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EVALUATION OF RESTORED PAVEMENT CUTS USING AN
ACCELERATED PAVEMENT TESTING MACHINE
Khalid Farrag, Ph.D., PE
Manager, Civil Engineering Research
Gas Technology Institute
1700 South Mount Prospect Road
Des Plaines, IL. 60018
ABSTRACT
An accelerated pavement loading machine (APLM) was used to apply repeated full-scale
wheel loads on bellholes restored using various backfills, compaction efforts, and
construction procedures in order to evaluate their performance in relation to the original
roadway.
The test sections were composed of a 6 inch (152 mm) asphalt concrete (A/C) layer and
10 inch (254 mm) stone-base layer on top of a silty-clay subgrade soil. Bellholes and
trenches were cut, excavated, and restored in order to model typical utility-cut
reinstatement procedures. Each bellhole was repaired differently to allow comparison of
the variables. The backfills used in the bellholes were native soil, sand, stone, and
flowable fill. The compaction efforts were controlled to obtain loose and dense backfill
densities. Plastic pipes were installed in the test sections to simulate a real utility cut.
The test sections were subjected to 9 kips (40 kN) repeated wheel loads from the
Accelerated Pavement Loading Machine (APLM). The loading machine applied the
wheel load at a speed of 6 mph (10 km/hr). The test sections were subjected to loading
for up to 125,000 passes or until the rut depth reached one inch.
Throughout loading, the surface deformation (rutting), pavement settlement, strains in the
asphalt layer, earth pressure on the top of the backfill, and strains in the pipe were
measured. The results showed that the effect of backfill type and traffic loads on the state
of stresses at the level of the pipe was negligible. It also demonstrated significant
variations of surface deformations due to the changes of backfill types and compaction
efforts.
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 2
INTRODUCTION
The repair and installation of utility pipes in roadways are necessary to provide public
services. Most of these activities are performed by cutting trenches and bellholes in the
roadway. Various types of backfill materials are used in the restoration of the cuts, such
as the originally excavated soil, select granular backfill, and flowable fill. Field
observations and monitoring programs have demonstrated the importance of selecting the
appropriate backfill and achieving the compaction levels which insure maximum density
(1).
The Gas Technology Institute (GTI) is performing an on-going full-scale experimental
program to evaluate the effect of the various types and properties of backfills on the
performance of repaired utility cuts in comparison to the adjacent roadway pavement.
Accelerated testing equipment has been commonly used to evaluate the effect of various
pavement properties, loading mechanisms, and temperatures (2, 3, and 4) on pavement
performance. The Accelerated Pavement Loading Machine (APLM) at GTI was used in
this experimental program to evaluate the effect of several types of backfill materials,
compaction efforts, and construction procedures on the performance of the restored utility
cuts.
The experimental program consisted of constructing pavement test sections with
instrumented bellholes. This paper presents the test sections, instrumentation, and the
results of the experimental program which consisted of testing nine bellholes with
various types of backfill and compaction efforts.
TEST SECTION
The roadway test section consisted of a 6 inches (152 mm) asphalt concrete (A/C)
pavement and 10 inches (254 mm) stone-base layer on top of a silty-clay subgrade. A
drainage layer was placed at 8 ft below the pavement surface to control soil moisture in
the base and subgrade layers. The drainage layer consisted of one-foot deep open graded
stone separated from the subgrade by a fabric layer. Perforated drainage pipes were
placed inside the stone and were connected to a drainage pump. Figure 1 shows a view of
the test section during construction.
A total of 9 bellholes were constructed in the roadway test section. The bellholes were cut
in the pavement using a saw cutter to dimensions of 3 ft (1 m) by 3 ft. The subsurface
materials were excavated to 3 ft below the pavement surface and were replaced by
various types of backfill materials. Four-inch (102-mm) diameter plastic pipes were
installed in the bellholes in order to simulate a utility cut. Table 1 shows the properties of
the backfills used in the testing program and Figure 2 shows a schematic diagram of the
bellholes in the roadway test section.
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 3
The backfills were compacted at various compaction and moisture levels as shown in
Table 2. The backfill in most of the bellholes was placed and compacted to the bottom of
the asphalt layer (Figure 3-a). The original roadway section was replicated in bellhole 8.
In this bellhole, the silty-clay subgrade was reused as a backfill and a 10-inch think stone-
base layer was compacted to the bottom of the asphalt layer (Figure 3-b).
The bellhole sections were paved using 6-inches thick A/C of the same properties as the
original pavement. A tack coat layer was placed on the edges of the cut and below the
new A/C layer. The restoration of the bellholes was performed few days after the
construction of the roadway section to ensure the age of the restored A/C in the bellhole
was similar to the original pavement. Figure 4 shows the placement of the asphalt layer
in the bellholes.
