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Rates of weathering and temporal changes in strength of bedrock of marine terraces in Boso Peninsula, Japan
171
Rates of weathering and temporal changes in strength of bedrock of marine terraces in Boso Peninsula, Japan Shoichi Hachinohe ~, Nobuaki Hiraki
b Takasuke
Suzuki r
Railway Technical Research Institute, 2-8-38 Hikari-cho, Kokubunji, Tokyo 185-8540, Japan b Sanko Consultant Co. Ltd, Tokyo, Japan Institute of Geosciences, Chuo University, Bukyo-ku, Tokyo 112-8551, Japan
Accepted for publication 24 May 1999
Abstract
The weathering rates of bedrock of the dated erosional marine terraces were examined for two definitions: (1) the rates at which the thicknesses of weathered zones with different strengths increase with weathering time and (2) the rates at which the strength of weathered materials at an arbitrary depth decrease with time. The bedrock of terraces in the study area consists mainly of Tertiary sandstone and mudstone. The weathering profiles were observed from drill cores on three levels of terrace surfaces, i.e. 2850, 290 and 70 years B.P. in the emergence age of terraces, which is regarded as the weathering time of bedrock. The degree of weathering grade was described by the residual strength ratio, which is defined as the ratio of the needle penetration hardness of weathered part to that of fresh part of bedrock. The weathering rates of the first definition do not decrease monotonically, but decelerate logarithmically with time. Zones with lower weathering grades have faster weathering rates than those with higher weathering grades. With respect to the weathering rates of the second definition, mudstone starts to weather earlier than sandstone, but is exceeded by the latter after a certain time has elapsed, because mudstone has a higher slaking susceptibility. 9 1999 Elsevier Science B.V. All rights reserved. Keywords: Needle penetration hardness; Marine terrace; Soft rock; Tertiary sedimentary rock; Weathering rates
1. I n t r o d u c t i o n
Prediction of long-term changes in the properties of bedrock due to weathering has recently been a concern of civil engineers in relation to the stability of large-scale structures such as high cutslopes, huge bridge-anchors, underground power stations and radioactive waste repositories (e.g. Kojima, 1992). Most previous papers dealing with weathering rates, however, have not necessarily been relevant to or useful for engineering purposes, * Corresponding author. Fax: + 81-3-3817-1880.
E-mail address: [email protected] (T. Suzuki)
because most of the data reported have not been based on the direct measurements of the thickness of weathered materials of bedrock in the field or the exact determination of the duration of time for weathering, as reviewed for example by Ollier (1969), Kukal (1990, pp. 73-82), Selby (1993, pp. 155-156) and Matsukura (1994). Such a gap in the study of weathering rates stems from the difficulty in determining both the thickness of weathered materials and the weathering time. The main reason for the difficulty is that weathered materials are easily eroded away on hill slopes in particular, and hence the weathering time is hardly determined.
172
In order to overcome the above-mentioned difficulty, the weathering rates of bedrock under the dated marine-erosional terrace surfaces were investigated in the present study. This is because the thickness of the weathered part of terrace bedrock can be regarded as the whole thickness of materials that have been weathered since the emergence of terrace, and the age of the emergence is assumed to be equal to the duration of weathering.
2. Method
2.1. Studyarea Data were obtained for the bedrock that forms erosional marine terraces in the Boso Peninsula, south of Tokyo, Japan (Fig. 1). The terraces had been uplifted abruptly and repeatedly in association with great earthquakes. They are divided into five levels named the Numa I, Numa II, Numa III, Numa IV (Genroku) and Taisho surfaces, and their ages of emergence date back to around 6150 years, 4350 years, 2850 years, 290 years (1703 A.D.) and 70 years (1923 A.D.) B.P. respectively (Nakata et al., 1980). These ages were regarded as the duration of time for the weathering of
bedrock. The heights of former shorelines of these terraces in the study area are about 23 m, 17.5 m, 12.5 m, 5.5 m, and 1.5 m respectively. The bedrock of the terraces is the Pliocene marine sedimentary rock, which is divided into the Shirahama Formation, Shiramazu Formation and Mera Formation (Kotake, 1988). The Shirahama Formation consists mainly of tuffaceous sandstone interbedded with occasional thin beds of mudstone and conglomerate, whereas the Shiramazu Formation and the Mera Formation consist mainly of mudstone with thin beds of unconsolidated sandstone. Each bed varies from 5 cm to 3 m in thickness. These strata strike east-west in general and vary from 18 to 70 ~ in dip. In the present paper, the weathering rates were examined for the sandstone in the Shirahama Formation, and the mudstone in the Shiramazu Formation and Mera Formation. The bedrock of the terraces is covered with a veneer of alluvium (a sand bed including gravel) ranging from 0.1 to 1.6 m in thickness.
