INTRODUCTION & PHASE SYSTEM Dr. Professor of Civil Engineering S. J. College of Engineering, Mysore 1.1 Geotechnical Engineering Why? 1. We are unable to buil castles in air (yet)! 2. Almost every structure is either built on or built in or built using soil or rock.. Mechanics of Soils an Rocks is the basis of Geotechnical Engineering 4. Geotechnical problems involve: Stability Deformations Water flow Soil is an un-cemente aggregate of mineral grains an ecaye organic matter (soli particles) with liqui an gas in the empty space between the soli particles forme by weathering of rocks in the top surface of earth crust. Fig. 1 represents a portion of soil mass comprising of soli particles an voi space. Voi space is mae up of liqui (water) an/or gas (air). Fig. 1 : Soil mass, a conglomeration of soli particles an voi space
Soil is an important construction material that is 1. Olest. 2. Cheap or available free of cost many a times.. Most complex, yet having interesting properties. 4. Moifie to suit the requirements, many a times. Soil is use 1. to manufacture bricks, Tiles or earthenware. 2. as founation material.. to construct ams an embankments. 4. to fill hollow zones behin retaining walls, low lying areas etc. 1.2 Why soil is complex? The following properties of soil make it perhaps the most complex construction material. 1. Porous 2. Polyphasic. Permeable 4. Particulate 5. Heterogeneous 6. Anisotropic 7. Non-Linear 8. Pressure Level Depenent 9. Strain Level Depenent 10. Strain Rate Depenent 11. Temperature Depenent 12. Unergoes volume change in shear
Yet, soil possesses some Interesting properties that relate with human beings, namely, 1. Colorful 2. Sensitive. Possesses Memory 4. Changes its properties with time 1. What is Geotechnical Engineering? It is an integration of Physics, Earth Science, Soli Mechanics, Geology an Hyrogeology. Soil Mechanics provies the theoretical basis for escribing the mechanical behaviour of earth materials. Geotechnical Engineering involves application of theory of soil mechanics to a variety of fiel problems. For most other engineering isciplines, the material properties are well-efine or can be controlle. But, in Geotechnical Engineering, material properties are highly variable an ifficult to measure with a reasonable egree of accuracy. Geotechnical Engineering is among the younger branches of Civil Engineering. Yet, it has evolve over centuries. 1. Geotechnical Engineering is probably one of the most challenging engineering isciplines. 2. For a geotechnical engineer, no two ays at work are going to be similar.. Geotechnical engineering expertise is require in a vast variety of isciplines that inclues the oil an offshore inustry. 4. Being a relatively new iscipline, there is ample scope for innovation. 5. For a geotechnical engineer, achieving job satisfaction is never a problem. 6. Material properties must be measure for each new construction site. 7. Remember that geotechnical engineers eal with natural materials an there can be no quality control.
8. Groun consists of innumerable variety of particle sizes an minerals. 9. To make matter worse, engineering properties of earth materials are strongly influence by their past geological history that is normally unknown. Climatic conitions also influence these properties. 1.4 Sub-branches in Geotechnical Engineering The following are the sub-branches of Geotechnical Engineering. 1. Founation Engineering 2. Deep Excavation. Tunneling 4. Earth Pressure an Retaining Structures 5. Earth embankments 6. Stability of Slopes 7. Environmental Geotechniques 8. Earthquake Geotechnical Engineering 9. Groun Improvement technique 10. Rock Mechanics 11. Engineering geology
Fig. 2 represents the family of Geotechnical engineering. Founation Engg Earthquake Geotechnical Engg. Deep Excavations Environment al Geotechnics Geotechnical Engg. Tunneling Embankment s Slopes Retaining Structures Fig. 2 : Family of Geotechnical Engineering 1.5 Distinctions between Fine an Coarse Graine Soils Soil can be broaly classifie in to two types, namely Fine graine soil an coarse graine soil base on the size, shape an behaviour. Table 1 provies the important istinctions. Table 1 : Distinctions between Fine graine soil an coarse graine soil Fine Graine Soil Size of particle is less than 75 microns Coarse Graine Soil Size of particle is mores than 75 microns Silt & Clay belong to this group Properties are influence by surface area Attraction an boning between particles enable strength Mostly plate-like San, Gravel, Cobble, Bouler etc. belong to this group Properties are influence by gravity Dense packing, particle to particle contact enable strength Mostly roun, sub-roun, angular Voi ratio & water content can be very high Voi ratio & water content can not be very high Possess consistency (liqui, plastic & shrinkage) limits Consistency (liqui, plastic & shrinkage) limits are absent
1.6 Soil formation, a geologic Cycle Soil is forme from rock ue to erosion an weathering action. Igneous rock is the basic rock forme from the crystallization of molten magma. This rock is forme either insie the earth or on the surface. These rocks unergo metamorphism uner high temperature an pressure to form Metamorphic rocks. Both Igneous an metamorphic rocks are converte in to seimentary rocks ue to transportation to ifferent locations by the agencies such as win, water etc. Finally, near the surface millions of years of erosion an weathering converts rocks in to soil. 1.7 Soil Mass, a three phase system Fig. : Geological Cycle of Soil Soil mass comprises of soli particles an voi space. The voi space is fille with water an/or air. Hence, soil mass comprises of some volume of soli (soil particles), some volume of liqui (mostly soil water) an some volume of gas (air). Hence, the total volume of soil mass can be treate as a three phase system.
