Sabkha Soils: Problems And Solutions - Essay Example

Geotechnical hazards induced by Sabkha: problems and solutions Department of Civil and Environmental Engineering, Carleton The paper sought to discuss the nature of sabka soils and their impact on issues concerning to construction of different structures. The sabka soils mainly comprise of marine or coastal found close to the sea. The main objectives of the paper include creating an understanding of the behaviour of the sabka soils in terms of construction. The min findings of the paper have shown that there are numerous damages to structures such as buildings, roads as well as pavements that can be attributed to the type of sabka soils. Solutions such as soil treatment can be implemented to solve the problem. Adding cement to the soil when constructing is another viable method that can be implemented together with cut and filling methods. However, preloading and compression are viewed as practical, economic and easy to apply and research has shown that they are the best alternative solutions when constructing something on sabka soils. 1. Introduction Sabkha soils are often found in various parts of the globe particularly arid and semi arid places as illustrate in Fig. (1). Sabkha is an Arabic term that is used to describe coastal sediments recently developed and characterized by low bearing capabilities. Sabkha soils mainly contain enough quantities of clay that can enable them to swell or shrink as a result of the moisture content changes during different seasons due to various factors like ground water, rainfall as well as sea water. Sabkha soil tends to swell when it has absorbs enough moisture and it shrinks when it loses that moisture. Under certain conditions, Sabkha soils often exhibit significant swelling pressures that can result in damages to structures. Fig. (1) Geographical location of Sabkhas soils 1.1 Forms of Sabkha soils There are two main forms of Sabkha soils found in different areas namely: a) Marine or Costal Sabkha found close to the sea. Continental or Inland Sabkha soils are often found between sand dunes. Fig. (2) Different forms of Sabkhas 1.2 Objectives The objectives of this paper is; (a) To create better understanding of the behaviour of Sabkha soils. (b) To create an assessment of anisotropic behaviour of Sabkha soils to construction work. (c) To present the measures and solutions that can be implemented to mitigate and rehabilitate the damaged structures. 2. Background Damages to structures such as roads, buildings and other structures are common especially if they are constructed on Sabkha soils. Economic losses are often experienced and in some cases, life is even lost in the event of distress caused by the collapse or destruction of structures that are constructed on Sapka soils. As such, understanding the true nature of this type of soil is very important since it has a bearing on the welfare of human beings. Different elements of Sapka soils are discussed below: 2.1Nature of Sabkha Soil Sabkha soil is rich with salt deposits that result from evaporation of marine water in certain places and is often viewed as collapsible type of soil. When dry, this type of soil has good engineering qualities and is suitable for low lying settlements. Elements such as climate have a bearing on sabkha development especially in arid or semi arid areas characterised by thunderstorms. Temperatures vary from as low as 0 °C to as high as 50 °C. Humidity is often associated with wind direction as well as speed. High temperatures cause high rates of evaporation in the area involved. This often leads to salinity and increases the formation of shallow lagoons.  a) Sabkha soil comprising of cracks b) Sabkha soil with wet surface Fig. (3)(a-b) The appearance of Sabkha soils 2.2 Nature of Sabkha The most notable features of Sabkha soil include the presence of shallow and highly concentrated brines and a number of geotechnical characteristics in both the horizontal and vertical directions. These features are responsible for causing a number of engineering related problems to structures such as buildings and roads constructed on these soils. Sabkhas are also known as salt flats and they bear salt particles in arid climate sediments that cover vast coastal areas especially in the Middle Eastern and North African countries. The development of sabka soils is mainly attributed to to low wave energy that allows settlement of silt and clay particles to take place and these are then cemented by soluble .Sabkha sediments are mainly comprised of high void ratios as well as low dry densities. Thus, sabka soil is known for its high compressible material that has low resistance when it is wet which results in poor quality I terms of foundation material. There have been various techniques implemented such as stabilization as well as reinforcement to improve sabkha properties. Some of the measures include soil replacement, vibratory compaction and stabilization with lime and cement among others. Sabkhas have poorly cemented but impermeable soils due to flooding and quick evaporation and they are bordered by soft sand dunes. The high level of concentration of sea water and capillary discharge of ground water often result in formation of gypsum deposits, calcite, and aragonite. Most Sabkhas are believed to have been once minute sea inlets and close basins that formed as a result of evaporation. 