TEMPORARY EXCAVATION SUPPORT SYSTEM FOR A NEW RAILWAY BRIDGE AND HIGHWAY UNDERPASS IN PITTSBURGH Andrew G. Cusing 1, Member, ASCE and Cristoper J. Lewis 2, P.E., Member, ASCE ABSTRACT: Until recently, traffic exiting Pittsburg would use te West End Bridge to cross te Oio River only to become entangled in te West End Circle, wic consisted of two railway overpasses eac out-of-alignment wit te West End Bridge. A roadway improvement project involving te construction of a new railway bridge and igway underpass was undertaken to eliminate tis traffic circle and provide a direct connection between te West End Bridge and State Route 19. Brayman Construction Corporation as since completed tis project. Tis paper provides a summary of te design and construction sequencing of a temporary seet pile excavation support system tat was constructed to facilitate te completion of tis project. Te excavation support system consisted of opposing seet pile walls (soring and ancor walls) cross-tied wit prestressed tendons beneat te railroad corridor. Te general arrangement of tis system is sown in Figure 1. Te prestressed tendons limited te orizontal wall displacement to permissible levels specified by te railway owner, and te reuse of te pase one soring wall as te soring wall for te second pase of bridge construction eliminated te need to extract and re-drive seeting, resulting in reduced displacements and construction time. INTRODUCTION For te construction of te West End improvement project described above, railway traffic on te existing embankment could not be interrupted, except for a brief period to sift tracks for eac pase of construction. Since te excavation support system was offset approximately 3 m (10 ft) from te centerline of te nearest track, te railway owner specified tat te cumulative orizontal displacement at te top of te wall be limited to 19 mm (0.75 in) to control vertical settlement of te railbed. (1) Assistant Project Engineer, agcusing@dappolonia.com; (2) Principal Engineer, cjlewis@dappolonia.com; D Appolonia, 275 Center Road, Monroeville, PA 15146; pone: 412-856-9440
To satisfy tese stringent constraints, a continuous AZ-36 seet pile soring wall of 12.5 m (41 ft) maximum eigt was cross-tied beneat te railway corridor to an ancor wall consisting of discrete pairs of AZ-18 seet piling. Te use of prestressed tendon ties limited te orizontal movement of te top of te wall to 13 mm (0.5 in). Potos of te soring wall during construction are provided in Figures 2 and 3, and te key factors governing design and construction are described below. GEOTECHNICAL PROPERTIES AND RAILWAY LOADING Embankment materials at te site consist of random mixtures of soils, slag, cinders and rock fragments, wit a silty to clayey soil component predominating. Excluding outlier values, SPT N values between 10 and 35 blows per 305 mm penetration were typical. Te properties for tese embankment soils were adopted in accordance wit AREMA (2001) as follows: φ = 30 and γ = 17.3 kn/m 3 (110 pcf). Te railway loading applicable to te structural design of all wall components was governed by te Cooper E-80 configuration (AREMA, 2001), wit te maximum 356 kn (80 kip) axle load distributed over te minimum 1.52 m (5 ft) axle spacing and 2.59 m (8.5 ft) tie widt, resulting in a 90 kpa (1.88 ksf) vertical surface surcarge. Te orizontal component of tis vertical stress acting on te soring wall was conservatively calculated using a Boussinesq distribution for a frictionless, rigid wall, as prescribed by te railway owner for structural design. In analyzing te final condition, eac of te two tracks was loaded to te 90 kpa (1.88 ksf) vertical surcarge. However, for te construction conditions, te railway owner permitted te design load for te far track to be reduced to a vertical surcarge of 45 kpa (0.94 ksf). WALL CONFIGURATION AND GENERAL EXCAVATION AND CONSTRUCTION SEQUENCE Notably, te support system described erein constituted a more constructable and better performing alternate to te original concept, wic required continuous seet pile ancor walls and numerous tie rods, inerently imposed strict installation tolerances, and presumed sustained track outages wic were deemed unacceptable by te railway owner. Te alternate ancor wall was configured wit discrete pairs of seet piling and discrete wales to afford te contractor generous tolerances for tie installation. Also, strand ties were selected in lieu of tie rods to greatly reduce te number of ties and drilling requirements and provide better control of wall deflections. Additionally, te ancor wall alignment was sifted furter from te railroad tracks to enable te contractor to install te wall witout being restricted by railroad operations. Driving of Ancor and Soring Walls Te ancor wall seet piling (AZ-18, Grade 50) was first installed to te dept limits sown in Figure 1. After te railway tracks were relocated for te Pase 1 2
