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Specification for Bridge Design 825 Section 11 - Abutments, Piers and Walls 11.1 SCOPE This section provides requirements for design of abutments and walls Conventional retaining walls, anchored walls, mechanically stabilized earth (MSE) walls, and prefabricated modular walls are considered 11.2 DEFINITIONS Abutment - A structure that supports the end of a bridge span and provides lateral support for fill material on which the roadway rests immediately adjacent to the bridge Anchored Wall - An earth retaining system typically composed of the same elements as nongravity cantilevered walls that derive additional lateral resistance from one or more tiers of anchors Mechanically Stabilized Earth Wall - A soil-retaining system employing either strip or grid-type, or metallic or polymeric, tensile reinforcements in the soil mass and a facing element that is either vertical or nearly vertical Nongravity Cantilever Wall - A soil-retaining system that derives lateral resistance through embedment of vertical wall elements and support-retained soil with facing elements Vertical wall elements may consist of discrete elements, e.g., piles, caissons, drilled shafts, or auger-cast piles spanned by a structural facing, e.g., lagging, panels, or shotcrete Alternatively, the vertical wall elements and facing may be continuous, e.g., diaphragm wall panels, tangent piles, or tangent-drilled shafts Pier - That part of a bridge structure between the superstructure and the connection with the foundation Prefabricated Modular Wall - A soil-retaining system employing interlocking soil-filled reinforced concrete or steel modules or bins to resist earth pressures by acting as gravity-retaining walls Rigid Gravity and Semigravity Retaining Wall - A structure that provides lateral support for a mass of soil and that owes its stability primarily to its own weight and to the weight of any soil located directly above its base In practice, different types of rigid gravity and semigravity retaining walls may be used These include:  A gravity wall depends entirely on the weight of the stone or concrete masonry and of any soil resting on the masonry for its stability Only a nominal amount of steel is placed near the exposed faces to prevent surface cracking due to temperature changes  A semigravity wall is somewhat more slender than a gravity wall and requires reinforcement consisting of vertical bars along the inner face and dowels continuing into the footing It is provided with temperature steel near the exposed face Specification for Bridge Design 826  A cantilever wall consists of a concrete stem and a concrete base slab, both of which are relatively thin and fully reinforced to resist the moments and shears to which they are subjected  A counterfort wall consists of a thin concrete face slab, usually vertical, supported at intervals on the inner side by vertical slabs or counterforts that meet the face slab at right angles Both the face slab and the counterforts are connected to a base slab, and the space above the base slab and between the counterforts is backfilled with soil All the slabs are fully reinforced  A prefabricated modular wall consists of individual structural units assembled at the site into a series of hollow bottomless cells known as cribs The cribs are filled with soil, and their stability depends not only on the weight of the units and their filling but also on the strength of the soil used for the filling The units themselves may consist of reinforced concrete or fabricated metal 11.3 NOTATION Ab = surface area of transverse reinforcement in bearing (diameter times length) (mm2) (11.9.5.3) Am = maximum wall acceleration coefficient at the centroid (11.9.6.1) AReffi = area of reinforcement per vertical mm (mm2/mm) (11.9.6.2) As = total surface area (top and bottom) of reinforcement beyond failure plane, less any sacrificial thickness (mm2) (11.9.5.3) B = width of retaining wall foundation (mm) (11.9.7) B’ = offective width of retaining wall foundation (mm) (C11.9.4.2) b = width of bin module (mm) (11.10.4) b/i = reinforcement width for layer i (mm) (11.9.6.2) Co = uniaxial compressive strength of rock (MPa) (11.5.6) D60/D10 = uniformity coefficient of soil defined as ration of particle size of soil thast is 60 percent finer in size to the particle size of soil that is 10 percent finer in size (C11.9.5.3) d = fill above wall (mm) (11.9.7) Ec = thickness of metal reinforcement at end of service life (mm) (11.9.8.1) En = nominal thickness of steel reinforcement at construction (mm) (11.9.8.1) Es = sacrificial thickness of metal expected to be lost by uniform corrosion during service life (mm) (11.9.8.1) e = eccentrity of load from certerline of farndation (mm) (C11.9.4.2) Fr = friction component of resultant on base of foundation (N/mm) (11.6.3.1) fd = coefficient of resistance to direct sliding of reinforcement (11.9.5.3) f* = apparent coefficient of