evaluation of sample quality by non destructive and destructive methods

8/13/2019 Evaluation of Sample Quality by Non Destructive and Destructive Methods

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EVALUATION OF SAMPLE QUALITY BY NON-

DESTRUCTIVE AND DESTRUCTIVE METHODS

Hiroyuki Tanaka1and Vuthy Horng

2

1Professor, Hokkaido University, Japan,

Tel. +81 11 706 6193, Fax. +81 11 706 7204, Email: [email protected] of Technology of Cambodia (Formerly PhD Student, Hokkaido University)

Tel. +855 78 59 83 83, E-mail:[email protected]

Abstract

Effects of sample disturbance on the undrained shear strength were investigated from samples with

various qualities, retrieved by different types of samplers at the Takuhoku site, Sapporo, Japan.Sample disturbance, caused by difference in sampling tube geometry, was evaluated by two

nondestructive methods: measurement of residual effective stress (p'r) by ceramic disc; and shear

wave velocity (Vs), and thus maximum shear modulus (GBE), by bender element. Sample qualitywas also evaluated by three types of destructive test, i.e., shear tests: unconfined compression, fall

cone and triaxial recompression tests. Geometry effects of the sampling tube, for example,thickness of the tube wall, edge angle, and existence of a piston were carefully examined. It was

found from these studies that the small edge angle of a tube sampler is important to obtain highquality sample. In addition, the existence of a piston does not have a significant effect on thestrength properties.

Keywords:Clay, Drilling, Sample Disturbance, Sample Quality, Sampling Tube, Shear Modulus,Site Investigation, Suction

Introduction

High quality samples are required to interpret in situ soil properties, such as permeability,

compressibility, and shear strength characteristics, in order to provide designs that are not

overly conservative and decrease the cost of construction. Geotechnical properties of soils

are estimated from either in situ or laboratory tests. One of the most important restrictions

of laboratory test results is sample disturbance. Over the last few decades, considerable

efforts have been made to improve sampling techniques, including designs of the sampling

tubes, to correct soil parameters, for example, compressibility and shear strength measured

from poor quality sample.

Traditionally, sample quality has been assessed by the following values and features: i)

shear strength, strain at failure, and Youngs modulus from unconfined compression or

triaxial tests; ii) the shape of the e-logp' curve, where e is the void ratio and p' is theeffective consolidation pressure, preconsolidation pressure, or compression index from

oedometer test; iii) volumetric strain caused by recompression to the in situ effective stress

(Andresen and Kolstad, 1979), or the ratio e/e0, where e is the change in void ratio

during the recompression process to the in situ effective stresses and e0 is the initial void

ratio, which was proposed by Lunne et al. (1997). However, these criteria do not possess

an absolute value, but are strongly dependent on the properties of the local area (for

example, see Tanaka, 2000). If sample quality is examined for every sample along the

sampler, these evaluation methods are time consuming and costly. The methods described

above are destructive methods used to determine sample quality, thus further laboratory

testing to determine soil properties cannot be carried out after revealing high or low sample

quality.

Invited paper


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Two nondestructive methods were used to evaluate sample quality, measurements of

suction (sometimes called residual effective stress) using a ceramic disc and maximum

shear modulus by bender element (BE) test. After sample extrusion, the sample was placed

on a ceramic disc with a high air entry value. Following suction measurement, the sample

was embedded with BE plates at the bottom and the top, and the shear wave travel time (t)

was measured, from which the shear wave velocity (Vs) and maximum shear modulus (GBE)were calculated. Tests were performed on every sample in the sampling tube, including the

upper and lower parts, which are in general not used for mechanical testing, as they are

considered to be disturbed. After carrying out these non-destructive tests, three kinds of

mechanical tests were carried out as destructive tests,: unconfined compression test (UCT),

fall cone test (FCT), and triaxial recompression test (CKoUC).

To obtain soil sample with different sample quality, various samples were prepared,

based on the Japanese Standard Sampler. In this study, these geometries of the Standard

Sampler are changed to identify main factors governing sample quality of soft clayey soils.

Therefore, it is imperative that more systematic and efficient methods be employed to

identify main factors of geometry design and mechanisms of the sampling tube influencing

sample quality of soft clayey soils. The Takuhoku site, located near Sapporo, Japan, waschosen as a test site due to the near-uniform ground conditions with depth.

Samplers Used in this Study and Sampling Site

Samplers

Samplers used in this study are indicated in Table 1. The first sampler, which is the

standard tube currently used in Japan, consist of an inside diameter of 75 mm, a length of

1.0 m (the sample length is 0.8 m), and an edge angle of 6 o. The thickness of the tube wall

is 1.5 mm, which corresponds to an area ratio of 8.2%. Material of the sampling tube is

stainless steel. More details of this sampler may be referred to JGS (1998) and Tanaka et al.(1996). In this study, more geometrically different tube samplers consisting of the same

inside diameter of 75 mm were designed. The fourth tube sampler, 90oF10, has edge angle

of 90oand wall thickness of 10 mm, resulting in an area ratio of 60.4%. The last sampler,

6oF1.5 (O), is the 6oF1.5 without using fixed piston (an open drive sampler) during

sampling.

