Aşırı yüklemeler altında toprakarme istinat yapılarının tasarımı
Design of mechanically stabilized earth (MSE) walls under excessive loadings
Yazar: TAHİR YILDIZ
Danışman: PROF. DR. HÜSEYİN YILDIRIM
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Toprakarme istinat duvarları, konvansiyonel dayanma yapılarına alternatif olarak son
40 yılda yaygın bir şekilde uygulama alanı bulmaktadır. Esnek yapıları nedeniyle
büyük deformasyonları karşılayabilmesi, sismik kuvvetler etkisi altında yüksek
performans göstermesi, imalat kolaylığı sağlaması ve ekonomik olması yönü ile
karayolu, demiryolu, fabrika inşaatlarında, su ve liman yapılarında bu sistem tercih
edilmektedir.Gelişen dünya ile birlikte zorlayıcı topoğrafik ve jeolojik koşullar
altında mühendislik yapılarının inşa edilmesi bir zorunluluk olarak ortaya çıkmıştır.
Geleneksel sistemlerin ekonomik çözümler ortaya koyamadığı bölgelerde,
toprakarme istinat sistemleri önemli bir alternatif olmuştur. İnşa edilecek yapıların
ekonomik, işlevsel ve güvenli olması, toprakarme istinat sistemlerinin doğru tasarımı
ile mümkün olmaktadır. Taşıma gücü kontrolü, oturma analizleri, sistemde meydana
gelebilecek devrilme ve kayma gibi geoteknik problemler, toprakarme istinat
yapılarının analiz edilmesinde göz önüne alınması gereken başlıca kritik durumlardır.
Ortaya çıkan kompleks geometriler ve bu geometrilerin bir sonucu olarak meydana
gelmesi beklenen yüksek sürşarj yüklemeleri, istinat yapısının inşa edileceği
bölgedeki zemin koşulları ile bir arada düşünüldüğünde, bu etkiler altındaki
toprakarme istinat yapılarının tasarımının bir geoteknik problemi olduğu, bu
problemin çözümünün geoteknik mühendisliğinin temel yaklaşımları ile mümkün
olabileceği anlaşılmaktadır. Bu yaklaşımlar kapsamında, imalat ve tasarım süresince,
sistemin temel bileşenlerinden, donatı, dolgu ve yüzey kaplama elemanları gibi
yapısal elemanların ve çevresel koşulların doğru analiz edilmesi problemin doğru bir
şekilde anlaşılması ve çözümlenmesi açısından gereklidir. Söz konusu durum, bu tez
çalışması kapsamında, toprakarme istinat yapılarının tipleri ve genel hesap
yöntemleri ile ortaya konmuştur. Sonrasında bu yöntemler ışığında, palyeli
geometriler için hesap yaklaşımları araştırılmıştır. Derlenen veriler göz önüne
alınarak bir proje vaka analizi üzerinden, aşırı yüklemelere maruz kalacağı öngörülen
palyeli bir toprakarme istinat yapısının tasarımı araştırılmıştır. Tasarım kapsamında
ilk olarak, mevcut geoteknik koşullar için yapı davranışı incelenmiştir. Sistemde
meydana gelebilecek stabilite problemlerine karşı alınacak önlemler araştırılmıştır.
Sistemin, tasarıma uygun bir şekilde imalatının yapılması, analiz sonuçlarının
gerçeği yansıtması açısından son derece önemlidir. Bu amaçla, imalat sırasında,
dolgunun her bir tabakası üzerinde sıkışma kontrolleri yapılarak, elde edilen veriler,
imalat öncesinde yapılan deney sonuçları ile karşılaştırılmıştır. Karşılaştırmalı
sonuçlarla, dolgunun şartnamelerce belirlenen kriterlere uygunluğu doğrulanmıştır.

Abstract:
Retaining structures are indispensible elements of every highway and railway design.
Retaining structures are used not only for bridge abutments and wing walls but also
for slope stabilization and to minimize the amounth of embankments in construction
of different type of structures such as factories, dams and, railways. For many years,
retaining structures were almost exclusively designed as a conventional solutiols
which is made of reinforced concrete and with the gravity or cantilever type walls
which are essentially rigid structures and cannot perform well against significant
differential settlements unless founded on deep foundations. With increasing height
of the filling soil to be retained and poor foundation conditions, the cost of reinforced
concrete retaining walls increases rapidly.
Mechanically Stabilized Earth Walls (MSE) is costeffective reinforced-soil structures
that can accommodate much larger settlements than conventional retaining structure
systems. Using reinforcing elements in the soil mass, the strength of the soil can be
improved. Facing system is generally essential to prevent soil raveling between the
reinforcing elements. MSE system allows very steep slopes and vertical walls to be
constructed safely.
Different type of methods have been used since ancient times to improve soil. The
use of straw to improve the quality of adobe bricks had been observed since the
earliest human history. Many primitive people used wooden sticks to support mud
dwellings. French natives along the Bay of Fundy in Canada used sticks to reinforce
mud dikes for 200 years approximately during the 17th and 18th century. Some other
early examples of basic soil reinforcement methods include dikes of earth and tree
branches, which have been used in China and along the Mississippi River. Other
examples include wooden pegs for preventing of erosion and landslide control in
England, and bamboo or wire mesh, used for revetment erosion control. Soil
reinforcing can also be achieved by using live plant roots.
