10th Asian Regional Conference of IAEG (2015)

Fundamental research on the property assessment of ground improvement
utilizing the borehole wall imaging
Yasunori OOTSUKA(1), Hirokazu YASUTOMI(2), Katsuhiro TAJIMA(3) and Omer AYDAN(4)(1) Earth Scanning Association, Japan
E-mail:y.ootsuka@esa.gr.jp(2) Nippon Geophysical Prospecting Co, Japan,(3) Earth Scanning Association, Japan, (4) University of the Ryukyus, Japan
Abstract
In the field of ground improvement, various construction techniques have been developedto produce homogeneous solidified columns of well-defined dimension and strength. The
uniaxial compression strength obtained from laboratory soil tests of boring cores after theformed piles have been hardened is generally used to verify and control the quality of theimprovement piles. However, there is demand for techniques to quickly and continuously
identify the physical properties and mechanical properties associated with the improvement
piles that have been formed. In this study, we acquired borehole wall images throughinserting optical and ultrasonic borehole image scanner in the boring holes excavated in theimprovement piles that have been experimentally constructed, and have conducted uniaxial
compression tests using the boring cores of the improvement piles. We were able tocontinuously identify the shape and the distribution of both the void areas (unconsolidatedareas) and the solid areas (mixture of cement milk and soil in-situ). In addition, a trend
similar to that found between the porosity and the uniaxial compression strength of the core
specimen was also observed in the ultrasonic wave reflection intensity of the borehole walland the uniaxial compression strength of the core specimen. As a result, it was suggested
that it may be possible to quickly and continuously identify the uniaxial compression
strength of the improvement piles by means of the ultrasonic wave reflection intensity fromthe borehole walls.
Keywords: ground improvement pile, borehole image scanner, porosity, ultrasonic wave
reflection intensity
1.

Introduction

inclusive of both hard and soft consistencies thatwere formed under the test construction, and we
obtained borehole image data using optical andultrasonic borehole image scanner in the boring
holes. The results from the laboratory soil tests of thecore specimens exhibited a good correlation between
the porosity of the improved soil and the uniaxial
compression strength. In addition, the results of theimage analysis using the borehole wall image data
displayed a good correlation between the uniaxial
compression strength of the core specimen and theultrasonic wave reflection intensity of the boreholewalls. The results indicate that it is possible tocontinuously and quickly monitor and maintainawareness of the uniaxial compression strength of
the improved soil based on the borehole wall image
data. Some of the results of this research have beenpresented at the ARMS8 by Kawakami et al. (2014).

Ground improvement is a soft soil stabilization
method which mixes soft soil in-situ with solidified
material such as cement to produce soil-cement withhigher strength than the soil in-situ. As part of theoverall quality control for improved soil, we
generally verify that the target value for
improvement is reached upon the uniaxial
compression strength of the boring cores which wecollect after the formed piles finish hardening.
However, it is difficult to continuously monitor andmaintain awareness of the strength since laboratory
soil tests are limited in the area which is hardened tothe extent that it is possible to shape the testspecimen.
Therefore, in this study, we carried out core
boring on four improvement piles, which were110th Asian Regional Conference of IAEG (2015)

2.

Features of borehole image scanner

controller is operable in air temperature from 0 to40℃, enabling its application in most region of theworld.
The ODS probe is operable in temperature from0 to 40℃ and the USS probe even can be operated
from 0 to 50℃, so the probe can be used in mostnon-geothermal area. The USS probe can be used inborehole of diameters from 56 to 148mm while theODS probe can be used in borehole from 56mm to200mm in diameter. The 300m winch enables thesystem to be used in most boreholes for civilengineering purpose. Especially, ultrasonic borehole
image scanner details have been reported byKawakami et al. (2009) and Sato et al. (2009).

