Rockfill Embankment Settlement Sugarloaf Mountain Bridge Abutment and Hoover Dam Bypass (US-93) Scott A. Anderson and Nicholas J. LaFronz the Abutment 2 fill catches on the side-slope of an adjacent lower embankment for the Hoover Dam Access Road. The 1,700-ft-long embankment east of the east abutment is founded on moderately rugged bedrock terrain. Published studies of performance of spread footings on embankment are relatively rare. FHWA (1) presented a summary of Washington Department of Transportation studies of bridge spread footings on compacted fill, including on shot rock. Spread footings were instrumented and settlement was measured via borehole settlement devices and a SONDEX system at one site. The process of monitoring settlement before placement of superstructure and abutments was described, as was the use of jacked abutments. The cost of spread footings was approximately 50% to 65% that of typical pile foundations at the time of the study. The FHWA report (1) generally reported good performance of bridges on spread footings on the basis of monitoring data, including those founded on shot rock embankment, although the experience cited was primarily focused on smaller bridges and embankments. The authors indicated that many bridges tolerated 1 to 3 in. of differential settlement. Most of the measured settlement was compression of underlying foundation materials and not the embankments themselves. The authors concluded that (a) approximately 75% of observed settlement occurred during the construction period before placing the bridge deck, (b) post–deck settlement was typically less than 0.25 in., and (c) different methods of estimating settlement varied by approximately 0.4 in., on average. Subsequent sections of this paper present design considerations and development, specification and construction, and settlement monitoring of the Abutment 2 rockfill embankment. The Hoover Dam Bypass project (US-93) crosses the Colorado River between Arizona and Nevada below Hoover Dam, 30 mi southeast of Las Vegas, Nevada. The Arizona approach to the river includes the seven-span, 900-ft long Sugarloaf Mountain Bridge. The east bridge abutment footing is supported on a welded-wire–reinforced, special embankment fill zone within a 130-ft high rockfill embankment. Reinforced zone backfill consists of 4-in. maximum size select granular backfill; the remainder of the embankment consists of 2-ft maximum size rock. Backfill and rockfill were obtained from project roadway cuts, and construction and acceptance of the reinforced zone and rockfill embankment were in accordance with FHWA standard specifications and project special provisions. Postconstruction settlement of the embankment was monitored for 2 years with conventional survey monuments on abutment wingwalls and the embankment and multipoint borehole extensometers (MPBX) in the roadway shoulders. The monitoring period included 3 months before placement of the closure span and deck. Roughly 1 in. of settlement beneath the reinforced fill was measured during construction, with an additional 1⁄2 in. of settlement within 2 years after fill placement. Postconstruction settlement of the wingwalls was approximately 1 in. The rate of settlement decreased through the 2-year period, and settlement from construction and self-weight is nearly complete. Use of the MPBX was deemed partially successful in this application. US-93 is a designated North American Free Trade Agreement route. Increasing congestion and safety concerns because of existing switchbacks and the Hoover Dam crossing led to construction of the Hoover Dam Bypass by the Central Federal Lands Highway Division of the FHWA (CFLHD-FHWA). The bypass project crosses the Colorado River between Arizona and Nevada downstream of Hoover Dam, 30 mi southeast of Las Vegas, Nevada. The Arizona approach of the bypass is 1.7 mi long and includes the seven-span Sugarloaf Mountain Bridge. The finished grade of the bridge meets existing ground at the west abutment (Abutment 1) on the northeastern flank of Sugarloaf Mountain. It is a maximum of 160 ft above existing grade near the midpoint of the structure and is supported on a 130-ft high rockfill embankment at the east abutment (Abutment 2). Embankment side-slopes and fore-slope are at 11/2H:1V (steepest), where H:V is the slope ratio (horizontal to vertical), and the toe of BRIDGE FOUNDATION DESIGN CONSIDERATIONS During design development and design team (Hoover Support Team or HST) discussions with CFLHD and the Arizona Department of Transportation (ADOT), support alternatives for Abutment 2 loads, including both shallow and deep foundations, were evaluated. Deep foundation alternatives were dismissed because of numerous issues, including constructability, excessive lateral loading and deflection of columns, down-drag loading, and column verticality. Project earthwork estimates indicated that 80% to 90% of the embankment volume would be derived from roadway cuts in basalt, yielding mass material with rockfill characteristics. During design, it appeared that virtually all of the rock and soils from project roadway cuts would be suitable for use as embankment fill. The majority of the prospective embankment rockfill consisted of basalt, basalt flow breccia, dacite, rhyolite, and conglomerate. S. A. Anderson, FHWA, Federal Lands Highway, 12300 West Dakota Avenue, Suite 210, Lakewood, CO 80228. N. J. LaFronz, HDR Engineering, Inc., 3200 East Camelback Road Suite 350, Phoenix, AZ 85018. Corresponding author: N. J. LaFronz, Nick.LaFronz@hdrinc.com. Transportation Research Record: Journal of the Transportation Research Board, No. 2016, Transportation Research Board of the National Academies, Washington, D.C., 2007, pp. 3–12. DOI: 10.3141/2016-01 3 4 Transportation Research Record 2016 For the spread footing on fill option, immediate settlement (compression) of the embankment fill because of self-weight was estimated to be approximately 1⁄2% to 1% of the fill thickness or a maximum of approximately 8 to 16 in. for the 130-ft high fill, occurring during embankment construction. The magnitude of postconstruction differential settlement of the embankment and lateral spreading-related settlement was deemed insignificant, considering the high shear strength of rockfill on a bedrock foundation. Compression of the embankment because of applied abutment loads was estimated to be approximately 1⁄2 to 1 in. On the basis of AASHTO specifications (2), estimated tolerable differential settlement for the bridge end span was approximately 1⁄2 in., based on the span length of 128 ft and a tolerable movement (angular distortion between adjacent footings) of 0.004. This was not expected to be exceeded. The HST, in consultation with CFLHD and ADOT Bridge and Materials Groups representatives, selected the spread footing option based on anticipated performance, constructability, and cost. Initially, it was decided to modify the selected alternative by founding the abutment on a low-height, mechanically stabilized earth (MSE) wall on the crest of the embankment fill to more effectively distribute footing load, provide a stable upper portion of the embankment compared with embankment only, and aid in reducing magnitude of differential settlement. This scenario deviated from ADOT policy of using columns and deep foundations to support an abutment seat behind an MSE wall but was deemed preferable to placing the abutment directly on nonstabilized, unreinforced fill. However, geometric design constraints convinced the HST and CFLHD that inclusion of a reinforced fill zone beneath the bridge abutment footing would provide equivalent performance expected from the MSE system, without the added cost and construction issues. The selected design for Abutment 2 support consisted of a zone of welded wire-reinforced embankment fill, extending from the bottom of the footing to a depth of 25 ft below the spread footing base, within the limits of 1:1 lines extending from 5 ft outside the bottom edges of the footing laterally and longitudinally (Figures 1 and 2). Backfill for the reinforced zone consisted of 4-in. maximum size processed fill meeting the requirements for select granular backfill in the FHWA Standard Specifications FP-96 (3). An outer “wedge” of unreinforced rockfill encapsulates and protects the ends of the reinforcements and conceals the change in construction. REINFORCED FILL ZONE DESIGN Design of the reinforced fill zone was in accordance with methods for reinforced embankments (4). In the procedure, required total tensile load in the reinforcements at the base of the reinforced zone is determined (with a factor of safety) with a rotational slope stability method, the total load distributed into layers within the zone, and the required reinforcement layer strength assessed. Assumed 1H:1V outer slopes (nominal limits of the reinforced zone, corresponding to the abutment footing zone of stress influence) were used. A surcharge pressure of 4.2 kips/ft2 was applied to the top of the reinforced fill zone for the combined loading of the Abutment 2 spread footing and a traffic