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Geotechnical Engineering And Physical Properties Engineering Essay

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Burj Al Arab is the world's tallest iconic and most luxurious hotel. This building has received numerous recognitions around the world, and is mainly known by its design which resembles the shape of a sail boats mast.

This 321 meter high building was built on a man-made island only 280 meters from the coast, giving all visitors a 360o view of the bay. Construction began in 1993. Engineers created a surface with a layer of rocks, which is circled with a honey-comb pattern which protects the surface and foundation from erosion. The building contains more than 70 000 m2 of concrete and 9 000 tons of steel. It took only two years to construct the building and three years to construct the foundation on beach sand, making this a geotechnical wonder.8

Introduction

Geotechnical Engineering entails to obtain information of the physical properties of the soil and rock on a proposed site known as site exploration. This information combined with the mechanics of soil can assess the risks presented by the site conditions that must be concluded in the design process of foundations, earthworks and retaining walls.

Burj Al Arab has one of the rarest and most interesting foundations, earthworks and retaining wall phenomena's. This include building an island 280 meter off coast as a foundation for this 321 meter high sea shore wonder.1 Throughout this report we will look at which soils are found in Dubai, as well as the properties thereof including the construction of this hotel foundation.

Constructing the Burj Al Arab hotel

The design of the Burj Al Arab has been constructed with a specific geometry which supports the sail boat like design by protecting the building from changing wind loads. The outermost wall of the building has been constructed by the use of PTFE coated fibreglass which contains air gaps at regular intervals. This double curve membrane design is able to withstand wind pressures easily.3

Additional cables have been provided onto the structure to prevent any deflection in materials. On the full height of the building expansion joints were provided on the right side of the building to ensure the building can withstand the wind load pressures as well as the horizontal stresses that may occur during construction and operation. The material that was used for this sail boat like structure is not only robust but it also protects the buildings' interior from the sun by using light defusion.3

After seventy thousand cubic meters of concrete and ninety thousand tons of steel, this great engineering wonder is noted as the heart of Dubai. This spectacular feature of the Burj Al Arabhotel, with its two hundred and two rooms, is located 280 meter off shore from the coast of Dubai and recognised as the best in the world. 2

Concept Architect: Tom Willis-WrightimagesCAHJ41BO.jpg

Construction Engineer: WS Atkins and Partners Overseas - Multidisciplinary Consultancy.

Interior designers: Khuan Chew, Design Principal of KCA International (London).

Location: Jumeirah Beach Road, Jumeirah, Dubai, United Arab Emirates.

Date: 1994-1999

Style: Postmodern

Type/Structure: Luxury 7 stars* rating hotel/resort

Size: 321m x 280m (1,053 ft)

Medium: steel, glass, cement, steel cables, piles

From: Antonino Terranova. The Great Skyscrapers of the World. Special gatefold edition, page 269-279.

Figure 1: Burj Al Arab hotel

Dubai's Soil Profile

The topography of Dubai (which lies within the Arabian Desert) is different from the southern portion of the UAE. Its landscape consists of sandy desert patterns consisting mostly out of crushed shell and coral and is clean and white, whereas gravel dominates in the southern regions of the country. 2 When looking at the soil properties of Dubai, it's weak and will most probably move outwards in the case of any construction on it. See figure 1 Dubai soil map. 3

Studies also show that Dubai's possibility of a tsunami is minimal, due to the Persian Gulf water that is not deep enough to trigger a tsunami. Thus Dubai is classified as a stable zone, whereas the nearest seismic fault line is 120 km from the UAE, making it unlikely for Dubai to be hit by a seismic impact.2

http://www.gisdevelopment.net/proceedings/mapmiddleeast/2006/natural%20resource%20management/images/mme06138_3.jpg

Figure 2: Dubai soil map.

Foundation Problems

The moment when Dubai laid focus on the development of this world wonder, they knew it would be an engineering challenge. Many elements must be taken in consideration in geotechnical engineering to build the world's 15th tallest building on seabed, where its properties are known as a collapsible soil due to a lack of silt and clay.

