RIBBED MAT FOUNDATION DESIGN - İMOA lot of attempts are being made to control the swell-shrink...

RIBBED MAT FOUNDATION DESIGN

KABURGALI RADYE TEMEL TASARIMI

Aya AHMAD*1 Ayse BALKIS

2

ABSTRACT

Expansive soils which occurs in the arid and semi-arid climate regions of the world cause

serious problems on civil engineering structures. Such soils swell when given an access to

water and shrink when they dry out. A lot of attempts are being made to control the swell-

shrink behavior of these soils. The factors that affect the swelling potential of the expansive

soil mainly depends upon the properties of soil, environmental factors and stress conditions.

Each year, expansive soils causes damage to houses, roads, pipelines, and other structures.

This is more than twice the damage from floods, hurricanes, tornadoes, and earthquakes

combined.

Due to the extensive swelling clay occurrences of north-eastern parts of Nicosia, this area was

selected as the study area. The selection of the area was decided due to the presence of direct

outcrops of all formations containing marls of Miocene period, known for their high to very

high swell potential. Hamitkoy is a growing development area in north-east part of Nicosia

reputable of expansive soil damages to the existing buildings and roads, yet still a popular

area for the building industry on the island. This project investigated the soil profile of a

selected locality based on data from a UNOPS funded project and evaluated the soil

parameters required for the design of foundations, including the depth of active zone, swell

pressure, and surface heave.

The structural analysis of a two story residential building was carried out by SAFE. Between

the rib mat and flexible waffle mat, the ribbed mat has been selected to be designed by the

SAFE software. The flat plate mat is the commonly used foundation type in Northern Cyprus

which is constructed directly on the ground surface. This study aims to show that this is a

wrong practice due to damages caused to buildings by the center or edge lift or settlement of

mats due to swell-shrink behavior of soils, and to emphasize that a more suitable foundation

type, ribbed mat, should be adopted by the construction industry for the structural integrity, as

well as feasibility, economy and environmental concerns.

Keywords: expansive soil, ribbed mat foundation, SAFE program, construction failure

ÖZET

Dünyadaki kurak ve yarı kurak iklim bölgelerinde oluşan geniş topraklar, inşaat mühendisliği

yapılarında ciddi sorunlara neden olmaktadır. Bu tür topraklar, suya eriştiklerinde şişer ve

kuruduklarında büzülürler. Bu topraklarda şişme-büzülme davranışını kontrol etmek için bir

çok girişim yapılmaktadır. Toprağın şişme potansiyelini etkileyen faktörler esas olarak

toprağın özelliklerine, çevresel faktörlere ve gerilme durumlarına bağlıdır. Her yıl, şişen

*1 Master Student, Cyprus International University,[email protected] 2 Asst. Prof. Dr., Cyprus International University, [email protected]

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7. Geoteknik Sempozyumu 22-23-24 Kasım 2017, İstanbul

topraklar evlere, yollara, boru hatlarına ve diğer yapılara zarar vermektedir. Bu zarar sel,

kasırga ve depremlerden kaynaklanan hasarın iki katından fazladır.

Lefkoşa'nın kuzey-doğu bölgelerinde yaygın olan şişen kil nedeniyle bu alan çalışma alanı

olarak seçilmiştir. Bu alanlar yüksek veya çok yüksek şişme potansiyeli ile bilinen miyosen

dönemi marnları içeren tüm oluşumlar düşünülerek seçildi. Hamitköy, Lefkoşa'nın kuzey-

doğu kesiminde adadaki yapı endüstrisi için popüler bir alan olarak gelişen bir kalkınma

bölgesi durumundadır. Bu çalışma UNOPS tarafından finanse edilen bir projeden elde edilen

verilere dayanılarak seçilen bir lokalitenin toprak profilini araştırmış ve aktif bölge derinliği,

şişme basıncı ve yüzey akışı da dahil olmak üzere temellerin tasarımı için gerekli toprak

parametreleri kullanılmıştır.

İki katlı bir konutun yapısal analizi SAFE programı kullanılarak gerçekleştirildi. Kaburga

hasır ve esnek radye arasında, SAFE yazılımı tarafından tasarım olarak seçilmiştir. Düz plaka

hasır, doğrudan Kuzey Kıbrıs'ta zemin yüzeyine inşa edilen yaygın olarak kullanılan temel

tipidir. Bu proje, bina-zemin etkileşimini dikkate alarak şişme-büzülme davranışlarından

kaynaklanan temel merkez veya kenarlarda oluşan bozulmaları tanımlamak ve bu

uygulamanın dezavantajlarını belirtmektir. Bu tip zeminlerde daha uygun bir temel tipi olan

kaburgalı radye temelin ekonomik ve çevresel açıdan avantajlı olduğu savunulmaktadır.

Anahtar Kelimeler:şişen zemin, kaburgalı radye temel, SAFE program, yapı yıkımı

1. INTRODUCTION

Cyprus is the biggest island in Eastern Mediterranean, and the third biggest island in the

Mediterranean Sea , and has an area of 9,251 km². It occupies a special geographic place as it

is at the junction between Eurasian , African , and Arabian plates and plays an important role

for understanding the geology of this region.

Due to its geologic evolution, a large part of the island of Cyprus is covered by clayey and

marly formations bearing bentonitic clays to montmorillonite group of clays. Cyprus may be

divided into six geological zones according to geological evolution and emplacement of its

geological units (Revised frm GSDA 1995).