surcharge. Nominal reduction factors for creep, installation damage, and durability and ageing were applied to the computed total (ultimate) tensile strength to determine the allowable long-term tensile strength. Welded wire reinforcement meeting the requirements of ASTM A82/AASHTO M32 and M55 was selected for reinforcement, with tensile strength ( fy) of 75 ksi, allowable tensile strength (0.8fy) of 60 ksi, and allowable long-term tensile strength of 26.7 ksi. Reinforcement consisted of 12 × 12–W4 × W4 welded wire reinforcement located on 1-ft vertical spacings, extending both laterally and longitudinally to the limits of the 1H:1V zone from 5 ft outside the footing edge for the full width of the zone. Adjacent welded wire panels were fastened with approved fasteners (hog rings or tie wire). LIMIT OF FILL REINFORCEMENT EXTEND TO 1H:1V LINE PROJECTED FROM BOTTOM EDGE OF FOOTING (TYPICAL BOTH UP-AND DOWN-STATION) SUGARLOAF MOUNTAIN BRIDGE ABUTMENT 2 EL. 1389' BOTTOM OF FOOTING 1 FINISHED SLOPE 25' 1 EL. 1364' BOTTOM OF REINFORCED FILL ZONE 1.5 REINFORCED FILL ZONE (SEE TYPICAL DETAIL 1) 1 C L INTERIM US 93 EMBANKMENT FILL ABANDONED GRADE AT C L US 93 EMBANKMENT FILL FIGURE 1 Cross section at Abutment 2. Anderson and LaFronz 5 CL US 93 & CONSTR. CL LIMIT OF FILL REINFORCEMENT EXTEND TO 1H:1V LINE PROJECTED FROM BOTTOM EDGE OF FOOTING (TYPICAL BOTH UP-AND DOWN-STATION) 47'2" 47'2" ABUTMENT 2 FINISHED SLOPE 1 25' 1 1.5 EL. 1389' BOTTOM OF FOOTING 1 1 EL. 1364' BOTTOM OF REINFORCED FILL ZONE REINFORCED FILL ZONE (SEE TYPICAL DETAIL 1) 1 1.5 1 EMBANKMENT FILL CL INTERIM US 93 EMBANKMENT FILL FIGURE 2 Cross section at Abutment 2 (looking along US-93 construction centerline). The design included a 15% increase in the wire diameter of welded wire reinforcement over that determined by the design procedure— essentially providing “sacrificial” additional reinforcement against corrosion. Further, the FP-96 Standard Specifications (3) and special contract requirements (SCRs) for the select granular backfill included electrochemical requirements (minimum resistivity, pH range, and maximum sulfate and chloride content) verified by testing. ROCKFILL EMBANKMENT AND REINFORCED FILL ZONE CONSTRUCTION Gradation, placement, and compaction of the rockfill were performed in accordance with FP-96 (3) modified by the SCRs. SCR provisions included pushing individual rock fragments and boulders greater than 24 in. in diameter to the outside face of the embankment slope and embedding the particles into the adjacent slope face without stacking or piling (nesting). All boulders were reduced to less than 48 in. in greatest dimension; boulders and fragments not used on the embankment face were distributed within the embankment to prevent nesting and the interstitial voids filled with finer material. Rockfill was placed in 24-in. maximum thickness lifts and compacted primarily with a Caterpillar CP653 paddle foot roller (vibratory mode) and a specified number of passes. Rockfill embankment within approximately 400 ft of the abutment was designated as a special embankment placement zone, for which additional water for compaction was required. For this zone, rock was defined as material containing 50% or more (by volume) rock particles greater than 3 in. in diameter, visually verified, with the contracting officer’s optional verification by sieving the material through a grizzly screen. Watering for compaction was achieved by applying water as the material was excavated and end dumped before grading into lifts to help ensure that optimum moisture content was achieved throughout the full depth of each 24-in. (maximum) lift. Simply applying water to the surface of each lift was unacceptable. SCR requirements for moisture content of the fine fraction of embankment fill (material passing the No. 4 sieve) were optimum moisture content to 2 to 4 percentage points above optimum. The reinforced fill zone is located within the special embankment placement zone directly under the abutment footing. It consists of select granular backfill with 100% passing the 4-in. sieve, 75% to 100% passing the 3-in. sieve, and 0% to 15% passing the #200 sieve. The backfill was compacted in 6-in. lifts with a Caterpillar 634 steel drum roller. The contractor initially used both method compaction and density tests and subsequently solely density tests to verify sufficient compaction. Water was applied as described previously for the special embankment placement zone. Figures 3 and 4 depict embankment construction and welded wire reinforcement placement. EXPECTED POSTCONSTRUCTION SETTLEMENT Postconstruction differential settlement will affect bridge end span performance and determine the need for compensatory jacking at Abutment 2. Magnitude of postconstruction embankment FIGURE 3 End dumping of rockfill. 