LOADING MACHINE
The repeated wheel loads were applied using the APLM at GTI. The machine applied a
running dual-tire wheel load of 9 kips (40 kN) using an electro-hydraulic loading system.
The machine is 10 feet wide, 28 feet long, and the length of its loaded path is about 16
feet (Figure 5). The length of the loading path was suitable for evaluating the reinstated
trenches and bellholes in small controlled sections while reducing the possible variability
of soil properties.
The wheel loads were applied at speeds up to 6 mph (10 km/hr). Although the speed was
less than the ones commonly used in larger full-scale testing machines of 10 to 12 mph
(4), it allowed the application of the load at comparable frequency due to the shorter
length of the loading area. Furthermore, the low speed of the wheel modeled a more
realistic loading rate in streets and urban roads where the maximum speed of traffic is
less than 40 mph.
The machine could apply the wheel loads in both uni-directional (with the wheel in
contact with the pavement in one direction) and bi-directional modes. The loading tests
on the bellhole sections where preformed with the machine in the bi-directional mode.
Figure 6 shows the APLM machine in the test section.
INSTRUMENTAION OF THE TEST SECTIONS
Horizontal Inclinometers
The settlement of the pavement section was monitored using a horizontal slope
inclinometer. The inclinometer pipe was placed at a depth 2 ft (0.6 m) below the asphalt
layer during construction. The locations of the horizontal inclinometer pipes are shown
in Figure 2. A ‘Slope Indicator’ inclinometer probe was used to measure the total
settlement of the roadway. Measurements were recorded approximately every 25,000
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 4
passes and the results are shown in Figure 7. The results show the overall settlement
under the pavement section. The sensitivity of the device was not sufficient to monitor
the variations between the bellholes and the original roadway pavement.
Vertical Pressure
The development of vertical pressure at the top of the backfill during wheel passing was
monitored using horizontal earth pressure cells. The cells were 4-inch (100-mm) diameter
‘Geokon’ semiconductor strain gauge type and were able to read a maximum pressure of
50 psi (345 kPa). The cells were placed at the top of the backfill in two Bellholes. A thin
sand layer was placed above the cell in order to insure full soil contact and the A/C layer
was placed and compacted over the sand cover. Figure 8 shows the placement of the earth
pressure cells.
The earth pressure cells were calibrated by correlating known loads to the voltage output
readings. The calibration loads were applied in a controlled laboratory environment by
placing the cells in the same backfill used in the bellholes.
The initial measurements were recorded after placement of the A/C layer and the pressure
measurements were recorded after each 25,000 wheel passes. Typical vertical pressure
measurements are shown in Figure 9. The results show the peak increase of earth
pressure at every wheel pass. The average change of vertical pressure with passes is
plotted in Figure 10. The results show a slight reduction in vertical pressure with the
increase in the number of load passes. The reduction can be due to the hardening of the
asphalt layer, thus resulting in lower pressure transmitted to the cell; or due to the
settlement of the backfill, which results in a loss of contact pressure underneath the
surface of the pressure cells.
Strain Gauges in the Asphalt Layer
Strain gauges were placed inside the A/C pavement layer in some of the bellholes. Strain
gauges type ‘Dynatest Past- II’ were placed over a thin layer of bituminous mix. A hot
asphalt layer about 2 inches (50 mm) thick was placed over the gauges and compacted
manually using a steel plate. Figure 11 shows the placement of the H-gauges in the
asphalt layer.
Figure 12 shows the development of strains in the asphalt layer from the initial loading to
10,000 wheel passes. The figure shows a reduction of elastic and permanent strains with
the increase of the number of wheel passes.
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 5
Strain Gauges on Embedded Pipe
Strain gauges were installed on the surface of the 4-inch plastic pipe embedded at a depth
3 ft (1 m) under the pavement in Bellholes 5 and 6. Figure 13 shows the locations of the
gauges and the measurements at the application of 110,000 wheel passes. The spikes in
the curves where measured when the wheel passed over the location of the gauges and
they represent the elastic strains due to wheel loads. The figure shows that the maximum
increase in the elastic strains was at the top of the pipe (Gauge T3) and was less than 0.1
percent.
Figure 14 shows the increase of permanents strains with the number of wheel passes. The
strains in locations B2 and B4 (inside the bellholes) were comparable up to 45,000
passes. Gauge B4 showed a loss of strains at higher wheel passes, possibly due to
adhesive failure. Measurements in gauges B5 and B6 in the roadway outside the bellhole
were comparable and slightly less than the strains the bellhole. Further measurements
need to be taken to verify the repetitivity of strain measurements. However, the results in
general show that both elastic and permanent strains were small and the effect of wheel
loads on the plastic pipe is negligible.
SURFACE DEFORMATIONS
The surface deformations in the bellholes were measured using a profilometer. The
measurements were compared with the ones in the roadway adjacent to the bellholes.
