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.



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