Forensic Investigation and Remediation of Pavement Performance ...

Daoulas, Elfino, Nair, and Nelson FORENSIC INVESTIGATION AND REMEDIATION OF PAVEMENT PERFORMANCE IMPACTED BY GROUNDWATER SEEPAGE: A CASE HISTORY IN VIRGINIA

John Daoulas, P.E Senior Geotechnical Engineer Virginia Department of Transportation 1401 East Broad Street, Richmond, VA 23219 Phone: 804-221-7011 Fax: 804-328-3136 [email protected] Mohamed Elfino, Ph.D., P.E. Assistant State Materials Engineer State Pavement & Geotechnical Manager Virginia Department of Transportation 1401 East Broad Street, Richmond, VA 23219 Phone: 804-328-3173 Fax: 804-328-3136 [email protected] Harikrishnan Nair, Ph.D. * Assistant Concrete Program Engineer Virginia Department of Transportation 1401 East Broad Street, Richmond, VA 23219 Phone: 804-328-3059 Fax: 804-328-3136 [email protected] Sean Nelson Pavement Engineer Virginia Department of Transportation 1401 East Broad Street, Richmond, VA 23219 Phone: 804-328-3174 Fax: 804-328-3136 [email protected]

Re-Submit Date: November 12, 2010 Word Count: 5400 + 2250 Number of Tables: 0 Number of Figures: 9 * Corresponding Author

TRB 2011 Annual Meeting

Paper revised from original submittal.

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ABSTRACT Inadequate drainage of surface and/or subsurface water can have a significant impact on pavement performance and long-term maintenance costs. This paper presents how groundwater (subsurface water) can negatively impact the performance of pavement functionality if not adequately controlled. Forensic methods used to identify the sources of groundwater and measures that are taken to remediate the problem are explained in this paper. Forensic investigation revealed that, in the design of pavements, serious consideration must be given to regional groundwater conditions and the impact it can have on both the integrity and functionality of the pavement, subsequently the safety and comfort of the traveling public.




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INTRODUCTION Pavement deterioration and/or premature failure can result from the presence of water in the pavement structure. Water may enter the pavement through surface infiltration, cracks, joints, and through movement of subsurface water. Subsurface water may be present in the pavement system because of areas of high groundwater table, interrupted aquifers and springs, subsurface flow, and capillary action [1]. Excessive moisture in the pavement structure may cause one or more of the following: a reduction in resilient modulus of unbound subgrade/subbase materials, creation of weak layers by movement of unbound fines into flexible pavement subbase/base courses, reduction of strength during frost melt, stripping in asphalt pavements, hydroplaning, and icing of the pavement surface. Inadequate drainage of surface and subsurface water can have a significant impact on pavement performance and long-term maintenance costs. This paper presents a case history that demonstrates how groundwater (subsurface water) can negatively impact the pavement performance if not adequately controlled. The case history presented herein is located at Route 656 (Sliding Hill Road) between Route 1 to the west and the intersection of Route 656 and Route 637 to the east, in Hanover County, Virginia. In the winter of 2005, water was seeping up through the pavement and freezing on the pavement surface. This caused a safety hazard and several accidents occurred that were attributed to icing of the road in the areas of water seepage. An evaluation of the pavement subdrainage system and preliminary cores of the wet pavement were done in the early spring of 2005. A detailed investigation of the seepage problem followed and described in the following sections. PROJECT DESCRIPTION A new interchange along I-95 was constructed to facilitate relocation of Rte 656 (Sliding Hill Road) crossing I-95 to replace the original I-95/Route 656 interchange located just south of the project site that has since been demolished. The new Interchange consist of eight on-ramps and off-ramps that provides access to Route 656 from the north and south bound lanes of I-95. The project was designed in the late 1990’s and construction started in 2002. Paving of Route 656 was completed and opened to traffic in the fall of 2004 to satisfy public demand. Water seepage was evident shortly after the roadway was opened to traffic within isolated areas along Route 656 on both the east and west sides of the interchange with the east side being the worst case. The case history provided in this paper focuses on the work performed on the east side (the effected area was approximately 200 ft (61 m) long and three lanes wide). An aerial photograph and vicinity map of the project site are provided in Figures 1(a) and 1(b). Route 656 East of I-95 Water was observed seeping out through the surface of the asphalt pavement along the westbound lane of Route 656 between stations 158+00 and 160+50. The approximate area of water seepage is shown in Figures 2(a) and 2(b). The pavement in this area is superelevated (south to north) and consists of three lanes each way that are separated by a raised concrete median. Seepage was observed in all three of the westbound lanes (WBL). No seepage was observed in eastbound lanes (EBL). A graded ditch is located along the north edge of the pavement at the toe of a cut slope. The pavement and ditch slopes from east to west at about 1.6% to 4.4 % grades. From the project plans, up to 20



