settlement mitigation of a distressed embankment in …docs.trb.org/prp/16-4179.pdf · settlement...

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Ruttanaporamakul et al.

Settlement Mitigation of a Distressed Embankment in Texas by

Utilization of Lightweight EPS Geofoam Material

Pinit Ruttanaporamakul, Ph.D.

Box 19308, Department of Civil Engineering

The University of Texas at Arlington

Arlington, Texas 76019

Anand J. Puppala, Ph.D., P.E.

Distinguished Professor




Aravind Pedarla, Ph.D.

Faculty Research Associate




Tejo V. Bheemasetti, Ph.D.

Post-Doctoral Fellow

Department of Civil Engineering, Box 19308



Richard S. Williammee, Jr., M.S., P.E

District Materials Engineer

Fort Worth District

Texas Department of Transportation

P.O. Box 6 Fort Worth, TX 76115

Word Count: text = 3800; figures = 3000 (12 figures); tables = 250 (1 tables)

Total Words: 7050 words

A Paper Submitted for Possible Publication in

Journal of Transportation Research Board

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ABSTRACT 1

Approach slab settlement occurring at the start of a bridge super structure is one of the most 2

common problems in many states across USA, resulting in rider discomfort and unsafe riding 3

conditions. Federal and State transportation agencies continue to spend millions of dollars 4

annually to repair the ‘bump’ problem. Major factors contributing to these settlements are the 5

long term compressibility of backfill materials as well as the erosion of backfill material. This 6

paper documents a distress occurred at the approach slabs on each end of the US 67 bridge over 7

SH 174 in Johnson County, Cleburne, Texas. This approach slab had experienced more than 16 8

in. (406 mm) of settlement in 16 years since its initial construction. Current study highlighted the 9

factors causing distress in the embankment and evaluated the remedial technique adopted at this 10

site. Native fill material at the top of the embankment was replaced with lightweight expanded 11

polystyrene (EPS) geofoam blocks. This technique considerably reduced the magnitude of 12

overburden stresses transferred to the underlying layers as well as the erosion of embankment fill 13

soil. This remediation also mitigated further settlements of the embankment fill and foundation 14

soils. Field monitoring studies using inclinometers and pressure plates have been conducted at 15

regular time intervals for a period of three years to study the long term performance of EPS 16

geofoam under live traffic. The long-term settlements of the rehabilitated embankment were 17

predicted by using the field measured data with the hyperbolic method. These studies were 18

validated using measured field settlement data. Based on the current study, the effectiveness of 19

adopting EPS geofoam as the embankment fill material near a bridge approach slab is evaluated. 20

Keywords: Bump at the end of the bridge, Embankment settlement, Bridge approach 21

embankment, Expanded Polystyrene (EPS) geofoam 22

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INTRODUCTION AND BACKGROUND 1

The existence of the differential settlement or ‘bump’ at the end of bridge, not only presents an 2

unsafe driving condition but also results in agencies spending millions of dollars on maintenance 3

costs annually. A survey carried out in Kentucky by Hopkins and Deen (1) revealed that 78% of 4

several hundred highway bridge approaches required some maintenance activities to remedy the 5

problem of the bump at the end of the bridge. Briaud et al. (2) reported that 30 percent of the 6

bridges in Texas (i.e., 13,800 out of 46,000 bridges) encountered the problem of differential 7

settlement at the bridge ends and at least USD 100 million was spent on annual maintenance cost 8

for state departments of transportation (DOTs). Seo (3) presented that approximately USD 7 9

million was spent annually on the bump problem repairs in Texas. Recent studies show that 10

many state DOTs have insufficient funds for the maintenance costs spent on bridge repairing 11

activities (4). This trend clearly shows the strong need for effective and durable construction 12

techniques for approach slab embankments near bridges. 13

Several studies were conducted to identify the possible cause of the ‘bump’ at the end of 14

the bridge problem and also many studies focused on designing the most effective techniques for 15

mitigating the ‘bump’ problem (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13). Researchers concluded 16

that the primary factors of the settlement problems of the bridge approach pavements are void 17

development from backfill collapse, backfill material consolidation, severe backfill erosion, poor 18

soil compaction and construction practices, poor surface and subsurface water management. 19