The collapse phenomenon can be defined as a soil which can withstand somewhat large stresses, with little settlement at low in situ moisture content which will show signs of a decrease in volume and associated settlement with no increase of load if the moisture content rises. Therefore the change in volume goes hand in hand with the change in the soil structure.

It is thus evident that a number of conditions need to be met before collapsing begins: 6

The soil must have a collapsible fabric in its structure. This is where the specified soil has a high void ratio and yet has relatively high shear strength at low moisture content due to a coating (Colloidal) around each grain.

Partial saturation is essential. This is where collapse settlement will not occur in soils which are located under the water table.

Increase of moisture content. This could be seen as the cause for the collapse to take place. With the increase of moisture the colloidal coating loses its strength and thus forces the grains to a denser state with reduction in void ratio.

Subjected to an imposed load greater than their overburden pressure before collapse can take place. This is only applied to certain collapsible soils.

The typical problem associated with a collapsible soil to a building is that although it is dependent on the increase of the moisture content, collapse can take place years after construction has taken place. 6 Large magnitude settlements can occur beneath lightly loaded structures as well as collapse settlement is regularly localised due to defects in foundation, drainpipe leakage and where ponding occur during rainfall.

The engineering properties which most affect the cost of a construction are strengthening compressibility. Both can be enhanced by reducing the voids in the soil. Water must be displaced from the saturated soils in order to reduce the void volume. This can take months if the permeability of the type of soil present is low.

The following engineering solutions to the mitigation of the collapse problem are listed below: 6

Avoidance by stopping the triggering mechanism (increase in moisture). This can be made possible by ensuring that water does not penetrate the collapsing soil horizons.

Design for collapse. This could be possible in certain scenarios to design a structure which could withstand the predicted collapse settlement.

Chemical stabilisation. This is to make use of a stabilising agent which could reduce the settlement.

Piled or pier foundation. This is used only when the soil comes from a transported origin which means that the bedrock is covered with a shallow layer, making it possible to rather build on piers or piles.

Removal and compaction. This could be done by removing the collapsible soil to a certain depth and replace it through compacting the removed soil in layers.

In situ densification by surface rolling. Surface rolling can be done by making use of an impact or vibrating roller for compaction.

Soil Properties

Beach sand is one of many soils that have a collapsible grain structure, where its surface contains large quantities of calcium carbonate which in more defined terms are remains of microscopic plants and animals that thrive on nutrients in the water surface, where it ultimately settles to the floor.

The strength and the behaviour of this soil are thus dependent on the calcareous particles which it contains. These grain particles' is well rounded due to it being rotated and shaped by the waves and is poorly graded (i.e. having a narrow particle distribution). This contributes to the high void ratio, meaning that the soil is very loose and can be seen as not a good bonding material. 6

Table 1: Transported soil and possible engineering problem.

Soil Type

Transported Soil Type

Agency of transportation

Source

Problem to anticipate

Beach Sand

Littoral deposit

Waves

Mixed

Collapsible grain structure

Tests to be performed on beach sand

A large amount of data can be generated from soil, but it can all be wasted if the most important step of sampling is not carried out properly. Thus, in order for an analysis to be of significance to a proposed project, it should represent the bulk material of the site. Additionally, soil samples must be taken in abundance and at random, to ensure that the overall characteristics of the soil are effectively represented. See table 2 for properties beach sand.

The following tests were used in classifying beach sand (collapsing soil structure):3

Particle size distribution. This test is performed to measure the particle size distribution of the soil sample by passing it through a set of sieves. This is in order to produce a grading curve for the soil, which is used to find out its classification. The solid particles in a soil can have different shapes and sizes, and these characteristics thus have a significant effect on its engineering behavior. By making use of this test one can clearly note whether the soil is well or poorly graded. As for beach sand it is known to be a collapsible soil due to it having a poorly graded grain structure and affected by an increase in moisture.

Atterburg limits. This test makes use of three separate tests namely Liquid limit test, Plastic limit test and Shrinkage limit test. This test is used to determine a relationship between the soils consistency and its moisture content. If the soil has low moisture content, it would aim to break before deformation takes place, whereas if the moisture content is too high, the soil will deform more easily. This test is of great importance due to it having an impact on settlement underneath a proposed structure. The test can be used to distinguish between the presence of silts and clays. This is important as silt has much less cohesion than clay.