3. Damages that are caused by SapkhaSoils Sapkha soils pose a number of hazards to the structures constructed on top of them. There have been considerable changes to Sapkha soils in terms of volume and surface elevations and properties according to different climatic conditions over the years especially from dry and the opposite. This characteristic of the soil cause extensive damage to structures that include buildings, roads, airports as well as walkways among other structures. Changes in water and salt levels due to human activities play an active role in modifying the environment and this contributes to the salts levels contained in Sabkha soils. The main problem related to these soils I that of swelling of soil underneath buildings or other concrete structures which result in cracking. 3.1 General Problem of Sabkha The major distinct feature of Sabkha soils is that they contain poor mechanical properties. They have low bearing resistance though highly compressible and they pose serious geotechnical problems. Crossing the vast areas covered by sabka soils is one of the major geotechnical problems often encountered by people in most cases. On top of that road construction in these areas is costly and very difficult. The changes in water level during different seasons as well as high salt content in sabka sols also pose significant problems to pavement and building construction. Fig. (4) Problems caused by Sabkha soils 3.2 Building Small buildings are also vulnerable to damages if they are constructed on sabka soils because they not able to resist differential settlement of the foundation compared to larger structures. The differential movements of the foundation not uniform movements are responsible for the structural damage to the buildings. Corrosion and cracking are the major forms of damage to structures such as columns, slabs, foundations, beams and building walls. The damage to the structures can be minor or severe depending on the magnitude of hazard posed. The expenses of repairing the damaged structures can be extensive and in some cases can even exceed the original cost of erecting the structure. The most severe damage were caused by constructing structures on dry sabka soils but were later followed by wetting of the soils. Fig. (5) The effect of Sabkha soil on buildings 3.3 Roads Variations in thickness of sabka soils often result in pavement destruction. Level of water variation and high percentage of salt also leads to destruction of the pavements. Bumpy road surfaces and a number of patches can indicate the distribution of asphalt. In extreme case, the whole road surface collapses. Thus, pavement stabilization includes mechanical, chemical as well as other physical treatments that can be implemented in order to reduce the effects of sabka soil destabilisation during construction. (a) Subsidence and tension cracks (b) Sinkhole in front of a building. Fig. (6)(a-b)Sever distresses of roads 4. Solutions to Geotechnical Problems Related to Sabkha Soils. In its condition, sabka may not be adequate to be used as a foundation material to support different types of structures and is also not capable of supporting quite a number of many engineering structures. This makes it necessary to treat the soil in order to enhance engineering properties before they are constructed.there are quite a number of soil treatment techniques that can be implemented in order to solve the problems related to sabka. Precompression or preloading have many advantages since they are practical and economical to apply. However, the only disadvantage is that these methods are time consuming. 4.1 Cut and Fill method Cut and filling method is the preferred method for Sabkha treatment for road construction due to the fact that it is easy and simple to apply in instances where cutting and filling are used as reinforcement to increase the bearing capacity of the soil. When applying this method, weak sabka soil is excavated and replaced by different stonegrading, like igneous rocks ranging from large diameters of 20 cm approximatelydown to smaller ones of about 5 cm and this depends on the nature and structure of rock to facilitate reduction of water pressure since rock fragments are incorporated in weak soil by compaction methods. The layers of sand form the sub base followed by compacted rocks where the layers of pavement will be laid on since these are coarse and capable of binding the pavement as illustrated in figure (7). Fig. (7) Schematic diagram for cut and fill method 4.2 Geosynthetics for Sabkha Geosynthetics methods to stabilize soil have been used for quite a long time. Special pressed paper has also been used to reinforce the soil over a long period of time about a thousand years. Later on, the Romans have also used organic materials such as straw mats to stabilize road structures. The other materials to reinforce the roads were invented during the past century and these include steel and polymer to reinforces and stabilize the structures in a bid to minimize natural friction caused by the nature of soil used. In actual fact, there are mainly two geosynthetic products that are used in the road foundation for Sabkha soils which include: • Geotextiles (a): a planar, permeable, polymeric nonwoven textile material • Geogrids (b): a planar, polymeric structure consisting of a regular, open network of well connected tensile components whose openings are much larger than its constituents. On top of that, there are also geogrid+geotextilecomposites which provide a 2-in-1 solution (c). a-Geotextiles b-Geogrids c- Geogrid + Geotextile Fig. (8) (a-b-c) Geosynthetics for Sabkha Fig.(9) Using Geosynthetics in unpaved roads Fig. (10) Using Geosynthetics in paved roads 4.3 Reinforced Embankments on Piles There are different systems that can be implemented in order to prevent consolidation of settlements when the natural soil is likely to consolidate. A reinforced embankment on piles is feasible and a practical solution when a foundation is needed and allowing for zero settlement during construction and during service life of the structure. The piles are arranged in-situ forms and are installed at high speed where the maximum depth can be up to 17 m. This system helps prevent any consolidation settlement because each pile is installed in such a way that it reaches the good bearing capacity layer. Fig. (10) Installing the piling system for an embankment on piles. 