1、Section 5 RETAINING WALLS Part A GENERAL REQUIREMENTS AND MATERIALS 5.1 GENERAL Retaining walls shall be designed to withstand lateral earth and water pressures, including any live and dead load surcharge, the self weight of the wall, temperature and shrinkage effects, and earthquake loads in accord
2、ance with the general principles specified in this section. Retaining walls shall be designed for a service life based on consideration of the potential long-term effects of material deterioration, seepage, stray currents and other potentially deleterious environmental factors on each of the materia
3、l components comprising the wall. For most applications, permanent retaining walls should be de- signed for a minimum service life of 75 years. Retaining walls for temporary applications are typically designed for a service life of 36 months or less. A greater level of safety and/or longer service l
4、ife e., 100 years) may be appropriate for walls which support bridge abutments, buildings, critical utilities, or other fa- cilities for which the consequences of poor performance or failure would be severe. The quality of in-service performance is an important consideration in the design of permane
5、nt retaining walls. Permanent walls shall be designed to retain an aestheti- cally pleasing appearance, and be essentially maintenance free throughout their design service life. 5.2 WALL TYPE AND BEHAVIOR Retaining walls are generally classified as gravity, semi- gravity, non-gravity cantilever, and
6、 anchored. Examples of various types of walls are provided in Figures 5.2A, 5.2B, and 5.2C. Gravity walls derive their capacity to re- sist lateral loads through a combination of dead weight and lateral resistance. Gravity walls can be further subdivided by type into rigid gravity walls, mechanicall
7、y stabilized earth (MSE) walls, and prefabricated modular gravity walls. Semi-gravity walls are similar to gravity walls, ex- cept they rely more on structural resistance through can- tilevering action, with this cantilevering action providing the means to mobilize dead weight for resistance. Non- g
8、ravity cantilever walls rely strictly on the structural resis- tance of the wall and the passive resistance of the soillrock, in which vertical wall elements are partially embedded in the soilhock to provide fixity. Anchored walls derive their capacity through cantilevering action of the vertical wa
9、ll elements (similar to a non-gravity cantilever wall) and ten- sile capacity of anchors embedded in stable soil or rock below or behind potential soilhock failure surfaces. 5.2.1 Selection of Wall Type Selection of wall type is based on an assessment of the magnitude and direction of loading, depth
10、 to suitable foundation support, potential for earthquake loading, presence of deleterious environmental factors, proximity of physical constraints, wall site cross-sectional geometry, tolerable and differential settlement, facing appearance, and ease and cost of construction. 5.2.1.1 Rigid Gravity
11、and Semi-Gravity Walls Rigid gravity walls use the dead weight of the structure itself and may be constructed of stone masonry, unrein- forced concrete, or reinforced concrete. Semi-gravity can- tilever, counterfort, and buttress walls are constructed of reinforced concrete. Rigid gravity and semi-g
12、ravity retain- ing walls may be used for bridge substructures or grade separation. Rigid gravity and semi-gravity walls are gen- erally used for permanent wall applications. These types of walls can be effective for both cut and fill wall applications due to their relatively narrow base widths which
13、 allows ex- cavation laterally to be kept to a minimum. Gravity and semi-gravity walls may be used without deep foundation support only where the bearing soilhock is not prone to excessive or differential settlement. Due to their rigidity, excessive differential settlement can cause 111 112 HIGHWAY
14、BRIDGES 5.2.1.1 s REI NPORCI NC A= LEVELI ffi MSE wu WITH MODULAR PRECST CONCRETE FACING PANELS GEOSVNTNTI C KIP CONCRETE REI WORCI NC FbCI ffi CRONULOR FI LL JE MSE WALL WITH GEOSYNTHETIC REI NFORCEMNT ANO CI P CONCRETE OR SHOTCRETE FACING MSE WLILL WITH SEGMNTN CONCRETE BLOCK F AC1 NG FIGURE 5.2A
15、Typical Mechanically.Stabilized Earth Gravity Walls cracking, excessive bending or shear stresses in the wall, or rotation of the overall wall structure. 5.2.1.2 Nongravity Cantilevered Walls Nongravity cantilevered walls derive lateral resistance through embedment of vertical wall elements and supp
16、ort retained soil with facing elements. Vertical wall elements may consist of discrete vertical elements (e.g., soldier or sheet piles, caissons, or drilled shafts) spanned by a struc- tural facing (eg, wood or reinforced concrete lagging, precast or cast-in-place concrete panels, wire or fiber re-
17、inforced shotcrete, or metal elements such as sheet piles). The discrete vertical elements typically extend deeper into the ground than the facing to provide vertical and lateral support. Alternately, the vertical wall elements and facing are continuous and, therefore, also form the structural fac-
