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Evaluation and Recommendations
for Flowfill and Mechanically Stabilized
Earth Bridge Approaches
Naser M. Abu-Hejleh, Dennis Hanneman, Trever Wang, and Ilyess Ksouri
the extension of wing walls along the roadway shoulders and use of
concrete approach slabs and high-quality granular backfill behind the
abutment. The rigid, heavily reinforced approach slab is supported
by the bridge abutment wall at one end and a sleeper slab foundation
at the roadway end (Figure 1). It allows for a gradual distribution of
any approach slab settlement and a smoother transition between
the bridge and the approaching roadway. The typical CDOT bridge
approach system (Figure 1) includes a foundation soil layer in which
subsurface investigations are conducted, a roadway embankment fill
soil layer placed above the foundation soil, a high-quality granular
backfill material placed behind the abutment wall and beneath the
approach and sleeper slabs, and installation of surface and internal drainage systems as well as an expansion joint device. Most of
CDOT’s new bridges are constructed with an integral abutment system in which the abutment and the superstructure are rigidly connected to eliminate or reduce joints in the bridge superstructures.
Temperature cycles are more critical in integral abutments because
expansion and contraction of the bridge superstructure could lead to
lateral displacement of the approach backfill and slab. To address
this issue, a very small gap or a compressible collapsible material
(e.g., expanded polystyrene) is incorporated between the abutment
fill and the bridge abutment, and an expansion bridge device is placed
on top of the sleeper slab (see Figure 1).
Since 1992, CDOT has used three new alternatives for the abutment backfill to alleviate the bridge bump problem: flowfill bridge
approaches, mechanically stabilized earth (MSE) Class 1 backfill bridge approaches, and MSE Class B (porous) backfill bridge
approaches (see Figure 1).
In November 1992, CDOT began using flowfill (a low-strength
concrete mix) backfill at the abutment wall to reduce the approach
settlements (see Table 1). The self-leveling ability of flowfill allows
it to flow to fill voids at the hard-to-reach zones (curved and cornered
zones). Also, it experiences negligible settlement after curing. A total
of 110 bridges were constructed with the costly flowfill abutment
backfill ($76 per yd3 in 2005) from 1993 to 2001.
Standard details for MSE abutment Class 1 backfill were introduced
by CDOT on May 21, 2000. Most of the reinforcements are geofabric
wrapped around each layer of compacted backfill at the back face of
the abutment (Table 2), but geogrid (stiffer) reinforcements are considered in some situations to stiffen the backfill. The use of MSE
Class 1 backfill ($37 per yd3) as a lower-cost alternative to flowfill
has been a growing practice in Colorado. A total of 14 bridges were
constructed with MSE Class 1 backfill between 1999 and 2003. The
reinforced fill behind the abutment is used to build a vertical, selfcontained wall capable of holding an approximately vertical shape
(2) that forms an air gap between the abutment and the retained fill
(Figure 1).
To alleviate the common bridge bump problem, the Colorado Department
of Transportation (CDOT) has employed three new alternatives for bridge
abutment backfill since 1992: flowfill, mechanically stabilized earth (MSE)
using well-graded granular Class 1 backfill (reinforced soil mass as in
MSE walls), and MSE using free-draining Class B filter material. However, the occurrence of bridge bump problems is still reported. A study
evaluated CDOT current practice for design and construction of bridge
approaches and then developed recommendations to improve this practice
(improve performance and reduce costs) on the basis of the results of the
following: (a) best practices for bridge approaches collected from CDOT
staff and reported in the literature, (b) evaluation of the performance
and cost-effectiveness of Colorado’s MSE and flowfill bridge approaches,
and (c) identification of the causes of significant bridge approach settlement problems observed in some of Colorado’s MSE and flowfill bridge
approaches. Evaluation procedures and forensic investigations were
developed and applied to obtain the information needed for the first two
items. Flowfill should remain a viable alternative for certain field and
construction scenarios that justify its higher costs. MSE approaches
with both Class B and Class 1 backfill materials should be routinely used
in future CDOT projects with documentation of their performance and
cost (construction and repair costs) for a future evaluation. Comprehensive recommendations are presented to mitigate the observed bridge
approach settlement problem; the most important recommendations
are for improved support and drainage systems for the sleeper slab
where the settlement problem occurs.
A bump often develops at the end of a bridge near the interface
between the abutment (often supported by deep foundations) and the
approaches that often settle more than the abutments. Bridge bumps
cause uncomfortable rides, create hazardous driving conditions, and
require costly, frequent repairs with traffic delays. Numerous investigations have been undertaken during the past decades to address
this problem (1).
Before 1992, measures taken by the Colorado Department of
Transportation (CDOT) to alleviate the bridge bump problem included
N. M. Abu-Hejleh, FHWA Resource Center, Suite 301, 19900 Governors Drive,
Olympia Fields, IL 60461. D. Hanneman, Geotechnical Engineering Group 3,
Bureau of Reclamation, Denver Federal Center, Denver, CO 80225-0007. T. Wang,
Colorado Department of Transportation, Denver, CO 80222. I. Ksouri, Materials
and Geotechnical Branch, Colorado Department of Transportation, Unit A,
4670 Holly Street, Denver, CO 80216. Corresponding author: N. M. Abu-Hejleh,
Naser.Abu-Hejleh@fhwa.dot.gov.