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feet of cut was needed to grade along the baseline of Route 656 between stations 146+25 and 162+00. Standard Virginia Department of Transportation (VDOT) UD-4 pavement underdrains [2] were installed along the westbound lane of Route 656 and along the ramps to I-95. The typical pavement section shown on the plans for the relocated Route 656 generally consists of about 9.5 inches (238 mm) of asphalt concrete overlying about 8 inches (200 mm) of Cement Treated Aggregate (CTA) up to about Station 162+00. Beyond this point the CTA was replaced with VDOT dense graded aggregate Type I, 21A [3]. The paved shoulders consist of about the 3.5 inches (88 mm) of asphalt overlying about 8 inches (200 mm) of VDOT dense graded aggregate base, Type I size 21B [3].

Site

14 15 16 17

(a) (b) FIGURE 1 Aerial photograph and vicinity map of the project site.

EBL

158+00

Water Seepage 18 19 20 21 22

159+00

160+00

160+50

Water Seepage Schematic

(a) (b) FIGURE 2 (a) Pavement section showing water seepage and (b) water seepage schematic.



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OBJECTIVES Objectives of this study are: 1. Develop forensic investigation protocol to: (a) Determine the source of water seepage causing wetting and icing of the pavement surface (b) Determine the failure mechanism 2. Develop remediation plan 3. Document the Lessons Learned FORENSIC INVESTIGATION PROTOCOL Pavement Underdrain Inspection As an initial step, all pavement underdrains on the project were inspected to determine if they were functioning properly. The inspection showed that some of the drain pipes had been damaged, but not severe enough that the underdrain system was not functioning and causing the pavement wetting. By eliminating the underdrains as a potential cause, the investigation progressed, and it was decided more information were needed to determine the source of the water. Data Collection As part of further investigation the following details were looked at: • Surface Features • Topography • Regional Geological and Groundwater Conditions • Site Subsurface and Groundwater Conditions (Previous Investigation Data) • Conditions uncovered during construction (interviews and documentation review) Site Features and Area Topography The section of I-95 for the new interchange site is located in a low-lying marsh area that is bordered on the east side by Lickinghole Creek. Stony Run Creek crosses through the western portion of the interchange site, beneath I-95, and intersects with Lickinghole Creek in the southeast quadrant of the site. Stony Run Creek and Lickinghole Creek are tributaries of the Chickahominy River located just south of the old interchange site. The topography on the west side of the interchange consisted mainly of rolling terrain varying between elevation (El) 135 to El 185 within the upland areas of the site, and between El 120 and El 130 along Stony Run Creek. The topography on the east side of the interchange slopes moderately to steeply downward to Lickinghole Creek from about El 200 to El 120. The side slopes along the eastern portion of the site contain a number of natural ravines and drainage swales. According to the project plans, a narrow drainage swale had traversed across the relocated section of Rte 656 at about the eastern limit of seepage area that was filled in during construction of the roadway. To the east of the project site the grades along Rte 656 begin to level out with both heavy commercial and residential development on both sides of the roadway.