Table 1. Main characteristics and dimensions of tube samplers used in this study

FieldSamplers

Edge

Angle

(o)

Tube

Thickness

(mm)

Area

Ratio

(%)

Piston

Sampling

Depths

(Upper

Layer)

(m)

Sampling

Depths

(Lower

Layer)

(m)

6oF1.5 6 1.5 8.2 Yes 13 22

6oF10 6 10 60.4 Yes 12 21

90oF1.5 90 1.5 8.2 Yes 14 23

90oF10 90 10 60.4 Yes 11 20

6oF1.5(O) 6 1.5 8.2 No 15 24


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Sampling Site

Sampling was carried out at the Takuhoku site, Sapporo, Japan. The detailed properties

may be referred to Horng et al. (2010) and are briefly mentioned in this paper. The main

geotechnical properties of this site are shown in Fig. 1. The deposits consist of 5 m fill and

peat followed by a 4.5 m silty sand deposit, overlying the clay layers investigated in this

study. A sandy silt layer at a depth of 15 to 18.5 m separates the soil profile into the upperand lower clay layers. Sampling was carried out at two different depths: the upper (10~15

m) and lower (20~24 m) clay layers as indicated in Fig. 1. The ground water table is

located at about 3 m below the ground surface. The natural water content varies between

60 and 70 % and the plasticity index (Ip) is about 45~53 and 50~63 for the upper and lower

clay layers, respectively.

The yield consolidation pressure (py), which was measured by CRS oedometer at a

strain rate of 0.02%/min (3.310-6/s), is somewhat lower than the in situ effective

overburden pressure ('vo), which is calculated by assuming that the pore water pressure

distribution is hydrostatic. The fill material at ground surface was placed in the 1960s,

thus it is believed that the sampling clayey soil is still undergoing consolidation. Moredetails of this investigation can refer to Horng et al. (2010).

Figure 1. Takuhoku soil profiles


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The field vane test (FVT), using a vane blade of 40 mm in diameter and 80 mm in

height and the piezocone test (CPT) were carried out to measure mechanical properties of

the site. From the CPT test, the undrained shear strength was calculated using the relation

su(CPT)=(qt-vo)/Nkt, where qt is the point resistance of the piezocone, vo is the total

overburden pressure, andNktis the cone factor. By equating the undrained shear strengths

of CPT and FVT, the cone factor Nkt was able to be calculated. In this site Nkt wasestimated to be 11.5. The undrained shear strengths from the unconfined compression test

(UCT) are also plotted in this figure, where the soil samples were retrieved by the Japanese

standard fixed piston sampler (6oF1.5 in Table 1). The mean undrained shear strengths for

the upper and lower sampling depths are approximately 20 kPa and 40 kPa, respectively.

Laboratory Testing Methods

Suction Measurement

When a soil sample is extracted from the ground to the atmosphere, some amount of the

effective stress remains in the soil sample in the form of negative or suction pressure.

Ideally the value of the residual effective stress or suction (p'r) should be equal to the mean

in situ effective confining pressure (p'm=('vo+2'ho)/3), where 'voand 'hoare the in situ

vertical and horizontal effective stresses, respectively. However p'ris generally somewhat

smaller than the in situ p'mdue to sample disturbance caused by the process of sampling,

transportation, storage, extrusion from the sampler, and preparation of the specimen for

laboratory testing. Thus, the residual effective stress can be a soil parameter for the

evaluation of sample quality to compare with the in situ p'm. However, measurement or

estimation of 'ho is rather difficult. Therefore, in this paper, 'vo will be normalized, as

explained in further detail below.

The apparatus used for measuring suction is illustrated in Fig. 2. The air entry value of

the ceramic disc was 240 kPa. Before testing of p'r, the ceramic disc and the connectionbetween the ceramic disc and transducer were completely de-aired and saturated. After

placing the specimen on the ceramic disc, the negative pore water pressure gradually

decreased and became constant. The constant absolute value of this negative pressure

measured by a pressure transducer was defined as the suction value or residual effective

stress (p'r). The time duration for a constant value ofp'rin this study was approximately 20

minutes. However, all specimens were placed on the ceramic disc for about one hour.

During the suction measurement, a specimen was wrapped by plastic film and covered by

an acryl box to avoid loss of water content. The measurement was done under atmospheric

condition and cavity pressure (larger than 100 kPa) was not generated since the suction

value was less than 100 kPa for all samples in this study.