Transportation Research Record: Journal of the Transportation Research Board,
No. 2045, Transportation Research Board of the National Academies, Washington,
D.C., 2008, pp. 51–61.
DOI: 10.3141/2045-06
51
52
Transportation Research Record 2045
Bridge Expansion Device
Concrete Approach Slab
Approach Roadway
Sleeper Slab
Min 4'
Deep Foundations
3" low density EPS or
collapsible void
Abutment Wall Supported by
Bridge Deck
Flowfill or MSE Backfill
Slope level changed
over time, ranging
from 2H:1V to 1H:1V
Drainage layer and collector pipe
Embankment
Over foundation soil layer or original ground level
FIGURE 1
Typical details for CDOT flowfill or MSE bridge approaches.
In the past few years, Class B filter material (Table 2) has replaced
the Class 1 backfill in construction of 10 bridges ($57 a cubic yard
in 2005). Class B filter material was preferred over Class 1 backfill
because it is more free draining, is less susceptible to wetting-induced
softening or collapse, is less erodible, has less fines that could clog
drainage systems and requires less compaction effort.
Although construction of bridge approaches by using flowfill and
MSE backfill is believed to have reduced occurrences of bridge bumps
overall, significant problems are still reported. Field performance
information on the adopted abutment backfill systems (since 1992) and
other measures used by CDOT to alleviate the bridge bump problem
was needed.
• Evaluate the performance of Colorado’s MSE and flowfill bridge
approaches and estimate the average total unit cost (construction
and repair) needed to maintain acceptable performance level over
their entire service life (for comparison between flowfill and MSE
approaches). Information needed for this evaluation was obtained
from the CDOT Bridge Management Section, the CDOT Engineering
Estimates and Market Analysis Unit, CDOT regional maintenance
offices, and field visits.
• Determine the causes of the significant approach settlement
experienced in some of Colorado’s bridges constructed with MSE
and flowfill approaches. A forensic investigation was performed to
identify the magnitude and causes of the approach settlement and to
evaluate if this settlement has more or less ended, or if significant
future settlement potential remains.
OVERVIEW OF STUDY INVESTIGATION
The objective for this study was to evaluate CDOT current practice
for design and construction of bridge approaches and then to develop
recommendations to improve this practice on the basis of the results
of the following tasks:
• Document the CDOT current practice that has evolved since
1992 for the geotechnical investigation, design, construction, and
repair of bridge approaches and the comments and suggestions collected from CDOT staff and reported in the literature improve this
practice (1).
TABLE 1
Materials Requirements, Flowfill Backfill
Ingredient
lb/yd3
Cement
Water
Coarse aggregate (AASHTO No. 57 or 67)
50
325 (or as needed)
1,700
Fine aggregate (AASHTO M6)
1,845
CDOT Research Report 2006-2 provides the complete details of this
study (3). This paper provides the key findings and recommendations
of that study.
PERFORMANCE AND COST OF MSE
AND FLOWFILL BRIDGE APPROACHES
Evaluation Procedure
Condition states are used by CDOT bridge inspectors to describe
only the structural conditions of the bridge approach concrete slabs
as good, fair, or poor. Another rating system was developed in this
study to reflect the range of settlement experienced by the bridge
approaches at the sleeper slab:
1. Best or smooth approach with no to very slight bump problem
when the sleeper slab experiences settlement less than a maximum
tolerable settlement of 1 in. Note that an NCHRP Synthesis Report
indicates that a change in the slope of the approach slab of 1/200 is
tolerable (1), and for a typical approach of 15 ft, this corresponds to
a settlement of slightly less than 1 in.
Abu-Hejleh, Hanneman, Wang, and Ksouri
TABLE 2
53
Materials Requirements, Class 1 and Class B Backfills
% Mass Passing
Gradation Requirements
Class 1 Backfill
50 mm
37.5 mm (3/4 in.)
19 mm (3/4 in.)
Sieve # 4
Sieve # 16
Sieve # 50
Sieve # 200
100
Class B Filter Material
100
30–100
20–60
10–30
0–10
0–3
10–60
5–20
Other construction requirements for Class 1
Liquid limit (%)
Plasticity index (%)
Dry unit weight (kN/m3)
Other material requirements
consist of free-draining sand,
gravel, slag, or crushed stone.
≤35
≤6
>95% of AASHTO T-180
2. Slight to moderate bump problem when the sleeper slab has
settled more than 1 in. and less than 2 in. This condition is categorized
as a slight problem for relatively long approach slabs (>25 ft) or
when the speed limit is low (<40 mph) to a moderate problem for
relatively short approach slabs (<20 ft) or when the speed limit is
high (>50 mph).