18、ing. Typical continuous vertical wall elements include piles, precast or cast-in-place concrete diaphragm wall panels, tangent piles, and tangent caissons. Permanent nongravity cantilevered walls may be con- structed of reinforced concrete, timber, and/or metals. Temporary nongravity cantilevered wa
19、lls may be con- structed of reinforced concrete, metal and/or timber. Suit- able metals generally include steel for components such as piles, brackets and plates, lagging and concrete reinforce- ment. Nongravity cantilevered walls may be used for the same applications as rigid gravity and semi-gravi
20、ty walls, as well as temporary or permanent support of earth slopes, excavations, or unstable soil and rock masses. This type of wall requires little excavation behind the wall and is most effective in cut applications. They are also effective where deep foundation embedment is required for stabilit
21、y. Nongravity cantilevered walls are generally limited to a maximum height of approximately 5 meters (15 feet), unless they are provided with additional support by means of anchors. They generally cannot be used effectively where deep soft soils are present, as these walls depend on the passive resi
22、stance of the soil in front of the wall. 5.2.1.3 DIVISION I-DESIGN 113 5.2.1. twnL BIN wu PREcnst CONCRETE CRIB wu ION WALL PREcnsi CONCRETE BIN unLL FIGURE 5.2B Qpical Prefabricated Modular Gravity Walls Anchored Walls Anchored walls are typically composed of the same el- ements as nongravity canti
23、levered walls (Article 5.2.1.2), but derive additional lateral resistance from one or more tiers of anchors. Anchors may be prestressed or deadman type elements composed of strand tendons or bars (with corrosion protection as necessary) extending from the wall face to a ground zone or mechanical anc
24、horage lo- cated beyond the zone of soil applying load to the wall Bearing elements on the vertical support elements or fac- ing of the wall transfer wall loads to the anchors. In some cases, a spread footing is used at the base of the anchored wall facing in lieu of vertical element embedment to pr
25、o- vide vertical support. Due to their flexibility and method of support, the distribution of lateral pressure on anchored walls is influenced by the method and sequence of wall construction and anchor prestressing. Anchored walls are applicable for temporary and per- manent support of stable and un
26、stable soil and rock masses. Anchors are usually requirec. ldr support of both temporary and permanent nongravity cantilevered walls higher than about 5 meters (15 feet), depending on soil conditions. Anchored walls are typically constructed in cut situa- tions, in which construction occurs from the
27、 top down to the base of the wall. Anchored walls have been success- fully used to support fills; however, certain difficulties arising in fill wall applications require special considera- tion during design and construction. In particular, there is a potential for anchor damage due to settlement of
28、 back- fill and underlying soils or due to improperly controlled backfilling procedures. Also, there is a potential for unde- sirable wall deflection if anchors are too highly stressed when the backfill is only partially complete and provides limited passive resistance. The base of the vertical wall
29、 elements should be lo- cated below any soft soils which are prone to settlement, as settlement of the vertical wall elements can cause de- stressing of the anchors. Also, anchors should not be lo- cated within soft clays and silts, as it is difficult to obtain 114 HIGHWAY BRIDGES 5.2.1.3 Mortar Rub
30、blo Mamonry Rolntorood Concroto Cantilovor I Rlald Gravity Wall Sornl-Crmv I t y Wall Rolntorood Concroto Countorfort Slurry or Cyllndor Pl10 Soml-Cravlty Wall Non-gravity Cantllovor Wall Soldlor PlIo Tloback Wail FIGURE 5.2C Typical Rigid Gravity, Semi-Gravity Cantilever, Nongravity Cantilever, and
31、 Anchored Walls adequate long-term capacity in such materials due to creep. 5.2.l.4 Mechanically Stabilized Earth Walls MSE systems, whose elements may be proprietary, employ either metallic (strip or grid type) or geosynthetic (geotextile, strip, or geogrid) tensile reinforcements in the soil mass,
32、 and a facing element which is vertical or near vertical. MSE walls behave as a gravity wall, deriving their lateral resistance through the dead weight of the re- inforced soil mass behind the facing. For relatively thick facings, such as segmental concrete block facings, the dead weight of the faci
33、ng may also provide a significant contribution to the capacity of the wall system. MSE walls are typically used where conventional gravity, cantilever, or counterforted concrete retaining walls are considered, and are particularly well suited where substantial total and differential settlements are