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A cut slope and drainage ditch is located on the north side of the WBL of Rte 656 (east side). Groundwater was observed throughout the course of the investigation (May through October) seeping out along the side slope several feet above the top of the drainage ditch. Although not shown on the project plans the section of the ditch just to the west of the area of seepage problems was lined with concrete. According to the Contractor, who was still on site at that time, the ditch was lined with concrete to keep it from eroding due to the volume of water that was flowing in the ditch at the time of excavation. The remaining unpaved section of the drainage ditch to the east was saturated and heavily vegetated with cattails, a marsh plant that requires a constant source of water to sustain itself. Another feature of interest was a depression located north beyond the area at the top of the cut slope and outside the limits of the roadway construction. The depression extended over a relatively large area and was several feet in depth that contained no outlet. It could not be determined if the depression was man-made or a natural condition. Another site feature observed was an 18-inch storm sewer line that crossed Rte 656 just to the east of the seepage area at about Station 160+50. According to the plans, the invert of the storm water line is between 10 and 20 feet below the pavement grade. There were no obvious indications of excess groundwater built up in the area of the storm line in the form of seepage. Research of the construction records for this project proved to be of little value. Most of the records were found to be missing and/or poorly maintained. The inspection firm initially involved in the project was later replaced by a second firm. In interviews with both VDOT and the Contractor, it was discussed that during excavation of this area groundwater was encountered that required the use of temporary ditches and other dewatering measures to allow for construction of the pavement. The design geotechnical engineer was not involved during the construction of this project. Area Geological and Hydrologic Conditions The I-95/Route 656 interchange is located within the western boundary of the Atlantic Coastal Plan and just east the Fall Line (roughly along I-95). The Coastal Plan consists, within the area of the project site, of recent to Pleistocene age alluvial deposits overlying Miocene age marine deposits of the Calvert Formation and residual soil derived from the chemical and physical weathering of underlying parent material, Petersburg Granite Formation. Recent alluvial soils consist primarily of clay and sand and located mainly within the low-lying areas along the creek and swamps along I-95. Terrace deposits that are also alluvial soils consist of poorly sorted mixture of sand, clay and gravel located mainly with in the upland areas of the site. The Calvert Formation consist mostly of “Marine” clays typically comprise of highly plastic clays and elastic silts and are considered problem soils due to their potential for slope instability, poor drainage and shrink-swell characteristics. The Calvert Formation typically underlies the terrace deposits in the upland areas of the site. In the low-lying areas of the site, the majority of both the terrace and Calvert Formation soils have been eroded away to the underlying residual soils and replaced with more recent alluvial deposits.




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The highly impervious clay soil of the Calvert Formation creates an upper aquifer to form in the terrace deposits above this formation. The groundwater gradient generally follows the site topography sloping downward towards the creeks and river located in the lowlying areas of the site. This general hydrologic condition was typical of the groundwater conditions uncovered in the borings performed during design of this project. Previous Geotechnical Data At the time of the geotechnical investigation performed for design of this project [4], groundwater was encountered 1 to 4 ft below the planned pavement grades for the portion of Route 656 on the east side of I-95. However, it was noted in the study that drilling was conducted during a dry time of the year and that groundwater levels may rise above the design pavement grades in some areas of deep cuts. Therefore, it was recommended a drainage layer (blanket) be installed beneath the entire pavement along Route 656 between stations 147+00 and 153+00, along Ramp A1 between stations 344+00 and 357+00 and along ramp E1 between stations 374+00 and 380+00. However, the subdrainage recommendations contained in the geotechnical study was not included in the project plans nor were there any construction records that indicated a drainage layer was installed. FORENSIC INVESTIGATIONS Site and Subsurface Investigation - Phase 1 After completion of the reconnaissance, discussed in the sections above, a forensic investigation of the site described below was performed in May 2005. Preliminary Assessment Based upon the information obtained during the site reconnaissance the following preliminary assessment of the possible sources of the pavement seepage was determined: • •

•

Leakage from the existing 18-inch storm sewer line that extends beneath the EBL and WBL of Rte 656 at about Station 160+50. Surface and/or groundwater infiltration migrating into the effected area through the 21A/Cement Treated Aggregate (CTA) interface at about Station 162+00 and/or surface water entering along openings in the pavement along the concrete median between the EBL and WBL Groundwater infiltration either in the form of seepage flow from the cut slope along the north side of the WBL and/or from beneath the pavement originating up gradient to the site from the east.