3. Significant to large bump problem. The sleeper slab has settled
more than 2 in. This rating is categorized as a safety hazard if the
settlement is larger than 3 in.
For each bridge, any expended repair costs on the approaches and
required repair costs to bring their performance to a smooth approach
(rating of 1) were estimated. All these costs were generated in Year
2005 dollars. For all the bridge structures constructed with flowfill,
MSE Class 1 backfill, and MSE Class B filter materials the following
information is collected:
• The number of bridge approaches was determined. The total
volume of placed backfill was then estimated on the basis of the
number of bridges and typical geometry.
• The total expended and required repair costs (2005 dollars)
were estimated and then used to calculate the total repair costs and
unit repair cost per cubic yard of backfill (total repair costs/volume
of placed backfill).
• The 2005 unit repair cost was added to the given 2005 unit construction cost of the backfill to determine the current total 2005 unit
cost of the backfill. This average unit cost represents a lower-limit
estimate of the unit cost if no additional repair will be needed for the
approaches during the remaining service life of the approaches.
• Assuming that the same rate (or trend) of repair costs incurred
in the past will be incurred in the remaining service life, the 2005
unit repair cost of the bridge approaches over their entire design life
of 40 years was computed. This unit repair cost was added to unit
construction cost to obtain the total (construction and repair) 2005
unit cost of backfill over the entire design life. This total unit cost
represents an upper-limit estimate of the total unit cost of backfill
over the design life as long as the rate of future repair costs will not
exceed the cost of past repairs.
The two unit costs evaluated in the last two steps allow comparison of MSE and flowfill bridge approaches (these costs are for equal
acceptable performance of these approaches).
Evaluation Results
The performance and cost results presented in this paper for MSE
approaches are based on limited data and therefore should be considered with caution. A more scientific evaluation of MSE approaches
should be made after 5 or 10 years of collecting additional data (when
both the volume and the average service life of MSE approaches have
increased). Key conclusions (see Tables 3 and 4) are as follows:
• Most of the flowfill and MSE bridge approaches constructed by
CDOT since 1993 are performing well with no major settlement or
cracking problems.
• Most of the significant settlement problems for the flowfill
approaches occurred to the older bridge approaches constructed before
1994 when CDOT started using the flowfill. If these older bridge
approaches were not considered in the cost analysis, the upper limit
of the total unit cost would drop from $176/yd3 to around $80/yd3.
• Of 28 bridge approaches constructed with MSE Class 1 backfill,
four approaches at two bridge structures have experienced significant
TABLE 3
Performance Results
Type of Abutment Backfill
Flowfill
MSE Class 1
Backfill
MSE Class B
Filter Material
202 (98)
28 (14)
20 (10)
24
20
Performance Results
Number of bridge
approaches
(no. of bridges)
Rating Based on Sleeper Slab Settlement
With no or minimal bridge
bump problem
With slight to moderate
bridge bump problem
With severe bridge
bump problem
183
13
6
4
Rating Based on Inspection Records of CDOT Staff Bridge
Good
Fair
Poor
182
20
26
2
20
54
Transportation Research Record 2045
TABLE 4
Cost-Effectiveness Results
Type of Abutment Backfill
Flowfill
Number of bridge approaches (no. of bridges)
2005 unit construction cost of approaches per yd 3, $
Volume of placed back fill (yd 3)
Average number of service years until 2005
2005 expended repair costs ($)
2005 required repair costs ($)
2005 total repair costs ($)
2005 unit repair cost ($) per yd 3 of the backfill
2005 unit cost ($) per yd 3 lower limit
2005 unit repair cost per yd 3 ($) over the service life of 40 years
Total 2005 unit repair cost per yd 3 ($) over the service life of 40 years
202
76
40,400
7.7
264,500
514,000
778,500
19.27
95.27
100.23
176.23
approach settlement problems. Construction problems caused the
failure of these MSE approaches, which could be avoided in the
future with improved construction specifications and inspection for
the MSE backfill. Generally, MSE backfill is more sensitive to
construction problems than flowfill.
• The use of MSE Class 1 backfill is cost-effective only if the rate
of repair of MSE approaches will decline significantly in the future. In
this case, it is anticipated that its unit cost over the entire service life
would be much less than that for flowfill. However, if the past rate
of repair for these approaches continues in the future, the unit cost
of MSE abutment backfill would remain very high.
• The MSE Class B filter material has the lowest unit cost over
the entire design life. This is because no repair was reported for
the MSE Class B approaches and their current performance is
adequate.
MSE Class 1
Backfill
MSE Class B
Filter Material
28
37
5,600
4
69,500
50,000
119,500
21.34
58.34
214.16
251.16
20
57.5
4,000
2
0
0
0
0
57.5
57.5
allow thawing of frozen soils; increased soil moisture contents soften
the soils, that is, reduce its strength and increase its deformability).
• Creep soil movements, associated with both cohesive fill and
foundation soils.
• Lateral movement of side walls (MSE wall must laterally move
to mobilize the tensile resistance of its reinforcements).
