10th Asian Regional Conference of IAEG (2015)

Characteristic of loosen rock mass had in the dam site based on the hard sandstonelayer in the CretaceousKazumi YASUMOTO(1), Yoshiya HITOMI(1), Jun YAMAZAKI(1)(1) Docon Co., Ltd.
E-mail:ky1448@docon.jp
Abstract
In the dam site based on hard bedrock, loosen rock mass by the development of
the matter such as sheeting joints, and it may be with a problem in the construction ofthe dam. The investigated dam site in investigation site, a hard sedimentary bedrockof the Cretaceous is distributed, and characteristic joints develops in the loosen rockmass. As an example of the evaluation method of the loosening introduced to evaluate
index by measuring quantity of opening of the joint, but was not able to find an
effective evaluation technique without having correspondence with the boreholefindings in information in the investigation site. Therefore the authors performed adetailed description about the above-mentioned characteristic joints. Moreover, theevaluation technique of the loosen rock mass adopted analytical technique to connect
borehole camera analysis with the detailed observation of the drilled core andsummarized about a property of distribution, structure and features of the joints in thedam site. As a result, a characteristic of the joints for the advance of loosening andthe distribution range of the joints is understood, and the loosen rock mass range ofthe dam site is distinguished. And moreover, new evaluation technique of loosen rockmass is also supported to geomorphic history viewpoint.
Keywords: loosen rock mass, topography-related joint
1.

Introduction

topography, geological features and influence of theexternal force.
To understand of the loosening generally used forthe evaluation technique that paid attention toquantity of opening of the joints. However, in theinvestigation site, it became clear not to support theevaluation of loosening that the above-mentioned
technique to the borehole findings. Therefore, it isnecessary investigate new evaluation technique forunderstanding of the loosening. The authors
evaluated the loosening by the evaluation techniqueof loosen rock mass that combined borehole-camera
analysis with detailed drilled core observation.
The investigation site, local topography feature
have the V-shaped valley, and both sides slope forms
the same slope of dips around 35 degrees together(Fig.1). Gentle slope is confirmed both sides slope,and terrace sediments are distributed over theposition that situated 10 meters high from theriverbed. Brecciated rocks less than 50 centimeters
distributed over the surface of the earth, and therolling stone of around 2 meters is rarely confirmed.

Sheeting joints with stress release inter the rockmay develop in the site distribute to hard bedrock. In
case of the development of the matter such as
sheeting joints in the dam site, and it may be with aproblem for the strength and deformability of thebedrock in the construction of the dam. This isbecause a property of the bedrock changes by joints
formed by a progress process of the loosening, and itis necessary to investigate the loosening for an
appropriate evaluation of the bedrock.
In general, a loosening is explained that “Thestate that, as a whole, it is easy to transform it while
producing outbreak, an opening and a gap of thecrack by transformation, volume increase and density
decrease, and so on that due to stress release,gravitation, weathering, and keeping a state of the
bedrock and being both easy to deform and an
inelastic property grew high (e.g., Sasaki et al.,2005)”. However, it is necessary to evaluate theloosen rock mass in each site because a progress styleof the loosening varies according to the condition of1

10th Asian Regional Conference of IAEG (2015)

The lithological units in the site are consist
mainly of hard sandstone layers, and mudstone andsandy mudstone layer are interbedded 10 meters
thickness of the sandstone unit. The conglomerates
accompanying the base of sandstone are usually lessthan 2 meters thickness. The geological structure
forms the monoclinal structure that trends north anddips around 70 degrees west. In the site, the conditionof the dam foundation rock is investigated 10 adits
and 119 drilled core by boreholes for the purpose ofconfirming it.

purpose of understanding the change of the joint
condition to the depth direction, it is investigated thatthe wall surface observation in the adit andobservation drilled core by borings.

2.

