Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Proctor Tests

[Subject Prev][Subject Next][Thread Prev][Thread Next]
On 27 Jan 2005, at 7:05, THunt(--nospam--at) wrote:

> Gail,
> I remember looking into this a long time ago when we called for modified 
> proctors and received standard proctors from the lab.  As I remember, the 
> standard proctor and the modified proctor are very similar laboratory 
> tests with the standard proctor using more energy (a larger or heavier 
> hammer?).  Both can be used however the desired or accepted percent 
> compaction will be different depending on the test method.  I do not 
> believe there is a direct correlation but if I remember right a 90% 
> standard proctor is roughly about the same as a 95% modified proctor. With 
> this in mind, I seem to also remember that the standard proctor correlates 
> better with field compaction when using very large rollers whereas the 
> modified proctor correlates better when using smaller rollers, plate 
> compactors, and powder-puffs.  As already suggested, best to check with a 
> real geotech.
> Thomas Hunt, S.E.
> ABS Consulting

The Modified Proctor (ASTM D1557) uses a 10 lbs hammer that drops
18 inches while the Standard Proctor (ASTM D-698) uses a 5.5 lbs 
hammer that drops 12 inches.  The Modified Proctor uses 5 lifts to fill 
the soil mold while the Standard Proctor uses 3 lifts.  In a 4 inch mold 
the number of  drops for each test is 25 and for the 6 inch mold the 
number of drops of the hammer is 56 (for both test methods).  This 
yields a compactive engergy of the Modified Proctor of 56,250 ft*lb/ft^3
and a compactive energy of the Standard Proctor of 12,375 ft*lbs/ft^3.

As a general rule, 90% of the Modified Proctor is roughly equal to 95%
of the Standard Proctor in terms of the in-place density of the soil being
compacted.  With this relationship in mind, I usually specify 90% of
Modified Proctor where I don't have a building or road over the fill, and
I specify 95% of Modified Proctor under buildings and roads.  This spec.
will usually get the contractors attention and they tend to be more
diligent with their efforts.

Since the Standard Proctor was developed by Mr. Proctor in 1933, I
suspect that the test method was a reflection of the machinery that
was in common usage to attain compaction at that time as much as
it was a function of the needs for the structures.  As heavier structures and
bigger machinery have become more common, a Modified Proctor spec.
is easier to attain and more needed for the larger loads.

I would definitely us the Modified method under highways, airfields for
large aircraft, high-rise structures, dams, hospitals, and other critical
or high load bearing structures.  I have seen specifications for 98% of
a Standard Proctor under critical structures, and while this is less than
95% of the Modified Proctor it is fairly close and will probably work.

As a side note, it is fun to take a nuclear gauge out to test the compaction
of the soil and set it to return the relative compaction on the read-out
screen.  Sometimes in soils such a roadbase, it's very easy to exceed the
optimum compaction density of the Proctor test, giving a relative compaction
of greater than 100%.  This usually gets very funny looks from the contractors,
since we tend to think of 100% as the maximum attainable.  I then get to
explain how they just put more effort into the soil than is done in a Proctor
test and that they compacted the soil more than in the Proctor, and that with
even bigger equipment and more passes they would get even denser soil.  More
experienced contractors will try to minimize the efforts of compaction to just
comply with the requirements, as that saves them time and money.

Lloyd Pack, P.E.

> GSKWY(--nospam--at) 
> 01/26/2005 07:47 PM
> Please respond to
> <seaint(--nospam--at)>
> To
> seaint(--nospam--at)
> cc
> Subject
> Proctor Tests
> Are there are guidelines for when you should specify a standard Proctor 
> and when you should specify a Modified Proctor?
> Is it  based on type or magnitude of loading?
> Gail Kelley

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at:
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*   Questions to seaint-ad(--nospam--at) Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: 
******* ****** ****** ****** ******* ****** ****** ********