Investigation Forensic investigation performed during this phase consisted of six (6) standard penetration test (SPT) borings and four (4) pavement cores. Three (3) of the borings were drilled along the toe of cut slope (DL-1, DL-2, DL-3) and three borings at the pavement cores (C-1, C-8 and C-9). A boring was not performed in C-10. Water monitoring wells were installed in each of the borings. The wells consisted of a 1-1/4 inch PVC pipe with a screen at the bottom; the area around the screen was backfilled with sand. In the grassed




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areas the remaining annular space above the screen to the ground surface was backfilled with soil to prevent surface water from filtering into the well. The wells installed in the pavement core borings were backfilled with sand to about the bottom of the CTA layer. The annular space from the bottom of the CTA to top of pavement was sealed with bentonite to prevent both surface water and water contained in the pavement structure from entering into the well. Stabilized water level readings recorded in these wells indicate the groundwater gradient to slope in an easterly direction. At the time of drilling the groundwater was encountered at a depth of about 2.5 ft in boring DL-3 and about 6.0 ft in borings DL-1 and DL-2. Groundwater was encountered at the pavement surface to a depth of about 3.0 ft in the three pavement core borings. The soils encountered in borings DL-1 and DL-2 generally consisted of clay (CL and CH). In boring DL-3 silty sand (SM) was encountered to a depth of about 7 ft with underlying silt (ML). Preliminary Conclusions The following preliminary conclusions were determined based upon the information obtained during the Phase I investigation. o The invert of the 18-inch storm line is well below the depth of the groundwater elevations recorded the borings and cores. Therefore, leakage from the storm water pipe was deemed as an unlikely source of water. o Core C-10 located at about Station 161+40 indicate no water seepage in or directly beneath the pavement; pavement seepage was located about 150 ft east of the CTA/21A interface; and the water seepage remain constant over an extended period of time during both dry and wet weather conditions. Given these factors it was deemed unlikely the main source of water was due to surface water infiltration in the area of the median or water seepage into the effected area originating at the interface of the CTA and 21A pavement structures. o Based on the information obtained during the reconnaissance and field investigation performed, it was concluded that the source of water was most likely due to groundwater infiltration at some point beneath the pavement and migrating upward through the pavement structure. Based on the limited data available it would appear the groundwater flow originated up gradient to the east; however, infiltration from the north along the cut slope was also believed to be a possibility. Intermediate Remedial Measures At this point there was no conclusive way to determine exactly where and how the groundwater was infiltrating into the pavement structure. In lieu of installing an extensive subdrainage system beneath the pavement it was recommended that a modified VDOT Underdrain Type 1 (UD-1) [2] be installed along the northern edge of the pavement along with a trench drain installed perpendicular across the WBL at about Station 160+50 that would tie into the UD-1. The hope was the UD-1 and trench drains could be installed deep enough to cut off the likely groundwater flow from the northerly and easterly directions into the pavement area. However, there were restriction of the



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length and depth the UD-1 could be installed due to underground utilities in the area as well as the elevation at the outlet end of the UD-1 drain pipe. In addition, the lateral trench was not desired since it would require impacting traffic with no guarantee that the drain, if installed, would completely cut off the groundwater flow. The UD-1 was installed by the Contractor during the first part of June 2005. Only a limited amount of groundwater seepage was observed by the Contractor into the trench during installation of the UD-1. In the meantime, ongoing efforts were being carried out to assess the impacts the groundwater seepage had on the pavement structure. PAVEMENT ASSESSMENT Pavement Cores Figure 3 shows CTA and asphalt cores that were typical of the pavement conditions for this site. There were a total of ten (10) cores taken throughout the entire project, but only four (4) were concentrated in the problematic area; C-1, C-8, C-9, and C-10. All cores exhibited adequate pavement depths as called for on the plans. The surface and intermediate course averaged at 3.5” (88 mm), the base mix averaged at 5.6” (140 mm), and the cement treated aggregate (CTA) layer averaged at 9.2” (230 mm). Based upon the cores, the pavement in place appeared to be structurally sound. All cores were extracted to subgrade, and water was observed entering between the interface of the Asphalt Base Mix (BM-37.5) and the CTA layers at a rapid rate, with an exception of C10 (C10 remained dry). The interface between the BM-37.5 and the CTA has been known to be fairly weak due to the large size aggregate and relative high permeability of the BM-37.5 base mix material. Cores C1, C8, and C9 also exhibited debonding of the surface mix from the intermediate mix, but the CTA layers were in excellent conditions and relatively impermeable. Surface cores were tested for density and results ranged between 91.2% and 86% which is an indication of low density and will allow water to seep to the surface. The reverse would also be true in allowing surface water to infiltrate down into the pavement structure. However, as stated earlier, given the constant flow of water seepage during extended periods of dry weather, and drainage conditions observed along the surface of the pavement precluded the likelihood that surface water infiltration as a main contributor to the seepage problems that exist in this area. No distresses were apparent along Rte. 656, but there was cause for concern because of the water conditions that existed within the pavement. FWD testing and GPR testing were arranged to quantify the structural integrity of the pavement structure and potentially locate additional water within the pavement structure and subgrade.