Characteristic of the loosen rock mass ininvestigation site
2.1 Characteristic of the loosen rock mass
The parallel joint (topography-related joint)develops in the bedrock formed by the stress releaseof the slope, and, in the investigation site, the loosenrock mass that formed by deformation of gravity to
the slope lower part. The loosen rock mass ischaracterized by the development of the joints of thereverse dip to topography, a large opening of the joint
and a turn of the bedrock which transformed blocking(Fig.2). In the investigation site neighborhood, small
collapse often occurs that affected by a heavy rain
and the earthquake.
A characteristic of the loosen rock mass have a
deep connection with joints represented bytopography-related joint. It was necessary to confirm
distribution and a condition of the joints forunderstanding of loosening. Therefore, for the2.2 Characteristic of the topography-related joint
As for the topography-related joint that is one of
the factors to cause the loosening, the continuity ofjoint is confirmed higher in sandstone than muddy
rocks. The structure of joints is inclined parallel to aslope in the slope part and low-angle in the riverbed

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Table 1 Change bedrock condition to the depth direction each factor of examination in the adit.
Factor of examination
Quantity of opening andcontinuity of the joint

Structure of the joint dip

Change bedrock condition to the depth direction(Result of the wall surface observation in the adit)
The opening joint increase in number and quantity of opening insuperficial part. The opening joint is rarely confirmed in a deep part.
The joint continues in superficial part, and it becomes discontinuous
as deepen, and the opening joint is not distributed at around 10-20
meters depth in a deep part.
The topography-related joint and the joint of the reverse dip to
topography are distributed in superficial part. The reverse dip to
topography is not distributed at 10-20 meters depth in a deep part.

Frequency of the joint

Frequency of joint is distributed more than five per one meter insuperficial part. Frequency of the joint is decrease as deepen.

Degree of weatheringbrown

The bedrock has weathering brown generally in superficial part. A ratioof weathering brown is decrease as deepen.

Seismic wave velocity ingroundIn superficial part, it is less than 1 km/sec and fast as deepen.
However, it is difficult to let the velocity support the bedrock condition.

Water permeability into
bedrock

High water permeability into bedrock is confirmed to the deep part.

part. The bedding plane dips dowm stream side, andstructure is different from topography-related joint.
The frequency of joints is distributed on thesuperficial part and becomes infrequent as deepen.
The joint is formed of multiple joints with abrown oxidation zone (weathering brown). The joint
performs divergence and convergence (transfer)having undulation.

correspond with the loosening that confirmed in theadit findings. Thus, the understanding of theloosening came to need rearranging by new
evaluation technique not to assume only quantity of
opening of the joint an evaluation standard.
Therefore the factor that related to loosen rockmass was extracted by the wall surface observation in
the adit (Table 1). These factors were examined
whether an evaluation of the loosening was possible
as dam foundation rock by a method to compare eachfactor.
Of these factors, the depth direction was able tounderstand the joint condition other than seismic
wave velocity and water permeability. In addition, theloosening condition and correspondence may not takefrequency and weathering brown of the joint as factor
of becoming it because different factors such as thedislocation participate. As a result of theaforementioned reasons, it shows that classificationof the loosen rock mass in the site (Table 2).

3. Investigation for the loosen rock mass
3.1 Investigation for the evaluation technique ofthe loosen rock mass
In the site, at much borehole points, the borehole
wall is observed by using “bored hole television
camera (borehole-camera)”. As for the understandingof the loosening, it was expected that can evaluate
based on opening quantity of the joint to conform byborehole-camera analysis. However, an opening of
the joint was reported in the deep part more than 100
meters depth in the site, and it became clear not toTable 2 Characteristic of classification of loosen rock massClassification ofloosen rock mass
Loosening part

Weakly loosening partLittle-non loosening
part

Condition of bedrock(Result of the wall surface observation in the adit)
The topography-related joint and the joint of the reverse dip to
topography distribute at random, and often opened and continued.
Some of them have quantity of opening more than 10mm. Weathering
brown of bedrock is generally confirmed.
Topography-related joint almost develops two-five per one meter. The
joint of the reverse dip to topography is not mostly confirmed. Although
the joint is rarely opened, the joint has not continuity. Weathering brownpart and fresh part are mixed.
Topography-related joint almost develops non-three per one meter. The
joint is closely contact. A fresh part is distributed widely.