Many techniques have been proposed to mitigate the differential approach slab settlement 20

(3, 4, 8, 11, 13, 14, 15, 16, 17). Some of which are improvement of foundation soil, improvement 21

of backfill material, design of bridge foundation, design of approach slab and provide effective 22

drainage and erosion control methods (11, 13). 23

In the current research, the embankment for the bridge of a 40-ft. (12 m) high situated on 24

US 67 over SH 174 in Johnson County, Texas, had experienced more than 16 in. (406 mm) of 25

settlement in 16 years, since its construction. 26

From initial site analysis, it was found that the major factor contributing to the settlement 27

is the consolidation of the foundation soil due to its self-weight. Apart from current ground 28

improvement techniques, novel light weight fill materials such as expanded polystyrene (EPS) 29

geofoam material was found to be optimal for the current rehabilitation project and were 30

evaluated for performance in this research. 31

Expanded Polystyrene (EPS) 32

According to ASTM D 6817-07, EPS geofoam is defined as a block or planar rigid cellular foam 33

polymeric material used in geotechnical engineering applications. The first use of this material 34

was in 1972 for the construction of an embankment adjacent to a bridge founded on piles in 35

Norway (18). However, the use of EPS geofoam for lightweight fill in the US dates back to the 36

1980’s (19). The method of improvement of embankment backfill using expanded polystyrene 37

(EPS) geofoam material to mitigate the settlement problem has been well adopted worldwide 38

(18, 19). Because of its lightweight property, using EPS geofoam as an embankment fill can 39

reduce the loads on underlying soils and consequently, minimize the total settlement of the soils 40

and differential settlement at the bridge ends (20). The density of the EPS geofoam varies from 41

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0.70 to 2.85 pcf (11.2 to 45.7 kg/m3), which is approximately 100 times lighter than most soils 1

and at least 20 to 30 times lighter than other lightweight fill alternatives (20). 2

Previous literature reported that EPS geofoam blocks can be effectively used in highway 3

embankment construction (18, 21, 22). Frydenlund and Aaboe (18) evaluated the embankment 4

construction for a temporary bridge, the Lokkeberg Bridge, in Norway, where site investigation 5

indicated that the foundation soil with low bearing capacity. EPS geofoam blocks were 6

considered for use as the fill material for the bridge approaching embankment. After being open 7

to traffic for 12 years, deformation of 6 cm (2.4 in) was all that was observed in the EPS 8

embankment, and most of the deformation occurred during the construction phase. 9

In a more recent application, EPS geofoam was adopted by the Utah Department of 10

Transportation (UDOT) in reconstructing a 27.4 km (17.03 miles) portion of Interstate Highway 11

15 (I-15) in Salt Lake City, Utah (23). From past observations along I-15, settlements of up to 55 12

in. (1400 mm) were documented over a period of 30 years for embankment height ranging from 13

20 to 33 ft. (6 to 10 m) (24). The reconstruction involved the widening of the interstate 14

embankment from 8 lanes to 12 lanes, where an extensive deposit of compressible lake bottom 15

sediment exists. After successful construction, Bartlett et al. (2012) concluded that EPS geofoam 16

successfully reduced the soil settlement to 0.98 in. (25-mm) over a 5-year period after the 17

highway was open to traffic. 18

The objective of the current research is to evaluate the effectiveness of using lightweight 19