50

40

30

20

10

00 10 20 30 40 50 60

Time (min)

Suction

(kPa)

Sample 1Sample 2

Figure 2. Suction measuring system by ceramic disc


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Bender Element Test

Among devices measuring G, the bender element (BE) test is a simple and very fast

method. The details of the BE test have been described by several researchers (Viggiani

and Atkinson, 1995; Shibuya et al. 1997; Kawaguchi et al., 2001). After measuringp'r, the

BE test was performed to measure shear wave velocity (Vs) of the sample. The test

equipment and dimensions of BE is illustrated in Fig. 3. A pulse with various types ofwave forms such as sine and square over a wide range of frequency was input by a

function generator through the transmitter BE at the top of the sample. The wave

propagated through the sample and was detected by the BE receiver plate at the bottom.

The shear wave travel time (t) was defined as start-to-start between two instants of the

generated wave and the first deflection of the received wave (Kawaguchi et al., 2001). The

travel distance (L) was defined as tip-to-tip distance between the transmitter and the

receiver of BE plates (Viggiani and Atkinson, 1995). Thus, the shear wave velocity can be

calculated from Eq. (2);

Vs=L/t (2)

From the theory of shear wave propagation in an elastic body, the shear modulus from the

BE test (GBE) can be calculated from Eq. (3);

GBE=tVs2 (3)

(3)

wheretis the bulk density of soil.

Unconfined Compression Test (UCT)

The undrained shear strength (qu/2) from UCT is widely used for stability designs ingeotechnical engineering and this type of test is believed to be the most sensitive to sample

disturbance (Lacasse et al., 1985; Tanaka et al., 1996; Mitachi et al., 2001). UCT was

carried out by the Japanese Industrial Standard (JIS A 1216:2009): the specimen was

trimmed by a wire saw to the diameter and height of 35 and 80 mm, respectively. The

specimen was compressed at a constant strain rate of 1%/min.

Figure 3. Bender element equipments

BELength

(mm)

Width

(mm)

Thickness

(mm)

Transmitter 10.5 11.0 1.7

Receiver 8.0 11.0 1.7


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Fall Cone Test (FCT)

FCT has been used to measure the undrained shear strength mostly in Scandinavian

countries. FCT is not used in Japan for measuring the undrained shear strength, but for

determining the liquid limit, also obtained by the Casagrande test. In this study, FCT was

used to measure undrained shear strength, following the standard of the Japanese

Geotechnical Society (JGS 0142-2009) by using a cone with a tip angle of 60 oand mass of60 g. The cone was allowed to fall freely under its own weight from a position at rest with

the cone tip just touching the surface of the soil sample. The undrained shear strength of

FCT was calculated following the equation ofsu=k(mg/d2), where mis mass of cone (= 60

g),gis earth gravity acceleration, dis depth of penetration of cone tip into a soil specimen

after 5 seconds, and kis the cone factor depending on the angle of the cone tip. k=0.29

was assumed according to Wood (1990) for the cone angle of 60oin this study.

Triaxial Recompression Test (CKoUC)

The triaxial test specimen size was the same size as that of UCT in this test. A back

pressure of 200 kPa was applied in order to obtain high saturation. For estimating the insitu strength, a recompression test was employed, where the specimen was consolidated

under the same effective stress condition as that of the in situ. Note that the soil profile in

Fig. 1 shows that the objective sampling clay layers are under consolidation caused by the

filling. Thus, consolidation was done in the vertical direction by a pressure 0.8 times the

yield consolidation pressure, p'y, which was measured by the constant rate of strain

oedometer (CRS) test under a strain rate of 0.02 %/min. The reasons for using a coefficient

of 0.8 are to avoid the overestimation ofp'yfrom the CRS under relatively high strain rate

and possibility for overestimating the strength due to large consolidation pressure. The

coefficient of lateral earth pressure at rest (Ko) was estimated to be 0.55 from Ko-

consolidation triaxial test at the normally consolidated state. The consolidation stresses

were kept for about 24 hours. After complete consolidation, the specimen was shearedunder undrained conditions at an axial strain rate of 0.1%/min. The shearing was done until

the axial strain reached 15%.

Evaluation of Sample Quality by Non-Destructive Methods

Profiles of pr/voand GBE/Gfof the Best Samples

General

In order to characterize soil properties at the investigated site from a view point of sample

quality, p'r and GBE were measured on samples retrieved by the JPN standard sampler(designated as 6oF1.5 in Table 1) and the location of these samples is in the middle of the

sampling tube: i.e., the best quality is guaranteed, as described later. The variation of

p'r/'voandGBE/Gfratio to the depth is presented in Fig. 4.

It can be seen in this figure that the normalized values ofp'r/'voslightly increases with

depth, and its values are about 1/5 at the upper investigated depth and 1/3 at the lower

depth. Ladd and Lambe (1963) measuredp'rby the unconsolidated undrained (UU) triaxial

test and found that for the Kawasaki clay (a Japanese marine clay), the p'r/'pratio ranged

from 0.11 to 0.43 with an average value of 0.28, where 'pis the residual effective stress in

the perfect sample, and they reported that the 'pwas in the range of 0.560.05 of 'vo.

