foundation system of burj al arab

Behavior of Expansive Soil under Raft Foundation

Expansive soil shows volume changes upon change in moisture content in soil and movement is found to dominate in vertical direction. Plastic cohesive soils like CH are most susceptible to suffer volume change which is enriched with montmorillonite.

Lean clays like CL can also result structure damage when moisture content in soil is sufficiently large. These types of soils swell during wet season and shrink during dry seasons. 


These soils have an inherent fissure system that influences extent of volume changes within a season or a time frame depending on construction period or part of it. Numerous fissure permits run-off water to penetrate deep into soil strata. Desiccated zones increase depth of active volume change of soil, Za whereas fewer fissures prohibits flow of water which limits depth of penetration; thus depth of volume change within a season is also reduced. To analyze foundation performance due to soil movement, it is characterized depending on deformation mode

• Center lift

• Edge lift


Center lift:


In this type of movement mat is deformed upward at center relative to edge. This is due to increase in water content of soil and heave at center relative to foundation perimeter or reduction in moisture content and subsequent shrinkage near perimeter as compared to center. 


When foundation is constructed on ground surface, it prevents evaporation from ground surface; thus eliminate moisture loss due to transpiration from existing vegetation. The moisture content of soil, therefore, is increased, especially at center of mat foundation.

At the central portion of soil under mat, environmental variations have least effect. Soil outside or near perimeter have tendency to dry out during dry season which results settlement at edge relative to center.

Selection of Drilling Slurry for Pile Foundation

Drilling slurry selection is very important in a drilling project which is principally depends on ground condition. Different types of slurries may be required for a project to drill pile through diverse ground formation. Thus it is clear that different types of slurries may be required to drill pile on one contract because of variation in ground condition within a borehole or borehole to borehole.Some factors that dominate on making decision on selection of types of drilling slurry or slurries are listed below:

• Economy 


• Groundwater and ground contamination with disposal


• Air temperature


• Ground temperature 


• Type of formation through which drilling is conducting.

Behavior of drilling slurry is also affected by ground condition. The factors that are prominent:

• Alkalinity or acidity of ground water

• Grain size distribution of the soil

• Flow velocity of groundwater through ground formation

• Cohesion and cementation of soil

• Presence of clay structures or rock within ground formation


Considering these conditions, physical properties of drilling slurries can be modified or adjusted or suitable chemical additive can be added. As most of the drilling slurries cannot be disposed to ground or water without modification, they are reused frequently. These may reduce expense or difficulty of disposal to environment.

Drilling slurry, occasionally, is required for another pile followed by the completion of previous pile. It was reported that drilling slurry was successfully reused for another project. But such reuse of slurries should be part of plan submitted to engineer and as approved by him.

Contract specification usually doesn’t prohibit drilling slurry to be reused provided that it must confirm requirements of physical properties as described in specifications. If any contractor proposed to reuse particular or all drilling slurries used in projects conducted under different contract, they should provide test results for physical properties of drilling slurry (described in contract specification) as desired by the engineer before used in drilled hole.

It should keep in mind that drilling slurries degrade with time which is generally evaluated in months. Any drilling slurries must be cleaned properly before it is reused. In case of synthetic slurries, cleaning is performed by allowing adequate time to settle out contaminants. For mineral slurries, cleaning involve application of desanding units and use of chemical additives.

California department of transportation permits three types of drilling slurries, these are:

• Water
• Mineral slurries
• Synthetic slurries

Water:


Water is selected as drilling slurry only when ground formation encountered while drilling is good in terms of enough strong not to be subjected to significant deformation during drilling.

Mineral drilling slurries:


They are processed from different clay formations. The most popular clay formations are:

• Bentonite
• Attapulgite

Mineral slurries are suitable for almost all types of ground formations. The best performance is found in sands (cohesionless soils) and open graded gavels. While using in drilling cohesive formation like clays, caution should be taken, as these clays may become a part of mineral slurry which may rapidly alter physical properties of mineral slurry. Moreover, these clay particles may reduce filtration which may prevent formation of filter cakes.


Synthetic slurry:


Polymer slurry can be used to stabilize bore during excavation which is considered alternative to mineral slurries. Unlike mineral slurry, bentonite, they clog the pores exist in soil resulting a barrier. Polymer having different molecules are joined together and have capability to penetrate in silty or sandy soils up to considered depth

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