A typical soil mass Iealisation as three phase system Soil mass iealize as three phase system
W Weight V Volume s Soil grains w Water a Air v Vois Basic terminologies in a three phase system of soil mass Conversion from three to two phase system Fig. 4 : Soil mass as three phase system
1.8 Basic Definitions The following are the basic efinitions of soil. 1. Water Content (ω) 2. Voi Ratio (e). Porosity (n) 4. Degree of Saturation (S) 5. Air content (A c ) 6. Percentage air vois (n a ) 7. Bulk Density ( b ) 8. Dry ensity ( ) 9. Density of soil solis ( s ) 10. Saturate ensity ( sat ) 11. Density of water ( ω ) 12. Submerge ensity ( sub ) 1. Specific Gravity of Soil Solis (G) 14. Specific Gravity of Soil Mass (G m ) 15. Relative ensity (D r ) Each of the above efinition is efine with soil represente as three phase iagram. 1.8.1 Water Content (ω) 1. It is efine as ratio of weight of water to weight of solis. 2. It is also calle Moisture Content.. It has no unit. It is expresse in percentage or ecimals (for calculation purpose). 4. It inicates the amount of water present in the vois in comparison with weight of solis. 5. In ry soil, water content ω 0.
6. Clayey soil may possess very large water content leaing to unfavourable situation. 7. Water content of soil mass changes with season, being close to zero in summer an maximum uring rainy season. 8. It represents the amount of water present in soil mass. In ry soil, water content ω 0 9. Higher the water content, greater will be the vulnerability, especially in clayey soil. 1.8.2 Voi Ratio (e) 1. It is efine as the ratio of volume of vois to volume of solis 2. It has no unit. It is normally expresse in ecimals.. It inicates the amount of vois present in a soil mass in comparison with the amount of solis. 4. Normally, voi ratio of clayey soil will be large. 5. The more the voi ratio, more loose will be the soil mass an hence, less strong an less stiff. 6. It is not possible to etermine voi ratio in the laboratory. Hence, it is compute from other properties. 1.8. Porosity (n) 1. It is efine as the ratio of volume of vois to total volume of soil mass. 2. It has no unit. It is expresse in ecimals or percentage.. Its value ranges from 0 to 100 %(0 < n < 1).
4. Similar to voi ratio, it inicates the amount of vois in comparison with the total volume of soil mass. 5. In some countries, it is more familiar than voi ratio. But either can be use interchangeably in calculation. 6. Like voi ratio, porosity is compute an can not be irectly etermine in the laboratory. 1.8.4 Degree of Saturation (S) 1. It is efine as the ratio of volume of water to volume of vois. 2. It has no unit. It is usually expresse in percentage.. Its value ranges from 0 to 100 % (0 < S < 100 %) 4. It represents the amount of water present in the voi space of soil mass. 5. In ry soil, S 0 an in fully saturate soil S 100 %. Hence, uring summer S is close to zero, while uring rainy season, S is close to 100 %. In partially saturate soil, S lies between zero to 100 %. 6. It is compute an can not be irectly etermine in the laboratory. 1.8.5 Air content (A c ) 1. It is efine as the ratio of volume of air to volume of vois. 2. It has no unit. It is usually expresse in percentage.. Its value ranges from 0 to 100 % (0 < A c < 100 %). 4. It represents the amount of air present in the voi space of soil mass. 5. In ry soil, A c is 100 % an in fully saturate soil A c is 0 %. In partially saturate soil A c lies between 0 an 100 %.
