Laboratory Consolidation Test The laboratory consolidation test is conducted with an apparatus known as consolidometer consisting essentially of loading frame and consolidation cell in which the specimen is kept.
Porous stones are put on the top and bottom end of specimen. Consolidometer Apparatus Consolidometer shown in fig a The consolidation cell is two type 1. Floating or free cell, in which both top and bottom porous stones are free to move and shown in fig b. Fixed ring cell, in which bottom stone is fixed shown on fig b. The vertical compression of the specimen is measured by mean a dial gauge. Dial gauge reading taken after application of each increment of load at following total elapsed times :.
The consolidation test data are then used to determine the following. Void ratio and coefficient of volume change. Coefficient of consolidation. Calculation Of Void Ratio Void ratio at the end of each pressure increment can be calculated by two methods.
The change in void method is used only for fully saturated specimens, while the height of solids method can use for both saturated as well as unsaturated samples. Calculation Of Void Ratio a Height of solids method : The void ratio is calculated from the following relation:. H change in the specimen thickness under any pressure increment. Basic Definitions 1. Coefficient of Compressibility. The coefficient of compressibility av is defend as decrees in void ratio increase in effective stress.
It is equal to the slop of e curve at point under consideration. Basic Definitions 2. Coefficient of Volume Change The coefficient of volume change is defined as the volumetric strain per unit increase in effective stress. Basic Definitions 3. Compression index The compression index CC is equal to the slope of the linear portion of the void ratio versus logarithm of effective stress Thus. Compression index The compression index is extremely useful for determination of the settlement in field.
Terzaghi and Peck gave following empirical relationship for clays of low to medium sensitivity. The value of CC normally varies between. Basic Definitions 4. Expansion Index. The expansion index or swelling index Ce is the slope of the e-log plot obtained during unloading BEC in fig. Determination of Pre-consolidation Pressure 1. Determine the point E on the curve where the curvature is maximum.
Draw the tangent EF to the curve at E. Draw a horizontal line EG at E. Produce back the straight line portion CD of the curve and determine the point of intersection P at bisector. Draw the vertical PJ which cuts the log -axis at J. The point J indicates the pre consolidation pressure c. Casagrande construction for determining Pre-consolidation stress.
Computation of magnitude of final settlement. Determination of the time-rate of settlement. For the computation of final settlement, the coefficient of volume change or the compression index is required. Which is based on the plot between void ratio and the effective stress.
For the time-rate the Terzaghi theory is used. Computation of Immediate Settlement 1. Immediate Settlement of Cohesive soils Si. The value of ES is determined from the stress-strain curve obtained from triaxial consolidation- untrained test, with consolidation pressure equal to the effective pressure at depth from which the sample was taken.
Computation of Immediate Settlement 2. Computation of Immediate Settlement The value of the strain-influence factor Z varies linearly for a square or circular foundation. Computation of Immediate Settlement b Static cone penetration method. In this method ,the sand layer is divided into small layers. The settlement of each layer is estimated using the following equation, De Beer and Martens, Computation of Immediate settlement c Plate Load Test The settlement of footing can be estimated from the settlement of plate in the plate load test.
Where Bf is width of foundation in meters and Bp is width of plate also in meter, Sf is settlement of foundation. Standard penetration test can be used for the determination of the settlement on cohesion less soils. IS Part 1 gives a chart for calculation of settlement per unit pressure as function of the width of the footing and the standard penetration number. The settlement is in meter per unit pressure. Computation of Primary Settlement As the compression index of a normally consolidated soils is constant.
For such soils, it is more convenient to use compression index. Computation of Secondary Settlement It is difficult to evaluate secondary settlement.
It is analogues to the creep in steel when is overstressed and in the plastic state. The rate of secondary settlement is given by the secondary compression index ct , as defined below. Where e is change in void ratio between t1 and t2 It is equal to the slope of the final liner part of the curve drawn between the void ratio and log time show in fig.
Rate of Consolidation The Process of consolidation occur at the different rate in different parts of the specimen. As the drainage facilities are better at the upper and lower boundaries the progress of consolidation is rapid than the middle of the specimen. Theory of consolidation, has been developed for computing the rate of consolidation by Terzaghi based on assumption that the laws of hydraulics govern the decrees of pore water pressure. Tv is a dimensionless number called the time factor, Cv is known as coefficient of consolidation, d is drainage path, t is time corresponding to U, is coefficient of volume compressibility, is coefficient of permeability.
Accuracy of Foundation Settlement Prediction 1. The soil deposits are seldom isotropic and linearly elastic. The deposits are generally non-homogeneous. It is not possible to estimate the increase in stresses caused by loads. For estimation of immediate settlement, it is not possible to locate exactly the drainage faces.
For computation of immediate settlement, it is not possible to estimate the correct value of the modulus of elasticity. The rigidity of the foundation is usually neglected and the pressure distribution is assumed to be uniform.