3 m Top of Wall EL. 238.41 m Level 1 Ties & Walers EL. 235.37 m 14.5 m 7-Strand Tie 5 Level 2 Ties & Walers EL. 230.18 m 4-Strand Tie 3 Final Excavation EL. 225.91 m Railway Embankment Fill Bottom Tip EL. 224.39 m Bottom Tip EL. 220.12 m SHORING WALL (AZ-36) ANCHOR WALL (AZ-18) NOTE: Horizontal Tie Spacing = 2.5 m (TYP.) FIG. 1. Cross Section of Temporary Cross-Tied Seet Pile Soring and Ancor Walls, Pase 1 Construction, at a Maximum Soring Wall Heigt of 12.5 Meters detour, te soring wall seet piling (AZ-36, Grade 50) was driven to te dept limits sown in Figure 1, wit te top of te soring wall at El. 238.41 m (782 ft). Initial Excavation for Temporary and Permanent Free-Cantilever An initial 1.52 m (5 ft) ig excavation to El. 236.89 m (777 ft) was advanced in front of te soring wall to establis a construction platform and work area. Te railway owner would not permit a temporary (construction) cantilever condition greater tan 3.66 m (12 ft) ig or a permanent cantilever condition greater tan 3.05 m (10 ft) ig and restricted te exposed lengt of te temporary 3.66 m (12 ft) ig cantilever to 30.5 m (100 ft) prior to Level 1 tie installation. Recognizing tese restrictions, a limited excavation was conducted in front of te soring wall to El. 234.76 m (770 ft; a temporary 3.66 m (12 ft) ig cantilever) for installation of an initial section of Level 1 ties at El. 235.37 m (772 ft). Level 1 Strand Tie Installation and Prestressing An excavation in front of te ancor wall to El. 233.54 m (766 ft) was required to install te Level 1, 7-strand ties, wic were spaced every 2.5 m. 3
FIG 2. Soring Wall Under Railway Loading FIG 3. Soring Wall After Installation of Level 2 Ties 4
Te strand ties were prestressed to 0.45 GUTS, usually from te soring wall side. As te strand ties along a given section of wall were stressed and locked off, te excavation limits were extended by an equivalent lengt to advance te installation of te remaining Level 1 ties. Excavation for Level 2 Strand Tie Installation and Pre-stressing After incrementally installing te Level 1 ties, te Stage 2 excavation to El. 229.57 m (753 ft) and El. 228.96 m (751 ft) in front of te soring and ancor walls, respectively, commenced to enable te installation of te Level 2, 4-strand ties spaced every 2.5 m at El. 230.18 m (755 ft). Once te Level 2 ties were installed and prestressed, backfill was placed in front of te ancor wall to approximate te original grade of te embankment. Final Dept of Excavation, Soring Wall Side Te critical portions of te soring wall employed two tie levels, as previously described, and required a final excavation to El. 225.91 m (741 ft), as sown in Figure 1, resulting in a 12.5 m (41 ft) ig wall. Only one tie level (8-strand ties) was required for te remaining portions of te wall were te final excavation line ranged from El. 229.57 m (753 ft) to El. 228.20 m (748.5 ft). Te final excavation for te abutment construction was pased to closely coincide wit te initiation of foundation construction. HORIZONTAL DISPLACEMENT PREDICTIONS AND MEASUREMENTS As noted previously, te design of te excavation support system was mainly driven by te railway owner s requirements to limit disruption of train traffic and control orizontal wall displacements and settlement of te railbed. In te following subsections, orizontal wall displacements for te cantilever (bot temporary and permanent) and single tie level conditions are evaluated. In addition, te observed displacements are discussed. Predicted Displacement Temporary and Permanent Cantilever Conditions Te soring wall orizontal displacement predictions presented in tis section apply to te temporary and permanent cantilever conditions (i.e., Stage 1 excavation for tie installation and permanent free cantilever wall sections). Te metod used to predict tese cantilever wall displacements involved a two-step process. First, te maximum orizontal displacement at te top of te wall associated wit te development of te active eart pressure was determined, witout consideration of te effect of te railroad surcarge. Te relationsips reported by Cloug and Duncan (1991) were employed. As a consequence of te pre-loading effect of te railway traffic on te existing embankment soils, te relationsip for medium-dense sand was adopted. On tis basis, te orizontal displacement required to mobilize te active eart pressure is approximately 0.002 times te exposed cantilever wall eigt c. 5
Te additional effect of te railroad surcarge on te orizontal displacement was determined by introducing te factor f q, wic is computed as follows: f v q = 1+ = 1+ (1) σ σv were: = cange in orizontal stress at dept c (cantilever eigt) due to te orizontal component of te railroad surcarge; σ = orizontal effective stress at dept c due to te overburden (embankment) soil; v = cange in vertical stress at dept c due to te vertical component of railroad surcarge; and = vertical effective stress at dept c due to te overburden (embankment) soil. σ v Te values σ, σv, σ, and σ v are related to te coefficient of active eart pressure, K a as follows: σ 1 sin φ = = K a = (2) σ 1+ sin φ v v Te combined effect of active eart load and railway surcarge on te orizontal displacement (δ ) at te top of a cantliever wall of eigt c can ten be expressed as: δ = 0.002 f (3) c q A friction angle ( φ) of 30 and unit weigt (γ) of 17.3 kn/m 3 (110 pcf) were used to calculate K a and σ v, consistent wit te prescribed properties. Te results of tis two-step procedure are summarized in Table 1. TABLE 1. Summary of Predicted Horizontal Displacements at Top of Wall for te Temporary and Permanent Cantilever Conditions Exposed Cantilever Maximum Horizontal Displacement at Top of Wall, δ mm (in) Heigt c, m (ft) Temporary Cantilever Condition Permanent Cantilever Condition 3.66 (12) 12.78 (0.503) N/A 3.05 (10) N/A 14.33 (0.564) Concerning values of c = 3.66 and 3.05 m (12 and 10 ft), te predicted maximum orizontal displacements at te top of te soring wall for bot te 6