2.2. Definition of weathering rates The weathering rates are generally classified into the following three categories (e.g. Matsukura, 1994). The first definition is the rate dZ/dt at
Fig. 1. Distribution of marine terraces in the study area, south of Tokyo, Japan (Nakata et al., 1980). Drilling sites are indicated by open triangles for sandstone (S1-S 13) and open squares for mudstone (M 1-M 13).
173
which the thickness Z of weathered materials of bedrock increases with time t; the second is the rate dP/dt at which the rock properties P change with time t at an arbitrary depth; and the third is the rate of chemical denudation. Concerning the first definition, Suzuki and Hachinohe (1995) recently proposed empirical equations to express the rates of increase in thickness of four zones with different weathering grades; these were divided according to the visual observation of cut-slopes and drill cores and measurements of rock properties, including the pore-size distribution (PSD) for the bedrock of four levels of terraces in the same study area as the present one. However, the equations of Suzuki and Hachinohe (1995) were based on undifferentiated data for various kinds of lithology, such as sandstone, mudstone, conglomerate, alternated mudstone and sandstone, etc. In the present study, the weathering rates of both the first and second definitions were further re-examined on the basis of the changes in rock strength, PSD, mineralogical and chemical characteristics in the same study area as, but in a different way from, those by Suzuki and Hachinohe (1995). The weathering profiles were examined for drill cores obtained continuously from the top surface to a fresh part of the bedrock at 26 sites (13 sites for sandstone and 13 sites for mudstone) on the younger three levels of terrace surfaces, i.e. Numa III, Numa IV and Taisho (Fig. 1).
3. Occurrence of weathered materials and rock properties
3.1. Needlepenetration hardness In order to examine the changes in mechanical properties of bedrock due to weathering, it is preferable to test the weathering profiles continuously from the weathered part to the unweathered part for one parameter. The needle penetrometer (Model SH-70, manufactured by Maruto Co., Ltd, Tokyo) used in the present study can measure penetration loads Lp (kgf) and penetration depths Op (mm) simultaneously. The needle penetration
hardness Np (kgf/mm) is defined as
Np =Lp/Dp,
(1)
and Np is closely related to mechanical properties of rock, e.g. uniaxial compressive strength for a range from 0.3 to 40 MPa according to the test result by Maruto Co., Ltd. The uniaxial compressive strength of the fresh part of bedrock in the study area ranges from about 10 to 35 MPa for sandstone and from about 10 to 22 MPa for mudstone. Therefore, Np was adopted as an effective parameter to test the strength of bedrock from the weakest part to the hardest part in the study area. The needle penetration test was performed three times at the same depth at intervals of about 5 cm for all drill cores. Figs. 2 and 3 show examples of data of the needle penetration test for sandstone and mudstone respectively. For each of the weathered profiles, Np shows an almost constant value at the unweathered part, but decreases gradually as weathering proceeds. The depth where Np begins to decrease appears under the discolored zone of the weathering profile in general.