Daoulas, Elfino, Nair, and Nelson

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

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FIGURE 3 Asphalt and CTA cores and core showing water level rise.

Falling Weight Deflectometer (FWD) Testing In May 2005, FWD data analysis along Westbound Rte. 656 indicated that the pavement was structurally sound. Figures 4 shows the deflection data for sensors (D1, D7, and D9) spaced at 0 in, 48 in, and 72 in from the center of the load plate for the 9000 pound impulse load. Using the deflection and load data collected with the FWD and structure information from the cores, the in-place structural condition was evaluated using methods outlined in the 1993 AASHTO guide [5]. Cores indicated that the pavement structure in-place was 9 inches (225 mm) of asphalt over 9 inches (225 mm) of CTA. The as-planned typical section was designed to be 9.5 inches (238 mm) of asphalt over 8 inches (200 mm) of CTA, with a structural number of 5.36. FWD data showed average deflections to be 5 mils, which is a good indication of structurally sound pavement. The average effective structural number was calculated to be 6.91, which indicates the pavement section meets and exceeds the asplanned design. Along with quantifying the soundness of the pavement structure, the FWD data was to be used to potentially find other areas of water within the pavement structure and subgrade. The methodology behind this was to compare the deflections within the pavement from core locations that exhibited water within the pavement structure, and possibly use those deflections to determine other potentially problematic areas through out the project. This process did not work as planned because the high deflections within the pavement structure did not correlate directly with areas of water within the pavement. The FWD data concluded that the existing water within the pavement structure had not yet affected the structural integrity of the originally designed pavement structure.




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1 Atlee-Elmont Rte. 656 Lane 1 East Section FWD Deflection Analysis 8.00 7.00

C1 5.00

C10 4.00 3.00

D e f le c t io n ( M ils )

6.00

C9

2.00 1.00 0.00 138+10

139+10

140+10

141+10

142+10

143+10

144+10

145+10

146+10

147+10

148+10

149+10

150+10

151+10

152+10

153+10

154+10

155+10

156+10

157+10

158+10

159+10

160+10

161+10

162+10

163+10

164+10

165+10

Station (ft) D1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

D7

D9

FIGURE 4 FWD deflection data.

6. Ground Penetrating Radar (GPR) Investigation In June 2005, the GPR system used for the evaluation included a 2.0 GHz air-coupled horn antenna, a SIR-20 data collection system, and a wheel-mounted distance measuring instrument (DMI) [6]. The typical pavement section information was obtained from the cores prior to GPR scanning. By knowing the thickness of the various layers prior to scanning with GPR, it was easier to correlate the reflections indicated in the GPR data with the presence of a known physical boundary. The layer interfaces appear as brighter horizontal lines against a darker background. Analysis of the GPR data indicates layer interfaces at average depths of approximately 1.5, 3.5, 8.5, and 14.5 inches (38, 88, 213, and 363 mm) for Sliding Hill Road. Variations in the depth to the bottom of each layer were found to be approximately plus or minus 0.5 to 1.5 inches (13-38 mm) and are typical of paving operations. The reflection at the bottom of Layer 4 was transient in nature and was only found in certain portions of the project. As the dielectric constant of the cement treated aggregate (CTA) and crushed aggregate, for Sliding Hill Road and the underlying soil is expected to be similar, it was thought that a weak reflection would be found at the CTA/subgrade and crushed aggregate/subgrade interface. However, in certain locations (low-lying areas) the reflection was found to be strong. It was theorized that the stronger reflection occurs in these locations due to a significant moisture presence at the CTA/subgrade and crushed aggregate/subgrade interface. The fact that the stronger reflection was found in low-lying areas (and in areas where the cores showed water within the pavement), but not in areas well above the surrounding grade reinforced this theory. Figure 5 presents the results of the GPR data collection (along with FWD