temporary and permanent cantilever conditions are less tan te specified limit of 19 mm (0.75 in), wit values for te temporary condition approximating 13 mm (0.5 in). Tie Prestress and Predicted Wall Displacement - Single Tie Level In determining te required prestress for te upper tie level, te following incremental displacements for tree stages of construction were considered: Stage 1: Temporary 3.66 m (12 ft) cantilever wit ground surface at El. 238.41 m (782 ft) and cut to El. 234.76 m (770 ft) Displacement for tis stage is addressed in Table 1. Stage 2: Installation and Prestressing of Level 1 Strand Ties at El. 235.37 m (772 ft) Displacement reduction due to wall movement into te existing embankment. Stage 3: Excavation to El. 228.20 m (748.5 ft) Displacement at tis stage as two potential components: 3a.) Strand tie elongation (if te prestress level is less tan te actual induced reaction [presumed to be 0.6 GUTS]) 3b.) Structural cantilever movement (wit c = 3.05 m = 10 ft) above te tie level, wic occurs in response to te increase in railroad surcarge between te temporary and permanent conditions. Te average increase in orizontal surcarge pressure witin te top 3.05 m of te soring wall is approximately 4.8 kpa (0.1 ksf). Te calculations demonstrated tat a prestress level of 0.42 GUTS would control te maximum orizontal displacement of te top of te soring wall to less tan 19 mm (0.75 in). Terefore, a target prestress level of 0.45 GUTS was subsequently specified. Incremental and cumulative orizontal displacements for Stages 1 troug 3 are summarized in Table 2 for a prestress level of 0.42 GUTS. For sections of walls wit two rows of ties, and a final excavation to El. 225.91 m (741 ft), te effects of prestressing te bottom row of ties to 0.45 GUTS and te subsequent additional excavation were approximately equal and opposite, resulting in no appreciable influence on te resulting cumulative lateral displacement for te single tie condition provided in Table 2. Altoug prestressing te cross-tie tendons to te full design load of 0.6 GUTS would ave resulted in lower calculated orizontal wall displacements, tis option was not selected due to concern for potential strand overstress, especially considering tat a significant proportion of te design load was transient. Observed Displacements Te maximum orizontal displacement at te top of te soring wall during construction was no more tat 13 mm (0.5 inc), wic was somewat less tan te predicted value of 18.7 mm. Te conservative nature of te displacement estimate was most likely attributable to te fact tat te prescribed Cooper E-80 loading was 7
developed for steam locomotives, wic are generally eavier tan modern day diesel engines. TABLE 2. Summary of Predicted Incremental and Cumulative Horizontal Displacements at Top of Wall for Single-Tie Wall Section, wit 0.42 GUTS Tie Prestress Stage Baseline Horiz. Excavation Vertical RR Displacement @ El. m (ft) Surcarge T.O.W., mm (in) 1 Temp. Cantliever 234.76 Near: 90 kpa δ 1 = 12.8 (0.503) 2 Level 1 Tie Prestressing (770.0) Far: 45 kpa δ 2 = -23.3 (-0.916) 3 (a) Tie Elongation 228.20 Near: 90 kpa δ 3a = 28.9 (1.139) 3 (b) Added RR Surcarge (748.5) Far: 90 kpa δ 3b = 0.3 (0.012) Predicted Cumulative Displacement Single Tie Wall Section δ = 18.7 (0.738) SUMMARY Tis paper as summarized te design and construction sequencing of a temporary excavation support system consisting of opposing seet pile walls (soring and ancor walls) cross-tied wit prestressed tendons beneat a railroad corridor to facilitate te construction of a new railway bridge and igway underpass in Pittsburg s West End. Te orizontal offset of te proposed ancor wall from te soring wall, as sown in Figure 1, was greater tat set for fort in te original design, wic permitted continuous operation of railway traffic during all pases of construction, wit te exception of brief periods for track relocation for eac pase of bridge construction. Also, te use of prestressed tendon ties limited te orizontal wall displacement to permissible levels specified by te railway owner. In addition, te reuse of te soring wall from Pase 1 bridge construction as te soring wall for te second pase eliminated te need to extract and re-drive seeting, wic resulted in reduced displacements and construction time. REFERENCES AREMA (2001). Manual for Railway Engineering, American Railway Engineering and Maintenance-of-Way Association. Cloug, G.W. and Duncan, J.M. (1991). Eart Pressures, Foundation Engineering Handbook (Capter 6), Second Edition, Y.H. Fang, Ed., Van Nostrand Reinold, New York. 8