• Thermal longitudinal movements of integral abutment bridges.
The settlement problem becomes significant for compressible foundation soil layers or improperly placed fill soils (low compaction
level or low moisture content or both) or with drainage measures and
side walls that do not function as designed.
General Procedure of Forensic Investigation
The study attempted to collect and analyze the following information:
FORENSIC INVESTIGATION OF BRIDGE
APPROACHES WITH SEVERE
SETTLEMENT PROBLEMS
Comprehensive forensic investigations were conducted on five
bridge structures that experienced severe approach settlement problems (ranked 3). Limited forensics investigations were conducted on
approaches ranked 2. From this investigation, the study attempted to
determine the magnitude, causes, and sources (in fill or foundation
soils or both) of the postconstruction settlement of the sleeper slab
and evaluate whether this is a continuous (time-dependent) settlement
problem that will increase in the future or if it will cease soon after
construction (within 1 year). Figures 2 through 4 show some typical
information collected during the forensic investigation of the Salt
Creek Bridge.
A detailed description of possible causes of the bridge approach
settlement problem at the sleeper slab is thoroughly discussed by AbuHejleh et al. (3). The vertical compression of the fill and foundation
soils is primarily caused by the following:
• External loads of the added fills, constructed approach slab and
roadways, and traffic.
• Increased soil moisture and temperatures to levels not previously
experienced during construction (increased temperature can reduce
any apparent soil cohesions resulted from colder temperatures and
• Design, material, and construction records of all components
of the bridge approach structure;
• Magnitude, location, and time progress of settlement at the
approach;
• Condition of all components of the bridge approach systems;
• Subsurface geotechnical investigation; and
• One-dimensional settlement analysis on the foundation and
fill soils.
Design, Material, and Construction Records
Most of the information regarding design, material, and construction
for all components of the bridge approach structure was obtained from
the as-constructed plans and foundation report. It was critical to
obtain information on the placed fill soils (any added fill height, placed
fill densities and moisture contents, time of placement, etc.) and the
foundation soils (results and timing of subsurface investigations and
settlement computation results).
Magnitude, Location, and Time Progress
of Settlement
A digital road profiler was used in this study to obtain current
elevation profiles of the transition sections from the bridge deck to
Abu-Hejleh, Hanneman, Wang, and Ksouri
55
Measured Elevation Profile with Respect to the Bridge Abutment
Design Elevation Profile with Respect to the Bridge
Elevation with Respect to the Bridge Abutment
(inches)
5
4
3
2
1
0
0
10
20
30
40
50
60
70
80
-1
Profile Line along the Shoulder line of the West-South Corner of the Bridge
-2
Sleeper slab joint at distance of 20 ft from the bridge abutment
-3
-4
Distance from the Bridge Abutment toward Approaching Roadway (feet)
Elevation profile of Salt Creek Bridge approaches.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
FIGURE 3 Predicted settlements of sleeper slab caused by consolidation of
foundation clay layer.
6
6
5
5
4
4
3
3
2
2
1
1
0
Time Since Construction of Approach Slab Was Completed, Years
Consolidation Settlement of the Sleeper Slab (inches)
FIGURE 2
56
Transportation Research Record 2045
Applied Pressure (KSF)
0.1
1
10
0.0
Moisture Content = 3.3 percent
Dry Unit Weight = 111 pcf
-1.0
-2.0
-3.0
Compression on wetting
Compression-Swell (%)
-4.0
-5.0
-6.0
-7.0
-8.0
-9.0
-10.0
-11.0
-12.0
FIGURE 4
Influence of wetting on remolded soil sample of loosely compacted Class 1 backfill.
the approaching roadway. When these profiles were compared to the
design or as-constructed elevation profiles (Figure 2), the degree and
location of the approach settlement problem were identified.
Condition of All Components of Bridge
Approach Systems
Information was collected on the condition of all components of the
bridge approach systems, such as side walls (settlement, lateral displacement), approach slab (cracking, breaking), joints, and drainage
systems. Available information on the progress of any problem with
time is documented.
Subsurface Geotechnical Investigation
Two test holes were drilled around the sleeper slab at the location that
experienced the most settlement, one in the approach slab toward
the bridge and the other on the roadway side, and boring logs for
them were developed. Laboratory testing on recovered soil samples
allowed classification of the soil and measurements of its moisture
content and, in some cases, its unit weight. Problematic soil layers in
the fill and foundation soils were identified as those with N-values
of less than 10 blows per foot (bpf) for granular soils (described as
loose to very loose) and those with N-values of less than 8 bpf for
cohesive soils (described as very soft, soft, or medium stiff). For
problematic fill soils, measured field dry unit weights and moisture
contents were compared to the required density values and the
optimum moisture content.