friction at each reinforcement level (11.9.5.3) Specification for Bridge Design 827 Hm = incremental dynamic inertia force at level i (N/mm of structure) (11.9.6.2) H1 = equivalent wall height (mm) (11.9.5.2.2) H2 = effective wall height (mm) (11.9.6.1) hi = height of reinforced soil zone contributing horizontal load to reinforcement at level i (mm) (11.9.5.2.1) i = inclination of ground slope behind face of wall (DEG) (11.9.5.2.2) k = earth pressure coefficient (11.9.5.2.2) ka = active earth pressure coefficient (11.9.4) ko = at-rest earth pressure coefficient (11.9.5.2.2) L = spacing between vertical elements or facing supports (mm) (11.8.5.2) Lei = effective reinforcement length for layer i (mm) (11.9.6.2) l = length of mat beyond failure plane (mm) (11.9.5.3) Specification for Bridge Design 828 Is = point load strength index (MPa) (11.5.6) Mmax = maximum bending moment in vertical wall element or facing (Nmm or Nmm/mm) (11.8.5.2) N normal component of resultant on base of foundation (N/mm) (11.6.3.1) = Ncorr = SPT blow count corrected for overburden pressure (Blows/300 mm) (11.8.4.2) Np = passive resistance factor (11.9.5.3) n = number of transverse bearing members behind failure plane (11.9.5.3) Pa = resultant of active lateral earth pressure (N/mm) (11.6.3.1) PAE = dynamic horizontal thrust (N/mm) (11.9.6.1) Pb = pressure inside bin module (MPa) (11.10.4) Pi = horizontal force per mm of wall transferred to soil reinforcement at level i (N/mm) (1.9.5.2.1) PIR = horizontal inertial force (N/mm) (11.9.6.1) Pfg = pullout capacity developed by passive resistance per grid (N) (11.9.5.3) Pfs = pullout capacity per strip (N) (11.9.5.3) Ph = horizontal component of lateral earth pressure (N/mm) (11.6.3.1) PIR = horizontal inertia force (N/mm) (11.9.6.1) Pis = internal inertia force (N/mm) (11.9.6.2) Pv = vertical component of lateral earth pressure (N/mm) (11.6.3.1) p = average lateral pressure, including earth, surcharge, and water pressure, acting on the section of wall element being considered (MPa) (11.8.5.2) Qa = ultimate unit anchor resistance (N/mm) (11.8.4.2) qmax = maximum unit soil pressure on base of foundation (MPa) (11.6.3.1) Rn = nominal resistance (11.5.4) RR = factored resistance (11.5.4) SHi = horizontal reinforcement spacing for layer i (mm) (11.9.6.2) SPT = standard penetration test (11.8.4.2) T1 = limit state reinforcement tension (N) (11.9.5.1.3) T5 = tensile load at which strain in polymeric soil reinforcement exceeds percent (N) (11.9.5.1.3) w = width of mat (mm) (11.9.5.3) Specification for Bridge Design 829 x = spacing between vertical element supports (mm) (11.8.5.2) y = distance above base of foundation to location of P h (mm) (11.6.3.1) Z = depth below effective top of wall or to reinforcement (mm) (11.9.5.3) YP = load factor for earth pressure in Table 3.4.1-2 (11.9.5.2.2) Ys = soil density (kg/m3) (11.9.5.3)  = wall - backfill interface friction argle (DEG) (C11.10.1) = soil-reinforcement angle of friction (DEG) (11.9.5.3) = resistance factor (11.5.4) = internal friction angle of foundation soil (DEG) (11.9.5.2.2) = magnitude of lateral pressure due to surcharge (MPa) (11.9.5.2.1) = maximum stress in soil reinforcement in abutment zones (11.9.7) v = vertical stress in soil (MPa) (11.9.5.2.2) V1 = vertical soil stress (MPa) (11.9.7) V2 = vertical soil stress due to footing load (MPa) (11.9.7) f 1.4 SOIL PROPERTIES AND MATERIALS 11.4.1 General Where possible, backfill materials should be granular, free-draining materials Where clayey soils are used as backfill, drainage shall be provided to reduce hydrostatic water pressure behind the wall 11.4.2 Determination of Soil Properties The provisions of Articles 2.4 and 10.4 shall apply 11.5 LIMIT STATES AND RESISTANCE FACTORS 11.5.1 General Design of abutments, piers, and walls shall satisfy the criteria for the service limit state specified in Article 11.5.2 and for the strength limit state specified in Article 11.5.3 11.5.2 Service Limit States Abutments, piers, and walls shall be investigated for excessive displacement at the service limit state 11.5.3 Strength Limit State Design of abutments and walls shall be investigated at the strength limit states using Equation 1.3.2.1-1 for:  Bearing resistance failure, Specification for Bridge Design 830  Lateral sliding,  Excessive loss of base contact,  Overall instability,  Pull out failure of anchors or soil reinforcements, and  Structural failure 11.5.4 Resistance Requirement Abutments, piers, and retaining structures and their foundations and other supporting elements shall be proportioned by the appropriate methods specified in Articles 11.6, 11.7, 11.8, 11.9, or 11.10, so that their resistance satisfies Article 11.5.5 The factored resistance, RR, calculated for each applicable limit state shall be the nominal resistance, Rn, multiplied by an appropriate resistance factor, , specified in Table 11.5.6-1 11.5.5 Load Combinations and Load Factors Abutments, piers, and retaining