6. S + A c 1 7. It is compute an can not be irectly etermine in the laboratory. 1.8.6 Percentage air vois (n a ) 1. It is efine as the ratio of volume of air to total volume of soil mass. 2. It has no unit. It is expresse in percentage.. Its value ranges from zero to 100 %(0 < n a < 100 %). 4. It represents the amount of air present in the total volume of soil mass. 5. Always n a < A c. 6. It is compute an can not be irectly etermine in the laboratory. 1.8.7 Bulk Density ( b ) 1. It is efine as the ratio of total weight to total volume of soil mass. 2. In SI units, it is expresse as kn/m.. Its value normally ranges from 12 to 24 kn/m. 4. It inclues the weights of air, water an solis as a function of total volume of soil mass. It changes with season, being maximum uring rainy season an minimum in summer. 5. Bulk ensity of soil mass can be etermine experimentally. It is therefore use to compute other properties such as ry ensity an voi ratio. 1.8.8 Dry ensity ( )
1. It is efine as the ratio of weight of soil solis to total volume of soil mass. 2. In SI units, it is expresse as kn/m.. Dry ensity will always be less than or equal to bulk ensity of soil mass. 4. Dry ensity is inepenent of season. Hence, it is use in many esign calculations such as safe bearing capacity of soil. 5. Knowing water content an bulk ensity, ry ensity can be compute. 1.8.9 Density of soil solis ( s ) 1. It is efine as the ratio of weight of soil solis to volume of soil solis. 2. In SI units, it is expresse as kn/m.. It is always greater than ry ensity of soil. 4. It can not be etermine experimentally. Hence, it is compute from other parameters. It is use to calculate other properties such as specific gravity of soil solis. 1.8.10 Saturate ensity ( sat ) 1. It is efine as the ratio of total weight to total volume of soil mass when the soil is fully saturate. Hence, it is the bulk ensity of soil mass when S 1. 2. In SI units, it is expresse as kn/m. 1.8.11 Density of water ( ω ) 1. It is efine as the ratio of weight of water to volume of water. 2. In SI units, it is expresse in kn/m an can be taken as 9.8 kn/m.
. It is use in computation of other quantities. 1.8.12 Submerge ensity ( sub ) 1. It is efine as the net weight of weight per volume of soil mass in water. 2. In SI units, it is expresse as kn/m.. It is equal to saturate ensity minus ensity of water. 4. sub sat ω 5. It is also calle buoyant ensity. 6. In saturate soil, water exerts upwar pressure on soil. Net weight of soil particles acting ownwar will be actual weight of soil minus weight of water. 1.8.1 Specific Gravity of Soil Solis (G) 1. It is efine as the weight of soil solis to weight of equal volume of water. 2. Hence, it is the ratio of ensity of soil solis to ensity of water.. It has no units an is expresse in ecimals. 4. Normally, G of most soils varies from 2.6 to 2.75. Organic soils may have G up to 2. 5. G is etermine in the laboratory an is use to compute other parameters such as voi ratio. 6. Many a times, specific gravity means G
1.8.14 Specific Gravity of Soil Mass (G m ) 1. It is efine as the weight of soil mass to weight of equal volume of water. 2. It is also calle Apparent Specific Gravity.. It has no units an is expresse in ecimals. 4. Its magnitue is always smaller than that of G. 5. It is less commonly use in calculations. 1.8.15 Relative ensity (D r ) 1. It is also calle Density Inex. 2. It has no unit. It is expresse in percentage.. D r ranges from 0 to 100 %. 4. It is applicable for coarse graine soil such as san an gravel. 5. It inicates whether the insitu ensity of soil is close to loosest or ensest state. 6. D r e e max max e e min 7. In terms of ry ensity, relative ensity is given as follows. D r max max min min 8. When D r 1, soil in its ensest state an when D r 0, soil is in its loosest state. Table 2 : Influence of Relative ensity on Soil State
Relative Density (%) State of Soil 0 to 20 Very Loose 20 to 40 Loose 40 to 60 Meium ense 60 to 80 Dense 80 to 100 Very Dense 1.9 Problems an Solutions Problem 1 A natural soil mass has a bulk ensity of 18 kn/m an water content of 8 %. Calculate the amount of water require per cubic meter of soil to raise the water content to 18 %. What will be the egree of saturation at this water content? Assume voi ratio to be constant an take G 2.7. (July 2006) 8 Marks Data b 18 kn/m ω 8 % G 2.7 ω 9.8 kn/m (assume) b 16.67kN / m 1+ ω G e ω 1 0.587 W s V