The components for ODS (Optical DigitalScanner) system and USS (Ultra Sonic Scanner)
system of borehole image scanner are shown in Fig.
1. As shown in the figure, both systems consist ofprobe, controller, depth encoder, remote control
winch and a note pc. Except the probe and thecontroller, all the other parts are common, so it isvery convenient to change the system from one toanother by only substituting the probe and thecontroller.
Specifications of both the ODS and USS are
shown in Table 1. As shown in the table, theUSS Controller

ODS Controller

Depth encoder
Remote control winch

USS Probe

Notebook Computer

ODS Probe

Fig. 1 System components for ODS and USS
Table 1 Specification of borehole image scannerArticleController

Probe

ItemsOperation temperaturePower supplySizeImage output
Operation temperature
Water proof
Borehole diameter
Size
Resolution
Ground water
Peformance

WinchOthers

Sensor rotation
LengthCable
Operation
Saving data
Depth pulse encoder
Survey speed

ODS (Optical Digital Scanner)
USS (Ultra Sonic Scanner)
0～40゜Ｃ (without moisture)
AC100V (ODS 200m system AC240V available)
272 ×252×45 mm
NTSC (RCA connector ) USB 2.0
0～40゜Ｃ (without moisture)
0～50゜Ｃ500m
φ56mm ～ φ200mm
φ56mm ～ φ148mm
φ50mm L = 1030mm
φ50mm L = 930mm
depth 0.25mm (min)depth 0.25mm (min)horizontal 360/720pixel
horizontal 360pixel
water or dry conditionwater condition
Borehole projected image 180/360degree
frequency 1.0 MHz
built -in orientation sensor
full color
black and white
no rotation
600 revolutions/min
200m & 300m
5 core,kevlar cablePC
HDD/external video recorder
HDD
0.25mm/Pulse
0～216m/ h(standard 54m/h)
0～36m/h210th Asian Regional Conference of IAEG (2015)3.

Overview of field tests

formed in a manner so that we could see the effects
associated with the specification of mechanicalperformance and mixing means, targeting volcanic
cohesive soil as shown in Table 2. The period inwhich these improvement piles were formed wasapproximately half a year prior to the fieldmeasurements performed in this study. We collectedboring cores in the position about 0.4 m (half of theradius) away from the center of the ground
improvement pile and acquired borehole wall images
with ODS and USS equipment in the same borehole.

We thought it ideal to use man-made soil that had
some variation in hardness as the target soil for thisfield test. Therefore, we decided to use four test
improvement piles. We knew in advance that thesepiles had some variation in uniaxial compression
strength due to being formed changing agitating andmixing specifications.
The construction conditions varied for each of
the improvement piles which were experimentally

Table 2 Construction conditions and test results for the ground improvement piles
Items

Construction
conditions

Test resultsof the cores
after piles
were formed

CASE 1

CASE 2

CASE 3

Target Soil Type

Volcanic cohesive soil

Cement Adding
Quantity

250 kg/m3

Improved depth
Improvementof Smooth Rotation
Drawing Speed

Core Solidification

Variation of UniaxialCompression
StrengthCASE 47.0mAbsence
Low

PresenceHighHigh△△(including(including somesome〇〇
unconsolidated
unconsolidatedparts )parts )The solidified area satisfies the target strength of500 kN/m2（Strength on 28th day）
Slightly large

3Low

Small

Slightly large

10th Asian Regional Conference of IAEG (2015)

4．Relationship between physical and mechanicalproperty

We show the maximum reflection intensity in agrayscale with 256 shades. The lighter shades ofgray depict higher reflection intensity, whereas thedarker shades of gray depict lower reflectionintensity.
The ODS image shows multiple clumps of voidarea (unconsolidated area) that were left behindduring the consolidation process. These clumps ofsoil appear to be darker on the USS image due to thetexture being softer than that found in the improved
and hardened clumps of soil. In addition, the tracesof scratches running in a vertical direction that werethought to be formed when the boring core tubes
were moving up and down were indistinct on theODS image. It is suggested that we can know thedifference of both hardness and asperity of theborehole wall by USS image.

Physical and mechanical laboratory tests areperformed using the boring cores. Fig. 2 shows therelationship between the wet density and the uniaxial
compression strength, and Fig. 3 shows therelationship between the porosity and the uniaxial
compression strength. From both of these figures, wecan see general relationships in which the uniaxialcompression strength increases in conjunction withincreases in wet density, and increases inconjunction with decreases in porosity.
Fig. 4 shows an example of both ODS and USS
borehole wall image which were measured in thesame respective location. The waveforms shown on
the right side are depictive of ultrasonic reflection
waveforms obtained at the circles in the USS image.

