土工合成材料加筋擋土墻段

1、 Geotextiles and Geomembranes 19(2001359386Geosynthetic reinforced segmental retaining wallsRobert M. Koerner a, *,Te-Yang Soong ba Geosynthetic Research Institute, Drexel Uni v ersity, Philadelphia, PA 19104, USAb Earth Tech Consultants, Inc., 36133Schoolcraft Road, Li v onia, MI 48150USAReceived 2

2、3December 2000; received in revised form 9March 2001; accepted 24May 2001AbstractSegmental retaining walls (SRWs(primarilythose with precast concrete block facing reinforced by geogrids or geotextiles are in a period of enormous growth. Estimates are that 35,000of these walls exist and that they cov

3、er the entire range of practical wall heights. This paper gives a perspective of the evolution of retaining walls in general, and follows with results of a recent cost survey. It is seen that geosynthetic reinforced walls are the least expensive of all wall categories and at all wall heights. Three

4、design methods are then compared to one another with respect to their details and idiosyncrasies. This is followed by a numeric example showing that the modied Rankine method is the most conservative, the FHWA method is intermediate, and the NCMA method is the least conservative. A survey of the lit

5、erature is included where it is seen that there have been approximately 26walls which suered either excessive deformation or actual collapse. The overwhelming causes for these cases of poor performance were (ibacklling with improperly draining ne grained soil and (iicontractors deciencies which coul

6、d have been avoided with proper quality control and inspection. The paper, which reects North American practice, closes with a discussion of possible concerns most of which are under active investigation. Clearly, continued strong growth for geosynthetic reinforced SRWs is justied. r 2001Elsevier Sc

7、ience Ltd. All rights reserved. Keywords:Segmental retaining walls; Modular block walls; Wall costs; Wall design; Field behavior 1. IntroductionIt is quite understandable that retaining wall design and construction has occupieda pivotal position in the historic development of geotechnical engineerin

8、g. Retaining *Correspondingauthor. Tel.:+1-610-522-8440;fax:+1-610-522-8441.E-mail address:(R.M.Koerner.0266-1144/01/$-see front matter r 2001Elsevier Science Ltd. All rights reserved.PII:S 0266-1144(01 00012-7walls (alongwith earth dams were among the rst soil-related st

9、ructures to be considered as both critical and permanent insofar as their service life was concerned. Along with this signicance came a variety of retaining wall types, design methods, and related construction methodologies. Over time, the classical gravity retaining walls transitioned into reinforc

10、ed concrete types, some with buttresses and counterforts. These were then followed by a variety of crib and bin-type walls. A paradigm shift occurred in the 1960s with the advent of mechanically stabilized earth (MSEmasses, i.e., reinforced layers of soil allowing for modular construction, which was

11、 clearly recognized as being advantageous in most situations. The reinforcement was initially steel straps, and subsequently welded wire mesh provided an alternative. Wall facings varied from metallic-to-reinforced concrete-to-segmental units of a variety of types and shapes. By the 1980s this MSE t

12、echnology segued into polymeric reinforcement using geogrids, geotextiles and polymer straps. Thus, at the present time there exists four categories of wall types, each with subcategories. They are somewhat arbitrarily grouped as follows:1. Rigid and/orgravity walls a. concrete cantileverb. concrete

13、 cantilever with buttresses/counterfortsc. rubble masonry d. cylinder pilese. soldier piles and tiebacks2. Prefabricated and compartmentalized gravity walls a. metal binsb. precast concrete bins c. precast concrete cribs d. gabions3. MSE F with metal reinforcement a. precast concrete facing panels b

14、. cast-in-place facingc. segmental retaining walls (SRWs(modularblock facing 4. MSE F with geosynthetic reinforcement a. precast concrete facing panels b. cast-in-place facingc. segmental retaining walls (SRWs(modularblock facingThis paper focuses on the fourth category, namely MSE-walls with geosyn

15、thetic reinforcement. While a variation exists in the type of polymer reinforcement (e.g.,geogrids, geotextile, and even polymer straps, it is the facing which is an ever-evolving transition. MSE geosynthetic reinforced wall facing are of the following types:*wrap-around facing *timber facing*welded

16、-wire mesh facingR.M. Koerner, T.-Y. Soon g /Geotextiles and Geomembranes 19(2001359386360*gabion facing*precast full-height concrete facing*cast-in-place full-height concrete facing*precast panel wall facing units*SRWs; also called modular concrete block wallsIt is this last type of wall facing tha