4.4 Adding Fine Gravel and Cement Sabkha sediments are often characterized by high void ratios and low dry densities hence upon wetting, these soils are known for possessing highly compressible material with low bearing resistance which make them to be considered as the poorest foundation material. In most cases, saline soils are often associated with quite a number of geotechnical problems as a result of the presence of digenetic salts of different structures, shapes and compositions and shallow saline ground waters. Saline soil is considered as an inferior material for construction. Due to these characteristics, some of the pavements located on saline flats have shown various forms of deterioration that range from cracking, reveling, rutting and they have also been characterised by formation of landslides near built roads. This makes these soils highly risky for construction since they are susceptible to quite a number of natural hazards. Saline soils often present high rigidity and high sheer strength in natural conditions but they change radically in the event of water action that triggers huge localized settlement in civil works. Many studies are performed about geotechnical behavior of saline soil in civil projects in different countries have indicated that saline soil possesses a high collapse potential that is attributed to dissolution of sodium chlorides, leaching of calcium ions and soil grain adjustment in different circumstances 4.5 Sabkha Preloading The preloading of sabka is often viewed as practical, economical and easy to apply and the cocept has been in existence for a long time back dating to as long as 1930. Busiman (1936) made a presentation of a study on a long duration settlement test at the First International Conference on Soil Mechanics and Foundation Engineering. Johnson (1970) also used the term pre-compression in order to improve soil density and he suggested that sand drains can be used to reduce the time of consolidation. Though the method seems to be simple and straight forward, there are quite a number of pitfalls that can lead to failure of the process (Ramli Mohamed 1992). However, it has been discovered that preloading often called precompression, has the many advantages such as being practical, economical, and easy to apply. Since the early fifties, this concept has been widely applied. Earth fills method is considered the most common method for applying the required preloads (Johnson, 1970). 4.6 Using Lime A lot of studies have shown that settlements decrease due to the increase in lime content in the Sabkha soil. Thus, adding lime or cement in the range of 2.5 to 100kg in weight helps increase soil weight strength by 20 times compared to its original weight. When all fine grained soils are mixed with water and lime, they show cation exchange reaction in short term and pozzolanic reaction in long term. Thus, the cation reaction takes place immediately after adding water to lime in the soil which results in decrease in plasticity and the exchange the structure of soil to flocculated structure. This reaction leads to change of clay structure of soil with cohesionless behaviour. On the other hand, the pozzolanic reaction causes improvement in resistance of soil that is treated with lime. 6. Conclusion The conclusion is based on the main points discussed in the paper and is as follows. a. Sabka soils are associated with problems that make construction difficult hence it is recommended to avoid any construction on them. b. Sabka soils are dangerous for use in any engineering purpose unless they are treated. c. Cut and filling method for filtration of water and reinforcement are ideal to increase the bearing capacity of Sabkha soil. d. Prealoading method is also ideal since it is practical, economic and easy to apply and many people find it the best technique. e. Adding additives to Sabkha soils can be dangerous if proper measurements are not taken. f. The use of Geosynthetics is easy and fast to conduct during construction but effective for short period. 7. References Sabtan, M. Al-Saify& A. Kazi (1995) “Moisture retention characteristics of coastal sabkhas”. F. Ahtchi-Ali, and M.B. Vitiello, (2012) “Construction Methodology of Fill Placement over Sabkha”. Hassan Obaid Abbas, (2012) “Effect of Adding Fine Gravel and Cement on Settlement of Sabkha Soil”. Hassan Obaid Abbas, (2012) “Improvement of Sabkha Soil by Using Geomesh and Addition of Polycoat”. Omar A. Al-Harbi, Ghulam Hussain, M. Mujtaba Khan (2006)“Sedimentology, Mineralogy and Geochemistry of Al-Awshaziyah Inland Sabkha, Central Saudi Arabia”. Imtiaz Ahmad, (1997) Improvement of Eastern Saudi sabkha soils for road construction. S.A. Aiban and s.m. ali, “Nonwoven Geotextile-Sabkha And –Sand Interface Friction Characteristics Using Pull-Out Tests”. Mosleh A. Al-Shamrani and Abdulmohsin W. Dhowian, “Settlement-Time Behavior of Preloaded Sabkha Sediments”. Mohamed M. El-Omla, Hamdy A. Aboulela,(2012) “Environmental and Mineralogical Studies of the Sabkhas Soil at Ismailia—Suez Roadbed, Southern of Suez Canal District, Egypt”. Mohamed MehemmedShahin, (2009) “Road Construction on Sabkha Soils”. Abdulwahab Mohammed Mubarki “Improvement of JizanSabkha Soil by Using Lime”. Jan-Maarten Elias, (2004) “Building Roads on Sabkha Soils with Geosynthetic Systems”. 8. 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