6 Transportation Research Record 2016 basins and cross-drainage conveyances), concrete-lined drainage ditches located on fill, and relocated existing natural drainages (to below the embankment toe). Arid site conditions (annual precipitation of 6 in.) and a preponderance of precipitation in the form of shortduration, high-intensity, high-runoff thunderstorms, which can be routed off the embankment, indicate a limited source of water for embankment wetting. MONITORING SYSTEM AND RESULTS FIGURE 4 Placing initial layer of welded wire reinforcement. settlement depends on fill characteristics/composition, compacted density and water content, thickness and geometry, foundation compressibility, stress state before wetting, and degree of wetting (hydrocompression) (5, 6 ). Postconstruction settlement of the rockfill embankment was estimated at 0.25% to 0.50% of the total fill thickness, thought to represent a conservative upper bound. However, Noorany suggested there is no simple empirical rule to estimate fill settlement and no basis for assumed settlement strain of 0.50% to 1% (5). Noorany recommended conservative analysis of probable future wetting of the fill during design (5). In general, Noorany’s recommendations are applicable to the embankment rockfill considered here (5). Fill settlement mechanisms include primary and secondary compression and hydro-compression (7 ). Primary compression is expected to occur very quickly in partially saturated compacted fill, particularly given the significant coarse-grained fraction, and to be essentially complete by the end of construction. The magnitude of secondary compression was expected to be small; the estimated rate of settlement after primary compression is complete (coefficient of secondary compression, per log cycle of time) was on the order of 0.001 to 0.002 for the fill, corresponding to settlement of 1 in. or less in a 10-year period for a 100-ft high fill (7, 8). Therefore, the primary mechanism in long-term postconstruction settlement is hydrocompression, were embankment wetting to occur. Previous studies demonstrate that even well-compacted embankments undergo some amount of compression because of wetting (7 ). Variations in compacted density and water content significantly affect settlement magnitude, such that control of field compaction (both compacted density and moisture content) is critical to fill performance. Effects of the significant gravel- to boulder-sized fraction on settlement magnitude on wetting are thought to be threefold (6 ): (a) reduced compressibility, because individual oversize particles generally do not respond to wetting; (b) reduced overall degree of wetting, in that the oversize material (particularly at high overall rock content) tends to interfere with response to wetting of the fine soil fraction in the interstitial spaces; and (c) reduced compression (volume change) of the fine fraction because of wetting (effect difficult to predict). Design elements likely aid in limiting postconstruction wetting to direct precipitation and thereby minimize percolation of surface flows. These elements include roadway drainage provisions (catch Instruments were installed to monitor settlement to verify settlement predictions and to forecast magnitude and rate of additional settlement. Monitoring results were considered in scheduling girder placement and casting the deck for the bridge end span (Figure 5). Monitoring results currently are used to predict future settlement and to evaluate the need for jacking at the abutment seat before paving. The bridge span and abutment were designed to accommodate up to 4 in. of jacking. The monitoring system was designed to provide a degree of redundancy through the use of different survey methods to permit independent quality check on results and to help isolate poorly performing areas of the embankment (if necessary). Monitoring included the following instruments, which are located as shown in Figures 6 and 7: 1. Five rockfill survey monuments, consisting of steel pins embedded in 3- to 5-ft deep concrete monuments located on the embankment slope coincident with the reinforced fill base, monitored using optical survey methods 2. Four concrete survey monuments, consisting of standard benchmarks anchored to the top of the abutment