outer “wedge”
of unreinforced rockfill encapsulates and protects the ends of the
reinforcements and conceals the change in construction.
REINFORCED FILL ZONE DESIGN
Design of the reinforced fill zone was in accordance with methods
for reinforced embankments (4). In the procedure, required total tensile load in the reinforcements at the base of the reinforced zone is
determined (with a factor of safety) with a rotational slope stability
method, the total load distributed into layers within the zone, and the
required reinforcement layer strength assessed. Assumed 1H:1V outer
slopes (nominal limits of the reinforced zone, corresponding to the
abutment footing zone of stress influence) were used. A surcharge
pressure of 4.2 kips/ft2 was applied to the top of the reinforced fill
zone for the combined loading of the Abutment 2 spread footing and
a traffic surcharge. Nominal reduction factors for creep, installation
damage, and durability and ageing were applied to the computed
total (ultimate) tensile strength to determine the allowable long-term
tensile strength.
Welded wire reinforcement meeting the requirements of ASTM
A82/AASHTO M32 and M55 was selected for reinforcement,
with tensile strength ( fy) of 75 ksi, allowable tensile strength (0.8fy)
of 60 ksi, and allowable long-term tensile strength of 26.7 ksi. Reinforcement consisted of 12 × 12–W4 × W4 welded wire reinforcement located on 1-ft vertical spacings, extending both laterally and
longitudinally to the limits of the 1H:1V zone from 5 ft outside
the footing edge for the full width of the zone. Adjacent welded
wire panels were fastened with approved fasteners (hog rings or
tie wire).
LIMIT OF FILL REINFORCEMENT EXTEND TO 1H:1V LINE PROJECTED
FROM BOTTOM EDGE OF FOOTING
(TYPICAL BOTH UP-AND DOWN-STATION)
SUGARLOAF MOUNTAIN BRIDGE
ABUTMENT 2
EL. 1389'
BOTTOM OF FOOTING
1
FINISHED SLOPE
25'
1
EL. 1364'
BOTTOM OF REINFORCED
FILL ZONE
1.5
REINFORCED FILL ZONE
(SEE TYPICAL DETAIL 1)
1
C
L INTERIM US 93
EMBANKMENT FILL
ABANDONED GRADE AT C
L US 93
EMBANKMENT FILL
FIGURE 1
Cross section at Abutment 2.
Anderson and LaFronz
5
CL US 93
&
CONSTR. CL
LIMIT OF FILL REINFORCEMENT EXTEND TO 1H:1V LINE PROJECTED
FROM BOTTOM EDGE OF FOOTING
(TYPICAL BOTH UP-AND DOWN-STATION)
47'2"
47'2"
ABUTMENT 2
FINISHED SLOPE
1
25'
1
1.5
EL. 1389'
BOTTOM OF FOOTING
1
1
EL. 1364'
BOTTOM OF REINFORCED
FILL ZONE
REINFORCED FILL ZONE
(SEE TYPICAL DETAIL 1)
1
1.5
1
EMBANKMENT FILL
CL INTERIM US 93
EMBANKMENT FILL
FIGURE 2
Cross section at Abutment 2 (looking along US-93 construction centerline).
The design included a 15% increase in the wire diameter of welded
wire reinforcement over that determined by the design procedure—
essentially providing “sacrificial” additional reinforcement against
corrosion. Further, the FP-96 Standard Specifications (3) and special
contract requirements (SCRs) for the select granular backfill included
electrochemical requirements (minimum resistivity, pH range, and
maximum sulfate and chloride content) verified by testing.
ROCKFILL EMBANKMENT AND REINFORCED
FILL ZONE CONSTRUCTION
Gradation, placement, and compaction of the rockfill were performed
in accordance with FP-96 (3) modified by the SCRs. SCR provisions
included pushing individual rock fragments and boulders greater
than 24 in. in diameter to the outside face of the embankment slope
and embedding the particles into the adjacent slope face without stacking or piling (nesting). All boulders were reduced to less than 48 in.
in greatest dimension; boulders and fragments not used on the embankment face were distributed within the embankment to prevent nesting and the interstitial voids filled with finer material. Rockfill was
placed in 24-in. maximum thickness lifts and compacted primarily
with a Caterpillar CP653 paddle foot roller (vibratory mode) and a
specified number of passes.