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data) and highlights the presents of water (where a strong reflection at the Layer 4 interface was found).

Cores 1, 9, and 10 were taken from this region

8.00

7.00

5.00

4.00

GPR Detected Water between Sta. 140+75146+50



3.00

D e fle ction (M ils )

6.00

2.00

1.00

0.00 11 3+ 1 0

11 4+ 1 0 11 5+ 1 0

11 6+ 1 0

11 7+ 1 0

11 8+ 1 0 11 9+ 1 0

12 0+ 1 0 12 1+ 1 0

12 2+ 1 0

12 3+ 1 0

12 4+ 1 0

12 5+ 1 0 12 6+ 1 0

12 7+ 1 0

12 8+ 1 0

12 9+ 1 0

13 0+ 1 0

13 1+ 1 0

13 2+ 1 0 13 3+ 1 0

13 4+ 1 0

13 5+ 1 0

13 6+ 1 0 13 7+ 1 0

13 8+ 1 0

13 9+ 1 0 14 0+ 1 0

14 1+ 1 0

14 2+ 1 0

14 3+ 1 0 14 4+ 1 0

14 5+ 1 0

14 6+ 1 0

14 7+ 1 0

14 8+ 1 0

14 9+ 1 0

15 0+ 1 0 15 1+ 1 0

15 2+ 1 0

15 3+ 1 0

15 4+ 1 0 15 5+ 1 0

15 6+ 1 0

15 7+ 1 0

15 8+ 1 0

15 9+ 1 0

16 0+ 1 0

16 1+ 1 0 16 2+ 1 0

16 3+ 1 0

16 4+ 1 0

16 5+ 1 0

Station Lane 1 East of NB I-95

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Lane 1 West of SB I-95

D9 East of NB I-95

D9 West of SB I-95

FIGURE 5 GPR data analysis.

Site and Subsurface Investigation - Phase 2 In August 2005, the pavement seepage was reevaluated based upon the results of the monitoring program performed after installation of the UD-1 drain. Preliminary Assessment The effects on the pavement seepage due to the installation of the UD-1 were monitored over a period of about two months. There was a constant flow of water exiting the UD-1 during the monitoring period. Although the water seepage condition in the pavement appeared to improve, the seepage remained persistent in a number of areas. During the monitoring period the Contractor revealed that a field change was made to a portion of the pavement structure for EBL Rte 656 that was never documented. The modifications included replacement of the CTA base material with 21A aggregate and increasing the asphalt layer an additional 3 inches. The Contractor could not recall the information provided regarding the pavement modifications, but believed to occur between about Station 159+00 and Station 162+00. Given the conditions observed during the monitoring period and the information provided regarding the pavement modifications the preliminary assessment of the source of water was as follows:

1. Groundwater remains the primary source of pavement seepage. Although the UD1 appeared to be intercepting some of the groundwater flow, the majority of the