EPS geofoam to mitigate the settlement of the bridge approach embankment at US 67 site, Texas 20

and to predict the long term settlement behavior of EPS geofoam. 21

BRIDGE REHABILITATION AT US 67 22

US 67 bridge over SH 174 was constructed in 1995, using pre-stressed concrete beams and 23

abutments were supported by drilled shaft foundations. Approach slabs were constructed on the 24

embankments adjacent to the bridge ends. Clay soil with moderately high plastic index (PI) was 25

used as the embankment backfill, retained by the concrete block walls, as presented in FIGURE 26

1. 27

28

FIGURE 1: Schematic of the US 67 bridge over SH 174 in Johnson County, Texas 29

16 years since the initial construction, the approach embankments on each end of the 30

bridge had experienced more than 16 in. (406 mm) of settlements, as illustrated in FIGURE 2. 31

Several treatment methods, including overlays, foam slab jacking, and compaction grouting were 32

attempted and found not effective in mitigating the settlements. According to the field 33

investigations conducted in 2000, it was found that the significant factors contributing to the 34

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settlements experienced at this site include insufficient compaction of soils associated with 1

abutment construction, erosion and settlement of the embankment fill soil underlying the 2

approach slabs, and the movement of the concrete block retaining wall. 3

4

FIGURE 2: Settlement at US 67 bridge approach slab over 16 years since the initial 5 construction (Courtesy of TxDOT, 26) 6

SITE CONDITIONS OF TEST SECTION 7

BASED ON THE DATA OBTAINED FROM FIELD INVESTIGATIONS, THE 8

SUBSURFACE STRATIGRAPHY UNDERNEATH THE PAVEMENT SYSTEM 9

CONSISTS OF THE EMBANKMENT WHICH COMPRISES OF 30 FT. (9 M) OF CLAY 10

FILL OVER 10 TO 20 FT. (3 TO 6 M) OF NATURAL CLAY SOILS EXTENDED TO A 11

LAYER OF LIMESTONE. THE COMPRESSIBILITY PARAMETERS OF THE SOILS 12

OBTAINED FROM THE TEST ARE PRESENTED IN 13

. 14

TABLE 1: Variation of Soil Parameters at US 67 bridge site over a period (2000 to 2011) 15

Year 2000 2011

Parameter Unit Embankment

fill soil

Foundation

soil

Embankment

fill soil

Foundation

soil

Natural moisture

content, 𝜔

% 27.2 20.7 23.2 N/A

Dry unit weight, 𝛾𝑑𝑟𝑦 pcf 96.9 106.6 101.9 N/A

Liquid limit, LL % 61 53 N/A N/A

Plastic index, PI % 40 36 N/A N/A

Initial void ratio, 𝑒𝑜 - 0.739 0.580 0.654 N/A

Compression index,

𝐶𝑐

- 0.25 0.18 0.23 N/A

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Recompression index,

𝐶𝑟

- 0.10 0.06 0.07 N/A

Overconsolidation

ratio, OCR

- 1.00 10.29 10.10 N/A

Consolidation test was performed on the representative embankment fill and foundation 1

soil and test results showed that the natural foundation soils are overconsolidated (OCR = 10.29). 2

During this time period, the void ratio decreased with an increase in the dry unit weight. 3

DESIGN, CONSTRUCTION, AND INSTRUMENTATION OF EPS GEOFOAM 4

EMBANKMENT TEST SECTION 5

In 2012, the rehabilitation project on US 67 bridge over SH 174 was initiated by TxDOT to 6

repair bridge beams, and renew 150 ft. (46 m) of approach slabs and pavement on each bridge. 7

Approximately 35,000 ft3 (990 m

3) of EPS 22 geofoam blocks were placed at the top 6-ft. (1.8-8

m) of the bridge approach embankments. The use of EPS geofoam was expected to reduce the 9

settlements and erosion of the embankment fill and consequently reduce the approaching slab 10

settlement and bump at the end of the bridge problems. Preliminary design for the EPS 11

geofoam used in the US 67 bridge rehabilitation was conducted in accordance to NCHRP Report 12