17、t is seeing the largest growth at the present time. This comes about for a number of reasons, among which are, dry-cast fabrication of the blocks, ease of placement using manual labor, ease of geosynthetic connection to the facing, conformance to essentially any variation in line and grade, good tol

18、erance for irregularities, and (perhapsmost importantly outstanding aesthetics. The SRWs are being mainstreamed in large part, due to their pleasing appearance. In addition to a growing use by geotechnical consultants; building architects, landscape developers, golf and park superintendents, commerc

19、ial and private property owners, etc., have readily accepted these wall systems and have utilized them accordingly. Accompanying this expansion in the shear number of walls, it is understandable that the height of the walls is also increasing. No longer conned to low and medium heights, MSE walls wi

20、th geosynthetic reinforcement now compete with other wall types in all height categories. Twelve meter high walls, and above, exist at the present time. The largest (locatedin Taiwan is approximately 38m high. In the authors opinion, the profession nds itself in the midst of a massive transition fro

21、m a bevy of wall types to a predominance of MSE-walls with geosynthetic reinforcement, and particularly of the SRW-type. Such walls are the topic of this paper.The paper (basedlargely on North American experience will present a comparison of wall cost data, and then follow with a detailed comparison

22、 of three dierent design methodologies. It will then present a listing of poor performing wall case histories taken from the literature and from personal les, and nally present areas of concern where additional investigation is felt to be warranted.2. Wall costsRetaining wall cost surveys have undou

23、btedly been conducted by many public agencies, private users, design engineers, contractors and manufacturers over the years. The perspective for this cost survey, however, begins in 1973. This date follows closely the early use of MSE walls using steel straps. Lee (1973used categories of gravity wa

24、lls and crib/binand compared them to MSE walls with steel reinforcement. He furthermore subdivided the walls into high (H X 9.0m, medium (4.5o H o 9.0 and low (H p 4.5m height categories. The unit prices, on the basis of dollars per square meter of wall face, are given in Table 1. Readily seen is th

25、at MSE walls with metallic reinforcement are the least expensive wall of the types surveyed at all heights.The Lee survey was followed by one conducted by the VSL Corporation in 1981 which included welded wire mesh in the MSE (metalreinforced wall category. The R.M. Koerner, T.-Y. Soon g /Geotextile

26、s and Geomembranes 19(2001359386361cost data is also shown in Table 1. Here it is seen that all wall costs increased considerably in the 8-year interval between 1973and 1981, and that MSE walls with both steel straps and welded wire mesh remained the least expensive.Yako and Christopher (1988perform

27、ed a survey seven-years after the VSL survey which focused on MSE walls with geosynthetic reinforcement. The data appears in Table 1where it is seen that the data for the original three categories of walls was taken directly from the VSL study. The important part of the Yako and Christopher survey,

28、however, is that the MSE walls with geosynthetic reinforcement have been added and are seen to be the least expensive of all wall categories.Ten years later, Koerner et al. (1998conducted a survey which included all four wall categories so as to update the three earlier studies. The survey form was

29、sent to all fty U. S. Departments of Transportation. Of the 40-states responding, bid prices for the dierent wall types listed in the introduction were solicited and are included in the data base. As such, the data reects a nation-wide study of thousands of walls which were publicly funded; as oppos

30、ed to walls that were privately funded. Cost data on privately funded walls is just becoming available, but is seen to be still lower than the costs reported here, Koerner et al. (2001.The data from the 1998survey appears in Table 1and can be contrasted to the three earlier surveys. When graphing re

31、sults from these four surveys the dierences become even more apparent, see Fig. 1. Gravity walls are by far the most expensive, with crib/binwalls and MSE (metalwalls signicantly less expensive. Note that crib/binwalls are rarely over 7-m in height. It is also obvious that MSE (geosyntheticwalls are

32、 the least expensive of all wall categories and over all wall heights. Convergence, however, seems to beTable 1Comparison of past retaining wall costs with current (1998survey results (unitsare U.S. dollars per square meter of wall facing a Wall category Wall height (relativeLee et al. (1973VSL Corp

33、oration (1981Yako andChristopher (1988Koerner et al. (1998Gravityhigh 245377377I/Dmedium 230280280390low 225183183272MSE wallslowN/AN/A130223aNote:I/D,Inadequate data; N/A,not available at time of survey.R.M. Koerner, T.-Y. Soon g /Geotextiles and Geomembranes 19(2001359386362occurring within the tw