Rockfill embankment within approximately 400 ft of the abutment
was designated as a special embankment placement zone, for which
additional water for compaction was required. For this zone, rock was
defined as material containing 50% or more (by volume) rock particles greater than 3 in. in diameter, visually verified, with the contracting officer’s optional verification by sieving the material through a
grizzly screen. Watering for compaction was achieved by applying
water as the material was excavated and end dumped before grading
into lifts to help ensure that optimum moisture content was achieved
throughout the full depth of each 24-in. (maximum) lift. Simply
applying water to the surface of each lift was unacceptable. SCR
requirements for moisture content of the fine fraction of embankment
fill (material passing the No. 4 sieve) were optimum moisture content
to 2 to 4 percentage points above optimum.
The reinforced fill zone is located within the special embankment
placement zone directly under the abutment footing. It consists of
select granular backfill with 100% passing the 4-in. sieve, 75% to
100% passing the 3-in. sieve, and 0% to 15% passing the #200 sieve.
The backfill was compacted in 6-in. lifts with a Caterpillar 634 steel
drum roller. The contractor initially used both method compaction
and density tests and subsequently solely density tests to verify sufficient compaction. Water was applied as described previously for
the special embankment placement zone. Figures 3 and 4 depict
embankment construction and welded wire reinforcement placement.
EXPECTED POSTCONSTRUCTION SETTLEMENT
Postconstruction differential settlement will affect bridge end
span performance and determine the need for compensatory jacking at Abutment 2. Magnitude of postconstruction embankment
FIGURE 3
End dumping of rockfill.
6
Transportation Research Record 2016
basins and cross-drainage conveyances), concrete-lined drainage
ditches located on fill, and relocated existing natural drainages (to
below the embankment toe). Arid site conditions (annual precipitation of 6 in.) and a preponderance of precipitation in the form of shortduration, high-intensity, high-runoff thunderstorms, which can be
routed off the embankment, indicate a limited source of water for
embankment wetting.
MONITORING SYSTEM AND RESULTS
FIGURE 4
Placing initial layer of welded wire reinforcement.
settlement depends on fill characteristics/composition, compacted
density and water content, thickness and geometry, foundation
compressibility, stress state before wetting, and degree of wetting
(hydrocompression) (5, 6 ).
Postconstruction settlement of the rockfill embankment was estimated at 0.25% to 0.50% of the total fill thickness, thought to represent a conservative upper bound. However, Noorany suggested
there is no simple empirical rule to estimate fill settlement and no
basis for assumed settlement strain of 0.50% to 1% (5). Noorany
recommended conservative analysis of probable future wetting of
the fill during design (5). In general, Noorany’s recommendations
are applicable to the embankment rockfill considered here (5).
Fill settlement mechanisms include primary and secondary compression and hydro-compression (7 ). Primary compression is expected
to occur very quickly in partially saturated compacted fill, particularly given the significant coarse-grained fraction, and to be essentially
complete by the end of construction. The magnitude of secondary
compression was expected to be small; the estimated rate of settlement after primary compression is complete (coefficient of secondary
compression, per log cycle of time) was on the order of 0.001 to
0.002 for the fill, corresponding to settlement of 1 in. or less in a
10-year period for a 100-ft high fill (7, 8). Therefore, the primary
mechanism in long-term postconstruction settlement is hydrocompression, were embankment wetting to occur. Previous studies
demonstrate that even well-compacted embankments undergo some
amount of compression because of wetting (7 ). Variations in compacted density and water content significantly affect settlement magnitude, such that control of field compaction (both compacted density
and moisture content) is critical to fill performance.