3.2. Pore-sizedistribution The PSD was measured by a mercury intrusion porosimetry, AUTO-PORE #9200 manufactured by Micromeritics Co., USA, using the same method as that by Suzuki and Matsukura (1992). The 'significant range' of PSD reading is from 0.0046 to 316 mm. The PSD was tested at eight sites for sandstone and five sites for mudstone (Table 1). Pores were tentatively divided into four grades according to the pore diameter D: large ( 101"5> D~ > 100.5 ~tm), medium ( 100.5 > D~ > 10-~ ~tm), small ( 10 -0.5 > D r > 10-1.5 ~tm) and very small ( 1 0 - 1 5 > D ~ > 4 . 6 • 10 -3 ~tm). The pore volume per unit weight of rock specimen is denoted as V~, V~, V~ and V~ (cm3/g) for the four grades respectively. In general, the highest peak of the PSD for the unweathered part of sandstone appears in the D~ zone, then, as weathering proceeds, the value in the D~ zone becomes smaller and the clear peak migrates toward the D~ zone (Fig. 4). Similarly, a clear peak in the case of mudstone migrates from
174
PSD
NPH v. E 0 KI
~
_
-
~, ~
(
:
.....
i
~
,~:. :
:
i~ : i:~i i!,. . . . . . ', ~ ! ~:~~ , P
.
v,
v~
.-
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.. . . . . .
i.
i
:
:
:
"
-.&:..&;
. . . .
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v,
! >; i{ ! .... '
0
~=
XRF
L..?_ _. t.
.
i
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!
~ ~ i..*i ~'~ 9
:
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9149
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....... iI
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o
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.7.~~ i ......... ii/ii]
.&
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...
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......
i ............~i........~'~.....i----i----i--i---i--i!.!---!-~-.!i: 9
:
9 9. . . . . . . . . . .
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, 51
:
.
.
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.....~,,.......... ~,
. . . . . . . . . . . . .
.
......................
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...... !
10
0
...................
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Np (kgf/mm)
0.10
0.10
0.1
0
0.30
2
Pore volume(cm3/g)
10
40
Weight
percent
20
50
60
70
(%)
Fig. 2. Changes in rock properties due to weathering of sandstone (an example, at $4 site). NPH: needle penetration hardness, where open triangles show the data of discolored zones and filled triangles are those of undiscolored zones. PSD: pore-size distribution (for the ranges of V=, V~, Fv and F~ refer to the text). XRF: X-ray fluorescence analysis.
PSD
NPH v,
"g 00o:
I
v~
XRF Vx
V~
.....
i
o
" ]
.... Ii
!
' _
I
-~
~
0.4
;- ! "~-=:, 9 ::"',
t Mds
:
1
-.. ilit
~ o 0"~=2
10 Np ( k g f / m m )
0-3_1
0 '0.
0
0.20
Pore volume(cm3/g)
0.1
0
1
2
3
0
1 Weight
2
3 0 percent
10
2050
60
70
(%)
Fig. 3. Changes in rock properties due to weathering of mudstone (an example, at M8 site). NPH: needle penetration hardness, where open circles show the data of discolored zones and filled circles are those of undiscolored zones. PSD: pore-size distribution (for the ranges of V=, V~, Fv and V~ refer to the text). XRF: X-ray fluorescence analysis.
175
Table 1 Summary of the depths from the bedrock surface to the thickness to the discolored front Zac, the mean needle penetration hardness of unweathered part Npf and the analysis method: NPH is the needle penetration hardness; PSD is the pore-size distribution; X R D is X-ray diffraction analysis; XRF is X-ray fluorescence analysis. Open circles show those measured and crosses are those not measured for any drill core Lithology
Terrace surface
Sandstone
Taisho
Numa IV
Numa III
Mudstone
Taisho
Numa IV
N u m a III
Loc. no.
Zac (m)
Npf (kgf/mm)
weathering time (years B.P.) 70
290
2850
70
290
2850
Analysis method NPH
PSD
XRD
XRF
S1 $2
0.06 0.04
3.25 8.04
O O
O O
O Q
O O
$3
0.04
5.28
O
O
•
•
$8 Sll
0.05 0.09
5.03 4.94
O Q
x x
x 9
x x
S12
0.09
5.92
O
x
x
x
S13
-
2.56
O
O
x
O
$4 $5
0.70 0.78
8.47 8.94
O O
O O
O x
O x
$6
0.30
-
O
x
x
x
$7
0.50
-
$9
0.28
8.62
0 Q
0 0
x 0
0 0
S10
0.30
8.62
Q
x
x
x
M5 M7
0.02
3.45 3.83
O
x
x
x
M8 M9
0.05 0.12
4.95 3.27
9 9
9 9
9 9
9 9
O
x
x
x
M6 M12
0.13
3.78 2.71
O O
x x
x x
x x
M13
0.14
2.56
O
x
x
x
M1 M2
0.56 0.65
2.68 3.01
O O
x x
x x
x x
M3 M4
0.45 0.46
2.74 2.85
9 9
9 x
x x
x x
M10
0.55
5.18
Mll
0.85
5.26
9 9
9 0
9 9
9 9
smaller pore-diameters toward larger ones, as weathering proceeds (Fig. 4).