34、o dierent MSE types (metaland geosynthetics in the high wall height category.This latter survey also generated statistical data in providing a mean value (whichwas plotted in Fig. 1, standard deviation and variance. This information is provided in Table 2, where it is seen that the standard deviatio

35、n in data is highest with gravity walls, intermediate with crib/binand MSE (metalwalls, and the least with MSE (geosyntheticwalls. Variance values, however, are similar in all wall categories.Table 2Statistical data for retaining wall costs from Koerner et al. (1998survey a Wall categoryWall height

36、(mWall cost in dollars(mÀ2 Variance (%MeanStd. Dev. Gravity walls>9.0I/DI/DI/D4.59.039012933o 4.52729836MSE (metal>9.0385122324.59.038112633o 4.534113540MSE (geosynthetic2236730aNote:I/D,Inadequate data.R.M. Koerner, T.-Y. Soon g /Geotextiles and Geomembranes 19(20013593863633. Elements o

37、f design and numeric exampleThis section presents essential elements in the design of geosynthetic reinforced retaining walls. It is illustrated for SRWs, but is similar for other facing types as well. Embodied in the design are both external and internal considerations;*external stability issues:*m

38、ass sliding on foundation soil *bearing capacity of foundation soil *overturning about the toe of the wall *internal stability issues:*reinforcement spacing and tensile overstress *soil pullout length *facing connection overstressThese six considerations will be analyzed using (ia modied Rankine app

39、roach as illustrated by Koerner (1998,(iithe Federal Highway Administration approach, FHWA (1998and (iiithe National Concrete Masonry Association approach, NCMA (1997.A numeric example of the design of a 7-m, high SRW will follow so as to illustrate the dierences in results using the three methods.

40、The design comparisons and numeric example will use symbols and denitions as illustrated in Fig. 2.Fig. 2. Identication of terms used in the design of geosynthetic reinforced retaining walls.R.M. Koerner, T.-Y. Soon g /Geotextiles and Geomembranes 19(20013593863643.1. External stability issueTable 3

41、presents the calculation methods for determining the coecient of active earth pressure from which the active earth pressure force is calculated, i.e.,*for soil:P ¼0:5g H 2K a*for surcharge:P ¼qHK awhere P is the active earth pressure (kN/m,g the unit weight of backll soil (kN/m3, H the hei

42、ght of wall (mand q the surcharge behind wall (kN/m2. Table 3also presents the inclination angle at which P acts on the reinforced soil zone. Note that in all methods P acts at H /3above the foundation soil for soil pressure, and H /2for surcharge pressure.Rankines analysis is the simplest but also

43、the most restrictive. It can only include a horizontal thrust (whichis probably not accurate for soil versus soil so it is modied in Table 3to include an inclination angle. The FHWA and NCMA methods use the Coulomb analysis which can handle earth pressure inclination angles other than horizontal. Im

44、portantly, the Coulomb analysis can include backslopes and batter walls. Table 3illustrates the wide range of wall variations that are included in the FHWA and NCMA design methods.Table 4presents the various calculation methods to arrive at a factor-of-safety (FSvalue for sliding of the entire MSE m

45、ass (facing,drainage soil zone and reinforced soil zone along the foundation soil or rock. A horizontal force summation is used in the process. All three calculation methods require the resulting FS to equal or exceed a value of 1.5.Not illustrated in Table 4is the possibility of sliding within the

46、MSE soil mass,e.g., along an individual geogrid or geotextile layer and exiting between adjacentfacing elements. The situation is handled in the same manner but with obvious dierences in the interface friction values and the respective forces.Table 5presents the various methods to calculate load ecc

47、entricity on the basis of the drainage/reinforcedsoil zone where it interfaces with the foundation soil. This load eccentricity is then used in the calculation of the MSE-mass bearing pressure (BP,as well as the foundation soil bearing capacity (BC.The result of this ratio is the FS-value which must

48、 exceed, or equal, either 2.0or 2.5using the various methods illustrated.Table 6presents the various design methods used for the calculation of a FS-value against overturning of the MSE mass about the toe of the wall. It uses the earth pressures at their respective inclinations and locations to obta

49、in the overturning moment. When compared to the resulting or stabilizing moment the ratio results in a FS-value. This value must exceed, or equal, 2.0, except for FHWA which feels this mode of failure is unlikely and that the calculation is not necessary.3.2. Internal stability issuesTable 7presents