Consolidation and creep characteristics of the problematic foundation and fill soil layers were determined by using undisturbed Shelby
tube samples. Water was added at the beginning of the consolidation test to saturate the samples. Thus, the predicted consolidation
settlement reflects the total settlement resulting from the increased
applied vertical loads, and increased soil moisture content to 100%
saturation level. This would overestimate the total soil settlement if the
in situ soil is not saturated. To investigate the influence of just water
on soil settlement of granular fill soils (called collapse), remolded
soil samples were prepared at various density levels and with moisture
contents on the dry and wet sides of the optimum. After the samples
were consolidated to a vertical stress close to the stress that occurred
in the field, they were inundated to measure the resulting hydrocompression strain [following the procedure suggested by Coduto (4)].
Additional stress increments were then applied as in the conventional test procedure.
One-Dimensional Settlement Analysis
For foundation soil layers, the study estimated the settlement generated
during and after construction due to the placed fill and loads applied
on the sleeper slab (Figure 3). Construction time was estimated (on
the basis of construction records) because the emphasis was on postconstruction settlement. Fill settlement occurs because of self-weight
consolidation of the fill and because of loads applied on top of the
sleeper slab. Fill self-weight consolidation occurs after compaction is
completed because of the weight of the overlying soil layer, which can
be significant for high and poorly compacted fills. Although granular
soils typically experience all of their settlements during or shortly
Abu-Hejleh, Hanneman, Wang, and Ksouri
after construction, some of these settlements may be delayed until the
thawing and wetting season is over, that is, until the soil is subjected
to its highest moisture or temperature so any softening (or collapse)
responses would then appear. Within a year of construction completion, the soil is subjected to one cycle of thawing or wetting, and it
is expected that most of the consolidation settlement in granular
fills would occur. This is not necessarily the case for cohesive fills
(especially if they are high and not adequately compacted). Their
consolidation times could be much longer than the construction time
and could occur even longer than 1 year after construction completion. Additionally, continuous wetting and drying cycles of cohesive
fills would lead to increased penetration depth of infiltrating water,
resulting in continuous softening and settlement of cohesive fill
with time.
Forensic Investigation Results at Five Bridges
Sources of Bridge Bump Problems
The sleeper slab, where settlement occurs and causes the bridge bump
problem, is supported by only 4 ft of high-quality flowfill or MSE
backfill material (see Figure 1), so the main sources of the settlement
are the embankment fill and foundation soils. This finding suggests
that the type of abutment backfill (flowfill versus MSE Class 1 backfill) should not have a significant role in the performance of the bridge
approaches.
57
on top of the original ground, the greater its self-weight consolidation
and the larger the load applied on the foundation soil layer.
Construction Problems of Placed Abutment and Embankment
Fill Materials Most of the bridge approach settlement problems
occurred shortly after construction (within 1 year after completion)
and sometimes during construction before the project was accepted.
Fill settlement often causes a quick settlement problem (ceased within
a year after construction completion).
• Inadequate compaction of the fill; it is also difficult to construct
firm and well-compacted side slopes for the embankment fill.
• Construction during the cold season leading to frozen fill and
low compaction level.
• Placement of compacted granular and cohesive fill soils on the
dry side, leading to soil compression on wetting (see Figure 4).
• Placement of low-quality soil and flowfill materials that do not
meet standards.
Settlement may be caused by
presence of a compressible clay foundation soil layer not detected
during the subsurface investigation or adequately addressed in the
geotechnical evaluation, or softening of the foundation soil layer
caused by a rise of the groundwater table or infiltration of surface water
(presence of a structure could increase exposure of the foundation
soil layer to water).
Settlement of Foundation Soils
Causes of Approach Settlement Problems
Salt Creek Bridge, CO-50, Pueblo
Failure of the installed drainage systems to
prevent surface water and groundwater from reaching the fill and
foundation soil layers is a common factor in almost all the bridge
approaches that experienced settlement problems. In some bridge
approaches, the settlement problem occurred at the location of water
flow lines. It appears to be common for the water to infiltrate into
the embankment fill at the expansion joint and cracks, which then
leads to softening of the fill soils. This softening appears to be worst
at the interface between a granular soil layer and an underlying
cohesive soil layer where water accumulates. At this zone, slurry-like
soil zones (N-values of zero) were encountered during the subsurface
geotechnical investigation. In some cases, surface water runoff washed
out the embankment material, creating voids underneath the approach
and roadway slabs.
A settlement of about 4 in. at the sleeper slab was estimated on the
basis of digital profiling results presented on Figures 2–4. Note the
sudden change in the slope (rotational problem) at the abutment and
the sleeper slab joints. The added fill varied across the widened bridge
and resulted in the observed differential settlement pattern across the
approaches. A very loose backfill not meeting the required compaction level was identified for a depth that extends 20 ft below the
surface. Collapse of the granular fill on wetting after construction
completion was suspected and was confirmed with the test results
presented in Figure 4. It was estimated that the wetting settlement is
very significant for loose granular backfill (around 5 in. in 20 ft of
fill). The test results indicate that compression on wetting remains
of concern for well-compacted fills placed on the dry side. Other
possible contributing factors to the observed settlement are the presence of a slightly compressible sandy lean clay foundation layer (see
Figure 3), lateral displacement of the upper MSE wall, and failure
of the drainage measures.