529 (19). The wheel loads of a standard truck used in a bridge design, HS-20 (AASHTO, 2012), 13

was considered in the design. 14

The approximate analysis of soil settlement showed that in order to reduce the settlement 15

of the embankment to be less than 1 in. (25 mm), almost all of the existing backfill soil has to be 16

replaced by EPS geofoam. However, excavating and replacing all embankment backfill soil is 17

neither practical nor economical. From NCHRP report guideline and the preliminary design, the 18

height of 6 ft. (1.8 m) of EPS geofoam was adopted. With this depth of EPS geofoam, about 2.5 19

in. (63.5 mm) settlement was estimated to be occurred on the embankments. 20

The rehabilitation work started in January 2012 and was completed at the end of February 21

2012. The pavement structure along with underlying 10 feet of embankment material was 22

removed. The underdrain systems were installed at the bottom of the excavation. Following that, 23

2 to 6 in. (50 to 152 mm) of a sand-leveling layer was compacted on the underdrain systems and 24

then, the 6-ft. (1.8-m) high stack of three layers of the EPS22 geofoam was installed on the 25

compacted sand blanket, as illustrated in Error! Reference source not found.. To protect the 26

EPS geofoam from water infiltration, a layer of impermeable geomembrane was used in 27

encapsulating the installed geofoam blocks, as presented in FIGURE . Finally, about 2 ft. (0.6 28

m) height of the pavement structure, including lightweight aggregates, flex base, hot mix asphalt 29

concrete (HMAC), and concrete pavement, was constructed on top of the embankment. 30

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FIGURE 3: Retrofitting of approach slab using EPS geofoam blocks 3

Utilization of EPS geofoam as the backfill material not only enhanced reduction in 4

settlement but also decreased the overall cost of construction. This is due to the easy handling 5

and reduced labor and installation costs of EPS geofoam blocks, without any necessity of having 6

heavy compaction equipment (11). To evaluate the effectiveness of EPS geofoam material in 7

mitigating the settlement problem of the embankment system, the test site was instrumented with 8

horizontal inclinometer device for monitoring the vertical movement of the embankment. During 9

the rehabilitation, the total four inclinometer casings of diameter 3.34 in. (8.5 cm) were placed 10

on top of the EPS geofoam layer (i.e., about 2 ft. (0.6 m) below the pavement surface. The length 11

of each casing is more than 22 ft. (6.7 m). The locations of the installed inclinometer casings are 12

shown in Error! Reference source not found.. 13

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FIGURE 4: Location of the inclinometer casings installed at the test embankment 2

Settlement Behavior of the EPS Geofoam Embankment Test Section 3

The vertical movements at the top of EPS geofoam layer were monitored at every 2 ft. (0.6 m) 4

intervals. Collected data is presented in the form of graphs plotted between the vertical 5

displacements, recorded cumulatively from the initial reading at the time of casings installation 6

(01/30/2012). Error! Reference source not found. to 8 present the data of the vertical 7

movements measured from the horizontal inclinometer casings US 67 _ 1 to US 67_4, 8

respectively. Due to some damage caused during initial construction, casing US 67_2 had to be 9

replaced. Data from Figure 6, US 67_2 were missing in the time range of November 2013 to 10

May 2014. The variation of maximum vertical movements of the test embankment with time, 11

measured at the middle of pavement, is presented in FIGURE . The plot includes two sets of the 12

displacement data, which are the total vertical displacement and the post-construction vertical 13

displacement. Post-construction vertical displacement can be determined by subtracting the 14

settlement that occurred from initial 28 days of construction from the total vertical settlement 15

that had occurred at that time. It can be observed from the plots that during almost three years 16

after opening to traffic, less than 1.5 in. (38 mm) of post-construction settlement had occurred. 17

US 67_1

US 67_2

US 67_3

US 67_4

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FIGURE 5: Vertical displacement data plot from horizontal inclinometer US 67_1 2

