2.4.2 Sample disturbance

This section will discuss the three types of soil samples that can be obtained during the subsurface
exploration.

In addition, this section will also discuss:

• sampler and sample ratios used to evaluate sample disturbance; 
• factors that affect sample quality
• and transporting, preserving, and disposal of soil samples.

Types of soil samples

There are three types of soil samples that can be recovered from borings:

Altered Soil (also known as Nonrepresentative Samples).  During the boring operations, soil can
be altered due to

• mixing or 
• contamination. 

Such materials do not represent the soil found at the bottom of the borehole and hence should not be used for visual classification or laboratory tests.

Some examples of altered soil are as follows:

• Failure to clean the bottom of the boring.  If the boring is not cleaned out prior to sampling, a soil sample taken from the bottom of the borehole may actually consist of cuttings from the side of the borehole. These borehole cuttings, which have fallen to the bottom of the borehole, will not represent in situ conditions at the depth sampled.

• Soil contamination.  In other cases, the soil sample may become contaminated with drilling fluid, which is used for wash-type borings.

mud

• Soil mixing.  Soil or rock layers can become mixed during the drilling operation, such as by the action of a flight auger.  For example, suppose varved clay, which consists of thin alternating layers of sand and clay, becomes mixed during the drilling and sampling process. Obviously laboratory tests would produce different results when performed on the mixed soil as compared to laboratory tests performed on the individual sand and clay layers.

Flight auger

• Change in moisture content.  Soil that has a change in moisture content due to the drilling fluid or from heat generated during the drilling operations should also be classified as altered soil.

• Densified soil.  Soil that has been densified by over-pushing or over-driving the soil sampler should also be considered as altered because the process of over-pushing or over-driving could squeeze water from the soil.                                                                                                   Figure 2.12 shows a photograph of the rear end of a Shelby tube sampler. The soil in the sampler has been densified by being over-pushed as indicated by the smooth surface of the soil and the mark in the center of the soil (due to the sampler head).

Disturbed Samples (also known as Representative Samples).  It takes considerable experience and
judgment to distinguish between altered soil and disturbed soil.

In general, disturbed soil is defined as soil that has not been contaminated by material from other strata or by chemical changes, but the soil structure is disturbed and the void ratio may be altered.

In essence, the soil has only been remolded during the sampling process.

For example, soil obtained from driven thick-walled samplers, such as

•  the SPT spilt spoon sampler, 
• or chunks of intact soil brought to the surface in an auger bucket (i.e., bulk samples) 

are considered disturbed soil.

Disturbed soil can be used for:

• visual classification 
• as well as numerous types of laboratory tests.

Example of laboratory tests that can be performed on disturbed soil include

• water content, 
• specific gravity, 
• Atterberg limits, 
• sieve and hydrometer tests, 
• expansion index test, 
• chemical composition (such as soluble sulfate), 
• and laboratory compaction tests such as the Modified Proctor.

SPT disturbed sample

Undisturbed Samples.  Undisturbed samples may be broadly defined as soil that has been subjected to no disturbance or distortion and the soil is suitable for laboratory tests that measure;

• the shear strength,
• consolidation, 
• permeability, 
• and other physical properties of the in situ material. 

As a practical matter, it should be recognized that no soil sample can be taken from the ground and be in a perfectly undisturbed state. 

But this terminology has been applied to those soil samples taken by certain sampling methods. 

Undisturbed samples are often defined as those samples obtained by slowly pushing thin-walled

tubes, having sharp cutting ends and tip relief, into the soil.

Undisturbed soil samples are essential in many types of foundation engineering analyses, such as

the determination of allowable bearing pressure and settlement. 

undisturbed soil sample

Sampler and Sample Ratios Used to Evaluate Sample Disturbance

Figure 2.13 presents various sampler and sample ratios that are used to evaluate the disturbance potential of different samplers and of the soil samples themselves. 

For soil samplers, the two most important parameters to evaluate disturbance potential are

• the inside clearance ratio 
• and area ratio,

 defined as follows:

So that they can be expressed as a percentage, both the inside clearance ratio and area ratio are

typically multiplied by 100.

Note in Fig. 2.13 that because common terms cancel out, the area ratio can be defined as the volume of displaced soil divided by the volume of the sample.

In general, a sampling tube for undisturbed soil specimens should have:

• an inside clearance ratio of about 1 percent 
• and an area ratio of about 10 percent or less. 

Having an inside clearance ratio of about 1 percent provides for tip relief of the soil and reduces the friction between the soil and inside of the sampling tube during the sampling process. 

A thin film of oil can be applied at the cutting edge to also reduce the friction between the soil and metal tube during sampling operations. 