Loading tests started in the fall of 2002. The tests stopped during the freeze-thaw period
of the Winter and Spring of 2003 and continued again from May through August 2003.
The surface deformations of the pavement sections increased with the increase of
temperature during the testing periods. Surface measurements in the roadway (between
the bellholes) are shown in Figure 15. The figure shows the vertical deformations at the
various seasonal temperatures after the application of 50,000 and 100,000 wheel passes.
In order to separate surface deformations caused by the variation in backfill from the
deformations caused by temperature changes, the deformations in the bellholes were
compared only with the adjacent roadway which was loaded in the same period.
Surface deformations of the bellholes are plotted along with the ones in the adjacent
roadway in Figure 16. The figure shows the rutting after the application of 100,000 wheel
passes. The loading of Bellhole 4 (dense clay backfill) stopped at 50,000 passes as rutting
reached one inch.
The figure shows the variations of surface deformations due to backfill type and
compaction effort. The bellholes without stone-base layers experienced more settlements
than the adjacent roadway sections. Bellhole 8, which was restored using the original
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 6
configuration of a dense silty-clay backfill and a stone-base layer, compared well with the
adjacent highway.
As expected, surface deformations in the bellholes with lower compaction was higher
than the ones with highly compacted backfill. The amount of increase in deformation
varied according to soil type and relative compaction.
The results also show that the deformations of Bellhole 2 (flowable fill) were less than
the adjacent roadway settlement and, consequently, not compatible with roadway
deformations.
CONCLUSIONS
The performance of various types of backfills and compaction efforts in restored
bellholes was evaluated under full-scale repeated wheel loads up to 125,000 passes.
Surface deformation, earth pressure, and pipe strains were monitored in order to evaluate
their effect on long-term pavement performance.
The strains in the asphalt layer and the vertical pressure measurements decreased with the
increase of number of wheel passes. The reduction can be attributed to hardening of the
asphalt layer and backfill settlement.
The strain gauges were installed on the plastic pipe monitored the increase in pipe strains
under the repeated wheel loads. These strains, however, were elastic and the permanent
deformations in the pipe were negligible.
The rut depths of the bellholes were evaluated with respect to the ones in the adjacent
roadway sections. The results demonstrated the effect of backfill type and compaction on
the performance of the restored bellholes.. The bellhole which was restored to its original
configuration with stone-base layer compared well with the adjacent roadway.
ACKNOWLEDGMENT
The testing program was performed at the Pavement Research Facility at Gas Technology
Institute under the research program GTI-30742 funded by the New England East Coast
Distributors Group.
REFERENCES
1. Manual for Controlling and Reducing the Frequency of Pavement Utility Cuts,
Federal Highway Administration, Report Number FHWA-IF-02-064, 2002.
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 7
2. Epps, A., Walubita, L., Hugo 3, and Bangera, N., Comparing Pavement Response
and Rutting Performance for Full-Scale and One-Third Scale Accelerated
Pavement Testing
Transportation Research Board, Washington, D.C., January 2001.
3. Harvey, J., and Popescu, L., Accelerated Pavement Testing for Rutting
Performance of Two Caltrans Overlay Strategies, In Transportation Research
Board, National Research Council, Washington, D.C., January 2000.
4. Saleh, M, Steven, B., and Alabaster, D., Three-Dimensional Non-Linear Finite
Element Model for Simulating Pavement Response, In Transportation Research
Record 1823, TRB, National Research Council, Washington, D.C., 2003, pp. 153-
162
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 8
LIST OF TABLES
Table
1 COMPACTION PROPERTIES OF THE BACKFILL MATERIALS
2 TYPES AND PROPERTIES OF THE BACKFILLS
LIST OF FIGURES
FIGURE
1 Placement of the drainage layer at the bottom of the test section
2 Schematic of the pavement sections with the reinstated bellholes
3 Schematic of two cross-sections of the bellholes
4 Pavement of the bellhole sections after compaction of backfill
5 Schematic of the Accelerated Pavement Loading Machine (APLM)
6 View of the Accelerated Pavement Loading Machine (APLM)
7 Vertical displacement of the pavement from the Horizontal Inclinometer
measurements
8 Placement of the pressure cell at the top of the backfill
9 Measurement of vertical pressure on top of backfill during repeated loading