Drainage Problems
Construction often proceeds as follows: after the bridge abutment and roadway within (often)
50 ft of the bridge abutment are constructed, a sleeper slab and an
expansion device on top of it are installed, and the remaining approach
system (approach slab and roadway) is then placed (see Figure 1).
The grades of the as-built bridge and roadway approaches do not
exactly match the design elevations (because of settlement of the
bridge abutment and approaching roadway during construction),
creating in some cases a built-in bump at the end of construction.
The problem is worsened if the expansion joint and the sleeper slab
are placed per the design grades and are not based on grades of the
constructed bridge and approaching roadways.
Built-In Bump at End of Construction
The bridge bump problem occurred in
almost all cases when high fills are placed. The higher the added fill
Placement of High Fill
I-70–I-225 Bridge Approaches
The I-70–I-225 bridge was constructed in 1994 and since then the
approaches have not stopped settling. The variation in the longitudinal and transverse stiffness of the foundation systems resulted
in a noticeable differential settlement pattern along and across the
approaches. A very soft soil layer (almost like slurry, very high
water content) was detected in the cohesive fill that extends 17 to
23 ft from below the top of the wall. It is impossible that this layer
was placed in this form during construction. Most likely, the backfill was poorly compacted during construction and not placed as
58
required by CDOT construction specifications. Failure of the surface and internal drainage measures was observed and is the main
cause of the settlement problem. Many of the wall’s drainage pipes
were clogged. Most likely, surface water continues to infiltrate the
cohesive fill and soften it over time. It was concluded that the future
growth of this soft cohesive zone could lead to continued settlement
problems.
CO-287 Bridge Approaches
Settlement at the CO-287 bridge approaches is attributed to the presence of a very soft foundation clay soil layer that apparently was not
detected in the subsurface geotechnical investigation. If the top of
the foundation soil layer was desiccated at the time of the subsurface
geotechnical investigation, this soil layer would have been described
as stiff. During construction, the project personnel may have thought
it to be firm and safe to build on. While the bridge has been in service,
surface water has infiltrated through the permeable embankment
material and accumulated on top of the foundation soil layer, perhaps
softening it over time. It is also possible that softening resulted from
the rise of the groundwater table, which is located at a relatively shallow depth. Water can easily soften a cohesive soil layer that derives
its strength from desiccation, not from a mechanical compaction or
consolidation of overlying loads.
Structure E-19-Z Approaches, US-36,
East of Bennett
The embankment at the US-36 approaches east of Bennett was constructed during the winter months (the fill was frozen), and maintenance personnel observed sudden and rapid settlement of the newly
constructed embankment once the ground thawed in late spring.
Continued settlement has occurred because of loss of fines via water
intrusion into the embankment and the probable breakdown of intact
claystone nodules found in the fill. This has occurred at the sleeper
slab expansion joints where water has flowed into and under the
approach and sleeper slabs.
Structure E-17-PR Approaches,
I-76 at 136th Avenue
The lab results for the I-76 approaches at 136th Avenue showed very
soft and excessively moist embankment materials that get stiffer
and drier with depth. This indicates infiltration of water from the
surface into the embankment materials. The combination of excessive moisture in the embankment and repetitive traffic loading induce
settlement of the embankment material. Localized erosion underneath the approach slab was observed during the site investigation.
Erosion of the embankment material will lead to further settlement
of the approach and roadway slabs unless the drainage system is
corrected.
RECOMMENDATIONS
Following are recommendations to improve CDOT current practice
for construction of bridge approaches.
Transportation Research Record 2045
Abutment Backfill and Fill Embankment
Flowfill should remain a viable alternative for certain difficult field and
construction conditions where specific features of flowfill warrant
the additional costs. Flowfill is recommended for consideration in
filling and closing voids in areas where compaction is difficult, as in
phased construction of walls and around an embankment slope, fast
track and critical construction projects, where the excavation presents
a deep, awkward hole to fill in a minimum amount of time. Add to
the construction specification a new requirement to vibrate the flowfill. Tighten the quality assurance requirements to ensure uniform
flowfill. Pay the contractor per the plan design volumes, not the placed
quantities.
MSE approaches with both Class B and Class 1 backfill granular materials should be routinely used in CDOT future projects and
should extend at least 4 ft below the sleeper slab. For embankment
fill, avoid soils having high silty content, especially directly beneath
the MSE backfill, because such soils become unstable with high
moisture content.
Additional recommendations follow:
• Monitor and document performance and costs of these approaches
for future evaluation as performed in this study.
• Enhance quality assurance procedures of the placed fill by
increasing the frequency of compaction testing and tightening
requirements for not allowing any frozen fill.
• Compact the fill materials wet of the optimum. If future wetting of the fill is expected and cannot be avoided, consider the
following recommendation only if it is feasible from a construction
standpoint. After compaction is completed, spray the fill with water
to slightly increase its moisture content. This is suggested to reduce
postconstruction fill settlement caused by increase in its soil moisture
content.