Effects of the significant gravel- to boulder-sized fraction on settlement magnitude on wetting are thought to be threefold (6 ): (a) reduced
compressibility, because individual oversize particles generally do
not respond to wetting; (b) reduced overall degree of wetting, in that
the oversize material (particularly at high overall rock content) tends
to interfere with response to wetting of the fine soil fraction in the
interstitial spaces; and (c) reduced compression (volume change) of
the fine fraction because of wetting (effect difficult to predict).
Design elements likely aid in limiting postconstruction wetting to
direct precipitation and thereby minimize percolation of surface
flows. These elements include roadway drainage provisions (catch
Instruments were installed to monitor settlement to verify settlement predictions and to forecast magnitude and rate of additional
settlement. Monitoring results were considered in scheduling girder
placement and casting the deck for the bridge end span (Figure 5).
Monitoring results currently are used to predict future settlement and
to evaluate the need for jacking at the abutment seat before paving.
The bridge span and abutment were designed to accommodate up to
4 in. of jacking.
The monitoring system was designed to provide a degree of redundancy through the use of different survey methods to permit independent quality check on results and to help isolate poorly performing
areas of the embankment (if necessary). Monitoring included the
following instruments, which are located as shown in Figures 6 and 7:
1. Five rockfill survey monuments, consisting of steel pins embedded in 3- to 5-ft deep concrete monuments located on the embankment
slope coincident with the reinforced fill base, monitored using optical
survey methods
2. Four concrete survey monuments, consisting of standard benchmarks anchored to the top of the abutment wingwalls, also monitored
by optical survey
3. Four multipoint borehole rod extensometers (MPBXs) consisting of five or six anchors (number dependent on fill thickness), spaced
25 ft vertically, each connected to a stainless steel extension rod
encased within a polyvinyl chloride (PVC) sleeve, with surface standpipe and measurement head. Anchors were individually grouted
within boreholes over a range of depths within the embankment fill,
with the lowermost anchor anchored to bedrock at approximately 5 ft
below foundation grade. Surface completions were flush mounted
FIGURE 5
Placing end-span girders.
Anderson and LaFronz
FIGURE 6
7
Plan layout of MPBXs and settlement monuments with limits of reinforced fill zone.
with frame and cover over the instrument head and protected from
low-speed construction traffic with bollard posts (Figure 8).
MPBXs were monitored with a depth micrometer at the measurement head, and measurement head reference plates were surveyed by
optical methods. Instruments were installed as soon as practical, such
that instruments were installed at intervals. Initially, rockfill survey
monuments were placed, with the embankment at partial height. Concrete survey monuments were installed near the end of construction,
just after casting of wing walls. Finally, the MPBXs were installed,
because they fell within a work area for casting the abutments and
setting girders.
MPBXs were read and monuments surveyed approximately twice
weekly after installation and subsequently at a decreasing frequency.
By approximately 2 years after embankment construction, all instruments were being surveyed or read quarterly. Monitoring results indicated embankment settlement generally within the expected range.
The results permit prediction of future settlement. Results indicate that
only a portion of the settlement was immediate (i.e., occurring dur-
ing construction) and that settlement is ongoing at slow and progressively decreasing rates 2 years after construction. Postconstruction
settlement of less than 0.1% of the fill height was observed in 2 years,
and current settlement rates are such that total postconstruction
settlement of less than 0.15% is expected.
Specific observations are discussed and presented in the following
paragraphs and figures. The plotted data were “smoothed” using a
three-point running average to better indicate trends over the 2-year
monitoring period.
Monuments at the reinforced fill zone base were installed before
embankment completion, such that they record the response of the
lower 90 ft of embankment to loading of the upper 40 ft of fill,
which was placed within a 60-day period. As such, these are the
only instruments that recorded settlement both during and after
construction. Figure 9 presents this response, in which rate of
settlement decreases significantly after approximately 300 days.
After nearly 1,000 days, total settlement is between 0.9 and 1.7 in.,
with a differential settlement of almost one-half the maximum
total settlement.
