Compressibility and One Dimensional Consolidation of Soil

Transcription

1 Compressibility and One Dimensional Consolidation of Soil ONE DIMENSIONAL SETTLEMENT BEHAVIOUR Initial condition U= u0 u0 is hydrostatic pore water pressure Effective stress, σo' Undrained loading U = uo+δu Δu: excess pore water pres Δσ= Δu, Δσ =0 σ = σo' After consolidation (Drained condition) Δu =0, Δσ = Δσ σ = σo + Δ σ

2 Compressibility and One Dimensional Consolidation of Soil Consolidation is the gradual reduction in volume of a fully saturated soil of low permeability due to drainage of some of the pore water, the process continuing until the excess pore water pressure set up by an increase in total stress has completely dissipated. When thin soil layers covering a large area are loaded vertically, the compression can be assumed to be one dimensional (zero lateral strain). Embankment loading on a layered soil Embankment Soil layer 1 x Soil layer 2 z Rock

3 Embankment loading on a layered soil If the embankment is wide compared to the depth of the soil layer then the restraining effect of adjacent columns of soil prevents horizontal movement. Thus there is vertical strain but no horizontal strain Before loading After loading The accuracy of this assumption (one dimensional) depends on the relative dimensions of the loaded area and thickness of the soil layer. If the area is relatively large and the thickness of the soil layer relatively small then the assumption of one dimensional conditions (zero lateral strain) will be reasonable.

4 The one dimensional consolidation testing procedure was first suggested by Terzaghi. In the laboratory, the soil is compressed in a special device called an oedometer or consolidometer. The characteristics of a soil during one dimensional consolidation or swelling can be determined by means of the oedometer test. The schematic diagram of a consolidometer is shown below: Loading cap Load Displacement measuring device Cell water Soil sample Porous disks Schematic diagram of an oedometer

5 Measurement of soil properties The oedometer apparatus Load Displacement measuring device Cell Loading cap water Soil sample Porous disks

6 The following points may be noted: The soil is loaded under conditions of no lateral strain (expansion), as the soil fits tightly into a relatively rigid ring. Uncontrolled drainage is provided at the top and bottom of the specimen by porous discs (two way drainage). A vertical load is applied to the specimen and a record of the settlement versus time is made. The load is left on until all settlement ceases (usually 24 hours although this depends on the soil type, impermeable clays may take longer). The load is then increased. The sequence of loading is as such: P/P= 1 (the vertical stresses might be e.g. 20, 40, 80, 160 kpa). When the maximum load is reached, the soil is unloaded in several increments. If desired reloading can be carried out. At each step time settlement records are made. The relationships between voids ratio and effective stress, and settlement and time are found from the test. The methods by which these are obtained will be explained in the laboratory classes. One dimensional soil behaviour Voids ratio, e 2 A The general shape of the plot of void ratio of the sample against pressure is given below. AB corresponds to initial iti loading of the soil. BC corresponds to an unloading of the soil. CD corresponds to a reloading of the soil. Upon reloading the soil beyond B the soil continues along the path that it would have followed if loaded from A to D. 1 C Recompression curve Unloading Curve (expansion) B D Log 10 (effective stress, σ )

7 Terminology Preconsolidation stress (pressure) The maximum effective vertical stress that has acted on the clay in the past is referred to as preconsolidation pressure σ p. Over - consolidated A soil is called over-consolidated (OC) if: Current Effective Stress < Preconsolidation Stress Terminology Preconsolidation stress (pressure) The maximum effective stress which has been applied to an element of soil Over - consolidated (whose present effective overburden pressure, σ 0 is less than that which the soil has experienced in the past). A soil is called over-consolidated (OC) if: < Current Effective Stress Preconsolidation Stress Normally consolidated A soil is called normally consolidated (NC) if: Current Effective Stress = Preconsolidation Stress

8 If you define σ 0 as the present effective vertical stress acting on the clay, When σ 0 = σ p Normally consolidated clay When σ 0 < σ p Over consolidated clay The OCR is defined as the ratio of the preconsolidation pressure to the current effective overburden pressure. Overconsolidation ratio, OCR = σ p/σ 0 If OCR = 1 Normally consolidated clay OCR > 1 Over consolidated clay The shape of the curves are related to the stress history of the clay Precompression line Virgin compression line The plot shows that a clay in the overconsolidated state will be much less compressible than the same clay in a normally consolidated state.

9 Estimating the preconsolidation pressure e D A C F E B σ pc log (σ ) Steps in the construction are given below: Determine the point of maximum curvature A. Draw a tangent to the curve at A, i.e. line AB. Draw a horizontal line at A, i.e. line AC. Draw the extension of the straight line (normally consolidated) portion of the curve DE. Where the line DE cuts the bisector (AF) of angle CAB, is the preconsolidation stress.