COMPACTION CONSOLIDATION II

ECG426 – SOIL MECHANICS

COMPACTION AND CONSOLIDATION OF SOILS 2/2

LEARNING OUTCOMES Week 6 : Coverage : Field density measurements, field compaction and specification. Consolidation – theory and principles.

Learning outcomes: At the end of this lecture/week the students would be able to: i. ii.

Discuss the different field density methods Compare specifications for field compaction

iii.

Explain the theory and principles of consolidation

COMPACTION & CONSOLIDATION OF SOILS FIELD DENSITY

CONSOLIDATION THEORY FIELD COMPACTION

COMPACTION OF SOILS

OUTLINE of PRESENTATION 3.4

Field density measurements

3.5

Field Compaction and Specification

3.6

Consolidation – Introduction & definition

3.7

Theory of Consolidation

COMPACTION OF SOIL Field density measurements

COMPACTION OF SOILS Field density methods FIELD DENSITY MEASUREMENTS …..cont..

Field density can be determined using the following tests : 3.4.1 The Core Cutter Method

3.4.2 The Sand Replacement Method 3.4.3 The Immersion in Water Method 3.4.4 The Water Displacement Method 3.4.5 The Nuclear Method

FIELD DENSITY MEASUREMENT Core Cutter method

FIELD DENSITY MEASUREMENT Core Cutter Methodmethod

3.4.1

THE CORE CUTTER METHOD

FIELD DENSITY MEASUREMENT Sand Replacement Method

FIELD DENSITY MEASUREMENT Sand Replacement Method

Sand Replacement Apparatus

FIELD DENSITY MEASUREMENT Immersion in Water Method

3.4.3

THE IMMERSION IN WATER METHOD

Suitable for cohesive @ stabilised soils  where an irregular intact lump of soil has been obtained.  



The lump sample is weighed (Ms) & after coating with paraffin is weighed again (Mw).

The coated sample is then suspended in water & submerged mass recorded (MG).

Mass of parafin wax, Mp = Mw – Ms If the density of the paraffin wax is p & density of water is w ; then

Volume of specimen,Vs  and

Bulk density, b  Ms Vs

Mw - MG - MP w P

FIELD DENSITY MEASUREMENT Water Displacement Method

FIELD DENSITY MEASUREMENT The Nuclear Method

3.4.5

THE NUCLEAR METHOD

The nuclear apparatus is a versatile device to rapidly obtain the unit weight & water content of the soil nondestructively. Soil particles cause radiation to scatter to a detector tube and the amount of scatter is counted. The scatter count rate is inversely proportional to the unit weight of the soil. If water is present in the soil, the hydrogen in water scatters the neutrons and amount of scatter is proportional to the water content. The calibration is used to determine the unit weight and water content.

Consist of a small shielded radiator source & detection.

COMPACTION OF SOILS Field Compaction and Specification 3.5

FIELD COMPACTION AND SPECIFICATION

The amount of compaction achieved on site will depend upon:(a) The amount of compactive effort 

The type and mass of plant used



The number of passes



The layer thickness per pass

(b) The field water content – may need to be modified in dry condition (c) The type of soil

FIELD COMPACTION Suitability of Compaction Plant

FIELD COMPACTION Specification & Quality control

FIELD COMPACTION Specification & Quality control (B)

Method Specifications

A procedure is laid down, giving a specific type of plant; its mass, the frequency of vibration (if applicable).

The thickness of each layer & the number of passes to be made on each layer.

Suitable for soils wetter than the moisture content.

optimum

Consolidation 3.6 Introduction & Definition ď ś When a saturated layer is subjected to a stress increase, the pore water pressure is suddenly increased. ď ś In sandy soils that are highly permeable, the drainage caused by the increase in the pore water pressure is completed immediately. Pore water drainage is accompanied by a reduction in the volume of the soil mass, which results in settlement. Because of rapid drainage of the pore water in sandy soils, immediate settlement and consolidation occur simultaneously.

Consolidation …..Introduction & Definition –

cont….

 When a saturated compressible clay layer is subjected to a stress increase, the elastic settlement occurs immediately. Because the hydraulic conductivity of clay is significantly smaller than that of sand, the excess pore water pressure generated by loading gradually dissipates over a long period. Thus the associated volume change (i.e., the consolidation) in the clay may continue long after the immediate settlement. The settlement caused by consolidation in clay may be several times greater that the immediate settlement.