3.3. Mineralogical and chemical properties Mineral analysis was done by the X-ray diffraction method (XRD) for bulk powder samples using a Rigaku RAD-C. Chemical analysis was carried out for SiO2, TiO2, A1203, FeO" (FeO+Fe203), MnO, MgO, Ca9 Na20, K20 and P 2 0 5 by the X-ray fluorescence (XRF) analysis method using a Rigaku 3270. Observation under a polarization-microscope and scanning electron microscope (SEM) clarified changes in
the shape of minerals due to weathering. Based on this investigation, the'general characteristics of the mineralogical and chemical properties of sandstone and mudstone in the study area are summarized as follows. 3.3.1. Sandstone
Sandstone contains quartz, feldspar, smectite, kaolinite and zeolite. As weathering proceeds, Na20 starts to decrease (e.g. at around 2 m deep from the bedrock surface, Fig. 2), whereas other chemical components show no systematic changes. The diffraction peaks of smectite for the glyceroltreated specimens have a lower peak in shallower
176
Sandstone 0.03 . . . . . . . .
o.o -
I...... ~ '
_2_
!. . . . . . .
5(0.65)[
~
I. . . . . . .
Mudsto n e w. . . . . . .
7
!
i
i
IJII
103
i
i
_i
l
II
102
101
l
i
, i
0
I..... ;' ' '
10 ~
10 1
l
10 .2
Pore diameter, D ( F m)
I
----
10 .3 103
102
101
10 ~
10 1
10 .2
10 "3
Pore diameter, D ( ,u rn)
Fig. 4. E x a m p l e s o f the c h a n g e s in the differential p o r e - v o l u m e d i s t r i b u t i o n d u e to w e a t h e r i n g o f s a n d s t o n e (at $4 site in Fig. 2) a n d m u d s t o n e (at M 8 site in Fig. 3). F o r the r a n g e s o f V~, Va, V~ a n d V~ refer to the text.
parts than in the deeper parts (Fig. 5). This means that the crystallinity of smectite becomes lower due to weathering. Surface features of feldspar in the weathered part change, compared with the fresh part, to have a number of large cracks around 1 lam in width (Figs. 6 and 7). 3.3.2. Mudstone
Mudstone contains quartz, feldspar, smectite, kaolinite, illite and calcite. The peaks of calcite and feldspar at the top of the weathered part are markedly lower than those in the deeper part of the weathered zone (Fig. 8). This trend was seen with all drill cores of mudstone. C a d tends to decrease in the weathered part (e.g. at 0.08 m deep, Fig. 3), whereas other chemical components show no systematic changes. A lot of calcite is contained in both the unweathered part and the highly weathered part, but some grains of calcite are dissolved in the weathered part. In addition, pyrite was not specified by the XRD method for sandstone and mudstone.
3.4. Mechanisms of decrease in rock strength
Based on the above-mentioned occurrence and properties of weathered materials, decreases in rock strength are considered to result mainly from the increases in the pore volume of larger pores due to weathering (Fig. 4), because the larger the pore volume of larger pores in the weathered bedrock is, generally the smaller the strength becomes (Yamashita and Suzuki, 1986). In the case of sandstone, increases in pore volume of medium-size pores (V~) ma3? be mainly due to the formation of large cracks on the feldspar surface, as supported by Figs. 6 and 7. In the case of mudstone, on the other hand, increases in pore volume of medium- and small-size pores (V~ and V~) may have resulted from the dissolution of calcite as well as the formation of large cracks on feldspar surfaces due to weathering; hence the initial size and content of calcite may be the main variables controlling changes in the PSD due to weathering, because the dissolution of calcite and
177
Fig. 5. Changes in X-ray diffractograms of oriented and glycerol-treated bulk samples (G.t.) due to weathering of sandstone at the S1 site: Sm, smectite; Z, zeolite; A, the front of discoloration; B, the depth where R~ becomes almost constant. Figures on the right-hand side show the depth (meters) from the bedrock surface.