wingwalls, also monitored by optical survey 3. Four multipoint borehole rod extensometers (MPBXs) consisting of five or six anchors (number dependent on fill thickness), spaced 25 ft vertically, each connected to a stainless steel extension rod encased within a polyvinyl chloride (PVC) sleeve, with surface standpipe and measurement head. Anchors were individually grouted within boreholes over a range of depths within the embankment fill, with the lowermost anchor anchored to bedrock at approximately 5 ft below foundation grade. Surface completions were flush mounted FIGURE 5 Placing end-span girders. Anderson and LaFronz FIGURE 6 7 Plan layout of MPBXs and settlement monuments with limits of reinforced fill zone. with frame and cover over the instrument head and protected from low-speed construction traffic with bollard posts (Figure 8). MPBXs were monitored with a depth micrometer at the measurement head, and measurement head reference plates were surveyed by optical methods. Instruments were installed as soon as practical, such that instruments were installed at intervals. Initially, rockfill survey monuments were placed, with the embankment at partial height. Concrete survey monuments were installed near the end of construction, just after casting of wing walls. Finally, the MPBXs were installed, because they fell within a work area for casting the abutments and setting girders. MPBXs were read and monuments surveyed approximately twice weekly after installation and subsequently at a decreasing frequency. By approximately 2 years after embankment construction, all instruments were being surveyed or read quarterly. Monitoring results indicated embankment settlement generally within the expected range. The results permit prediction of future settlement. Results indicate that only a portion of the settlement was immediate (i.e., occurring dur- ing construction) and that settlement is ongoing at slow and progressively decreasing rates 2 years after construction. Postconstruction settlement of less than 0.1% of the fill height was observed in 2 years, and current settlement rates are such that total postconstruction settlement of less than 0.15% is expected. Specific observations are discussed and presented in the following paragraphs and figures. The plotted data were “smoothed” using a three-point running average to better indicate trends over the 2-year monitoring period. Monuments at the reinforced fill zone base were installed before embankment completion, such that they record the response of the lower 90 ft of embankment to loading of the upper 40 ft of fill, which was placed within a 60-day period. As such, these are the only instruments that recorded settlement both during and after construction. Figure 9 presents this response, in which rate of settlement decreases significantly after approximately 300 days. After nearly 1,000 days, total settlement is between 0.9 and 1.7 in., with a differential settlement of almost one-half the maximum total settlement. 8 Transportation Research Record 2016 U.S. 93 CONST. CL 35' Lt. LIMITS OF ABUTMENT 2 FOOTING & WALLS (SHOWN FOR CLARITY ONLY) 35' Rt. 1410 1400 FINISHED SUBGRADE (EL. VARIES - SEE TABLE) FINISHED EMBANKMENT SLOPE 1390 1380 REINFORCED FILL ZONE LIMIT OF WELDED WIRE REINFORCED FILL ZONE (SHOWN FOR CLARITY ONLY) 1375 1370 ELEVATION (FT) 1360 EL. 1360 FINISHED EMBANKMENT SLOPE ANCHOR POINT (TYPICAL FOR ALL MPBX) 1350 1350 1340 EMBANKMENT FILL 1330 1325 MULTIPOINT BOREHOLE EXTENSOMETER (MPBX) (TYPICAL) 1320 1310 1300 1300 EMBANKMENT FILL 1290 TIP POINT ANCHOR MIN. OF 5FT BELOW EXST. GRADE (TYPICAL FOR ALL) 1280 1275 EXST. GROUND 1270 1260 180 160 140 120 100 80 60 40 20 0 20 40 60 80 100 120 140 160 180 DISTANCE (FT) FIGURE 7 Cross section of MPBX installations (looking along US-93 construction centerline). Results for MPBX No. 1 are shown in Figure 10. Because each rod records the settlement between its anchor and the ground surface, there being no plausible mechanism for heave at intermediate depths, deeper rods that indicate less settlement than shallower ones were assumed to have malfunctioned and are not shown. Possible sources of error or malfunction include an overly stiff grout column, slippage of anchors, or rods bending in compression. Each of these mechanisms could result in underestimation of settlement; therefore, the presented results are interpreted to represent a lower bound on actual settlement. The results show that early settlement occurred at a higher rate FIGURE 8 vault. MPBX surface completion with measurement head