8
Transportation Research Record 2016
U.S. 93
CONST. CL
35' Lt.
LIMITS OF ABUTMENT 2 FOOTING & WALLS
(SHOWN FOR CLARITY ONLY)
35' Rt.
1410
1400
FINISHED SUBGRADE
(EL. VARIES - SEE TABLE)
FINISHED
EMBANKMENT SLOPE
1390
1380
REINFORCED FILL ZONE
LIMIT OF WELDED WIRE
REINFORCED FILL ZONE
(SHOWN FOR CLARITY ONLY)
1375
1370
ELEVATION (FT)
1360
EL. 1360
FINISHED
EMBANKMENT
SLOPE
ANCHOR POINT
(TYPICAL FOR ALL
MPBX)
1350
1350
1340
EMBANKMENT FILL
1330
1325
MULTIPOINT BOREHOLE
EXTENSOMETER (MPBX)
(TYPICAL)
1320
1310
1300
1300
EMBANKMENT FILL
1290
TIP POINT
ANCHOR MIN. OF 5FT
BELOW EXST. GRADE
(TYPICAL FOR ALL)
1280
1275
EXST. GROUND
1270
1260
180
160
140
120
100
80
60
40
20
0
20
40
60
80
100
120
140
160
180
DISTANCE (FT)
FIGURE 7
Cross section of MPBX installations (looking along US-93 construction centerline).
Results for MPBX No. 1 are shown in Figure 10. Because each rod
records the settlement between its anchor and the ground surface,
there being no plausible mechanism for heave at intermediate depths,
deeper rods that indicate less settlement than shallower ones were
assumed to have malfunctioned and are not shown. Possible sources
of error or malfunction include an overly stiff grout column, slippage
of anchors, or rods bending in compression. Each of these mechanisms
could result in underestimation of settlement; therefore, the presented
results are interpreted to represent a lower bound on actual settlement. The results show that early settlement occurred at a higher rate
FIGURE 8
vault.
MPBX surface completion with measurement head and
deep in the embankment than near the top and that after approximately
600 days, the rate of settlement was roughly equivalent at all depths,
possibly suggesting material characteristics varying with depth.
Sensitivity of optical survey of MPBX reference plates is insufficiently precise for such small magnitudes of settlement, but the
results shown in Figure 11 generally confirm the rates and magnitude of observed settlement for the MPBXs. After approximately
700 days, total settlement was between 0.8 and 1.1 in., with a differential settlement of less than 30% of the maximum total settlement.
These fill settlement results were very consistent with the measurements on the concrete wingwalls shown in Figure 12, in terms of
both total and differential settlement, and suggest that the reinforced
fill zone provides a stable platform that effectively limits differential
settlement.
Overall, performance of the MPBXs was disappointing, with only
approximately half the rods functioning satisfactorily, mostly for
MPBX No. 1 (Figure 10) and No. 3. Figure 13 presents MPBX No. 3
results on a semilog plot. These results are consistent with MPBX
No. 1, when plotted in semilog form. The results suggest that a lower
rate of secondary compression can be estimated from the slope of
the data trend after approximately Day 300. These results suggest
approximate future settlement of 0.2 to 0.6 in. in the next 25 years
and approximately the same in the subsequent 250 years, assuming
no other mechanism is in effect.
Figure 14 depicts all optically surveyed monuments plotted against
a log-time scale. These results exhibit the decreasing rate of settlement through time, further supporting the secondary compression
response and suggesting the same range of possible interpretations
of future settlement as the MPBXs. This plot also indicates the range
of values at different times, which directly corresponds to the differential settlement at that time. Differential settlement is lowest at the
wingwalls (representing the abutment seat) where tolerance for differ-
Anderson and LaFronz
9
0.0
Complete Backfill of Abutment (6/23/2004)
Monument
Set Span #7 Girders (6/30/2004)
-0.5
Change in Elevation (inches)
No. 1
No. 2
No. 3
No. 4
No. 5
Substantial
Completion
of Deck
(12/1/2004)
-0.0
-0.5
Complete Reinforced Fill Zone (5/13/2005)
Begin Reinforced Fill Zone (4/27/2004)
-2.0
0
100
200
300
400
500
600
700
800
700
800
900
1000
Days
FIGURE 9
Settlement recorded at bottom of reinforced fill zone.