34、an- ticipated. The allowable settlement of MSE walls is lim- ited by the longitudinal deformability of the facing and the performance requirements of the structure. MSE walls 5.2.1.4 DMSION I-DESIGN 115 have been successfully used in both fill and cut wall ap- plications. However, they are most effe
35、ctive in fill wall applications. MSE walls shall not be used under the fol- lowing conditions. 0 When utilities other than highway drainage must be constructed within the reinforced zone if future access to the utilities would require that the rein- forcement layers be cut, or if there is potential
36、for material which can cause degradation of the soil re- inforcement to leak out of Ure utilities into the wall backfill. 0 With soil reinforcements exposed to surface or ground water contaminated by acid mine drainage, other industrial pollutants, or other environmental conditions which are defined
37、 as aggressive as de- scribed in Division II, Article 7.3.6.3, unless envi- ronment specific long-term corrosion or degradation studies are conducted. 0 When floodplain erosion may undermine the rein- forced fill zone or facing column, or where the depth of scour cannot be reliably determined. MSE w
38、alls may be considered for use under the fol- lowing special conditions: 0 When two intersecting walls form an enclosed angle of 70 or less, the affected portion of the wall is de- signed as an internally tied bin structure with at-rest earth pressure coefficients. 0 Where metallic reinforcements ar
39、e used in areas of anticipated stray currents within 60 meters (200 feet) of the structure, a corrosion expert should eval- uate the potential need for corrosion control require- ments. 5.2.1.5 Prefabricated Modular Walls Prefabricated modular wall systems, whose elements may be proprietary, general
40、ly employ interlocking soil- filled reinforced concrete or steel modules or bins, rock filled gabion baskets, precast concrete units, or dry cast segmental masonry concrete units (without soil reinforce- ment) which resist earth pressures by acting as gravity re- taining walls. Prefabricated modular
41、 walls may also use their structural elements to mobilize the dead weight of a portion of the wall backfill through soil arching to provide resistance to lateral loads. Prefabricated modular systems may be used where conventional gravity, cantilever or counterfort concrete retaining walls are consid
42、ered. Steel modular systems shall not be used where the steel will be exposed to surface or subsurface water which is contaminated by acid mine drainage, other industrial pol- lutants, other environmental conditions which are defined as aggressive as described in Division II, Article 7.3.6.3, or whe
43、re deicing spray is anticipated. 5.2.2 Wall Capacity Retaining walls shall be designed to provide adequate structural capacity with acceptable movements, adequate foundation bearing capacity with acceptable settlements, and acceptable overall stability of slopes adjacent to walls. The tolerable leve
44、l of wall lateral and vertical de- formations is controlled by the type and location of the wail structure and surrounding facilities. 5.2.2.1 Bearing Capacity The bearing capacity of wall foundation support sys- tems shall be estimated using procedures described in Ar- ticles 4.4,4.5, or 4.6, or ot
45、her generally accepted theories. Such theories are based on soil and rock parameters mea- sured by in situ and/or laboratory tests. 5.2.2.2 Settlement The settlement of wall foundation support systems shall be estimated using procedures described in Articles 4.4,4.5, or 4.6, or other generally accep
46、ted methods. Such methods are based on soil and rock parameters measured directly or inferred from the results of in situ and/or labo- ratory test. 5.2.2.3 Overall Stability The overall stability of slopes in the vicinity of walls shall be considered as part of the design of retaining walls. The ove
47、rall stability of the retaining wall, retained slope, and foundation soil or rock shall be evaluated for all walls using limiting equilibrium methods of analysis such as the Modified Bishop, simplified Janbu or Spencer methods of analysis. Aminimum factor of safety of 1.3 shall be used for walls des
48、igned for static loads, except the factor of safety shall be 1.5 for walls that supportabutments, build- ings, critical utilities, or for other installations with a low tolerance for failure. A minimum factor of safety of 1.1 shall be used when designing walls for seismic loads. In ail cases, the su
49、bsurface conditions and soillrock proper- ties of the wall site shall be adequately characterized through in-situ exploration and testing and/or laboratory testing as described in Article 5.3. Seismic forces applied to the mass of the slope shall be based on a horizontal seismic coefficient kh equal to one- half the ground acceleration coefficient A, with the verti- cal seismic coefficient k, equal to zero. 116 HIGHWAY BRIDGES 5.2.2.3 It must be noted that, even if overall stability is satis- factory, special exploration, testing and analyses may be required for bridge abutments or retainin
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