Compaction. Densification of soil by removal of air using mechanical energy

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1 Compaction Densification of soil by removal of air using mechanical energy

2 Compaction vs. Consolidation CONSOLIDATION: REDUCTION OF SOIL VOLUME UNDER STRESS/LOADING. STRESS OR LOADING CAUSES 1) DEFORMATION OF SOIL PARTICLE 2) RELOCATION OF SOIL PARTICLE 3) EXPULSION OF WATER OR AIR ( IN MOST PRACTICAL CASES, ITS WATER)

3 Effect of moisture content Water added act as softening agent Soil slip onto each other and into a more dense packed position Beyond certain m%, dry unit weight is reduced because the moist/water takes up spaces of soil!!!

4 Optimum Moisture Content Unit Weight Water d Moisture content, m

5 Proctor compaction test Extension HAMMER 25 BLOWS MOULD

6 Standard & Modified Proctor Original Ilustration by Prof. Bengt B. Broms in Foundation Engineering

7 Test Concept Soil is mixed with varying amount of water then compacted. Unit wt W V m W weight of compacted soil in the mold V m volume of mold Moisture content w% is determined in lab.

8 Plotting dry density against w% Dry density is calculated as: d 1+ w% 100 Plot the various d vs. w% d w%

9 S100 and 80 curve S100% is the ZAV curve zero air void ZAV plot can be calculated using: zav w + Both ZAV and S80 can also be calculated using: w 1 G s w d S d w S80 w% 1 100% Gs Zav

10 Specs. For Field Compaction Typical requirements: 90-95% MDD Specs. normally in term of relative compaction Cr. Type of works Building platform or road Upper 150mm of sub-grade below roadway Dam Minimum C R 90% 95% 100% C R d ( ) d max lab 100%

11 Site Procedure Soil is normally compacted in layers of mm. Constant check of field density should be carried out to ensure compliance. Rate of construction in embankment work should be control to prevent build up of pore pressure Heavier roller gives better compaction.

12 Site Procedure (cont d) For embankment work, the best procedure is to compact a trial area and measure the dry density. When relative compaction is satisfactory, the number of roller passes is used for the actual embankment.

13 How to determine field density? Field density tests Sand Cone Test/Sand Replacement Method Rubber balloon test Nuclear density test Water ring test Drive cylinder test

14 Sand Replacement Method ASTM D1556 or BS 1377 W dry W Test hole filled with std. sand field 1+ w% 100 dry W dry V Compacted fill

15 Example 1: Proctor compaction test Compacted Soil + mold Mass of tray Mass of tray + wet soil Mass of tray + dry soil Mass of mold 2031 g, Volume of mold 9.44 E-4 m3 1) Compute d and w% for each data point and plot results 2) Calculate S80 and S100 using Gs ) Determine MDD and w o

16 Calculate moisture content for each data points: M M w% wet dry 100% M M w dry tray % Moisture content 100%

17 Dry density W V m / 9.44E kn/m3 Remember W mass x gravity Gravity 9.81 m/s2 W1 ( ) x kg.m/s2 or N kn w% d kn / m 3

18 Summary of data points (kn/m3) w 4.5% 7.5% 10.1% 12.9% 16.4% d (kn/m3)

19 S80 and S100 zav w w + 1 G s w S w d 1 100% G s Use d 16, 18 and 20 (i.e. within data points range) w 80% 100% %

20 S80 and S100 data points d S80 S % 24.1% % 17.3% % 11.9%

21 Chart and Report Dry Density (kn/m3) MDD curve s80 s OMC Moisture content (%)

22 Example 2: Sand Replacement test Determination of Dry Density of soil on site BS 1377 Specs. Requirement of Compaction ratio is 90% MDD of ex.#1 Initial mass of sand: kg Remaining sand after pouring: kg Mass of soil from hole: kg Moisture content w 7.0 % Density of sand: 1490 kg/m3 Volume of cone: 1.114E-3 m3 Calculate d and Compaction ratio.

23 Example 2: Solution ) ( / / ) 10 (1.114 ) 10 ( m kn w m kn V W kn g M W m V V V m V M M d hole soil soil soil cone hole cone hole hole cone wetsoil hole cone sand

24 Example 2:cont d Relative compaction What if CR < 90%? C R d ( ) d max lab 100% 100% 91.36% Ripping, mixing and re-compacting Add water OK

26 Relative density In place density compared to its laboratory max and minimum density Max compacted in laboratory Min loosely filled into a test container Then compacted and loose density including void ration can be calculated D R D R % d ( % d e e (max) max field ) max e e or d field min (min) d (min) 100% d d (max) ( field )

27 Typical values Loose Relative density (%) <35 Unit Weight (kn/m3) <14 Medium dense Dense Very dense >85 >20

28 Consolidation and Settlement The weight of any structure on the earth will result in stresses being imposed on the soils below the level of the base or foundation of that structure. The deformation that develop in the soil because of these stresses cause dimensional changes in the soil volume, with the result that the structure undergoes settlement. The extent of foundation settlement that will actually occur is related to the bearing pressure (stresses) imposed on the soils and the stress-strain properties of the soil. D.F. McCarthy

30 Normally consolidated and overconsolidated clay NORMALly consolidated the present Po is the maximum pressure that the soil was subjected since the past. OVER consolidated the present Po is LESS THAN the maximum past pressure. Preconsolidation pressure (Pc) the maximum effective past pressure Po Pc Normally consolidated Po < Pc Overconsolidated

31 Overburden pressure The effective overburden pressure (Po) at any depth is determined by accumulating the weights of all layers above that depth as follows: 1) Soil above water table multiply the total unit wt by the thickness of each respective soil layer above the level. 2) Soils below the water table reduce the total unit wt by the wt of water (1000 kg/m3) i.e. use the effective unit wt.

32 Pc 1) Locate point a at the minimum radius of the curve 2) Draw a horizontal line ab 3) Draw the line ac tangent at a 4) Draw the line ad bisector of angle bac 5) Project straight line portion of the curve to intersect ad the intersection is the Pc [Das (1998) after Cassagrande] Chart from IKRAM (2002)

33 Oedometer Test Particular Sample Measurements: (change of) Height Applied Load General Derived Relationship: Void Ratio Applied Stress h

CE 240 Soil Mechanics & Foundations Lecture 3.3. Soil Compaction (Das, Ch. 5)

CE 240 Soil Mechanics & Foundations Lecture 3.3 Soil Compaction (Das, Ch. 5) Class Outlines Soil compaction introduction Standard Proctor Compaction Test Effect of Compaction Energy Modified Proctor Compaction