The modern methods of soil reinforcement for retaining wall construction were
developed by the French architect and engineer Henri Vidal in the early 1960s. His
research led to the invention and engineering phenomena of Reinforced Earth, a
system in which steel strips are used for reinforcing the soil mass. Today, MSE walls
are the wall of choice in most fill situations, and MSE walls are used extensively in
worldwide.
MSE wall systems has many advantages compared to the classical systems by means
of their flexibility, high seismic performance, easy construction facilities and cost saving parameters. Main application areas can be classified as, highways, railways,
factory constructions, hydraulic and shore structures.
Cost saving advantage of MSE walls are the main reason of choosing this system in
engineering applications. Site specific costs of a MSE structure depend on many
parameters, such as cutfill amount, wall-slope size and type, soil type, backfill
material quality, facing type, temporary or permanent applications. It has been found
that MSE walls with precast concrete facings are usually less expensive than
conventional type of retaining structures.
Construction in extreme environmental and geological conditions has appeared as an
obligation with the development of industry and urban cities. MSE wall has been
preferred as an alternative of the conventional systems where the classical methods
can not be applied properly. The main properties of a structure, listed as economy,
safety and functionality, can be provided by the correct design of MSE walls.
MSE walls are usually designed according to Load and Resistance Factor Design
(LRFD) method. In LRFD, the external and internal stability of the MSE wall is
evaluated at all appropriate strength limit states.Besides, overall stability and
lateral/vertical wall movement are evaluated at the service limit state, as well.
Extreme case load combinations are used to design and analyze for conditions such
as vehicle impact, excessive surcharge loadings and seismic effects.
For the internal stability analysis, tensile and pullout resistance of the reinforcement
is checked. The structural resistance of facing elements and connection parts should
be checked, too. Limiting eccentricity, sliding and bearing resistance cases should be
considered for the external stability check, according to LRFD design of the MSE
walls. Overal stability and compound stability cases are mostly related to global
stability.
The external stability of an MSE wall is maintained assuming that the reinforced soil
mass acts as a rigid body. This is because, the wall facing and the reinforced soil act
as a coherent block with lateral earth pressures acting on the back side of that block
in the proper design. This is called coherent gravity method.
Internal stability analysis depend on the soil-reinforcement interaction, tensile
resistance of the reinforcement and durability of the reinforcing strips. Calculations
are listed respectively as determination of the maximum factored load for each
reinforcement, comparison of this maximum factored load to the factored pullout
resistance and to the factored tensile resistance of the reinforcement for all
applicable strength, service, and extreme event limit states.
The major geotechnical engineering problems such as bearing capacity, differential
or total settlement, overturning and sliding should be taken into consideration during
the design of a retaining wall. These conditions are valid for MSE walls as well. The
design of MSE wall should be examined as a geotechnical problem when this issue
meets with the environmental effects with the complex geometries and excessive
surcharge loadings. The solution of this problem is possible with the logical
determination of construction components and environmental conditions during the
design and application on site.This master thesis is prepared to explain the typical systems of MSE types with
general calculation hypothesis. The design methods for tiered MSE walls are
researched.
After that, a case study has been investigated including a tiered MSE wall faces with
excessive surcharge loadings. The design of MSE wall is explained with the
consideration of geotechnical conditions. Structural behavior is questioned. The
precautions against possible stability problems are explained in detail.
Mechanical and electrochemical properties of backfill material is also explained. The
results of necessary tests are compared with the limit values in the related codes and
standarts. Suitable fill material affects the wall performance by the durability of the
structure in the long term.
The general terms of typical design of Mechanically Stabilized Earth Walls are valid
for this project. On the other hand, assumptions for the analysis should be
reconsidered because of the complex geometry of the structure. The excessive
loadings caused by trucks, cranes and storage area require special solution that
consider the situation as a geotechnical engineering problem. Therefore, analysis
carried on by this point of view.
Soil improvement is an important topic for the structures constructed on poor
foundation soils. This project has been examined for soil improvement. It is clear
that, MSE wall system can not be designed properly without any soil stabilization
application under excessive loadings with poor subsoil conditions. Therefore,
suitable improvement system was chosen for this application. Project details were
explained. The stability analysis before and after the improvement were compared in
order to show the efficiency of stabilization method.
It is important that, construction works and site applications should be compitable
with numerical analysis of the design. For this purpose, each layer of the fill has been
tested by its compaction behavior. Compaction test results have been compared with
the tests carried out before the construction. The validity of test results has been
shown by comparing the results with the interval values noted in the standarts.
MSE wall design is a complete geotechnical engineering problem if the structure is
expected to resist excessive surcharge loads with the poor subsoil conditions.
Therefore, the design of the MSE wall is a complex task that requires detailed site
investigation and a series of analysis. All of the stability conditions for static and
seismic cases should be confirmed. The proper design of the structure should be
based on this methodology.