0.2
Substantial Completion of Deck (12/1/2004)
0.1
0.0
Change in Elevation (inches)
-0.1
-0.2
-0.3
Rod No. 1
Rod No. 2
Rod No. 3
Rod No. 4
-0.4
-0.5
-0.6
-0.7
-0.8
Begin Reinforced Fill Zone (4/27/2004)
-0.9
Complete Reinforced Fill Zone (5/13/2005)
-1.0
Complete Backfill of Abutment (6/23/2004)
-1.1
Set Span #7 Girders (6/30/2004)
-1.2
0
100
200
300
400
500
Days
FIGURE 10
Settlement recorded at MPBX No. 1.
600
900
1000
10
Transportation Research Record 2016
0.2
Substantial Completion
of Deck (12/1/2004)
0.1
0.0
Change in Elevation (inches)
-0.1
MPBX No. 1
MPBX No. 2
MPBX No. 3
MPBX No. 4
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
Begin Reinforced
Fill Zone (4/27/2004)
-0.9
Complete Reinforced Fill Zone (5/13/2005)
-1.0
Complete Backfill of Abutment (6/23/2004)
-1.1
Set Span #7 Girders (6/30/2004)
-1.2
0
100
200
300
400
500
600
700
800
900
1000
Days
FIGURE 11
Settlement recorded at MPBX reference plates.
0.2
Begin Reinforced
Fill Zone (4/27/2004)
0.1
Substantial
Completion
of Deck
(12/1/2004)
0.0
Change in Elevation (inches)
-0.1
Monument
No. 1
No. 2
No. 3
No. 4
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
Complete Reinforced Fill Zone (5/13/2005)
-0.9
Complete Backfill of Abutment (6/23/2004)
-1.0
Set Span #7 Girders (6/30/2004)
-1.1
-1.2
0
100
200
300
400
500
Days
FIGURE 12
Settlement recorded at top of wingwalls.
600
700
800
900
1000
Anderson and LaFronz
0.2
Begin Reinforced
Fill Zone (4/27/2004)
0.1
0.0
Complete Reinforced
Fill Zone (5/13/2005)
-0.1
Change in Elevation (inches)
11
-0.2
-0.3
Rod No. 1 (Depth 0 - 28 ft)
Rod No. 2 (Depth 0 - 53 ft)
Rod No. 3 (Depth 0 - 78 ft)
Rod No. 4 (Depth 0 - 103 ft)
-0.4
-0.5
-0.6
-0.7
Complete Backfill
of Abutment (6/23/2004)
-0.8
-0.9
Set Span #7
Girders (6/30/2004)
-1.0
Substantial Completion
of Deck (12/1/2004)
-1.1
-1.2
10
100
1000
Days
FIGURE 13
Settlement recorded at MPBX No. 3.
0.2
Begin Reinforced
Fill Zone (4/27/2004)
0.1
0.0
-0.1
-0.2
Change in Elevation (inches)
-0.3
-0.4
-0.5
Bottom of Reinforced Fill
Wing Wall
MPBX Reference Plates
-0.6
-0.7
-0.8
-0.9
Complete Reinforced
Fill Zone (5/13/2005)
-1.0
-1.1
Substantial
Completion
of Deck
(12/1/2004)
-1.2
Complete Backfill
of Abutment (6/23/2004)
-1.3
-1.4
Set Span #7
Girders (6/30/2004)
-1.5
-1.6
-1.7
-1.8
1
10
100
Days
FIGURE 14
Approximate data bounds for all settlement monuments.
1000
12
ential settlement is least and greatest at locations on the embankment
slope, outside and below the reinforced fill zone.
