CHAPTER ONE
1.1 INTRODUCTION
The Marine Clays,1 also referred to as Marumsco soils, have been a source of difficulties for property owners for many years. They occur in parts of Virginia, Washington D. C., and Maryland. In Fairfax County, the Marine Clays occur on steep hilly land in the southeast of the county, mainly in widespread areas east of Interstates 95 and 395. The clays can cause problems in several ways, ranging from major structural damage in houses to poor drainage in yards. Fortunately, most types of structural damage can be repaired. In addition, homeowners can eliminate some causes of potential problems in Marine Clay.
Marine clay is very sensitive to change the stress system, moisture content and system chemistry of the pore fluid. Geotechnical engineers feel a necessity to improve the behavior of these deposits using anyone of the available ground improvement techniques for the construction of foundations.
The marine clays are highly compressible soft clays and also it exhibits moderate swelling when comes in contact with moisture. This behaviour is due to the presence of clay minerals with expanding lattice structure. The marine clay is very hard when it is dry but loses its strength on wetting. The marine clay got cracks as shown in the plate 1 on drying and in the worst cases the width of the cracks is almost 250 mm to 500 mm and travel down to 1.00m beneath the ground level. The consistency limits of the marine clay are as follows:
Liquid limit is 89%, plastic limit is 47%, shrinkage limit is 16% and specific gravity is 2.62 (Basak et.al:2009). Some types of damage are much more common than others. The various types of damage can occur singly or in combination with each other. House problems usually develop slowly at first and then become more serious as years go by. For example, minor cracks in a basement wall of a ten-year old house may progressively worsen over the next five years, eventually resulting in wall failure. Damage has been noted to occur in less than five years to greater than 30 years after construction.
Although the types of damage caused by Marine Clay can be grouped into a few categories, no two houses are alike. Building and foundation design, age of the house, soil characteristics, yard grading, vegetative plantings, and homeowner maintenance history make each diagnosis of the problem unique. Correctly identifying the problems usually requires a thorough investigation of the soil and structural conditions by a qualified professional. Repairs should be based on a careful analysis of the soil conditions under and around the building.
As some homeowners have discovered, an incomplete or improper repair will likely result in recurring problems in the future. For this reason, consult a qualified engineer experienced in soil and foundation investigations for design and inspection of difficult repairs.
1.2 PROBLEM OF THE STUDY
The problem of marine clay is that it shrinks and swells during natural changes in soil moisture. The clays shrink during dry periods of the year and swell during wet periods. Slight changes in moisture content are sufficient to cause detrimental shrinking and swelling. The pressures that the clays exert upon swelling can far exceed the strength of basement walls and even the weight of a house's wall and footing. Uneven settling can cause cracking and warping of the foundation and frame.
Problems tend to be more common in some Marine Clay areas than others. The Marine Clays contain a variable mixture of fine-textured soils – clay and silt – with frequent discontinuous sand layers. The most troublesome areas occur on steeper slopes and where the content of clay and silt is much higher than other soil types.
Many houses located within Marine Clay may never have problems, while others will undergo foundation distress at some time in the life of the structure. Surface drainage is often a problem since water percolates very slowly through the clays and does not drain well from level yard areas.
1.3 USES OF MARINE CLAY SOIL
Marine clay soils are used all around the world for many different uses, such as ceramics, building material, including adobe. Clay layers in soils which can be used as an impermeable layer are very important for dumps or chemical spills as they have a very high absorption capacity for heavy metals.
1.4 AIM OF THE STUDY
The aim of this work is to discuss the application and the impact of marine clay soils on foundation design
1.5 OBJECTIVE OF THE STUDY
The objective of this work is to:
- Recognize potential geotechnical problems of the marine clay soil
- Obtain sufficient data to finalize soil selection for foundation design
- Determine parameters needed for preliminary design of foundation
- Estimates of critical soil parameters
- Potential sources of construction materials
1.6 PURPOSE OF THE STUDY
- Collect and analyze specific soil data to determine geotechnical parameters needed for final design of foundation.
- Specify which soil parameters are needed and at which locations
1.7 FACTOR IN DESIGNING A FOUNDATION
The primary factor in designing a foundation is an understanding of the ground conditions. Data is required Concerning:
- The density of the soil below foundation level,
- The cohesion (stickiness) of the soil,
- The effective pressure of the overburden soil at the level of the foundation (the weight of soil that lies above the level at which the foundation will be placed),
- The angle of shearing resistance (this relates to the ease with which one soil particle will 'slide' over another soil particle),
- The depth of the proposed foundation,
- The width of the proposed foundation footing,
- The shape of the proposed foundation.
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