Thus, the p'r/'vo ratio was round 0.14, which is quite low compared with this study.

Tanaka et al. (1996) studied residual effective stress due to sample disturbance and hasshown thatp'r/'vowas in the order of 1/5 to 1/6 for high quality samples. From the depth


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of 10 to 15 m (upper clay layer),the p'r/'vo in this study is approximately 1/5 the value

reported by Tanaka et al. (1996), but below 15m the ratio linearly increases and is larger

than 1/5.

In addition to the p'r/'vo ratio, GBE/Gf is also plotted in the same Fig. 4. Contrary to

p'r/'vo, the normalized ratio along the depth is relatively constant ranging from around 0.45

to 0.73 with an average of 0.57. It is interesting to compare the present results with thoseconducted by Landon et al. (2007). They measured BE shear wave velocity (VBE) in the

same manner as in this study for samples from Boston Blue Clay, retrieved by the

Sherbrooke, the fixed piston, the free piston, and the SPT split spoon samplers. The VBE

was normalized with Vf from the field seismic piezocone for comparing sample quality

among those samplers. But in this study sinceGBE/Gfare used to compare sample quality,

the normalized VBE/Vf of Landon are converted to GBE/Gf. They found that the GBE/Gf

ratios were in the range of 0.49~0.64, 0.42~0.49, 0.09~0.25, and 0.09~0.16 for samples

retrieved by the Sherbrooke, the fixed piston, the free piston and the SPT split spoon

samplers, respectively. It is revealed that the normalized ratios (GBE/Gf) obtained from this

study are as high as those of Sherbrooke samples conducted by Landon et al. (2007).

Location

Figs. 5 through 12 show a comparison of p'r/'voand GBE/Gf ratios measured by various

samplers with different geometries. The disturbance at the lower edge tip may be caused

by the suction created by withdrawal of the sampler. The upper part of sample in a

sampling tube may suffer from the borehole drilling or extensive straining by removal of

the overburden pressure. In addition, it is inferred that, because of long travel distance, the

upper part of sample is damaged by frictional force between the inside wall and the sample

during both sampling and extrusion.

Figure 4.p'r/'voand GBE/Gfof the best quality samples along the depth (Takuhoku)


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Effect of Edge Angle

Figs. 5 and 6 show the effects of the edge angles of the sampler. Fig. 5 shows test results

from the upper clay layer using two pairs of samplers with different edge angles: 6oF1.5

and 90oF1.5; 6oF10 and 90oF10. The same pairs of sampler were also used for the lower

clay layer as shown in Fig. 6. The first pair of the samplers, 6oF1.5 and 90oF1.5, have the

same dimensions, with the exception of the edge angles (6oand 90o, respectively). Fromthe figures it can be seen that the normalized values of p'r/'vo and GBE/Gf for samples

retrieved by 6oedge angle samplers are higher than those of 90ofor both clay layers. It is

interesting to note from the Figs. 5 and 6 that the difference in p'r/'voandGBE/Gffor the

thin wall samplers, 6oF1.5 and 90oF1.5, is much smaller than those of the thick wall

samplers, 6oF10 and 90oF10. It implies that the influence of the edge angle is more

pronounced on sample disturbance when the sampler has a thicker wall.

Effect of Area Ratio

Figs. 7 and 8 show the results of the two pairs of different area ratio samplers, 6oF1.5 and

6o

F10; and 90o

F1.5 and 90o

F10. The values ofp'r/'voandGBE/Gfof samples retrieved byboth 6o edge angle show no noticeable difference between 1.5 and 10 mm thickness

samplers. But in the case of a larger edge angle (90o), sample quality obtained by the thin

wall sampler (90oF1.5) is significantly better than that by thick wall sampler (90oF10).

The magnitudes of p'r/'vo and GBE/Gf of the above four geometrically different

samplers for the upper and lower clay layers of Takuhoku site are plotted together in Figs.

9 and 10, respectively. The average magnitudes ofp'r/'voandGBE/Gfin Figs. 9 and 10 are

listed in Tables 2 and 3, respectively. These figures and tables show that the standard

sampler, 6oF1.5, gives the best sample quality and that the thicker sampler with 90 oedge

angle (90oF10) gives the lowest sample quality. Increasing the area ratio from 8.2% to

60.4% or increasing the wall thickness from 1.5 to 10 mm causes no significant sample

disturbance for the small edge angle but profound disturbance for the large 90oedge angle.The mean values ofp'r/'voandGBE/Gffor 90

oF1.5 are 71% and 55%, respectively, larger

than 90oF10 in the upper clay layer and are 58% and 50%, respectively, in the lower clay

layer.

It is found from the field samplings that the sharp edge angle of a sampling tube is the

most important key factor to obtain good quality. Very large differences in sample quality

are seen between 6oF10 and 90oF10, compared with those of thin wall samplers, 6oF1.5

and 90oF1.5 (Figs. 5 and 6).

