Esnek üstyapıların projelendirilmesinde AASHO 1972 ve AASHO 1986 yöntemlerinin karşılaştırılması

Abstract

Karayolları Genel Müdürlüğü esnek yol üst yapıların projelendirilmesinde AASHO 1972 yöntemini kullanmaktadır. AASHO yöntemi Amerikan Karayolları Görevliler Birliği tarafından hazırlanmıştır, 1963.' de ilk baskısı yapılan AASHO yöntemi zamanla, geliştirilmiş ve İ972' de 2. baskısı ve 1986' da.3. baskısı yayınlanmıştır. Yöntemin oluşturulmasında 1956 - 1958 yıllarında yapılan- AASHO Deneme Yolundaki değişik malzeme ve tabaka kalınlıklı üstyapıların davranışlarının incelenmesi temel. teşkil etmiştir.. AASHO taraf andan 'en son olarak hazırlanana AASHO 1936 yönteminin üzerindeki çalışmalarının ülkemizde de başlaması önemlidir, Bu yüksek lisans tez çalışmasının ana amacı 1972 ve 1986 yıllarında yayınlanan bu. iki yöntemin -farklılıklarım açıklamak ve ülkemiz için öneriler sunmaktır. l» bölüm' de AASHO Yönteminin nasıl oluşturulduğu anlatılmaktadır. AASHO Deney Yolu çalışmaları hakkında b i l g i verilmektedir.. 2. bolüm' de esnek üstyapının özellikleri açıklanmaktadır. Bir esnek üstyapının nelerden oluştuğu gösterilmektedir.. 3. bölümle AASHO 1972, 4. bölümle de AASHO 1986 esnek yol üstyapısı projelendirme yöntemleri sunulmaktadır, 5. bölüm' de ise bu iki projelendirme yönteminin farkları sergilenmiştir»