Fig.2 Relationship between uniaxial compression
strength and wet density

Fig.3 Relationship between uniaxial compression
strength and porosity

Fig.4 Comparison of ODS and USS borehole image with reflection intensity of USS4

10th Asian Regional Conference of IAEG (2015)

Next, Fig. 5 shows the changes in the ultrasonicwave reflection intensity and the uniaxial
compression strength in the depth direction. The
ultrasonic wave reflection intensity is depicted as anaverage value taken from 10 cm length of the entire
circumference (360°) on the borehole wall, which isdefined as circumferentially averaged reflection
intensity. From Fig. 5, we can see a trend ofincreases in the uniaxial compression strength andthe ultrasonic wave reflection intensity as we movetowards the bottom of the boreholes in Case 1, Case2, and Case 4. However, in Case 2, the ultrasonicwave reflection intensity decreased in the areas withdepths between 5.5 m and 6.5 m, whereas theuniaxial compression strength conversely increased.
This is seemed to be due to the reflection intensity
decreasing in response to the diffuse reflections by
the turbidity of slime suspension near the bottom of
the borehole. In contrast, in Case 3, we can see theconstantreflection
intensity
anduniaxial
compression strength from the ground surface to thebottom of the borehole. This is due to the differenceof construction conditions as shown in Table 2.

Interval in whichreflection intensity
decreased due toslime suspension

Fig.5 Relationship between reflection intensity and uniaxial compression strength in the depth510th Asian Regional Conference of IAEG (2015)

Fig. 6 shows the relationship between theuniaxial compression strength and the ultrasonicwave reflection intensity. The circumferentiallyaveraged reflection intensity for the section where
the core specimens (length=10cm, diameter=5cm)
for laboratory tests were collected was used to see
the relationship between the uniaxial compression
strength and the ultrasonic wave reflection intensity.
In the relationship diagram shown in Fig. 6, we don’t
use two sets of uniaxial compression strength data inCase 2 since the ultrasonic wave reflection intensity
was very low due to slime suspension in the deeparea. From this figure, although there was a varianceespecially within the area of high reflection intensity,
we can see a tendency that the uniaxial compression
strength increases in conjunction with increase in theultrasonic wave reflection intensity. The significant
variance in the data is due to void area
(unconsolidated area) which was left behind in the
improved clumps. If there is a hard consolidated areain the rear side, the uniaxial compression strength ofthe core specimen will increase. It is seemed to be
one of the reasons of fluctuations.

compression tests. Hereafter, we are going to further
develop our research in establishing more effective
assessment techniques for the ground improvement
pile due to borehole wall images.
Acknowledgements
In this study, we received a great deal of support
from Raax Co., Ltd with regard to measurements andequipment to perform these field experiments using
borehole image scanner. Additionally, Dr.Naohiko
Tokashiki from the University of the Ryukyus
provided valuable feedback with regard to thedirection in which we further develop this researchin future. Finally, we would also like to express ourdeep gratitude and thanks to all of those who
provided support to this research.
References
Kawakami,A., Yasutomi,H., Murata,A., Nakahara,T.,Hiraki,H., Moriguchi,Y., Ootsuka,Y. and Tajima,K.
(2014): Comparison of Borehole Scanning Systems,
Optical Digital Scanner (ODS) and Ultrasonic
Scanner (USS) , Proceedings of 8th Asian RockMechanics Symposium,ISRM, pp. 2224-2232
Kawakami,A., Fukui,K.,Yasutomi,H. and Sato,S.
(2009): Ultrasonic borehole scanner (in Japanese),the 121th Conference SEGJ, Proceedings,
PP.99-101Sato,S. and Katusima,N.(2009): Some examples ofground evaluation by ultrasonic borehole scanner(in Japanese) , Geotech e-forum2009,
Thesis number 77

Fig.6 Relationship between circumferentiallyaveraged reflection intensity and uniaxial
compression strength
5．Conclusions
In this study, we performed field measurements
using optical (ODS) and ultrasonic (USS) borehole
image scanner in the boring holes in addition to
performing uniaxial compression tests as a means toconfirm the quality of the ground improvement piles
which were made by changing mechanical agitating
and mixing treatment conditions. The results showed
a good correlation between the porosity (volumeratio of unsolidified soil) and the uniaxial
compression strength of the core specimen.
Moreover, we found a similar correlation between
the circumferentially averaged reflection intensity ofultrasonic wave from the borehole wall and theuniaxial compression strength of the core specimen.
This indicates the feasibility of a technique that canquickly and continuously assess the quality of theground improvement piles, complementing uniaxial

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