50、 the various design methodologies to obtain the spacing of the geosynthetic layers so as not to overstress them. In each calculation a design R.M. Koerner, T.-Y. Soon g /Geotextiles and Geomembranes 19(2001359386365Table 3Coecient of active earth pressure and its inclination angleModied Rankine FHWA

51、 NCMAVertical wall (i.e.,o C 0 Vertical wall (i.e.,o p 101 Vertical wall (i.e.,o p 101(1Horizontal backslope (i.e.,b ¼0 (1Horizontal backslope (i.e.,b ¼0 (1Horizontal backslope (i.e.,b ¼ 0K a ¼tan 245À f i 2K a ¼tan 245À f i 2K a ¼1Àsin f ii¼tan 2

52、40;45Àf i ÞInclination (deg.=d p f i Inclination (deg.=01Inclination (deg.=01(2Inclined backslope (2Inclined backslopeK a ¼cos bcos b À cos 2b Àcos 2f ipcos b þcos 2b Àcos 2f i" #K a ¼cos bcos b À cos 2b Àcos 2f ipcos b þcos 2b Àcos 2f

53、 i" #Inclination (deg.=b Inclination (deg.=bR.M. Koerner, T.-Y. Soong / Geotextiles and Geomembranes19 (2001359386 366Batter wall (i.e.,o >101 Batter Wall (i.e.,o >101 K a ¼sin 2ðy þf Þsin 2y sin ðy þd Þ1þsin ðf i þd Þsin ðf i &#

54、192;b Þsin ðf Àd Þsin ðy þb ÞK a ¼sin 2ðf þo Þsin 2o sin ðo Àd Þ1þsin ðf i þd Þsin ðf i Àb Þsin ðo Àd Þsin ðo þb ÞInclination (deg.=d þ90Ày Inclination (de

55、g.=d ÀoR.M. Koerner, T.-Y. Soon g /Geotextiles and Geomembranes 19(2001359386367Table 4Sliding against foundation soil or rockModied Rankine FHWA NCMA General situation (1Horizontal backslope General situation(2Inclined backslopeFS ¼FH1:5FS ¼FHX 1:5FS ¼FHX 1:5where F ¼W m ;m

56、 ¼min. of (tanf f ; tan f r or tan r ,f f the friction angle of foundation soil,f r the friction angle of reinforced soil,r the friction angle of GS-to-soiland P H ¼P s þP qwhere F ¼ðW 1þW 2þP sin b Þm ;m =min.of (tanf f ; tan f r or tan r ,f f the friction an

57、gle of foundation soil,f r the friction angle of reinforced soil,r the friction angle of GS-to-soilIf reinforced soil controlsF ¼C ds ðW 1þW 2þq d L b Þtan f rIf drainage soil controlsF ¼C ds ðW 1þW 2þq d L b Þtan f dIf foundation soil controlsF 

58、8;C ds c f L þðW 1þW 2þq d L b Þtan f fÂÃP H ¼P cos ðd Ào ÞR.M. Koerner, T.-Y. Soong / Geotextiles and Geomembranes19 (2001359386 368Table 5Eccentricity and foundation soil bearing capacity Modied Rankine FHWA NCMAe ¼M ov Le ¼P ðc

59、os b ÞhÀP ðsin b ÞLÀW 2L12e ¼P s ðH ÞY s þP q ðH ÞY q ÀW 1X 1ÀLÀW 2X 2ÀLÀq d L b X q ÀLr ð1Þr ð2Þd bwhere M ov ¼HP s3þP q2e oLin soil e oLin rockFS ¼Bearing capacity ; BCBearing pr

60、essure ; BPX 2:0FS ¼Bearing capacity ; BCBearing pressure ; BPX 2:5FS ¼Bearing capacity ; BCBearing pressure ; BP2:0where BC ¼c f N c þ0:5ðL À2e Þg f N gBP ¼W þqLL À2ewhere BC ¼c f N c þ0:5ðL À2e Þg f N gBP ¼W 1þW 2

61、P sin bwhere BC ¼c f N c þ0:5g f ðL À2e ÞN g þg f H emb N qBP ¼W r ð1ÞþW r ð2Þþðq 1þq d ÞL bNote:FS X 2.0might be acceptableif there is good geotechnical report available.R.M. Koerner, T.-Y. Soong / Geotextiles and Geomembranes19 (2001359386 369Table 6