and deep in the embankment than near the top and that after approximately 600 days, the rate of settlement was roughly equivalent at all depths, possibly suggesting material characteristics varying with depth. Sensitivity of optical survey of MPBX reference plates is insufficiently precise for such small magnitudes of settlement, but the results shown in Figure 11 generally confirm the rates and magnitude of observed settlement for the MPBXs. After approximately 700 days, total settlement was between 0.8 and 1.1 in., with a differential settlement of less than 30% of the maximum total settlement. These fill settlement results were very consistent with the measurements on the concrete wingwalls shown in Figure 12, in terms of both total and differential settlement, and suggest that the reinforced fill zone provides a stable platform that effectively limits differential settlement. Overall, performance of the MPBXs was disappointing, with only approximately half the rods functioning satisfactorily, mostly for MPBX No. 1 (Figure 10) and No. 3. Figure 13 presents MPBX No. 3 results on a semilog plot. These results are consistent with MPBX No. 1, when plotted in semilog form. The results suggest that a lower rate of secondary compression can be estimated from the slope of the data trend after approximately Day 300. These results suggest approximate future settlement of 0.2 to 0.6 in. in the next 25 years and approximately the same in the subsequent 250 years, assuming no other mechanism is in effect. Figure 14 depicts all optically surveyed monuments plotted against a log-time scale. These results exhibit the decreasing rate of settlement through time, further supporting the secondary compression response and suggesting the same range of possible interpretations of future settlement as the MPBXs. This plot also indicates the range of values at different times, which directly corresponds to the differential settlement at that time. Differential settlement is lowest at the wingwalls (representing the abutment seat) where tolerance for differ- Anderson and LaFronz 9 0.0 Complete Backfill of Abutment (6/23/2004) Monument Set Span #7 Girders (6/30/2004) -0.5 Change in Elevation (inches) No. 1 No. 2 No. 3 No. 4 No. 5 Substantial Completion of Deck (12/1/2004) -0.0 -0.5 Complete Reinforced Fill Zone (5/13/2005) Begin Reinforced Fill Zone (4/27/2004) -2.0 0 100 200 300 400 500 600 700 800 700 800 900 1000 Days FIGURE 9 Settlement recorded at bottom of reinforced fill zone. 0.2 Substantial Completion of Deck (12/1/2004) 0.1 0.0 Change in Elevation (inches) -0.1 -0.2 -0.3 Rod No. 1 Rod No. 2 Rod No. 3 Rod No. 4 -0.4 -0.5 -0.6 -0.7 -0.8 Begin Reinforced Fill Zone (4/27/2004) -0.9 Complete Reinforced Fill Zone (5/13/2005) -1.0 Complete Backfill of Abutment (6/23/2004) -1.1 Set Span #7 Girders (6/30/2004) -1.2 0 100 200 300 400 500 Days FIGURE 10 Settlement recorded at MPBX No. 1. 600 900 1000 10 Transportation Research Record 2016 0.2 Substantial Completion of Deck (12/1/2004) 0.1 0.0 Change in Elevation (inches) -0.1 MPBX No. 1 MPBX No. 2 MPBX No. 3 MPBX No. 4 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 Begin Reinforced Fill Zone (4/27/2004) -0.9 Complete Reinforced Fill Zone (5/13/2005) -1.0 Complete Backfill of Abutment (6/23/2004) -1.1 Set Span #7 Girders (6/30/2004) -1.2 0 100 200 300 400 500 600 700 800 900 1000 Days FIGURE 11 Settlement recorded at MPBX reference plates. 0.2 Begin Reinforced Fill Zone (4/27/2004) 0.1 Substantial Completion of Deck (12/1/2004) 0.0 Change in Elevation (inches) -0.1 Monument No. 1 No. 2 No. 3 No. 4 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 Complete Reinforced Fill Zone (5/13/2005) -0.9 Complete Backfill of Abutment (6/23/2004) -1.0 Set Span #7 Girders (6/30/2004) -1.1 -1.2 0 100 200 300 400 500 Days FIGURE 12 Settlement recorded at top of wingwalls. 600 700 800 900 1000 Anderson and LaFronz 0.2 Begin Reinforced Fill Zone (4/27/2004) 0.1 0.0 Complete Reinforced Fill Zone (5/13/2005) -0.1 Change in Elevation (inches) 11 -0.2 -0.3 Rod No. 1 (Depth 0 - 28 ft) Rod No. 2 (Depth 0 - 53 ft) Rod No. 3 (Depth 0 - 78 ft) Rod No. 4 (Depth 0 - 103 ft) -0.4 -0.5 -0.6 -0.7 Complete Backfill of Abutment (6/23/2004) -0.8 -0.9 Set Span #7 Girders (6/30/2004) -1.0 Substantial Completion of Deck (12/1/2004) -1.1 -1.2 10 100 1000 Days FIGURE 13 Settlement recorded at MPBX No. 3. 