Fig. 6. Representative photographs of feldspar in sandstone (at the S1 site) under an optical microscope: (a) weathered (discolored) part; (b) fresh part.
decreases in CaO are recognized during the early stage of weathering of mudstone in the study area.
4. Weathering rates
4.1. Rates of increase in the thickness of each weathered zone with different strengths The needle penetration hardness Npf of the initial bedrock or the flesh part differs in different beds, ranging from 2.56 to 8.94 kgf/mm for sandstone and from 2.56 to 5.26 kgf/mm for mudstone (Table 1 ). In order to compare the weathering grades for all drill cores, therefore, the effect of the difference in Npf on the changes in the mea-
sured Np of the weathered part was corrected by converting the measured Np into the residual strength ratio R~ (%), which is defined as
R~=(N./N.O •
100,
(2)
where Np is the measured value and Npf is an averaged value for the fresh part of each drill core (Table 1). The relationship between the residual strength ratio Rs and the depth from the bedrock surface Z (Fig. 9) shows that R~ decreases with increases in the grade of weathering and that the longer the weathering time is, the larger the decrease in Rs is. The dispersion of data in Fig. 9 may be mainly
178
Fig. 7. Secondary electron image of feldspar of sandstone in the weathered (discolored) part at the S1 site.
due to the variety in the microtexture of the rock even in the fresh part. Regarding the temporal change in strength due to weathering as a mass diffusion process from the bedrock surface, therefore, the relationship shown in Fig. 9 is redrawn schematically in Fig. 10 and expressed by a function similar to that proposed by Hirano (1990) and Yokota ( 1992): R~ = R~o)
+ (Rsf Rs(o) ) ( 1 --
e - kZ ) ,
(3)
where Rs(o) is R~ at the top of the bedrock surface, Z is the depth from the bedrock surface, and k and R~f (= 100%) are constants. Eq. (3) leads to
, In [ 1 - Rs Rs,o,].
k=
Z
(4)
Rsf - Rs(o)
According to the data of Fig. 9, the constant k for both lithologies decreases with the weathering time t. The relationship between k and t is shown in Fig. 11 and is expressed as k=at
(5)
b,
where a and b are constants: a = 6 . 5 x 10 2, b = - 7 . 6 x 10 -a for sandstone and a=4.3 x 102, b = - 5.7 x 10-1 for mudstone. According to R~ for the bedrock surface, the relationship between Rsm) and t is shown in Fig. 12 and is expressed as (6)
R~
where
Fig. 8. Changes in X-ray diffractograms of oriented samples due to weathering of mudstone at the M8 site: Q, quartz; F, feldspar; C, calcite; A, the front of discoloration; B, the depth where Rs becomes almost constant. Figures on the right-hand side show the depth (meters) from the top surface of the bedrock.
c and
d are
constants:
c = 8 . 8 x 10 ~,
d = - 2.7 x 10-1 for d= - 6.7 x 10- 2 for Eqs. (5) and (6), R0 ing time t. By using bedrock surface to expressed by Z
sandstone and c = 2.4 x 101, mudstone. As indicated by and k decrease with weatherEq. (3), the depth from the the weathered zone Z is
1 ln[J~f~R~m) ] ~
~
k
,
(7)
!_ R s f - Rs
The depth Z differs depending on the weathering grade indicated by Rs. By using Rs as a parameter, the weathering profiles were tentatively divided into seven weathered zones, of which the bases or weathering fronts are from 30 to 90% at intervals
179
Numa A)
111
Numa
B)
t = 2,850 yrs.