It is difficult to obtain undisturbed samples of cohesion less soils. The semiempirical method do not give accurate results. References Rajan, Gopal and Rao, A. Arora, KR. Punmia, B. C Punmia. Open navigation menu. Close suggestions Search Search. User Settings. Skip carousel. Carousel Previous. Carousel Next. What is Scribd? Explore Ebooks. Bestsellers Editors' Picks All Ebooks. Explore Audiobooks.
Bestsellers Editors' Picks All audiobooks. Explore Magazines. Editors' Picks All magazines. Explore Podcasts All podcasts. Difficulty Beginner Intermediate Advanced. Explore Documents. Consolidation of Soil. Uploaded by Harpreet Singh. Document Information click to expand document information Description: geotechnical engineering.
Did you find this document useful? Is this content inappropriate? Report this Document. Description: geotechnical engineering. Flag for inappropriate content. The rate of secondary consolidation is very slow when compared with primary consolidation.
Reduction in volume is due to expulsion of water from the voids. The volume change depends on the rate at which water is expelled and it is a function of permeability. Thickness of the saturated deposit We are concerned with Measurement of volume change The time duration required for the volume change 6. The space between springs is filled with water the pistons are perforated to allow for passage of water.
Piezometers are inserted at the centers of different compartment to measure the pressure head due to excess pore water pressure. Terzaghi has correlated the spring mass compression process with the consolidation of saturated clay subjected to external load.
The springs and the surrounding water represent the saturated soil. The springs represent the soil skeleton networks of soil grains and water in the vessels represents the water in the voids. In this arrangement the compression is one dimensional and flow will be in the vertical direction.
In the lower compartment the volume of water remains constant since the flow is in upward direction. Due to flow of water in the upper segment there will be reduction in volume due to this springs get compressed and they being to carry a portion of the applied load. This signifies a reduction in excess hydrostatic pressure or pore water pressure and increase in effective stress in the upper segments.
Whereas there will be no dissipation of excess hydrostatic pressure in lower compartments. The isochrones indicate that with passage of time there is flow of water from the lower compartments leading to gradual dissipation of excess hydrostatic pressure.
Loose sand compresses more than dense sand. Loose and dense sand deposits tend towards the same void ratio Compression of fine grained soil Clay Fig. The magnitude of compression is also large. This sample is subjected to 1D consolidation in the lad under various pressure increments. Then a plot of void ratio versus effective stress is made as shown in Fig 7 and 8. Beyond this is the virgin compression line portion BC also called the normal compression line and the sample undergoes large compression.
BC — Virgin compression curve also called normal consolidation line 2. Sample undergoes Permanent strain due to irreversible soil structure and there is a small elastic recovery. The deformation recovered is due to elastic rebound 5.
When the sample is reloaded-reloading curve lies above the rebound curve and makes an hysteresis loop between expansion and reloading curves. The reloaded soils shows less compression.
The change in volume of soil is equal to volume of pore water expelled. The volume of pore water expelled is equal to change in volume of voids. Since compression is in one direction the change in volume is equal to change in height. Therefore Cv is not constant 2. The flow is assumed to be 1D but in reality flow is three dimensional 3.
The application of external load is assumed to produce excess pore water pressure over the entire soil stratum but in some cases the excess pore water pressure does not develop over the entire clay stratum.
Uniform excess pore water pressure with depth 1. Single Drainage Drainage at top and bottom impervious 2. Double Drainage Drainage at top and bottom Single Drainage drainage at top and bottom impervious Fig. The depth z 2. The excess pore water pressure u 3. The time t after application of loading The above variables are expressed in the form of the following non-dimensional terms as Tv The graphical solution of the above equation is as shown below Fig.
The compression index is used to determine primary consolidation settlement of normally consolidated soils. A high value of Cc indicates high compressibility and higher consolidation settlement. If the soil is compressed along BC and unloaded along CD and then reloaded along DC the effective stress close to point C will be the new preconsolidation pressure. Effect of Stress History Based on the stress history preconsolidation pressure soils are classified as 1.
Normally Consolidated Soils 2. Over Consolidated Soils 3. Under Consolidated Soils 6. These soils are susceptible to larger deformation and cause distress in buildings built on these deposits. Conduct an oedometer test on the undisturbed soil sample obtained from the field. Draw a tangent at the point of maximum curvature Point A Step 5. Draw a horizontal line AC Step 6. Draw the bisector line AD between the tangent and horizontal line Step 7.
Logarithm of time method 2. Square root of time method 3. Plot the dial reading of compression for a given pressure increment versus time to log scale as shown in fig. Let x be the difference in dial reading between P and Q. Project the straight line portion of primary and secondary consolidation to intersect at point T.
The time corresponding to point V is t50 i. Plot the dial reading and square root of time i.
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