Figure 1: Geological map of Cyprus (Revised from GSD, 1995)

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However , Cyprus clays can be divided into five main groups :

*Bentonitic Clays ( high swelling potential)

*Clays of Mamonia Complex (high swelling potential)

*Clays of Kythrea Group (high swelling potential)

*Clays of Nicosia Formation (high swelling potential)

*Alluvial Clays

Occurrences of swelling clays are the result of the alteration of other minerals. Chemical

weathering of materials As the montmorillonite percentage increases in the soil, the more

swelling - shrinkage ability of that soil and the more damage it can cause. This makes us face

three types of problematic soils on the island:

Expansive (active , shrinkage) soils , Evaporitic bedrocks, and Collapsible soils. Most of the

swelling clay problems occur continuously in arid and semi-arid regions due to the soil

expanding in the winter and shrinking in the summer. However, our region of concern is

Hamitkoy in the North-East of Lefkoşa region.

Figure 2: Google earth map of Hamitkoy, North-east of Lefkosa.

2. Borelog Selection and Soil Profile

In this study area “Hamitkoy ", there are 2 bore logs: SC5 and SK5 as seen in the map below.

To specify the location of each bore logs on Cyprus map, Google Earth program was used.

Google Earth is a program to view satellite imagery, maps, terrain, 3D buildings, galaxies far

in space, and the deepest depths of the ocean. However, it works with GPS coordinates (WGS

72 coordination system). So another program called PHOTOMOD GeoCalculator 4.4 was

used to convert the coordinates of bore logs from local cartesian coordinates to global GPS

(WGS 72) coordinates and then entered to the Google Earth program to obtain the following

map:

.

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Figure 3: UNOPS project borehole locations in Lefkosa (Google earth map)

One borehole bedrock.log was selected, SC5 by among these borehole logs. It can be

observed in Figures 4.4-4.5 that it consists of light brown color clay soil layer formations

ranging from 2.5 to 5.5 meters followed by greenish grey clay color formation all the way to

the bedrock.

Figure 4: Borehole logs SC-5 field data’s

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Figure 5: Core soil samples obtained from borehole SC-5

The figure above shows some borehole soil colors and how cohesive the soil is at each level

down to bedrock from borehole log SC-5.

3. Active Zone and Swelling Pressure Estimation More important factor in designing process of various structural foundations is determining

the depth of active zone. The active zone depth is the depth of the soil beneath a structure that

contributes to or has the potential to contribute to actual heave that takes place at some point

at the surface at any particular time. The active zone is time and spatially dependent

parameter.

In the UNOPS project, two piezometric borehole logs were monitored (SC5 and SK5).

However, information’s from SC5 borehole log which shows the depth of water table in

different seasons and thus gives an idea about the earth movement in the specific location of

the borehole log which is the closest of the rest to "Hamitkoy" is used. From Figure 6

approximate active zone in meters can be seen.

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SC-5 WATER LEVEL FLUCTUATION

Figure 6: Water fluctuation in SC-5 (UNOPS, 2002)

4. Design Procedure of Mild Reinforced Ribbed Mat Foundation

Soil properties such as Plastic Limit, PL, % / Liquid Limit, LL, %/Allowable soil bearing

pressure, qallow, psf/percentage of soil passing No. 200 sieve, %-#200 / Percentage of soil finer

than 2 microns, %-2µ, %/ Soil unit weight, pcf/Modulus of elasticity of the soil, Esoil, psi /

Subgrade friction coefficient, µ/ edge moisture variation distance, em, ft were used for the

analysis.

Table 1: SC2 and SC5 Laboratory Data

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Figure 7: Classification Chart

After calculating the activity ratio LL / %fc was calculated, and e γo is obtained using the

Zone-3 Chart using Fig 8. Then Suction Compression Index (γh) has been calculated by the

following formulas :

γh swell = γoeγo

(%fc / 100) and

γh shrink = γoe-γo

(%fc / 100).

After that S is calculated by S = -20.29+0.1555(LL)-0.117(PI)+0.0684(%-#200).

Unsaturated Diffusion Coefficient (α) will be calculated next by :

αswell = 0.0029-0.000162(S)-0.122(γh swell)

αshrink = 0.0029-0.000162(S)-0.0122(γh shrinkl)

Figure 8: Zone-3

Fabric Factor (Ff) is obtained by Table 2.

Modified Unsaturated Diffusion Coefficient (α ) is calculated by the following equations :

α swell = αswell (Ff) α shirnk = αshink (Ff)

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Table 2: Fabric Factor

Thornthwaite Moisture Index (Im) is calculated by using Fig 9 em is obtained based on Im

for center and edge lift.

Figure 9: Thornwaite Moisture Index

Figure 10: Thornthwaite Moisture Index (Im) – α’, Weighted Average of Modified

Unsaturated Diffusion Coefficient

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4.RESULTS

Based on the structural analysis of a two story residential structure and the soil data

obtained from a borelog obtained from swelling clays project funded by UNOPS

completed in 2002, the ribbed mat design provided a much more suitable solution as it

employs soil-structure interaction throughout the proposed procedure. This is the main

advantage over a flat plate mat, which is the commonly used foundation type, as it will be

subjected to seasonal effects by swelling-shrinking behavior of the prevailing soils. The

thickness of the mat is based on structural loads only, and the only geotechnical

consideration is the allowable bearing capacity, as settlement is not the issue under light

loads on expansive soils.

REFERENCES

[1] Braja M. Das (1983) , “Principles of Foundation Engineering ”.

[2] D.Nelson , D.J. Miller (1992), “Expansive Soils Problems and Practice in Foundation and

Pavement Engineering – J ”.

[3] Sam L. Altshuler, P.E., M.E., Board Certified Environmental Engineer. February, 2007.

“Lowering the Carbon Footprint When Using WAFFLEMAT System for Concrete Slab

Foundations.”

[4] Joseph E.Bowels (1988) , “Foundation analysis and design”, Third edition.

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