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flow was by-passing the drain either from the upland area to the northeast of the site and/or up gradient along the alignment of 656 to the east. 2. In light of the new information obtained regarding the modification made during construction to the EBL pavement an investigation was performed in this area to determine the actual extent of pavement modifications and the possiblity of surface and/or groundwater infiltration that may be contributing to wetting of the adjacent WBL pavement. Investigation A total of 15 additional pavement cores/borings were performed to further assess groundwater movement within and beneath the pavement structure within the WBL and concrete median. In addition, to determine the approximate area of the pavement modifications and moisture condition of the base aggregate beneath EBL of Rte 656. Five additional borings (B-1 through B-5) were drilled in the grassed areas at the top of the cut slope north of the WBL and in areas of between 200 ft and 400 ft east of the site along both sides of Rte 656. Water observation wells were installed in each of the borings in the same manner described in Phase 1. The borings were drilled to depths ranging between 5 ft and 30 ft. Pleistocene terrace deposits were encountered in all borings that generally consisted of sand (SC, SM, SC-SM, SP-SC, SW and SW-SC) with intermediate layers of lean clay (CL) and silt (ML). A layer of fat clay (CH) was encountered beneath the sand and lean clay layers in seven of the borings. The fat clay is also believed to be terrace deposits. The fat clay layer extends uniformly across the study area varying between El 183 to El 187 just below the groundwater elevations recorded in the borings. Preliminary Conclusion Based on the core data the modified pavement section along the EBL extends between about Station 158+90 to Station 162+00. The 21A base aggregate uncovered in this area was dry. There existed some water within the pavement subgrade in the area of the median; however it was not observed at the interface of the concrete median and underlying intermediate pavement layer. Therefore, surface water infiltration seeping in from the EBL or from beneath the concrete median did not appear to be a main contributor to the wetting of the WBL pavement. Water that was encountered in the cores performed along the median is believed to be groundwater. Based upon the groundwater readings taken and the resulting pattern of groundwater flow determined in this area the most likely source of the water infiltration into the pavement is from perched groundwater flow in the more permeable sand and lean clay layers above the fat clay and originating northeast of the site. The groundwater levels in the two borings performed at the far east end of the site were well below the levels recorded in the area of pavement seepage. Therefore it was not believed that the groundwater infiltration into the pavement originated from this direction.




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Intermediate Remedial Measures No remedial measures were performed at this time other than to continue monitoring of the wells and UD-1 drain to determine if conditions would change over time that may impact final remedial measures to be taken to correct this problem. FAILURE MECHANISM After the Phase 2 investigation was completed the area was monitored for an addition two (2) months. No changes to the conditions were noted and the groundwater levels remained relatively constant in the wells. In October 2005 four additional borings (A, C, D-1, and E) were performed along the anticipated groundwater flow line believed to be the source of infiltration into the pavement. The approximate location of the majority of the borings and pavement cores performed during the Phase 1 and 2 investigation and corresponding stabilized water level readings taken are shown on Figure 6. The resulting phreatic surface of the groundwater table developed from the water level readings recorded in the wells that is believe to be the main cause of wetting of the effected WBL pavement area is shown in Figure 7. From the data collected during this investigation, it was concluded that the failure mechanism that resulted in the pavement water seepage and an unsafe road conditions occurred due to groundwater infiltration that originated in the upland areas just to the northeast of the site. The ground depression in this area may have further contributed to the groundwater recharge in this area. The groundwater table is believed to intersect the bottom of the CTA at about El 160+50 with a sufficient head pressure to allow the water to migrate up through the cracks and construction joints of the CTA and through the more permeable asphaltic pavement structure to the pavement surface. The flow of groundwater was constant and did not appear to vary due to changes in weather. Therefore, left untreated the area of pavement seepage would likely have expanded to the east creating an even greater traffic hazard as well as lead to the premature deterioration of the pavement structure.




N

Legend WBL

- Cores/borings - Cores

UD-1 EBL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

No. B-3 A C D1-2 B-4 D1 C-9

GS/GW Ele.* 192.27/186.87 192.11/186.70 189.71/184.09 186.27/176.37 198.09/184.02 190.15/186.85 188.55/187.15

GS/GW Ele C-16 190.4/188.9 C-15 190.48/Dry C-26 191.13/Dry C-20a 190.32/Dry C-20 190.17/Dry C-14 188.92/188.19 C-17 188.24/187.62

C-23a C-23 B-5 D1-1 C-1 C-24 C-11

GS/GW Ele 185.84/Dry 185.72/Dry 197.67/181.0 183.15/175.85 187.02/186.44 186.04/184.54 186.15/Dry

C-18 C-8 C-12 C-19 C-25 C-21 C-13

GS/GW Ele 186.15/184.9 187.00/185.80 187.58/186.50 189.03/Dry 190.75/Dry 189.86/dry 188.47/187.63

*Ground Surface/Groundwater Elevation


FIGURE 6 Location of the borings and pavement cores (all stations are in ft).