The wall thickness, and thus area ratio, has long been considered to have significant

influence on tube sampling disturbance since Hvorslev (1949) pointed out its importance.

He realized that the penetration resistance of a sampler, the possibility of entrance of

excess soil, and danger of disturbance of the sample all increase with increasing area ratio.He also suggested that the area ratio of sampler should be reduced to not exceed 10 to

15% for open drive samplers, but it is possible that the allowable limit is higher for

samplers with a stationary piston, even though small area ratio generally causes slighter

disturbance. The Sub-Committee on Soil Sampling of International Society for Soil

Mechanics and Foundation Engineering (1981) reported that an area ratio of less than 13%

is generally recommendable, and up to 15% is acceptable, depending on soil conditions.

The largest permissible area ratios of 11, 10, and 13% are required in Japan, United

Kingdom, and United States, respectively, for their sampling standards. Matsumoto et al.

(1968 and 1969) reported test results from the comparative study at the Kinkai site, using

three samplers with area ratios of 2.7, 5.4, and 13.7%. Dimensions and other features of the

three samplers were identical to the JPN standard sampler. It was found that the undrained


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shear strengths (qu/2) and strain at failure (f) did not change due to the wall thicknesses of

the sampling tube. In this study, in addition to the Japanese standard sampler (6 oF1.5), two

10 mm thick wall samplers were used: 6oF10 and 90oF10. The area ratios of the thick wall

samplers are as much as 60.4%. Even though the area ratio increases more than sevenfold

from 8.2% to 60.4% and far larger than the studies of Matsumoto et al., the sample quality

is not significantly affected. On the other hand, if the cutting edge is not sharp, then thearea ratio must be as small as possible. That is, the area ratio is dependent on the edge

angle selected.

Effect of Piston

In this study, the effect of piston on sample quality was investigated, using the sampler

(6oF1.5(O)), which has the same geometrical features as the standard Japanese sampler. It

is believed that the largest advantage of the piston sampler is considered to have high

recovery ratio, because the piston can create a vacuum high enough to prevent the captured

sample from falling when the sampler is withdrawn from the bottom of the borehole.

The recovery ratio for each sampler, including 6oF1.5(O), is shown in Table 4. No

remarkable difference can be seen in the recovery ratio for 6oF1.5 and 6oF1.5(O) in the

upper clay layer. On the other hand, in the lower clay layer, the recovery ratio of 6oF1.5(O)

was as low as 79%. The high ratio in the upper clay layer may derive from disadvantages

of the open drive sampler as pointed out by Hvorslev (1949) and Osterberg and Murthy

(1979): i.e., due to poor cleaning of the borehole prior to sampling, or soil shavings on the

borehole wall, the sampling tube may collect soil along the wall of the borehole in the

process of lowering the sampler or soil cuttings deposited at the bottom of the borehole. It

is inferred that even within the upper layer, the recovery ratio of 6oF1.5(O) would be lower

than the observed value shown in Table 4, if the targeted sample was properly captured.

This debris can be seen by the low p'r/'vo and GBE/Gf values as shown in Fig. 11,

compared with the rest of samples from the same sampler.The test results of p'r/'vo and GBE/Gf for the 6oF1.5(O) are compared with those of

6oF1.5 in Figs. 11 and 12 for the upper and lower clay layers, respectively. It is believed

that the stationary piston is a key for collecting a high quality sample. However, a great

difference in p'r/'vo values between the stationary piston (6oF1.5) and the open drive

sampler (6oF1.5(O)) are not seen in both clay layers, except for the upper part of the

samples, where some reduction inp'r/'vois observed. On the other hand, theGBE/Gfvalues

of the stationary piston samplers are slightly higher than those of the open drive samplers.

The average values ofp'r/'voandGBE/Gffor the two samplers for the upper and lower clay

layers are summarized in Tables 2 and 3, respectively. Tanaka et al. (1996) also

investigated the effects of piston on sample quality at the Kinkai site. They showed that no

difference can be seen between the samples with and without piston from the unconfinedcompression and the laboratory vane shear tests.

Table 2. Upper clay layer of Takuhoku

Samplers 6oF1.5 6oF10 90oF1.5 90oF10 6oF1.5(O)

Area Ratio (%) 8.2 60.4 8.2 60.4 8.2

(p'r/'vo)average 0.152 0.159 0.132 0.038 0.150

(GBE/Gf)average 0.462 0.419 0.282 0.128 0.417


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Table 3. Lower clay layer of Takuhoku

Samplers 6oF1.5 6oF10 90oF1.5 90oF10 6oF1.5(O)

Area Ratio (%) 8.2 60.4 8.2 60.4 8.2

(p'r/'vo)average 0.241 0.212 0.190 0.080 0.240

(GBE/Gf)average 0.507 0.395 0.360 0.182 0.418

Table 4. Samplers used and their recovery ratios

Depth (m)Sampler

used

Recovery

ratio (%)

11.00~11.80 90oF10 98.7

12.00~12.80 6oF10 99.4

13.00~13.80 6oF1.5 99.4

14.00~14.80 90oF1.5 100

15.00~15.80 6oF1.5(O) 120

20.00~20.80 90

o

F10 100

21.00~21.80 6oF10 95

22.00~22.80 6oF1.5 97.5

23.00~23.80 90oF1.5 98.7

24.00~24.80 6oF1.5(O) 78.7

Correlation between prand GBE

It is anticipated from the test results of this study that GBE increases with increase of p'r.