3

FIGURE 6: Vertical displacement data plot from horizontal inclinometer 67_2 4

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Prediction of a Long Term Settlements of Geofoam replaced Test Embankment 1

Prediction of long term settlements in the test embankment is attempted using the Lin and Wong 2

(25) hyperbolic method. Based on the assumption that the rate of settlement is decreasing 3

hyperbolically with time, the relationship between the settlement and time can be presented by 4

the hyperbolic equation, as provided in following: 5

𝑡

𝑆 = 𝛼 + 𝛽(𝑡) (1) 6

where 𝑡 is time from the start of embankment fill (days); 𝑆 is measured settlement as any 7

specific time (mm); 𝛽 is gradient of the straight line between 𝑡 and 𝑡

𝑆 ; and 𝛼 is intersection of the 8

straight line on the 𝑡

𝑆 axis. 9

By plotting the average of total vertical movement at the middle of pavement with a 10

function of time-settlment ratio and using linear regression analysis, the values of parameters 𝛽 11

and 𝛼 can be estimated, as presented in FIGURE 3. 12

The equation for estimating the settlements magnitude at a specific time (𝑡) of the test 13

embankment can be generated by substituting the parameters 𝛽 and 𝛼 back into Eq. 1 and 14

rewriting Eq. 2 and 3 as follows: 15

𝑆 = 𝑡

(𝛼+ 𝛽𝑡) (2) 16

𝑆 = 𝑡

(7.5838 + 0.024 𝑡) (3) 17

According to Eq. 3, the plot of the predicted vertical displacement which will occur in the 18

test embankment at the specific time interval can be provided, as illustrated in FIGURE . 19

Similarly, it is possible to predict the long term post-construction vertical displacement of the 20

test embankment, as plotted in FIGURE 4. The field data and its best-fit curves are also plotted 21

in the figures. 22

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FIGURE 9: Variation of average vertical displacement of the test embankment versus time 2

3

FIGURE 3: Relationship between time-settlement ratio of the test embankment 4

with time 5

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FIGURE 11: Variation of total vertical displacement of the test embankment with time 2

3

FIGURE 4: Variation of post-construction vertical displacement of 4

the test embankment with time 5

It can be observed from the plots that the vertical displacement/settlement of the test 6

embankment predicted by hyperbolic method is in good agreement with the best-fit curve of the 7

measured field data from the horizontal inclinometer surveys. Based on the hyperbolic method, 8

the total and post-construction vertical displacements which will occur on the test embankment 9

at 10 years interval are predicted to be 1.5 in. (38 mm) and 1.3 in. (34 mm) respectively. This 10

clearly shows that minimal settlement from the compression of EPS geofoam material over the 11

10 year period, where the majority is contributed from post construction settlement. 12

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SUMMARY 1

From the monitoring studies conducted at US 67 embankment over the past three years and 2

corresponding settlement prediction analysis conducted, it is recommended to adopt EPS 3

geofoam material as an embankment fill where high compressible and collapsible subsoil exists. 4

EPS material proved to be effective in mitigating the vertical stresses transferred to the 5

foundation soil thereby reducing any further increase in settlement. 6

Settlement prediction analysis conducted is consistent with the monitored data obtained 7

from horizontal inclinometer until the three year period. The maximum post-construction vertical 8

displacement occurring at the center of pavement, atop the EPS geofoam layer of the 9

rehabilitated US 67 bridge embankment was about 1.0 to 1.1 in. (25 to 28 mm). 10

Total settlement of 1.5 in. (38 mm) and post-construction settlement of 1.3 in. (34 mm) 11

were predicted to occur on the test embankment after 10-year time period. These settlements are 12

considered low, as they can cause only a slight bump at the bridge deck-approach slab interface, 13

which does not affect the riding quality. In the current rehabilitation project only top 6 feet of the 14

embankment section is replaced with EPS geofoam blocks. Studies are being conducted to 15