Hence, for 1 m of soil mass, W s 16.67 kn If ω 8 %, weight of water 0.08W s 1. kn If ω 18 %, weight of water 0.18W s kn Hence, amount of water require per cu.m of soil 1. 1.67 kn 167 lt. ωg Se S 0.828 82.8% Problem 2 How many cu.m of soil can be forme with voi ratio of 0.5 from 100 m of soil having voi ratio of 0.7? (Jan 2006) 5 Marks Data e 1 0.7 V v1 0.7V s V 1 100 m V v1 + V s 1.7 V s V s 58.82 m e 2 0.5 V v2 0.5V s 29.41 m V 2 V v2 + V s 88.2 m Problem A ry soil has a voi ratio of 0.65 an specific gravity is 2.8. Fin its unit weight. Water is ae to the sample so that its egree of saturation is 55 % without any change in voi ratio. Determine the water content an unit weight. The sample is then submerge in water. Determine the unit weight when the egree of saturation is 90 % an 100 %. (Jan 2008) 10 Marks Data e 0.65 G 2.8 ω 9.8 kn/m (assume)
G ω 16.6kN / m 1+ e When S 55 %, ω 12.77 % ( 1+ ω) 18.75kN / m b When S 90 %, ω 20.89 % When S 100 %, ω 2.21 % Problem 4 In an earth am uner construction, the bulk unit weight is 16.5 kn/m at water content 11 %. If the water content has to be increase to 15 %, compute the quantity of water to be ae per cu.m of soil. Assume no change in voi ratio. Determine the egree of saturation at this water content. Take G 2.7. (Jan 2009) 10 Marks. Problem 5 Data b 16.5 kn/m ω 11 % G 2.7 ω 9.8 kn/m (assume) b 14.86kN / m 1+ ω W 14.86 kn per unit volume W w 1.6 kn per unit volume G e ω ωg S e 1 0.78 51.92% An unisturbe specimen of clay was teste in a laboratory an the following results were obtaine. Weight 2.1 N, Oven ry weight 1.75 N. Specific Gravity
of soil solis 2.7. What was the total volume of original unisturbe specimen assuming that the specimen was 50 % saturate? (Moel QP) 8 Marks Data W 2.1 N W s 1.75 N W w 0.5 N G 2.7 S 0.5 ω 9.8 kn/m (assume) W ω ω W s ωg e S 20% 1.08 ω G 1+ e W s V 12.72kN / m 1.75X10 V V 0.176 X10 - m 12.72 Problem 6 The maximum an minimum ry unit weights of san etermine in the laboratory are 21 kn/m an 16 kn/m respectively. If the relative ensity of san is 60 %, etermine the in-situ porosity of the san eposit. Take G 2.65. (June 2008) 6 Marks Data D r 60 % 0.6 max 21 kn/m min 16 kn/m 18.67 kn/m
Problem 7 D r G e ω e n 1+ e max 1 0.78 0.44 max min min For a soil in its natural state, voi ratio, water content an specific gravity are respectively 0.8, 24 % an 2.68. Determine bulk ensity, ry ensity an egree of saturation. If the soil is completely saturate by aing water, what woul be its water content an saturate ensity. Data e0.8 ω 0.24 G 2.68 ω 9.8 kn/m (assume) G ω 1+ e b 14.59kN / m ( 1+ ω) 18.09kN / m ωg S e 80.4% Problem 8 If S 1, ω 29.85 % ( 1+ ω) 18.95 kn / m sat In its natural conition, a soil sample has a mass of 22.9 N an a volume of 1.15 X 10 - m. After being completely rie in the oven sample weighs 20.5 N. Fin bulk ensity, water content, voi ratio, porosity, egree of saturation, air content, ry ensity an percentage air vois. Data W 22.9 N
V 1.15 X 10 - m W s 20.5 N G 2.7 ω 9.8 kn/m (assume) W 22.9 b V 1.15X10 ω 12.5 % 17.7 kn/m G e ω 1 0.495 e n 0.1 1+ e ωg S 68.5 % e 19.91 kn / m A c 1 S 1.65 % (1 n a ) G 1+ ωg n a 0.105 ω Problem 9 A moist sample of soil has a weight of 6. N an a volume of 00000 mm at a water content of 11 %. Taking G 2.68, etermine e, S an n a. Also etermine water content at which soil gets fully saturate. What will be the unit weight at saturation? Data W 6. N V X10 5 mm G 2.68
ω 11% w 9.8 kn/m W V b 21.1 kn / m b 19 kn / m 1+ ω G e ω 1 0.8 ωg S e 77.6% If S 1, then ω 14.18% G e ( 1+ ω) 21.69kN / m sat 1.9 Distinction between Mass Density an Unit weight or Weight Density In geotechnical engineering, it is common to use weight ensity or unit weight than mass ensity. It shoul be note that weight ensity epens on acceleration ue to gravity (g) an hence it is place epenent. However, g oes not change consierably from place to place within the engineering limits. Commonly, symbol ρ is use to represent mass ensity an symbol is use to represent weight ensity. Further, ρ * g. Table : Common mass ensities an Unit weights in Geotechnical Engineering Mass Density (kg/m ) Unit weight (kn/m )
Bulk ρ b b Dry ρ Soil Solis ρ s s Water ρ ω ω Saturate ρ sat sat Submerge ρ sub sub ρ ω 1000 kg/m ω 9800 N/m 9.8 kn/m