Geotechnical Engineering III CGE31-2A LECTUER NOTE CONSOLIDATION.pptx

This is a detailed lecture note on clay soil CONSOLIDATION

1. Geotechnical Engineering III COURSE CODE: CGE31/2A TOPIC : Consolidation Part A

2. LEARNING OUTCOMES After the completion of this topic the student should be able to find the relationship between ground movement and the stability of related structures by having a good understanding of the following: Types of ground movement and causes of settlement Immediate settlement (Elastic settlement/immediate settlement ) Consolidation settlement (Time dependence settlement ) Rate of consolidation The oedometer test Taylor’s method Casagrande’s method Compressibility of granular soils

3. INTRODUCTION When a compressive force is applied to soil mass, a decrease in its volume occur. The decrease in the volume of the soil mass under stress is called compression and the soil property in respect to this decrease of volume is called compressibility. When the voids are filled with air alone, the soil compression occurs rapidly because air is compressible.

4. INTRODUCTION CONTINUES In the saturated soil where voids are filled with incompressible water, the decrease in volume can take place only when water is expelled out of the voids. Such compression result from a long term static load and the escape of pore water is known as consolidation. According to Terzaghi "every process involving a decrease in the water content of a saturated soil without replacement of the water by air is called a process of consolidation“. The soil settlement is a downward movement due to the volumetric change in soil mass generally due to a compressive load.

5. Ground movements can arise from two major sources: I. Movements due to ground instability, caused, for example, by slope instability, geological voids, or subsidence due to the collapse of old mine works; and movements due to a changing stress-state, leading to volume changes within the soil. Dewatering, tree problems or loading of foundations can change stress- states. Type of Ground Movement and Causes of Settlement

6. II. Ground movement due to physical processes: 1- distortion settlement, , is the settlement that result from the lateral movements of the soil in response to changes in stress Distortion settlements primarily occur when the load is confine to a small area, such as a structural foundation, or near the edge of large loaded areas such as embankments.   Type of Ground Movement and Causes of Settlement Continues

7. 2- Consolidation settlement, , it occurs when a soil is subjected to an increase in stress and the individual particles respond by rearranging into a tighter packing. The change of total volume is due to the decrease of the volume of voids, . 3-Secondary compression settlement, , is primarily due to particle reorientation, creep and decomposition of organic materials. This particles reorientation causes a reduction in the volume of the voids, like consolidation settlement .   Type Of Ground Movement And Causes Of Settlement Continues

8. 4-The total settlement at ground surface, , = distortion settlement + Consolidation settlement + Secondary compression settlement Ground movement due to changes in vertical effective stress: 1- stress changes due to placement of a fill , when a fill is place on the ground, in the underlying soil increases due to the weight of the fill. When the point is beneath the central area of the fill, the we compute , by simply adding another layer to the   TYPE OF GROUND MOVEMENT AND CAUSES OF SETTLEMENT

9. 2- stress changes due to placement of an external load , External loads, such as structural foundations, also produce increases in . In this case , where is the induce vertical stress   Type of Ground Movement and Causes of Settlement Continues

10. 3- stress changes due to changes in the ground water table elevation , When groundwater water table changes from one elevation to some lower elevation, the pore water pressure, u, in the underlying soils decrease and the vertical effective stress, , increases.   Type Of Ground Movement And Causes Of Settlement Continues

11. Question 1: , A 1.52m fill thick is to be placed on a site underlain by medium clay, as shown in figure beside . compute and at point A.   TYPE OF GROUND MOVEMENT AND CAUSES OF SETTLEMENT Solution : =  

12. Immediate settlement(elastic settlement) occurs immediately after the load is applied. This is due to distortion (change in shape) at constant volume. There is negligible flow of water in less pervious soils. This is determined by elastic theory. : net foundation pressure, : poisson’s ratio , : young modulus of the soil : Influence factor : the width of foundation   IMMEDIATE SETTLEMENT Influence factor (I f ) table

13. Consolidation settlement It occurs due to expulsion of pore water from the voids of a saturated soil. In case of saturated fine grained soils, the deformation is due to squeezing of water from the pores leading to rearrangement of soil particles. : compression index, : initial void ratio , : Thickness of soil layer : Vertical stress due to apply load : initial effective overburden pressure before applying load   CONSOLIDATION SETTLEMENT

14. CONSOLIDATION SETTLEMENT Question 2: , Calculate the immediate settlement ,the consolidation settlement , and the total settlement of the raft foundation with area on figure below . The depth of the foundation is 2.5 metres. The pore water pressure is u=10kN/ , the unit weight of the clay , B: the width of the foundation, and  

15. Solution to question2 Solution: Calculate the immediate settlement( ) Calculate the consolidation settlement at A ( ) Calculate the consolidation settlement  

16. Deformation in soils often occur much more slowly, especially in unsaturated clays. Many years, or even decades, may be required for the full settlement to occur in a soil, geotechnical engineers often need to evaluate both magnitude and the rate of consolidation settlement. They are many theories that are used to predict the rate of settlement and they are as follows: Terzaghi’s Theory of consolidation, Consolidation settlement versus time computations, The coefficient of consolidation ( )   RATE OF CONSOLIDATION

17. Terzaghi’s Theory of consolidation: this theory of consolidation is based on certain on certain simplifying assumptions regarding the compressible stratum: 1-the soil is homogeneous 2- the soil is saturated( S=100%) 3- the settlement is entirely due to the change of volume ratio 4- Darcy law is valid 5- the applied load causes an instantaneous increase of vertical total stress . 6- immediately after loading, the excess pore water pressure 7-the coefficient of consolidation ,   RATE OF CONSOLIDATION CONTINUES

18. = coefficient of consolidation = hydraulic conductivity = hydraulic conductivity = Compression index = unit weight of water = vertical effective stress   RATE OF CONSOLIDATION CONTINUES

19. Terzaghi’s Theory of consolidation: The dimensionless measure of time is called the time factor and define as = maximun drainage distance (for single drainage, = thickness of the compressible stratum; for double drainage, =half thickness of the compressible stratum). = coefficient of consolidation , = time since application of the load.   RATE OF CONSOLIDATION CONTINUES

20. T he figure beside shows a soil profile shown. The time since application of the load t=10 years, after placement of the fi ll. Calculate the maximum drainage distance ,calculate the dimensionless measure of time and the vertical stress due to the fill   Question 3

21. Solution: the hydraulic conductivities of the SM and ML strata are much greater than that of CH strata, so the double drainage condition exists.   Solution to question3