Fig. 13 shows thep'rand GBEdata measured for the sample retrieved at the Takuhoku site.

It can be seen that GBEis strongly related top'r, although some scatter exists in this relation.

In order to examine the effects caused by a damage of soil structure, the relation measured

for samples retrieved by 6oF1.5 and 90oF10 is plotted using different symbols to

distinguish them from other samplers. It should be kept in mind that 6oF1.5 and 90oF10

samplers provided the best and the worst sample quality, respectively. If the scatter in the

relation between p'r and GBE is created by destruction of soil structure, then the relation

obtained from 6oF1.5 should be located in the upper part and that from 90oF10 should be

located in the lower part of the band of the p'r and GBE relation in Fig. 13. It can be

apparently recognized that points obtained from 6oF1.5 are upper 90oF10, though the

difference is small.


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Evaluation of Sample Quality by UCT and FCT

Test Result from UCT

Fig. 14 shows typical test results of UCT on samples from various types of samplers.

Vertical axis (/2/py) is the unconfined compressive stress normalized by the yield

consolidation pressure at the depth of the specimen. The normalizations are done to take

into account slightly different depths of samples for the comparative studies. Horizontal

axis is the axial strain, a(%). From both upper and lower layers in the figure it can be seen

that the stress-strain curves of the samples retrieved by 6oF1.5, 6oF10, and 6oF1.5(O) tube

samplers are very similar and their peak shear strengths are the largest compared with the

other two tubes, 90oF1.5 and 90oF10. 90oF10 samples show unusual stress-strain patterns,

implying severe disturbance by geometric designs of blunt edge angle and thick wall

thickness (90o and 10 mm). Their stress-strain curves move to the right hand side and

harden up to a final axial strain of 15% without showing any peak strength and the curves

are completely different from other tube samples. 90oF1.5 samples show stress-strain

curves which are similar to those of other 6

o

samplers, but their peak strengths aresomewhat smaller than those from 6oF1.5, 6oF10, and 6oF1.5(O) samplers.

UCT results for all samples are shown in Fig. 15, where Figs. 15 (a), (b), and (c) show

the effect of different geometric designs of tube samplers on qu/2, E50 (secant moduli at

50% strength), and f (the axial strain at the peak stress), respectively. These test results

show the same features from the previous studies: if a sample is disturbed, its stress-strain

curve exhibits small peak strength, smallE50, and large f(Lefebvre and Poulin, 1979; Oka

et al., 1996). It is interesting to note thatE50is much more sensitive to disturbance than the

peak strength. The test results show that sample quality is reduced when a large edge angle

tube sampler is used. Difference in the sample quality resulting from the edge angle is

more significant if the wall thickness of a tube sampler becomes larger (see 6 oF10 and

90o

F10). 6o

F10 tube, whose area ratio is increased to 60.4%, being higher than that ofstandard sampler (8.2%), does not affect the sample quality for the small edge angle, but

the difference in sample quality is significant for the large edge angle (see 90oF1.5 and

90oF10). It can be concluded from these results that once the edge angle of a tube sampler

is kept sharply small, a large area ratio can be permitted up to 60% without affecting

sample quality; on the other hand, once the angle becomes large the area ratio must be as

small as possible. Thus the area ratio is strongly dependent on the edge angle.

Unexpectedly, no signs of disturbance by collecting samples with the open drive sampler.

The stress-strain curves of both fixed piston standard and open drive tube samples show no

clear differences in sample quality. These results are consistent with the measurements of

suction and shear wave velocity in the previous study performed by Horng et al. (2010).

Test Results from FCT

Similarly, the same trends as those of UCT can be seen from the test results of FCT as

shown in Fig. 16. However, the difference in quality between samples of 6 oF1.5 and

90oF10 is less profound than that of UCT. Comparing with UCT (Fig. 15(a)), strengths

from FCT are larger than those from UCT. It should be noted from Hansbo (1957), who

used FCT to study shear strengths of soil collected by different samplers, that the cone

factor (k) also depends on the degree of disturbance from different samplers. However, k

in this study was assumed to be constant and dependent only on the angle of the cone tip

(Wood, 1990). As a result, the difference in sample quality among samples of different

samplers in FCT measurement is less significant than that in the UCT test.


