Soil Testing Frequently Asked Questions
What are the ASTM and
AASHTO standards written specifically for moisture and density testing?
ASTM D 2922 & AASHTO T238 Density of Soil and Soil-
Aggregate In Place by Nuclear Methods
ASTM C 1040 Density of Unhardened and Hardened Concrete In Place by Nuclear Methods
ASTM D 2950 Density of Bituminous Concrete In Place by Nuclear Methods
ASTM D 3017 & AASHTO T 239 Water Content of Soil and
Rock In Place by Nuclear Methods
What is the
difference between compaction and load bearing capacity?
What is soil?
Soil (earth) - sediments or other
accumulations of solid particles produced by the physical and chemical disintegration
of rocks. It is usually found on top of the parent rock formation. It is
classified by the grain size distribution of the particles. Coarse soil with
little or no fine particles (5 mm to 0.010 mm) is usually called sand. Slightly
finer material is classified as silt (0.010 mm to 0.005 mm). Really fine
material, finer than 0.005 mm, is classified as clay.
What does 95%
compaction mean?
95% Compaction - This term is usually found in the contract
specifications of a project. It is a value recommended by the structural or
geotechnical engineer that is necessary to ensure adequate mechanical
consolidation of structural soil backfill. The value refers to a percentage of
the oven dry density of a particular uniform soil material as determined in the
laboratory using one of the specified test methods. The two methods commonly
used are:
a. Standard Method; ASTM D 698, uses the equivalent energy
or
compactive effort of 14,176
ft.-lbs./sq.ft./1 ft. layer.
b. Modified
Method; ASTM D 1557, uses the equivalent energy or
compactive effort of 64,439
ft.-lbs./sq.ft./1 ft. layer.
What is the
difference between Standard and Modified compaction?
Standard laboratory compaction, ASTM D
698, AASHTO T 99.
Modified laboratory compaction, ASTM D 1557, AASHTO T 150,
The Modified
Compaction method requires 4.5 times more effort than required by the Standard
Compaction method. It therefore densifies the soil to
a greater degree, reducing the air voids and increasing the in-place density by
a nominal 5% over that attained by using the Standard Compaction effort.
The Modified
Compaction method is required where foundations are to be placed in the soil
backfill, and where minimal or no settlement can be tolerated by the structure.
What is meant by a
proctor test?
Proctor curve or compaction curve - is the curve showing the
relationship between the dry unit weight (density) and the water content of a
soil for a given compactive effort. It is named after
the soil engineer who developed this relationship.
How do you determine
the percent compaction?
Percent compaction is determined by dividing the the oven dry in-place density by the maximum density of
that soil determined in the laboratory.
Why are compaction
tests required?
Compaction tests are required to verify and certify that the
required densification has been achieved. Testing frequency is usually one test
for every 2000 sq. ft. (185 sq.m.) for each 6 in. (15
cm.) compacted lift.
Why would you need
more than one laboratory compaction test?
Depending on the project site, and the availability of fill
materials, a laboratory compaction test is required for each soil type or
borrow source being used on the project. The maximum density is affected by the
grain size distribution, and chemical/mineralogical makeup of the soil.
How can the required
percent compaction be attained?
The required percent compaction is attained by conditioning
the soil so that the moisture content is within +/-3% of the optimum moisture.
It is very important to proof roll the site before placing any backfill to
verify that there are no soft or unstable areas. Soft and unstable virgin subgrades prevent attaining the required degree of
compaction.
How does field
moisture affect the percent compaction?
Field moisture is the single most important factor, after
the compaction methods, in controlling the outcome of the compactive
effort - PROPERLY COMPACTED SOIL. The recommended field practice is to try to
get the field moisture within ±3% of the Optimum value by either adding water
or spreading it out to dry. If the soil is too dry or too wet it cannot be
compacted the the required density.
Can you compact
crushed stone?
Crushed Stone can be compacted. The degree to which it can
be compacted is controlled by the grading or size distribution, and the
compaction methods. The added advantage to using a crushed stone product, is that it can be used as structural fill when wet
weather conditions make it impossible to use a soil material.
How do you determine
the bearing capacity of virgin soil?
Bearing Capacity of virgin soil can be determined my a number of different methods.
a. Soil
borings using a split spoon sampler and a 130 lb. falling weight.
b. Plate
Load test, using a steel plate from 2 sq. ft. to 4 sq. ft. and a
heavy load.
c. Drop Bar Penetrometer, using
either a 1" bar with a 25 lb. weight, or
a 0.5" bar with a 5 lb., weight.
What is an in-place
density (field density) test?
An In-Place density test is just what you would expect - the
determination of the density of the material in-place. The classical method is
to remove an undisturbed soil sample, weigh it and measure it to determine its
volume and then determine moisture content by drying it. From the measured
values the dry density can be calculated.
How is the in-place
density determined?
In-place density tests are performed by using one of the
field methods listed below.
1. Sand Cone Method (ASTM D 1556)
2. Rubber Balloon Method (ASTM D
3. Nuclear Densitometer (ASTM D 2922)
What is the
difference between a Sand Cone test and a Nuclear Densitometer test?
The Sand Cone method and the Rubber Balloon method are
methods that determine real values. The nuclear densitometer is and instrument
that determines relative values, relative to the "standards" used to
calibrate it.
Why must you perform
three to five Sand Cone tests before you start taking Nuclear Densitometer
readings?
The reason for performing 3 to 5 sand cone tests in the same
exact location a nuclear densitometer test is taken - is to calibrate the
instrument to the soil or aggregate type being tested. A correction can then be
made to the indicated values from the nuclear densitometer.
Why can't I just use
a Nuclear Densitometer to test for compaction?
You can use just a nuclear densitometer only if it has been
calibrated for the exact same material that is being tested. Each soil or
aggregate has its own particular chemical/mineralogical makeup.
Does an in-place
density test measure the bearing capacity of soil?
NO! An in-place density test does not measure the Bearing
Capacity of a soil.
Does the Drop-Bar Penetrometer test measure the degree of compaction?
The Drop Bar Penetrometer does not
measure the degree of compaction. It is used to test undisturbed virgin soil.
It indicates the resistance to penetration, which is an indication of the
relative bearing value of the soil.
Why must the large
cobbles or boulders be removed from backfill?
Large cobbles or boulders, larger than 0.75 times the loose
thickness of the compacted lift should be removed. The presence of too many large rock makes it almost impossible to compact the
soil between adjacent rock.
Can top soil be used
for structural backfill?
Top soil is not recommended for use as structural backfill
because it contains an undetermined amount of organic matter that can continue
to decompose and cause future settlement of a structure.
Can the top soil be
left in place and structural backfill be placed on top of it?
The usual recommendation is to remove all top soil prior to
the placement of structural fill.
How can the required
minimum compaction be attained if the soil fill is too wet?
The Required Minimum Compaction of an excessively wet soil
can be attained by adding a drying agent such as quick lime and mixing it in
with the wet soil to attain a moisture value near the Optimum Moisture of
original soil.
Can I just test the
top surface to certify the compaction?
Testing the top surface will only certify the degree of compaction of the top one foot of the fill . In order to certify the entire depth of compacted fill, it is necessary to excavate test pits in one to two foot steps and test each step for compliance. The number of test pits is determined by the area of fill. One test pit for every 2500 sq. ft. of fill area.