CONSOLIDATION SETTLEMENT Terzaghi’s Theory of Consolidation Regarding the theory of consolidation, Terzaghi made the following assumptions :

The soil is fully saturated and homogeneous.



Both the water and the soil particles are incompressible.



Darcy’s law of water flow applies.



The change in volume is one dimensional in the direction of the applied stress.



The coefficient of permeability and volume compressibility remain constant.



The change in volume corresponds to the change in void ratio and ∆e/∆σ remains constant.

Consolidation Theory The rate of consolidation for a stratum of clay is affected by the following factors:

the permeability of the soil



the extent or thickness of the compressible soil and the distance that pore water pressure of the soil must travel to escape from the zone where pressures due to the foundation loading exist



the in-place void ratio of the compressible soil



the ratio of new loading to the original loading



the compression properties of the soil

CONSOLIDATION SETTLEMENT Piston & Spring Analogy

When a saturated soil is loaded, there will be an increase in the pore water pressure.

Subsequently it will be compressed because of the squeezing out of water from the voids

COMPRESSIBILITY OF SOIL Instantaneous increase in total stress, 

1



’ = 0

’ u CLAY (low permeability)

  ' u

3

2



Principle of effective stress

u =  Immediate (t = 0) stresses change.

Drainage does not take effect.

Water is incompressible

 ’ > 0 u < 

4

 ’ =  u = 0

Slow drainage cause excess pore water pressure to decrease gradually.

Stress carried by the soil solids (effective stress) increases.

Primary consolidation is the volume change (settlement) due to the expulsion of water that occupies the void spaces. This a gradual process of transferring excess pore water pressureto effective stress (Phase 1 to 4). This phase ends when the change in total stress equals the change in effective stress.

COMPRESSIBILITY OF SOIL Secondary consolidation : Volume change (settlement) due to plastic adjustment of soil fabrics under CONSTANT effective stress which occurs after complete dissipation of pore water pressure, u=0 (i.e. beyond phase 4).

Cont’d..

4 

’=

u=0

5



’=

u=0

6  ’=

7  ’=

u=0

u=0

Diagrams 5, 6 & 7 : Soil fabrics start to orientate and move to a denser state under CONSTANT effective stress and cause settlement.

Notes: This slide was reproduced from Md. Noor’s Lecture Note.

GRADUAL TRANSFER OF TOTAL STRESS FROM PORE WATER PRESSURE TO EFFECTIVE STRESS IN A CLAY LAYER DRAINED AT TOP AND BOTTOM • The increasein porewater pressure produceshydraulic gradient in the soil causing the pore water to flow.

• As porewater is dischargedthe solid particles consolidateand begin to carry part of the new load.

• This is a gradual transfer of the total stress from pore waterto soil solids.

 GWT

SAND H CLAY

Depth

SAND

COMPRESSIBILITY OF SOIL

What is then the fundamental difference in the process between : 

COMPACTION, and



CONSOLIDATION explain it in your own words …..and discuss it in the context of piston and spring analogy !!!

COMPRESSIBILITY OF SOIL

The difference between compaction and consolidation.

Compaction Soil particles are forced to rearrange into a closer state of packing with a reduction in volume and the expulsion of air.

Time-independent process.

Consolidation The reduction in volume due to the expulsion of porewater from the voids in the soil.

The rate of consolidation depends on the coefficient of permeability, k, of the soil.

Thence, it is a time dependent process.

Both processes are aimed to increase the soil strength and to avoid excessive settlement after construction.

Consolidation Read about an engineering feat with respect to the topic in Newsweek, June 18, 2001, entitled â&#x20AC;&#x153;Righting One Wrong Towerâ&#x20AC;?

How it all started ……

….. Over 800 years ago..

Consolidation for hundreds of yearsâ&#x20AC;Śan on going process ---a time dependent process

How a brave professor fixed (sort of) the world’s most crooked jewel (1996). At least for now… …extruding out the soft soil..to counter the uneven consolidation on both sides.

Righting One Wrong Tower……

Righting One Wrong Tower……

Righting One Wrong Tower……

Righting One Wrong Tower……

Righting One Wrong Tower……

Righting One Wrong Tower……

Righting One Wrong Tower……

Righting One Wrong Tower……