• Tighten the specifications for compaction of the top of the
foundation soil layer, where loosening and then compacting should
be required even if the top of the foundation soil layer appears to be
very stiff (most likely because of drying).
• Consider compaction specifications for Class B (filter material,
uniformly graded; see Table 2) similar to those established in CDOT
specifications for rocky embankments: spray the fill with abundant
water (to ease compaction) before compaction with a heavy vibrator
compactor for a minimum number of passes established per test
sections. Smaller and hand-operated compactors are required for fill
areas within 3 ft of the walls.
• If feasible, consider placement of temporary fill on top of the
approach fill soils (preloading) for the longest possible period. This
will reduce the postconstruction settlements of the sleeper slab.
Better Supporting Systems for Sleeper Slab
Two new systems are recommended:
• Placement of an MSE wall using Class-1 granular backfill under
the sleeper slab (Figure 5a). Currently, most of the high-quality
backfill (Figure 1) is placed under the rigid approach slab that does
not transmit load to the underlying backfill. The idea behind the
proposed system is to move the high-quality MSE backfill from
the abutment wall to an area around and below the sleeper slab where
the support is needed. This is a short MSE wall (around 10 ft high)
Abu-Hejleh, Hanneman, Wang, and Ksouri
59
Impervious Membranes with Collector Pipes
Concrete Approach Slab
3" low density EPS or
collapsible void
12" strip drain at 8'
spacing can be
attached to the EPS
Deep Foundations
Abutment Wall Supported by
Bridge Deck
Approach Roadway
Bridge Expansion Device
or Joint
Embankment soil (or
Class 2 Structural
Backfill) behind
abutment and on top of
foundation soil layer
MSE Class 1 Backfill
Drainage layer and collector pipe
(a)
Bridge Expansion Device
Concrete Approach Slab
Bridge Deck
Abutment Wall
3" low density EPS or
collapsible void
12" strip drain at 8'
spacing can be
attached to the EPS
Approach Roadway
Embankmen t soil (or Class 2
Structural Backfill) behind
abutment and on top of
foundation soil la yer
Impervious
Membranes with
Collector Pipes
Drainage layer and collector pipe
Driven Piles
(flexible)
(b)
Approach Slab
Approach Roadway
Sleeper Slab
3"-diameter half-circle PVC pipe
(c)
FIGURE 5 Recommended supporting systems and drainage details for sleeper slab: (a) placement of MSE wall under sleeper
slab, (b) use of Class 2 backfill and driven piles to support sleeper slab, and (c) placement of gutter and half-circle PVC pipe
to drain water.
60
installed under the sleeper slab that should be constructed without a structural facing, just geofabric wrapped around each layer of
compacted Class 1 backfill. At any level, place the layer of compacted
Class 2 backfill after placement of the MSE Class 1 layer. The contractor can select the appropriate procedure for construction of an
MSE wall under the sleeper slab, to be reviewed and approved by a
CDOT field engineer.
• Use of a Class 2 backfill (or roadway embankment) and driven
piles for support of the sleeper slab as shown in Figure 5b. Most of
the in situ soils meet the requirements for Class 2 backfill (general
fill consisting of almost any soil without deleterious materials). It
should be compacted to meet CDOT construction specifications.
Note that the expensive granular backfill (Class 1 backfill) and
reinforcements are eliminated in this system. It is recommended to
avoid the design of a stiff driven pile foundation system that would
not allow any settlement of the sleeper slab, which would only serve
to move the settlement problem farther from the bridge (to the roadway side of the sleeper slab). To do so, design the piles for a safety
factor of 1 and with design loads that include only dead (no live) loads.
Raise the sleeper slab by 0.5 in. to compensate for the anticipated
postconstruction settlement.
Drainage Measures
Better drainage measures are recommended, especially for the sleeper
slab:
• Collect and drain any surface water before it reaches and softens
the soil layers of the bridge approach system:
– Make it standard (not designer choice) to place a drainage
inlet at the end of a bridge deck before getting to the approach slab
(when appropriate).
– Make the drains grates larger, as some drains become plugged
because the openings are too small.
• Several measures to intercept the infiltrating surface water are
shown in Figure 5:
– Place an impervious membrane with a collector pipe under
the approach slab and an impervious membrane with a collector
pipe under the approach roadway (Figure 5b). This extended
membrane system will collect the water that penetrates the cracks
and joints.
– Limit the membranes to locations under the joints only (Figure 5a). This will lead to significant savings on the membrane
material, but it will not collect the water that could penetrate through
the cracks.
– Under the sleeper slab joints, place a gutter and a 3-in. halfcircle PVC pipe to drain water (Figure 5c). This is the perhaps
most cost-effective method for draining infiltrated water.
• Improve the internal drainage measures:
– Solve current problems with internal drainage pipes. In many
cases, it was noted that many of the walls’ drainage pipes were
clogged. Maintenance is important to keep these pipes functioning
as design intended.