structures and their foundations and other supporting elements shall be proportioned for all applicable load combinations specified in Article 3.4.1 11.5.6 Resistance Factors Resistance factors for geotechnical design of foundations are specified in Tables 10.5.4-1 through 10.5.4-3 and Table 1, for which:  Factors for soft rock are applicable for rock characterized by a uniaxial compressive strength, C0, less than 7.0 MPa or a point load strength index, Is, less than 0.30 MPa;  Factors for permanent walls are applicable for walls that have a specified service life greater than 36 months, walls in a highly aggressive environment, or walls where the consequences of failure are serious;  Factors for temporary walls are applicable for walls that have a specified service life less than or equal to 36 months and walls in a nonaggressive environment, where the consequences of failure are not serious; 831  Specification for Bridge Design Vertical elements, such as soldier piles, tangent piles, and slurry trench concrete walls, shall be treated as either shallow or deep foundations, as appropriate, for purposes of estimating bearing resistance, using procedures described in Sections 10.6,10.7, and 10.8 If methods other than those given in Tables 10.5.4-1 through 10.5.4-3 and Table are used to estimate the soil capacity, the performance factors chosen shall provide the same reliability as those given in these tables Table 11.5.6.1 - Resistance Factors for Retaining Walls 832 Specification for Bridge Design 11.5.7 Extreme Event Limit Stat The applicable load combinations specified in Table 3.4.1-1 shall be investigated Unless otherwise specified, all resistance factors shall be taken as 1.0 when investigating the extreme event limit state 11.6 ABUTMENTS AND CONVENTIONAL RETAINING WALLS 11.6.1 General Considerations 11.6.1.1 Loading Abutments and retaining walls shall be investigated for: Specification for Bridge Design 833  Lateral earth and water pressures, including any live and dead load surcharge;  The weight of the wall;  Temperature and shrinkage deformation effects; and  Earthquake loads as specified herein, in Section 3, and elsewhere in these Specifications The provisions of Article 3.11.5 shall apply For stability computations, the earth loads shall be multiplied by the maximum and/or minimum load factors given in Table 3.4.1-2, as appropriate 11.6.1.2 Integral Abutments Integral abutments shall be designed to resist and/or absorb creep, shrinkage, and thermal deformations of the superstructure 11.6.1.3 Load Effects In Abutments For computing load effects in abutments, the weight of filling material directly over an inclined or stepped rear face, or over the base of a reinforced concrete spread footing may be considered part of the effective weight of the abutment Where spread footings are used, the rear projection shall be designed as a cantilever supported at the abutment stem and loaded with the full weight of the superimposed material, unless a more exact method is used 11.6.1.4 Wingwalls And Cantilever Walls Wingwalls may be designed as monolithic with the abutments or as free standing, with an expansion joint separating them from abutment walls The wingwall lengths shall be computed using the required roadway slopes Wingwalls shall be of sufficient length to retain the roadway embankment and to furnish protection against erosion The vertical stems of cantilever walls shall be designed as cantilevers supported at the base 11.6.1.5 Expansion And Contraction Joints Consideration shall be given to measures that will accommodate the contraction and expansion of concrete walls 11.6.2 Movement at the Service Limit State 11.6.2.1 Abutments The provisions of Articles 10.6.2.2.3, 10.7.2.3, and 10.8.2.3 shall apply as applicable Specification for Bridge Design 834 11.6.2.2 Conventional Retaining Wall Criteria for tolerable movement criteria for retaining walls shall be developed based on the function and type of wall, anticipated service life, and consequences of unacceptable movements The provisions of Articles 10.6.2.2, 10.7.2.2, and 10.8.2.2 shall apply as applicable 11.6.3 Bearing Resistance and Stability at the Strength Limit State 11.6.3.1 General Abutments and retaining walls shall be proportioned to ensure stability against bearing capacity failure, overturning, and sliding Where a wall is supported by clayey foundation, safety against deepseated foundation failure shall also be investigated Stability criteria for walls with respect to various modes of failure shall be as shown in Figures through Where the horizontal earth pressure is computed using the Coulomb theory, and where the horizontal earth pressure is not applied directly to the back of the wall, a vertical component load acting on the vertical plane extending upward from the heel shall be considered Figure 11.6.3.1-1 - Earth Loads and Stability Criteria for Walls with Clayey Soils in the Backfill or Foundation (Duncan et al 1990)