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193 B-3

Approximate ground surface grade

191

189

Elevation (ft)

Water level in pavement 187

185

Perched groundwater table

183

181

1 2 3 4

162+95 162+03 61.37 ft.lt 26.86 ft.lt


160+60 13 ft.lt

160+07 24.25 ft.lt

159+01 23.39 ft.lt

158+24 8.91 ft.lt

FIGURE 7 Phreatic surface of the groundwater.


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Final Remediation Plans During development of the remediation plan the following points needed to be considered: (i) Public safety (ii) Time to install (iii) Cost (iv) Level of intrusiveness on existing pavement (v) Impact on traffic to construct (vi) Public perception and (vii) Potential risk of success. Based on the above points, several design options were considered. The final remedial plan developed recommended removal of the entire existing pavement structure to include the CTA within the effected area and undercutting the subgrade about 15-inches (375 mm) and replacing it with a new pavement section that included a drainage layer. The replacement section included 9.5” of asphalt pavement, 4” of aggregate, and a 19”drainage blanket of AASHTO No. 57 stone as illustrated in Figure 8. The 19” drainage blanket was connected to the UD-1 which was installed earlier during the investigation to provide an outlet for water collected within the drainage blanket.

Median

Shoulder 1.5” Asphalt surface Course, Type SM 9.5 D 2” asphalt Intermediate Course, Type IM 19 6” Asphalt Concrete, Type BM-25 4” Aggregate Material, Type 1, 21B Geotextile Drainage Fabric 19” of AASHTO # 57 Stone 8” perforated pipe

FIGURE 8 Rehabilitated Pavement Section (Not to scale) (1 inch= 25mm). The rehabilitation work was completed in 2007. The rehabilitated pavement section has not shown further water seepage problems since it was installed. Figure 9 shows the pavement section after 3 years (July 2010) of service life.




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Rehabilitated section

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

FIGURE 9 Final Rehabilitated Section. Conclusions and Lessons Learned Contrary to the popular belief that pavement cracks are the primary source of water infiltration into the pavement structure and the cause of accelerated deterioration, this project shows that subsurface seepage if not properly addressed can also act as a main culprit. Also, the freeze–thaw negative effect on the pavement performance is much higher when water exists within the pavement structure and is not drained out because of the absence of an adequate subsurface drainage system. The following are the lessons learned from this project. In design of pavements, serious consideration must be given to regional and local groundwater conditions and the impact it can have on both the integrity and functionality of the pavement (Public Safety) if not adequately controlled. Information about groundwater occurrence and soils characteristics for the design of subsurface drainage structures often require extensive field investigation and a subsurface exploration program. A proper geotechnical investigation should identify groundwater, the potential for variation, and provide recommendations that should be considered in both the design and construction of the project. The consequences of not considering this information will likely lead to improper performance and premature failure of the pavement. If groundwater conditions are different than those considered during design, the Designer needs to be notified immediately to address these changes. If not, ignoring these changes will likely result in problems to occur that will be more difficult and expensive to deal with at a latter date. Inspectors’ needs to be properly trained and the lines of communication well defined and maintained. Also, the groundwater conditions if observed needs to be well documented.



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References 1. Huang, Y. H. (1993). Pavement Analysis and Design. Prentice Hall, Inc., Englewood Cliffs, NJ. 2. VDOT 2008 Road and Bridge standards, Richmond, Virginia 3. VDOT 2007 Road and Bridge specifications, Richmond, Virginia 4. Geotechnical Study, Proposed I-95 and Atlee-Elmont Interchange by Schnabel Engineering dated April 30, 1999 5. Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, D.C., 1993 6. Brian K. Diefenderfer, Forensic Evaluation of Sliding Hill Road Using Ground Penetrating Radar, Technical Report, Virginia Transportation Research Council, July 2005.