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Table 3 Technique for judging of degree of the contact of the joint based on comparison of the two observation
Borehole-camera analysis
Degree of contact of the joint

Detailed drilledcore observation

Closely contact

Sandwiched rockfragment

OpenedEngagedClosedClosedClosedNot engaged

ClosedPossibility ofopened

Opened

3.2 Application to the dam site for evaluation
technique of looseningThe understanding of the joint condition of thewhole dam site had to make effective use of result byboreholes that investigated regionally. Therefore thefactors of quantity of opening and continuity andstructure of the joint that investigated effective
understand of the loosening in the adits.
Then, it is reported that degree of quantity of thejoint in observation of borehole-camera is notcorrespond with findings in the adit due to the joint
considered to a opening with a borehole camera isconfirmed as the closely contact joint in the adit
(Hitomi and Yamazaki, 2006). As a reason for that theclosely contact joint is evaluated by mistake as theopening joint, because of the rock fragment issandwiched between the joints in a adit is exfoliatedfrom borehole wall by mistaking water supply
pressure and contact the core sampler duringborehole investigation. That is to say, in case ofremaining the rock fragment to the joint by boreholecamera analysis and confirming the rock fragment
sandwiched between the joints by observation ofdrilled core, the joint is evaluated as possible toclosely contact.
Note the occurrence mentioned above, it is triedA

to classify the loosening by distribution and thecondition of these joints that the joint to relateloosening is extracted from various joints distributing
in dam site except for closely contact joints.
The evaluation of degree of the contact of thejoint is compared condition of the joint is based on
the borehole camera analysis with degree ofengagement of the joint is based on the detail coreobservation. Therefore, the evaluation of degree ofthe contact of the joint is judged from comparison of
the two observation techniques (Table 3).
The bedrock in the site is divided into three zones
based on evaluation technique of the loosen rockmass by Quantity of opening and continuity of thejoint, distribution of the joint of the reverse dip to
topography and frequency of the joint for the joint torelate loosening is extracted on Table 3(the joint
belong to possibility of opened and opened) (Fig.3).
Cross section of division of loosening are used
result of the evaluation of loosening in eachborehole (Fig.4).
The physical property of each loosening divisionindicated distributed concentrating each division.
That is to say, the weakly loosening part andlittle-non loosening part are judged adapted to thedam foundation rock.

southwardnorthward(m)Elevation
Fig. 4 Cross section of division of the loosening

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Fig. 3 Example of the evaluation of the loosening on borehole-camera analysis

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Evaluation of the loosening indicates slope
process of the site
In the site, the loosening that the topographyrelated joint formed by stress release in the slope,
make progress by the creep to the slope lower part by
the gravity. The talus is more progressive part ofloosening relatively early. In particular, the talus isdistributed thick in a south-facing slope affected by
the sunlight. In the undeveloped part of the joints of
the reverse dip to topography, loosening is little
developed, it forms erosion surface by recent
downward erosion. Distribution of loosen rock massthat divided into three zones in this paper greatlychange at altitude of terrace surface, the loosening is
undeveloped at the lower altitude. This characteristicof topography is indicated progress of downwarderosion by river.
5.

Conclusions
We understood a characteristic of the joint in theinvestigation site from a geologic and topographicalstandpoint and classified the joint, and understood acharacteristic of the joint in conjunction with thedevelopment of the loosening in the foundation rock.
And, the problems of the analytical technique using
quantity of opening of the joint are indicated bycomparison examination of the adit with observationof the core. The classification of loosen rock mass by
new technique relate to degree of the contact of thejoint is tried to divide. Note the classification
mentioned above is also supported to slope process
viewpoint. Therefore, evaluation of the loosen rockmass of the dam site is judged appropriately.
Acknowledgements
We express sincere gratitude to Y. Wakizaka andS. Anan (Pubric Works Reserch Institude) for helpful
suggestion, and also to K. Tomioka and Y. Tago
(Docon Co., Ltd.) for support during our field
investigation.
References

Hitomi, Y. and Yamazaki, J. (2006): An evaluation
method of slack bedrock by comparison of a boringcore. Japan society of engineering geology
conference 2006 collected papers, pp.505-508.
Sasaki, Y., Katayama, H. and Kurahashi, T. (2005):
An actual condition report and suggestion of theclassification tentative plan of loosen rock mass for
dam fundation rock. Engineering for dams, No.228,
pp.9-11.

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