The design of highway pavements involes a study of soils and paving materials, their behavior under load, "..- and the design of pavement to carry that load under all climatic conditions» All pavements derive their ultima te support -from the underlying subgrade? there-fore, a. knowledge o-f basic soil mechanics is essential. * Pavements will be divided into two categories. The first, the flexible pavement, consist of a relatively thin wearing surface built over a base course and sub- base course, and they rest upon the compacted subgrade. In contrast, rijid pavements &re made up of Portland ce ment concrete and may or may not have a base course bet ween the pavement and the subgrade. The thickness of the flexible pavement is meant to include all components of the pavement above the compacted subgrade. Thus, the Bitbbase, base, and wearing surface are the structural components of the pavement. The load-carrying capacity of -flexible pavement is brought about by the load-di sributing characteristics of the layered system. Flexible pavements consist of a series of layers, with the highest, quality materials at or r\ear the surface. Hence, the strength of a flexible pavement is a result of building up thick layers and thereby distributing the load over the subgrade, rather than by the bending action of a slab. Subbase course for flexible pavements are generally m a d e up of c h e a p, 1 o c a 1 1 y a v a i 1 at b I e m a t e r i a Is, w h e r e a s the base course are higher quality processed materials. In most case the base course consist of crushed stone a n d, i n s o m e i n s t. a n c e s, m a y c: o n t. a i n a s p h a 1 1. The purpose of a pavement is to provide a smooth surface over which vehicle may pass under all climatic conditions. In turn, the performance of pavement is affected by the characteristics of the subgrade. De sirable properties which the subgrade should possess in clude strength, drainage, case of compaction, permanency of compaction, and permanency of strength» Since sub- grades vary considerably, it is necessary to make a tho rough study of soils in place and, from this, to deter mine the design of the pavement. Soil is a highly var iable materials the interrelationship of soil texture, density, moisture content, and strength are complex, and in particular, behavior under repeated loads is difficult to evalute. Because o-f the complexity of the problem, it is not. possible to set down rules which wiil be s u i t a b 1 e t o r all c a s e s » The purpose o-F the bituminous wearing course is to provide a smooth riding sur -face that is resilient and that will resist pressures exerted by the tires. The wearing course should be watertight to prevent surface infiltration with subsequent, weakening of the subgrade, and in addition, it should be flexible so that it will not fail if consolidation of the subgrade or base course takes place. A problem continuall y f a c i n g a n e n g i n e e r is that dealing with procedures and teen i que s by which otherwise unsuitable soil may be improved by stabilization». In many instances subgrade soils that are unsatisfactory in their natural state can be altered by admixtures, by the addition of aggregate, or by proper compaction and thus made suitable for highway-subgratie construction. The cemeting materials which may be used include Portland cement, lime, a mixture of lime and flays. Portland cement has been used with great success to im prove existing gravel roads, as well as to stabilize na tural soils. ît can be used for base courses and sub- Das e of all types. Stabilisation of soil with cement consist of adding Portland cement to a pulverized soil and per mitting the mixture to harden. The factors which affect the physical properties of soil cement include soil type and, quantity of cement, degree of mixing, time of curing and, dry density of the compacted mixture. Stabilisation of soil with cutback asphalt, road tars, and asphalt emulsions is satisfactory -For coarse grained or granular soil, but its use for stabilising plastic soils is 1 i m i t ed. The study of AASHO in design of pavement began in 1950, The experimental design at AASHC Road Test cons - ructed in 1956- İ 958. The first method of AASHO has i s-sued in 1961. The second and third issue developed in 1972 and 1986 in rows,, These Guides base on observations at AASHO Road Test in Ottawa, Illinois. Each of the 50 states have developed methods of design for new consruction and for overlays. Many of the states have used information and procedures contained in the 1972 version of the AASHO Inter m Guide for Design of Pavement Structures,, It can be expected that these states will adopt information from this revision (1986) to the Guide as the basis for updating design procedures as seems appropriate.. VI THE BASIC DESIGN EQUATIONS USED FDR FLEXIBLE PAVEMENTS AASHO (1972) DESIGN EQUATION Loci T = 9,36 Log (SN+1) -0,20 + S3 ""? (SN+1) Log 1 + 0,372 (S - 3,0) R i 4,2 - P G = Log ( t ) t 4,2-1,5 SN = a*D + a*D + a*D 1 1 ^ *? ~Z T where T s predicted number of S, 2 ton equivalent.single-*- 8.2 ax lei oad app 1 i c at i on s, SN s structural number of pavement S s the value of strerrgth for roadbed soil i R ^. s the factor of region a si» layer coefficient i D - si, layer thickness (inches) i AASHO (1986) DESIGN EQUATION Log APSI 10 4.2-1.5 Log (W ) = Z*So+9.36*Loq (SN+1) -0.20+ _ - 10 8.2 R 10 0.40+ 1094 5,19 (SN+1 ) 2. 32*Log ( M ) -8. 07 Î0 R SN = a * D + a * D * m + a * D * m 1 1 T' C5 '~? ~? "^ ~? where W s predicted number of 8.2 ton equivalent single re o axle load applications, VII Z s standard normal deviate R So s combined standard error o-F the traffic pre diction and performans prediction, M '. s resilient modulus (psi ) R SN 3 structural number of pavement APSI ; difference between the initial design service ability inde;-;, Po, and the design terminal ser v i c eab i 1 i t y i n d ex, R s Reability g a si. layer coefficient i D i i» layer thickness (inches) i m s i» layer drainage coefficient i Traffic information required by the design equation used in these Guides included axle loads, axle config uration, and number of applications. The servi cebi 1 i ty of pavement is expressed in terms of the present servicebi 1 i ty index (PSI). The PSI is obtained from measurement of roughTTess and distress, e.g cracking, patching and rut depth, at a particular time during the service life of the pavement. The major factors influencing the loss of serviceability of a pavement are traffic, age, and enveroment. Each of factors 'has been considered im formulating the design requirements included in this Guide (1986) The issue of the AASHO 1986. Guide contains the following modifications to the 1986 version.