Dry density. This can easily be determined in a laboratory by measuring its physical dimensions and by weighing them. The dry density of a collapsible soil lies between 900-1600 kg/m3.

Oedometer test. When a structure is build on a soil it produces settlement due to compression within the soil profile, which depends on the soil's properties such as self-weight and also the type of load the soil is experiencing. This test makes use of a series of loads in order to measure the corresponding settlement of the soil. By knowing the soil's stress and strain properties will allow the prediction in settlement and swelling of the soil.

Collapse Potential test. This test is used to determine the collapse parameters in order to design accordingly. The CP (Collapse Parameters) is given in percentage, to determine the level of severity.

Triaxial test. This test is similar to the unconfined compression test, except that the sample is surrounded by a waterproof membrane and installed in a pressure chamber (cell). This test is thus performed to estimate the stress and strain parameters of the specific soil.

Permeability. This test is used to determine the ease of which water can flow through a soil profile, which is important for geotechnical engineers in projects.

Table 2: Soil properties of silt sand.9

Soil Properties

Beach Sand

Specific Gravity

2.64-2.66

Bulk Density (Mg/ M3)

1.82-2.15

Dry Density (Mg/ m3)

1.45-1.95

Void ratio

0.35-0.85

Liquid limit (%)

24-35

Plastic limit (%)

14-25

Effective cohesion (kPa)

75

Angle of friction (deg)

32-36

Construction of Burj Al Arab Foundation

Constructing a building on sea, an artificial island is needed to design and build the foundations. As many elements need to be taken into account to build an off shore structure, it is therefore important to ensure the protection of the foundation. This can only be done by evaluating all apposed loads to a structure. Seafloor stability regards to the bearing capacity and the sliding resistance thereof must be evaluated for static and combined static, operational and environmental (Like horizontal, vertical loading and overturning moments of the environment which have a return period of up to 15 seconds) loads. Structures with more or less a 150 meter depth could experience horizontal loads of 15-35% of the vertical loading, whereas the overturning moment can be ranging from 100 to 500 million kN/m.8 The change in vertical load during a storm can range from 10 - 40 % of the static vertical load. This means that the foundation needs to be strong to be able to obtain these loadings. Luckily these loadings were much less when Burj Al Arab was constructed, due to it only being 7 meters in depth.8IslandConstruction1.jpg

Figure 3: Piling of the Burj Al Arab hotel

The first step in constructing the island was to place 230 concrete piles (see Figure 3), each one 40 meter in length, which was drilled into the sea bed. The foundation is therefore held in place by the friction of the sand and silts along the length of the piles, rather than the conventional bedrock. The surface was then made by using large rocks that were put together in a specific concrete pattern (honey-comb pattern) which serves as a shield to minimise erosion to the foundation.

Making the platform on which the building would be constructed, tube files and sheet files were drilled deep into the sea to support various boundary rocks. Once this was completed the sea water was displaced to fill the inside with concrete slabs as seen on table 3. IslandConstruction2.jpg

The structure was then surrounded with a temporary concrete structure to protect the island and the base of the structure, which was filled with a concrete plug slab. Lastly the concrete walls was made where the main basement floor of the building was build (See Figure 4).3

Figure 4: Burj Al Arab Excavated Basement

Table 3: Foundation of the Burj Al Arab

Conclusion

The Burj Al Arab being a geotechnical wonder is due to its size as well as its ability to withstand the environment and the impacts thereof. This building is only carried by a sandy soil which contains broken shells. Its ability under shear strength and pressure is very low, especially since it is located 280 meters of shore. Thus the building stands only on piles which are held into place by just the friction between the soil and the piles, making this project one of the most remarkable foundation types in history. The design of this foundation on this type of soil is breaking barriers in the building industry, making Dubai a leading country in development.

Due to South Africa not being a first world country it is impossible for us to be compared to a city which encourages ultimate engineering in structural, hydraulic and geotechnical engineering. Therefore we cannot compare the infrastructures of their country with ours. South Africa could always strive to be a first world country by focusing on infrastructure as well as the inequality of societies. This would encourage tourism and affect the economy positively.


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