0.2 Begin Reinforced Fill Zone (4/27/2004) 0.1 0.0 -0.1 -0.2 Change in Elevation (inches) -0.3 -0.4 -0.5 Bottom of Reinforced Fill Wing Wall MPBX Reference Plates -0.6 -0.7 -0.8 -0.9 Complete Reinforced Fill Zone (5/13/2005) -1.0 -1.1 Substantial Completion of Deck (12/1/2004) -1.2 Complete Backfill of Abutment (6/23/2004) -1.3 -1.4 Set Span #7 Girders (6/30/2004) -1.5 -1.6 -1.7 -1.8 1 10 100 Days FIGURE 14 Approximate data bounds for all settlement monuments. 1000 12 ential settlement is least and greatest at locations on the embankment slope, outside and below the reinforced fill zone. Transportation Research Record 2016 zone and the reinforced fill zone and careful quality control. If control of percolation and runoff is achieved, the embankment is expected to undergo minor additional settlement and will provide an excellent foundation for the bridge abutment. CONCLUSIONS Although monitoring continues on a quarterly basis (for approximately 2 more years), the results collected to date permit several conclusions regarding performance of the rockfill embankment to be drawn. These conclusions will be reevaluated in 2 years, before paving and opening of the Hoover Dam Bypass: 1. The reinforced fill zone and special embankment placement zone focused attention on the quality and care in placement of rockfill in the most critical area of embankment, near the abutment. 2. Differential settlement is lower above the reinforced fill zone than below it and well within the limits of structure tolerance. 3. Settlement during construction is difficult to measure because new material is being placed as recently placed material is settling. Monuments at the base of the reinforced fill zone indicate the rate and magnitude of settlement as the upper embankment is placed and allow estimation of settlement during construction that could be used for material balance calculations. 4. Postconstruction settlement of the rockfill is minimal, as was expected, but it was not “immediate,” with approximately 1 in. of settlement occurring throughout the year after placement. 5. Settlement rate is greatest during construction and decreases through time. After approximately 2 years, a low log-linear secondary compression rate is established. Even longer-term observation is planned to confirm this trend in the data. 6. Settlement is essentially complete, with 0.1 to 0.7 in. (approximately 0.01% to 0.05%) additional settlement expected in the next 25 years and approximately the same amount in the subsequent 250 years (Figures 13 and 14). The range in these values reflects the uncertainty in the coefficient of secondary compression (see Conclusion No. 5). Surface settlement caused by placement and self-weight was at the lower end of the expected range before construction. This is interpreted to be the result of the special embankment placement ACKNOWLEDGMENTS Many individuals and organizations contributed to the Hoover Dam Bypass project and to the Sugarloaf Mountain Bridge and Abutment 2 rockfill embankment. The authors specifically acknowledge the work of HST (consisting of Jacobs Engineering, HDR Engineering, and AMEC Earth and Environmental), ADOT, and the design and construction branches of CFLHD. REFERENCES 1. Performance of Highway Bridge Abutments on Spread Footings. Report No. FHWA/RD-81/184. Office of Engineering Research and Development, FHWA, U.S. Department of Transportation, 1982. 2. AASHTO. Standard Specifications for Highway Bridges, 16th ed. AASHTO, Washington, D.C., 1996 with interims through 2001. 3. FHWA. Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects. FP-96. FHWA, U.S. Department of Transportation, 2001. 4. Geosynthetic Design and Construction Guidelines, Participant Notebook. National Highway Institute Course No. 13213. FHWA HI-95-038. FHWA, U.S. Department of Transportation, 1998. 5. Noorany, I. Structural Fills: Design, Construction and Performance Review. In Unsaturated Soil Engineering Practice (S. L. Houston and D. G. Fredlund, eds.). ASCE Geotechnical Special Publication 68. Geo-Institute of ASCE, Reston, Va., 1997. 6. Noorany, I., and S. L. Houston. Effect of Oversize Particles on Swell and Compression of Compacted Unsaturated Soils, in Static and Dynamic Properties of Gravelly Soils (M. D. Evans and R. J. Fragaszy, eds.). ASCE Geotechnical Special Publication 56. ASCE, Reston, Va., 1995. 7. Brandon, T. L., J. M. Duncan, and W. S. Gardner. Hydrocompression Settlement of Deep Fills. Journal of the Geotechnical Engineering Division of ASCE, Vol. 116, No. 10, 1990. 8. U.S. Department of the Navy. Soil Mechanics Design Manual 7.1. Naval Facilities Engineering Command (NAVFAC), Alexandria, Va., 1982. The Soil and Rock Properties Committee sponsored publication of this paper.