Compaction

Densification of soil by the application of mechanical energy

Compaction

Densification = increase in soil’s strength and stiffness

WHY?

Reduce settlement due to loads

Improve slope stability

Provide site uniformity

p p y

Improve foundation capacity

Provide site uniformity

Reduce hydraulic conductivity (dams)

Earth dam construction

History

R.R. Proctor (1930’s) – building dams in LA

Compactability is a function ofp y

h f h l h fthe water content of the soil at the time of compaction.

the “compactive effort” imposed (mechanical energy applied to the soil mass, [J/m3])pp , [ / ])

soil type namely the composition and gradation.

Lab: Proctor TestLab: Proctor Test“Impact test” to measure compactability

Modified Proctor Test1. Obtain soil to be tested and sieve it past the No. 4 sieve.

2 Determine the mass of the Proctor mold2. Determine the mass of the Proctor mold

3 Prepare several soil batches at different w (5% to 18%)3. Prepare several soil batches at different w (5% to 18%)

4. Divide the first batch into five portions. Compact each portion in the mold using the Modified Proctor Test Hammerthe mold using the Modified Proctor Test Hammer.

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6 Determine theM and V of the compacted soil (batch #1)

5. Weigh the mold with the compacted soil.

6. Determine the M and V of the compacted soil (batch #1).

7.  Use phase relationships to get ρ and ρd for batch 1.

VM

=1ρV1ρ

M ρ

1

11 1 wV

M sd +

==ρρ

1

8 and for batch 1 are the coordinates of one point [ ]8. ρd and w for batch 1 are the coordinates of one point: [w1, ρd1]. 

9. Get ρd and w for all batches following the procedure described for batch #1. 

10. Plot the points (one point per batch) and draw (by hand) the compaction curve.

ρd

Compact 1st batch and get   p gρd and w.

Compact 2nd batch and get dρd and w.

Do same for 3rd, 4th …

Sketch a curve

w

Compaction Curve ‐ LABρd

ρρd max

maximum dry density in the labdensity in the lab

w

ρd is for a specific compactive effort and method: IT DOES NOT

wopt optimum water content

ρdmax is for a specific compactive effort and method: IT DOES NOTnecessarily reflect the maximum ρd that can be obtained in the field.

sd 1=

ρρ

sGw 1

+

ρd ZAV ; S = 1

ww

Relative compaction

As engineers we

1. relay the compaction curve

and

2. prescribe a relative compaction p p

to the contractor that will compact the soil in the fieldcompact the soil in the field.

Relative compaction

Relative compaction:

drC ρ

=maxd

r ρ

Ex: if Cr = 95%, and ρdmax = 1760 kg/m3, then ρd = (1760)(0.95) = 1672 kg/m3

Typically, we prescribe Cr > 90%. for critical projects (for example, airport pavements) Cr can be as high as 100%

ExampleA soil was compacted in the lab via Modified Proctor. The maximum dry mass density was 1700 kg/m3 and the optimum water content was 10%. The project in question calls for a relative compaction of 95%.

You are charged to determine whether or not the contractor has compacted the soil appropriately. Thus, you go to the site and sample the soil at three locations; the sampling involves measurement of water content w and total mass density ρ.

In location 1, w = 10% and ρ = 1780 kg/m3

In location 2, w = 9% and ρ = 1820 kg/m3

In location 3, w = 10% and ρ = 1750 kg/m3

SOLUTION

Determine the relative compaction at each location, using

d =ρρ d

rC ρ=

wd +1ρ

maxdr ρ

Our analysis indicates that the relative compaction achieved in the third location p(location #3) is lower than 95%.

Therefore, we must urge the contractor to re-compact the soil in that general location.