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Figure 5. Effect of edge angle (upper layer)

Figure 6. Effect of edge angle (lower layer)


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Figure 7. Effect of area ratio (upper layer)

Figure 8. Effect of area ratio (lower layer)


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Figure 9. Combined effects of edge angle and area ratio (upper layer)

Figure 10. Combined effects of edge angle and area ratio (lower layer)

Relating to the Residual Effective Stress

Horng et al. (2010) measured residual effective stresses (pr) for the samples used in this

paper. Relations between pr and UCT parameters, qu/2, E50, and f are plotted in Figs.

17(a), 17(b), and 17(c), respectively. Relation between pr and strengths of FCT is also

plotted in Fig. 18. The important points from the figures are as follows:

(1)Both Figs. 17(a) and 18 show that strength is strongly related to pr. But qu/2 of UCT

indicates better correlation than that of FCT. This means that the strength from UCT is

more significantly governed bypr.

(2)E50 also correlates with pr, but it exhibits much more scatter than those between pr

and qu/2. Horng et al. (2010) tried to correlate pr with maximum shear modulus ofbender element (GBE). In addition, even though the past maximum stress was


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consideredin the relation ofprand GBE, scatters still existed in the same manner as the

relations betweenE50andpr.

(3)f is not influenced by pr unless pr/py become smaller than 0.1. pr/py=0.1 can be

considered the lowest boundary that fdoes not change by sample quality.

(4)Figs. 17 and 18 indicate that strength and E50values have strong relations with pr, i.e.,

the reductions of strength and E50may be partially explained by the decrease of pr.However, strains at the peak strength of UCT are constant with pr, except for those of

90oF10 whose values were assumed at large axial strains.

Figure 11. Effect of piston (upper layer)

Figure 12. Effect of piston (lower layer)


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Figure 13. Correlations betweenp'rand GBE.

Triaxial Recompression Test (CKoUC)

Stress-Strain Relation and Stress Path

As already indicated, strengths from the UCT and FCT are strongly influenced by thecurrent effective confining stress,pr, because these tests are carried out under unconfined

conditions. Therefore, when the in situ effective confining pressure is applied to the

specimen, the strength should be recovered. In triaxial test, this idea is called the

Recompression methodand was introduced by Berre and Bjerrum (1973). Lacasse et al.(1985) confirmed that the triaxial technique is able to correct for a large portion of

sampling disturbance. Of course, there is also criticism that the recompression technique

overestimates the strength caused by decreasing the void ratio due to the recompression.

This possibility may become significant when the sample is heavily disturbed.

Comparisons of typical stress-strain curves for specimens collected by different types

of tube samplers are shown in Fig. 19. It can be seen that the stress-strain curves and stress

paths of all samples from the tube samplers, 90oF1.5, 6oF10, and 6oF1.5(O) do not differ

remarkably from those collected by the standard Japanese sampler. On the other hand, the

stress-strain curves of the samples collected by the thick wall and large edge angle, 90oF10,

are totally different from those of other tube samplers. The noteworthy unusual patterns of

90o

F10 samples compared with those of other tubes are observed as follows: 1) 90o

F10samples show behaviors of strain hardening up to axial strain of 10%, which are similar to

their UCT counterparts, whereas other tube samples show the behaviors of strain softening

after well-defined peak shear stresses at small strains; 2) 90oF10 samples show lower shear

stresses at strains where the other samples reach the peak stress, but higher shear stresses at

large axial strains (>5%) than those of other samples. Tanaka (2000) also compared test

results from the recompression triaxial test for Bothkennar clay retrieved by JPN standard

and ELE 100 samplers and he made the same conclusions: i.e., the behavior of the ELE

100 samples is the same as that of 90oF10 in the present study, while the JPN sample

shows the same tendency of 6oF1.5, 90oF1.5, 6oF10, and 6oF1.5(O) samples. Similar

hardening behavior can also be observed from test results by Lunne et al. (2006) for

Scandinavian clays retrieved by NGI 54 mm.


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(a) Upper layer

(b) Lower layer

Figure 14. Typical test results of UCT.


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(a) Unconfined compression strength

(b) Secant moduli

(c) Strain at failure

Figure 15. Summary of test results of UCT


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Figure 16. Summary of test results of FCT

The stress paths are also plotted in Fig. 19, using diagram [(a+r)/2, (a-r)/2],

where aand r represent the axial and radial effective stresses, respectively. Similar to

the stress-strain curves above, the stress paths of 6oF1.5, 90oF1.5, 6oF10, and 6oF1.5(O)

show no profound difference in sample quality. On the other hand, the stress paths of the

low quality samples, i.e., 90oF10, rise up without any peaks until the end of test and excess

pore water pressure does not significantly build up. These patterns of the stress path are

somewhat different from those of the poor quality samples of Bothkennar retrieved by ELE

100 sampler or of Norwegian marine clays retrieved by NGI 54 mm as reported by Tanaka

(2000) and Lunne et al. (2006), respectively. Reasons for this difference may be attributedto soil characteristics of the sampling site or the features of samplers. At present, it cannot

be identified which factor is dominant.