evaluate the optimal placement of EPS geofoam blocks in the embankment section for effective 16

performance. 17

ACKNOWLEDGEMENTS 18

This study was sponsored by Texas Department of Transportation (TxDOT) under research 19

project No. 5-6022-01. The authors would like to sincerely thank the Project Committee for 20

providing their support and valuable guidance during the research. 21

REFERENCES 22

1. Hopkins, T. C. and Deen, R.C. (1969). “The Bump at the End of the Bridge.” Rep. No. 23

KYHPR-64-17; HPR-1(4), Kentucky Transportation Center, Lexington, Kentucky. 24

2. Briaud, J. L., James, R. W., and Hoffman, S. B. (1997). NCHRP synthesis 234: 25

Settlement of Bridge Approaches (the bump at the end of the bridge), Transportation 26

Research Board, National Research Council, Washington, D.C. 27

3. Seo, J. (2003). “The Bump at the End of the Bridge: An Investigation.” Dissertation 28

submitted in partial fulfillment of the requirements for the degree of the Doctor of 29

Philosophy, Texas, A&M University, College Station, Texas. 30

4. White, D. J., Mekkawy, M. M., Sritharan, S., and Suleiman, M. T. (2007). “Underlying 31

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of Constructed Facilities, 21(4), 273-282. 33

5. Stewart, C. F. (1985). “Highway Structure Approaches.” California Department of 34

Transportation, Sacramento, CA 35

6. Ardani, A. (1987). “Bridge Approach Settlement.” Rep. No. CDOT-DTP-R-87-06, 36

Colorado Dept. of Highways, Denver. 37

7. Tadros, M. K. and Benak, J. V. (1989). “Bridge Abutment and Bridge Approach Slab 38

Settlement, phase I.” Final Rep., Nebraska Dept. of Roads, Lincoln, Nebraska. 39

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8. Wahls, H. E. (1990). NCHRP Synthesis of Highway Practice No. 159: Design and 1

Construction of Bridge Approaches. Transportation Research Board, National Research 2

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9. Mahmood, I. U. (1990). “Evaluation of causes of bridge approach settlement and 4

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Norman, Okla. 6

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Slabs under Deteriorating Soil Washout Conditions.” Journal of Bridge Engineering, 14

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Bridge Approaches: Performance, Cost and Recommendations for Improvements.” 26

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17. Hsi, J. P. (2008). “Bridge Approach Embankments Supported on Concrete Injected 28

Columns.” Proceedings of The Challenge of Sustainability in the Geoenvironment, 29

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18. Frydenlund, T. E. and Aaboe, R. (2001). “Long term performance and durability of EPS 31

as a lightweight fill material.” EPS Geofoam 2001, 3rd International Conference, UT. 32

19. Stark, T. D., Arellano, D., Horvath, J. S., and Leshchinsky, D. (2004). “Geofoam 33

applications in the design and construction of highway embankments.” NCHRP Web 34

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construction of expanded polystyrene embankments." Construction and Building 40

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Copyright © Softoria 2006. 43

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evaluation of expanded polystyrene geofoam embankments for the I-15 reconstruction 45

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project, Salt Lake City, Utah.” Report No. UT-1X.XX, Utah Department of 1

Transportation, UT. 2

24. Negussey, D. and Stuedlein, A. (2003). “Geofoam fill performance monitoring.” Utah 3

Department of Transportation Research Division Report No. UT-03.17, UT. 4

25. Lin, Q.L., Wong, I.H. (1999). “Use of Deep Cement Mixing to Reduce Settlements at 5

Bridge Approaches.” Journal of Geotechnical and Geoenvironmental Engineering, 6

ASCE, Vol. 125(4),309. 7

26. TxDOT-FTW Committee (2002). “The Bump at the End of the Bridge.” Report to 8

District Staff., Texas Department of Transportation, Fort Worth District. 9

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