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.