Volume Change due to Recompression

Lunne et al. (1997) used the parameter e/e0 to quantify sample disturbance as shown in

Fig. 20, whereeis the change of void ratio during the reconsolidation back to the in situ

effective stress and e0is the in situ void ratio. Fig. 20 shows relation between e/e0andpr

from the recompression test, indicating the existence of strong correlation between them,

i.e., e/e0 decreases with the increasing pr. Tanaka and Tanaka (2006) also studied

relations betweenprande/e0, and found that correlation of the two parameters cannot be

recognized for eight different sites. It should be noted that e/e0 was measured by theoedometer test in their study, whereas e/e0 in this study was measured by the triaxial

recompression technique. It may be anticipated that in the case of the oedometer test, there

is a gap between the specimen and the oedometer ring so that the measurement of eis not

as accurate as the triaxial test. Another possible reason is that the soil properties are

different. In this study, the ground is underconsolidated, exactly normally consolidated, but

Tanaka and Tanakasdata contains various OCRs.

Fig. 20 shows that most of the samples retrieved by the Japanese standard sampler are

classified as Very good to excellent, whereas the samples from 90 oF1.5, 6oF10, and

6oF1.5(O) are Very good to excellent and Good to fair. e/e0 ratios for 90oF10 are

remarkably so large that the samples are classified as Poor according to Lunne et al.

(1997).


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(a) Unconfined compression strength

(b) Secant moduli

(c) Strain at failure

Figure 17. Relations betweenprand UCT parameters


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Figure 18. Relations betweenprand strengths of FCT.

(a) Upper layer

(b) Lower layer

Figure 19. Typical results of recompression CKoUC triaxial.


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As already mentioned, the most noticeable concern for the recompression test is the

overestimation of the strength due to decrease in the void ratio brought by reconsolidation

to the in situ stresses. Especially, if a sample is heavily disturbed and large eis observed,

there is possibility that the strength from the recompression might be larger than the in situ

strength. The normalized strengths (su/py) for all recompression test results in this study

are plotted withe/e0in Fig. 21, where two strengths from the case of 90o

F10 samples aredefined: strengths at the strain where the other samples show the peak strength (about 2 to

3 %) by the symbol (o); strengths at axial strain of 15 % by (). It can be seen thatsu/pydoes not increase with increasing e/e0, butsu/pyis nearly constant. Considering Figs. 20

and 21, it may be concluded that the recompression technique can restore the soil behavior

if the samples are at least Good to fair or better in terms of sample quality by Lunnes

criteria.In other words, the recompression technique is applicable when e/e0is less than

0.07 or whenpr /pyis greater than 0.10. Therefore, there is no possibility to overestimate

the strength for poor quality samples by the recompression test, as long as the strength is

not taken at a large strain.

Figure 20. Relations betweenprand volume changee/e0.

0 0.02 0.04 0.06 0.08 0.1 0.12 0.140

0.1

0.2

0.3

0.4

0.50.6

su

/p'y

e/e0

90oF10 (at large strains)

90oF10 (at small strains)

Other samplers

Figure 21. Relations betweene/eoandsu/pyof CKoUC triaxial


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Conclusions

This paper investigated the main geometrical features of samplers affecting sample quality.

Sample quality was evaluated using two nondestructive methods: residual effective stress

and maximum shear modulus, and destructive methods: unconfined compression test

(UCT), fall cone test (FCT), and CKoUC triaxial test. The results of this study showed that

sample quality is dependent on the geometry designs of the sampling tube, these are, edge

angle and area ratio. Comparative results of sample quality among different samplers were

obtained from samples of the same location inside the samplers. Important points found

from nondestructive methods are summarized as follows:

1) Small edge angle was very important for a sampling tube to minimize sample

disturbance.

2) The wall thickness, and thus, area ratio cannot be the sole and independent geometrical

factor affecting sample quality. A strong dependency on the edge angle was observed.

If edge angle was kept small enough, larger area ratio can be tolerated. In contrast, if

edge angle was increased, the area ratio must be specified and kept as small as

possible for a well designed and successful sampler.3) The effect of a piston played less significant role in disturbance for field sampling.

Comparative results of samples obtained from stationary pistons and open drive

samplers showed slight differences in sample quality.

4) Residual effective stress (p'r) and maximum shear modulus (GBE) were not

independent parameters but are closely related.

From mechanical tests:

5) The results in this study by UCT and FCT are very consistent with the nondestructive

tests.

6) Recompression technique in triaxial test, where the specimen is consolidated at the in

situ effective stresses, is used to overcome sample disturbance. However, if soilstructures of a soil sample are strongly destroyed such the case of 90oF10 samples in

this study, the technique cannot restore the undisturbed soil behavior. The sample

quality must be ranked at least in the category Good to fair or better from the criteria

of Lunne et al. (1997) or pr/pyis greater than 0.10 so as to duplicate the in situ soil

behavior.

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clays and representative sampling of coarser materials, Proceedings of the

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evaluation of sample quality by non destructive and destructive methods

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