Transportation Research Record 2016
zone and the reinforced fill zone and careful quality control. If
control of percolation and runoff is achieved, the embankment is
expected to undergo minor additional settlement and will provide an
excellent foundation for the bridge abutment.
CONCLUSIONS
Although monitoring continues on a quarterly basis (for approximately
2 more years), the results collected to date permit several conclusions
regarding performance of the rockfill embankment to be drawn. These
conclusions will be reevaluated in 2 years, before paving and opening
of the Hoover Dam Bypass:
1. The reinforced fill zone and special embankment placement
zone focused attention on the quality and care in placement of rockfill
in the most critical area of embankment, near the abutment.
2. Differential settlement is lower above the reinforced fill zone
than below it and well within the limits of structure tolerance.
3. Settlement during construction is difficult to measure because
new material is being placed as recently placed material is settling.
Monuments at the base of the reinforced fill zone indicate the rate
and magnitude of settlement as the upper embankment is placed and
allow estimation of settlement during construction that could be used
for material balance calculations.
4. Postconstruction settlement of the rockfill is minimal, as was
expected, but it was not “immediate,” with approximately 1 in. of
settlement occurring throughout the year after placement.
5. Settlement rate is greatest during construction and decreases
through time. After approximately 2 years, a low log-linear secondary
compression rate is established. Even longer-term observation is
planned to confirm this trend in the data.
6. Settlement is essentially complete, with 0.1 to 0.7 in. (approximately 0.01% to 0.05%) additional settlement expected in the next
25 years and approximately the same amount in the subsequent
250 years (Figures 13 and 14). The range in these values reflects
the uncertainty in the coefficient of secondary compression (see
Conclusion No. 5).
Surface settlement caused by placement and self-weight was at
the lower end of the expected range before construction. This is
interpreted to be the result of the special embankment placement
ACKNOWLEDGMENTS
Many individuals and organizations contributed to the Hoover Dam
Bypass project and to the Sugarloaf Mountain Bridge and Abutment
2 rockfill embankment. The authors specifically acknowledge the
work of HST (consisting of Jacobs Engineering, HDR Engineering,
and AMEC Earth and Environmental), ADOT, and the design and
construction branches of CFLHD.
REFERENCES
1. Performance of Highway Bridge Abutments on Spread Footings. Report
No. FHWA/RD-81/184. Office of Engineering Research and Development,
FHWA, U.S. Department of Transportation, 1982.
2. AASHTO. Standard Specifications for Highway Bridges, 16th ed.
AASHTO, Washington, D.C., 1996 with interims through 2001.
3. FHWA. Standard Specifications for Construction of Roads and Bridges
on Federal Highway Projects. FP-96. FHWA, U.S. Department of
Transportation, 2001.
4. Geosynthetic Design and Construction Guidelines, Participant Notebook.
National Highway Institute Course No. 13213. FHWA HI-95-038. FHWA,
U.S. Department of Transportation, 1998.
5. Noorany, I. Structural Fills: Design, Construction and Performance Review.
In Unsaturated Soil Engineering Practice (S. L. Houston and D. G.
Fredlund, eds.). ASCE Geotechnical Special Publication 68. Geo-Institute
of ASCE, Reston, Va., 1997.
6. Noorany, I., and S. L. Houston. Effect of Oversize Particles on Swell and
Compression of Compacted Unsaturated Soils, in Static and Dynamic
Properties of Gravelly Soils (M. D. Evans and R. J. Fragaszy, eds.). ASCE
Geotechnical Special Publication 56. ASCE, Reston, Va., 1995.
7. Brandon, T. L., J. M. Duncan, and W. S. Gardner. Hydrocompression
Settlement of Deep Fills. Journal of the Geotechnical Engineering Division
of ASCE, Vol. 116, No. 10, 1990.
8. U.S. Department of the Navy. Soil Mechanics Design Manual 7.1. Naval
Facilities Engineering Command (NAVFAC), Alexandria, Va., 1982.
The Soil and Rock Properties Committee sponsored publication of this paper.
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