10 Change of vertical pressure with wheel passes
11 Placement of the PAST-II strain gauges in the asphalt layer
12 Strain measurements in the asphalt layer
13 Pipe strains at the application of 110,000 wheel passes
14 Accumulated permanent strains in pipe with wheel passes
15 Temperature effect on surface deformations in the roadway adjacent to the
bellholes
16 Surface deformations in the bellholes and the adjacent roadway
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 9
TABLE 1 - Compaction Properties of the Backfill Materials
Max. Dry Optimum
Liquid Plastic
Soil Type Test Density Moisture PI
Limit Limit
(pcf) (%)
Native Soil AASHTO T-180 116 12 34 22 12
AASHTO T-99 114 15
Silty-Clay 32 17 15
AASHTO T-180 124 11
Sand AASHTO T-180 116 9 - - -
Stone AASHTO T-180 145 8 - - -
3
Note: 1 pcf = 16 kg/m
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 10
TABLE 2 – Types and Properties of the Backfills
Average Average
No. of
Bellhole Compaction Relative Moisture
Backfill Type backfill
No Effort Compaction Content
layers
(%) (%)
1 Flowable Fill 1 No compaction - NA
2 Flowable Fill 1 No compaction - NA
3 Silty-Clay 2 Loose 80 17
4 Silty-Clay 4 Dense 90 12
5 Sand 4 Loose 85 5
6 Sand 7 Dense 92 6
Silty clay +
7 4 inch sand cover 5 Dense 90 13
above pipe
Silty Clay +
8 2 Dense 85 16
10 inch base layer
1 layer loose +
9 Stone aggregate 2 Variable 6
1 layer dense
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 11
FIGURE 1- Placement of the drainage layer at the bottom of the test section
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 12
FIGURE 2 – Schematic of the pavement sections with the reinstated bellholes
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 13
FIGURE 3 – Schematic of two cross-sections of the bellholes
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 14
FIGURE 4 – Pavement of the bellhole sections after compaction of backfill
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 15
FIGURE 5 – Schematic of the Accelerated Pavement Loading Machine (APLM)
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 16
FIGURE 6 – View of the Accelerated Pavement Loading Machine (APLM)
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 17
-0.2
A/C Pavement Section Location of wheel loading on slab
-0.15
-0.1
Distance (ft)
S e ttle m e n t ( in c h )
-0.05
0 5 10 15 20 25 30 35 40 45 50
0
0.05
0.1
25,000 passes
0.15
75,000 passes
0.2 100,000 passes
125,000 passes
0.25
FIGURE 7 –Vertical displacement of the pavement from the Horizontal
Inclinometer measurements
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 18
FIGURE 8 – Placement of the pressure cell at the top of the backfill
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 19
9
Vertical P ressure (psi) 8
7
6
5
4
3
2
1
0
0 20 40 60 80 100 120
Time (sec)
FIGURE 9 – Measurement of vertical pressure on top of backfill during repeated
loading
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 20
9
8
7
pressure (psi)
6
5 Static pressure
Wheel pressure
4
3
2
1
0
0 20 40 60 80 100 120 140 160
No. of Wheel passes
FIGUR 10 – Change of vertical pressure with wheel passes
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 21
FIGURE 11 - Placement of the PAST-II strain gauges in the asphalt layer
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 22
1200
Initial Reading
1100
Strain (microstrain)
1000
900
800
at 10,000 passes
700
0 20 40 60 80 100 120
Time (Sec)
FIGURE 12 – Strain measurements in the asphalt layer
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 23
0.025
0.02 Elastic Strain
B2 under wheel load
0.015
Pipe Strain (%)
0.01
Permanent Strain
0.005 B5
B6
0
0 10 20 30 40 50 60
B4 70
-0.005
-0.01 T3
-0.015
Time(sec)
FIGURE 13 – Pipe strains at the application of 110,000 wheel passes
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 24
B2
B4
B5
B6
0.02
T3
0.015
0.01
Strain (%)
0.005
0
0 20000 40000 60000 80000 100000 120000
No. of Passes
-0.005
-0.01
FIGURE 14 – Accumulated permanent strains in pipe with wheel passes
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 25
1 80
50,000 passes Temperature
0.9
70
0.8 100,000 passes Sec 9
60
S e ttle m e n t ( in c h )
0.7
Sec 4 Sec 8 50
0.6
0.5 Sec 2 40
0.4
30
0.3
20
0.2 Sec 6
Secs 5&7 10
0.1
0 0
10/10 - 11/7 11/12 - 12/30 5/6 - 6/10 6/11 - 7/2 7/9 - 7/22 7/23 - 8/10
Loading Period (2002 - 03)
FIGURE 15 – Temperature effect on surface deformations in the roadway adjacent
to the bellholes
Evaluation of Restored Pavement Cuts Using an Accelerated Pavement Testing Machine 26
1.2
(stopped after 50,000 passes) Roadway
1
Bellhole Section
S e t t le m e n t ( in c h )
0.8
0.6
0.4
0.2
0
FF Dense Clay Loose Sand Dense Sand D. Clay + D. clay + Stone
pipe cover base layer
Backfill Type
FIGURE 16 – Surface deformations in the bellholes and the adjacent roadway
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