TECHNIQUES TO IMPROVE UNDISTURBED SAMPLING

GEOTECIJMCAL ENGINEERING MANUAL GEM-8

Revision #2

GEOTECHAICAL ENGINEERING B UREA tI

APRIL 2007

NEW YORK STATE DEPARTMENT OF TRANSPORTATION

GEOTECHNICAL ENGINEERING TMANLTAL: TECHNIQUES TO IMPROVE UNDISTURBED SA.IVIPLING

GEM-8 Revision 42

STATE OF NEW YORK DEPARTMENT OF TRANSPORTATION

GEOTECIINICAL ENGINEERING BUREAU

APRIL 2007

EB 07039

Page 1 of 11

T:A-BLE OF CONTENTS

1 .

PURPOSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

H .

SAMPLING INSIDE CASING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 1 .

Cause. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 2 .

Preventative Measure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

111 .

IMPROPER CLE ANOLJ-f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1 .

Cause. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Preventative lvleasure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

SAMPLING OF "SHEARED" MATERIAL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 l .

Cause. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 2 .

Preventative Measure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

OVERPRESS OF WASHED MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1 .

Cause. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 0 2 .

Preventative Measure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

I . PURPOSE

The purpose of thus project is to evaluate the effects of sampling disturbance on plastic clays . The project was carried out in cooperation with the Region 1 Geoteclmical Section .

During September and October, 1970 two 4 inch (100 n1m) diameter cased undisturbed holes were drilled in a cut section of New York Route 85 in Slingerlands . In the first hole, DNC-1, undisturbed samples were taken while deliberately using improper procedures . The second hole, DNC-2, drilled 30 ft. (10 m) away, was taken using the prescribed sampling procedures . The purpose of the two holes was to try to identity the causes for se-\%eral types of sample disturbance which are frequency encountered iii undisturbed samples . By visually comparing the "bad" sample with a "good" sample taken at the same elevation we are able to conclude that two main causes of unsatisfactory undisturbed samples are improper cleanout and inaccurate measurements .

The following diagrams and photographs show the results of improper sampling techniques and suggest a preventative measure for the driller in each case . The iiil'ornlation in this report is presented to assist the Geotechnical Engineer and drilling personnel in obtaining the highest duality undisturbed sample .

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Page 3 of 11

II . SAMPLING INSIDE CASING

1. Cause Inaccurate measurements may cause the sample to be taken partly inside the casing and partly outside (Figure 3) . The result is that the sample is highly arched in all directions (Figures I & 2) due to the driving of the, casing . Notice that the arched layers stop and horizontal layering begins inunediately below the bottom of the casing (Figure 2) .

2 . Preventative Measures Careful and accurate measurements should be taken on both the casing as it is being driven and the rods as the hole is being cleaned out and sampled. It is especially important in deep holes to check the elevation of the casing since small errors may add up to produce a significant discrepancy .

Fib e 1

Fit~tu"e 2

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Page 4 of 11

Thin ~N-allect Tube Sampler

-I" (1(a{I mm) Dia. Casing

--~

Disturbed Soil C'ausecl 1),% Dri-dng the Casing

Figure 3

Sampling Inside the Casing with Piston Sampler

EB 07-039

Page 5 of 11

31) in . (761) min)

Press 18 in . (-1-50 nom)

III . IMPROPER CLEANOUT 1. L'ause

If the soil which has been loosened up by the chopping bit is not completely cleaned out (Figure 5) the sample will look like Figure 4. The top part of the sample has been completely disturbed and is useless for laboratory testing . The recovery for the undisturbed portion is so small that adequate laboratory testing cannot be performed .

2 . Preventative Aleasures Accurate measurements should be taken while clearing out the hole.

Figure 4

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Page 6 of 11

-1 in. (100 111m) Dia. Casing

Disturbed Soil ('aused by Drivilla the t'asimy

4v

Improper Cleanout

Figure 5

Sampling xvith Shelby Head and Improper Cleanout

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Page 7 of 11

Thin Walled Tube _ Sampler

'Ithttut Piston)

Y-1 ill . ('760 mm)

Press .18 in. (-150 mm)

IV.SAMPLING 4F "SHEARED" MATERIAL

1. Cause If, during continuous undisturbed sampling operations. the hole is not cleaned out by the cleanout auger to the elevation of the bottom of the previous sample, soil will be forced by the piston into the vicinity of the next tube as shown in Figure 8 . If the sampler is pressed to the, proper elevation with the piston locked, the material in the second tube will be sheared (see Figures 6 & 7) before the sample is taken . The conical shear planes are characteristic of this procedure .

2 . Preventative Measure During continuous undisturbed sampling, the hole should be cleaned out with the cleanout auger after every tube has been taken. The sampler with the piston locked should never be forced to the desired depth . The, hole, should be cleaned out again. In some soft soils it may be necessary to advance the casing after each tube to eliminate this type of disturbance .

Figure 6

Fi

.u-e 7

EB 07-039

Page 8 of 11

EB 07-039

4 in. ~1 E{ mm)

Figure S

Corltitnious Sampling

Page 9 of 11

Disturbed Soil Caused bx7 Driving, the Casino

lVtaterial Bulged After Tube 1 Has Been Removed

Material Forced into Area }f Tube 2 1)1- Pressing with Piston Locked and Not Cleanincr Cut Prior to Sa ml)lin ,-v Tube 2.

V . OVERPRESS OF WASHED MATERIALS

1. Cause While sampling without a piston as shoxA~n in Figure 12, improper eleanout can result in an ovetpress . The resulting disturbance may take the form shown in Figure 10 . The sample was so severely oveipressed that the chopped up soil was reconsolidated and forced into the vent in the Shelby head . The reconsolidated soil (Figure 10) looks almost like a normal deposit ; hoNA7ever, close examination will reveal the conical opening through which the soil was forced. The entire sample is rendered useless for testing because it has been squeezed to such a degree. If for some reason the Shelby vent becomes plugged, the soil may be squeezed down the sides of the sample and out the bottom of the tube as shown in Figures 9 & 10 .

2 . Preventative Njeasure Accurate measurements should be taken when the hole is cleaned out . A cleanout auger - should be used with a high capacity pump to unsure that all droppings are washed out of the hole . If the piston had been used the driller would have been able to tell that the sampler was too high and could have taken appropriate action .

EB 07-039

Page 10 of 11

Fib e 9

Fit~tu-e 1 0

Thin lVallecl Tube Sampler (11, it11{flit Piston)

4 in. (1(K-) MM) Dia. Casino

Disturbed Soil Caused 1)v Drivin;y the (yasin y

Improper Cleanuut -_

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Page 11 of 11

Fig .u"e 12

Improper Cleanout & Improper Measurement