– Consider the installation of horizontal drainage systems (e.g.,
drainage layer with collector pipes) at the interface zone between
a granular soil layer and a cohesive soil layer. It was noted that
water always accumulates on top of the cohesive soil layer and
softens it, leading to an ongoing settlement problem.
Transportation Research Record 2045
Approach Slab
• The length of the approach slab should be related to the magnitude of the projected postconstruction settlement of the sleeper slab.
The length of the slab, L, should be selected to ensure that Δ < 0.005L,
where Δ is the difference between postconstruction settlement of the
sleeper slab and postconstruction settlement of the abutment wall
(the latter can be conservatively assumed to be zero).
• Smoothness requirements around the bridge expansion joints
should be applied. Currently, no such requirements are required
within 25 ft of each joint.
• The contractor should be required to report the as-built surveyed
grades of the bridge abutment and approach roadway, often constructed to within approximately 50 ft of the bridge abutment. Install
the expansion device and sleeper slab at an elevation that corresponds
to a smooth line between the end of the bridge and the end of the
roadway. This will ensure that the bridge approach system is constructed to match the as-constructed (not design) grades of the bridge
and roadway.
• If approved by the hydraulic, structural, and roadway engineers,
install the expansion device at an elevation higher by up to 1 in. (for an
approach slab 20 ft long) than the design (or straight grade elevation
as determined in the previous recommendations) to compensate for
the anticipated postconstruction settlement.
• Replacement or repair of a concrete approach slab is very costly.
In some cases in which the settlement problem would be significant
and would continue for extended periods (e.g., because of the presence of fat and thick clay foundation layer), elimination of the concrete
approach and sleeper slabs altogether should be considered. As a more
cost-effective alternative, full-depth asphalt approach slabs could
be considered with thin asphalt overlays added in the future as needed
to correct the settlement problem. Or, consider a concrete approach
slab topped with a thin asphalt overlay so that future adjustments
would be less costly. It is relatively inexpensive to correct the bridge
bump problem with thin asphalt overlays.
• Require a warranty by the contractor for bridge approaches
constructed in the first year or 18 months of service.
• CIP reinforced concrete wing walls are suggested because some
problems were encountered when MSE walls with block facings were
used for the wing walls.
• If a tiered MSE wall system is selected around and below the
bridge abutment wall, extend the reinforcements of the lower wall
beyond the leveling pad of the upper MSE wall.
• Improve CDOT details for bridge expansion joints to prevent
cracking of the concrete approach slab that leads to seepage of
surface water into the soil under the sleeper slab.
Foundation Investigation at Bridge Approaches
• Consider the following guidelines in selecting the locations for
the test holes:
– At the expected location of the sleeper slabs,
– At the sides of an existing bridge that will be widened (where
the foundation soil is not consolidated, as at the center of the
bridge), or
– Where the future fills height above the original ground level
is expected to be highest (highest potential for future consolidation
settlement).
Abu-Hejleh, Hanneman, Wang, and Ksouri
• Apply seasonal corrections to the collected standard penetration
test (SPT) data if they are collected during the dry or cold times of the
year. These corrections should take into account possible reduction
in the soil strength estimated from these SPT N data due to future
increase in the soil moisture (rise in the groundwater table) and
temperature.
• Estimate the magnitude and timing of the postconstruction settlements of the sleeper slab as discussed in the report of this study (3).
The fill soil settlement should be evaluated in addition to the settlement from the foundation soil layer. This is important for cohesive
fill soils and if the fill height is large (more than 20 ft).
ACKNOWLEDGMENTS
Funding for this study was provided by CDOT and FHWA. David
White of Iowa State University; George Hearn of the University of
Colorado at Boulder; Rich Griffin and Ahmad Ardani, formerly of
CDOT; Rene Valdez, Dennis Rhodes, Cheng Su, Skip Outcalt, Dean
61
Sandoval, and Dick Osmun of CDOT. Others provided in-depth
technical reviews of this report and offered comments.
REFERENCES
1. Briaud, J.-L., R. W. James, and S. B. Hoffman. NCHRP Synthesis 234:
Settlement of Bridge Approaches (The Bump at the End of the Bridge).
TRB, National Research Council, Washington, D.C., 1997.
2. Reid, R. A., S. P. Soupir, and V. R. Schaefer. Use of Fabric Reinforced
Soil Walls for Integral Bridge End Treatment. Proc., Sixth International
Conference on Geosynthetics, Atlanta, Ga., 1998, pp. 573–576.
3. Abu-Hejleh, N. M., D. Hanneman, D. J. White, T. Wang, and I. Ksouri.
Flowfill and MSE Bridge Approaches: Performance, Cost, and Recommendations for Improvements. CDOT-DTD-R-2006-2. Colorado Department
of Transportation, Denver, 2006.
4. Coduto, D. P. Foundation Design: Principles and Practices, 2nd ed.
Prentice–Hall, New York, 2001.
The Foundations of Bridges and Other Structures Committee sponsored publication
of this paper.
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