The purpose of having a low area ratio and a sharp cutting end is to slice into the soil with as little disruption and displacement of the soil as possible. 

• Shelby tubes are manufactured to meet these specifications and are considered to be undisturbed soil samplers. 
• As a comparison, the California sampler has an area ratio of 44 percent and is considered to be a thick-walled sampler.

Standard Practice for Thin-Walled Tube Sampling of Soils for Geotechnical Purposes (ASTM D1587)

TABLE 2 Suitable Thin-Walled Steel Sample TubesA

Outside diameter (Do):
in.  mm	2 50.8	3 76.2	5 127
Wall thickness:
Bwg	18	16	11
in.	0.049	0.065	0.120
mm	1.24	1.65	3.05
Tube length:
in.  m	36 0.91	36 0.91	54 1.45
Inside clearance ratio, %	<1	<1	<1

A The three diameters recommended in Table 2 are indicated for purposes of standardization, and are not intended to indicate that sampling tubes of intermediate or larger diameters are not acceptable. Lengths of tubes shown are illustrative. Proper lengths to be determined as suited to field conditions.

Figure 2.13 also presents common ratios that can be used to assess the possibility of sample disturbance of the actual soil specimen. 

Examples include the 

• total recovery ratio
• specific recovery ratio
• gross recovery ratio
• net recovery ratio
• and true recovery ratio. 

These disturbance parameters are based on the compression of the soil sample due to the sampling operations.

Because the length of the soil specimen is often determined after the sampling tube is removed from the borehole, a commonly used parameter is the gross recovery ratio, defined as:

where 

• Lg is gross length of sample, which is the distance from the top of the sample to the cutting edge of the sampler after removal of the sampler from the boring (in. or cm).
• H is depth of penetra-tion of the sampler, which is the distance from the original bottom of the borehole to the cutting edge of the sampler after it has been driven or pushed in place (in. or cm).

The closer the gross recovery ratio is to 1.0 (or 100 percent), the better the quality of the soil specimen.

                                            Factors that affect sample quality

It is important to understand that using a thin wall tube, such as a Shelby tube, or obtaining a gross recovery ratio of 100 percent would not guarantee an undisturbed soil specimen. 

Many other factors can cause soil dis-turbance, such as:

• Pieces of hard gravel or shell fragments in the soil, which can cause voids to develop along the sides of the sampling tube during the sampling process

• Soil adjustment caused by stress relief when making a borehole
• Disruption of the soil structure due to hammering or pushing the sampling tube into the soil stratum
• Tensile and torsional stresses which are produced in separating the sample from the subsoil
• Creation of a partial or full vacuum below the sample as it is extracted from the subsoil
• Expansion of gas during retrieval of the sampling tube as the confining pressure is reduced to zero
• Jarring or banging the sampling tube during transportation to the laboratory
• Roughly removing the soil from the sampling tube
• Crudely cutting the soil specimen to a specific size for a laboratory test

The actions listed earlier cause a decrease in effective stress, a reduction in the interparticle bonds, and a rearrangement of the soil particles.

• An “undisturbed” soil specimen will have little rearrangement of the soil particles and perhaps no disturbance except that caused by stress relief where there is a change from the in situ ko (at-rest) condition to an isotropic perfect samplestress condition 
• A disturbed soil specimen will have a disrupted soil 
  structure with perhaps a total rearrangement of soil particles.

When measuring the shear strength or deformation characteristics of the soil, the results of laboratory tests run on 

undisturbed specimens obviously better represent in situ 

properties than laboratory tests run on disturbed specimens.

                                                                 Transporting Soil Samples

During transport to the laboratory, soil samples recovered from the

borehole should be kept within the sampling tube or sampling rings. In order to preserve soil sam-ples during transportation, the soil sampling tubes can be tightly sealed with end caps and duct tape.

Shelby tube samples

For sampling rings, they can be placed in cylindrical packing cases that are then thoroughly sealed.

Bulk samples can be placed in plastic bags, pails, or other types of waterproof containers. 

The goal of the transportation of soil samples to the laboratory is to prevent a loss of moisture. 

In addition, for  undisturbed soil specimens, they must be cushioned against the adverse effects of transportation induced vibration and shock. 

The soil samples should be marked with: 

• the file or project number
• date of sampling
• name ofengineer or geologist who performed the sampling
• boring number
• depth

Other items that may need to be identified are as follows (ASTM D 4220-00, 2004):

1. Sample orientation (if necessary)

2. Special shipping and laboratory handling instructions

3. Penetration test data (if applicable)

4. Subdivided samples must be identified while maintaining association to the original sample

5. If required, sample traceability record