Taisho
IV C)
t = 290 yrs.
E N
9
~x.
9 ,-
1
CO
-;~
-
0.1
i
----~
...
0.2
1.5
]
0.05
l
0.15
I
~e
o O
,
0.05 0.5
o ,'-
.~,~
t = 70 yrs.
0
o
9
0.1
0.25 rn
2~--
E =
0.3
2.51
0.35 !
041 s.n,,,one t
I Sandston e
c~ 3
100
10
10 Strength
Numa
D)
Remaining
111
0.2 10
100 Ratio,
Rs
100
(%) Taisho
Numa E)
t = 2,850 yrs.
F)
t = 290 yrs.
t = 70 yrs.
0
0:
ol
A
E N
o
0.05 O.5
..
d
~
L :~
~ '
0.05
0.1 -
0.15
-
-}
r e t
o 0
1.5
m m E o ,-
~ 2L ~ t 2.5!
L
L
a
.o-~-
~
-,-!"
'
1 e
i
0.2
~ r
1
100
.
.
.
. .
.
. .
. .
.
o4 .
9
-
.e
J
0.15
0.3
. . . . . . . . . . . . .
0.35
3' 10
. ...
~.. -e.
.
18-
iq
L
~ Mudstone
....
0.25
.-i~ . . . .
0.1
Mudstone
0.4
0.2
100
10 Strength
Remaining
Ratio,
10
o-
9
. . . . =
I J
100
R s (%)
Fig. 9. Changes in the residual strength ratio Rs at different depths Z of sandstone and mudstone at three different levels of terraces, i.e. Numa III (2850 years B.P.), Numa IV (290 years B.P.) and Taisho (70 years B.P.). All data obtained from the drill cores on each level of terraces are plotted together on each diagram, excluding exceptional data such as those along major joints where bedrock is markedly weathered. Data for sandstone from: (A) $4, $5, $9, S10; (B) $8, S11, S12, S13; (C) S1, $2, $3. Data for mudstone from: (D) M1, M2, M3, M4; (E) M6, M12, M13; (F) M5, M7, M8, M9. Open symbols show the data of discolored zones and filled symbols show those of undiscolored zones. The bedrock surface is the same as the bottom of the veneer of alluvium.
180
Fig. 12. Relationship between Rs(o~ in Eq. (6) and weathering time t for sandstone and mudstone. Data for the cores of which the top parts were broken with drilling were eliminated. Fig. 10. A schematic diagram of the relationship between the residual strength ratio Rs and the depth from bedrock surface Z. The suffix (0, t,) of Rs means R~ at the bedrock surface (Z = 0) at a given weathering time t.
of n = Rs into Eq. (7) gives 1 ln~Rsf--R~~ ]
Z.:~ L
Rsf--/'/
(8) "
By using Eq. (8), the weathering fronts indicated by Z, are plotted against the weathering time t in Fig. 13 separately for sandstone and mudstone. When the weathering grade indicated by n is constant, differentiation of Eq. (8) gives the rates of increase in the thickness of Z, as = dt
,{ iRsf-Rso,l -In
k2
k'+
Rsf - n
Rs(o) - Rsf
t
. (9)
Fig. 11. Relationship between coefficient k in Eq. (5) and weathering time t for sandstone and mudstone.
of 10% in Rs. The depth from the bedrock surface to each of the weathered zones thus divided is designated as Z,, in which the suffix n is Rs. Defining the weathering grade as n, substitution
The values of dZ,/dt thus defined are the weathering rates at which the thickness of weathered zones with different strengths increases with time or the weathering rates of the aforementioned first definition. It shows that the weathering rates do not decrease monotonically, but exponentially with time (Fig. 14), and that the zones with lower weathering grades have faster weathering rates than those with higher weathering grades.
181
.'..'."
10'
"
90%
N=
~
80%
0
'~
60%
~z
9 5o%
"" :" .,," --:--".".
..-.~....
"
90%
~
80%
9
70%
-"""
. .
.
/////.-..
:~:
o
40%
<>""
o
40%
"
*
30%
"
9
30%
o'-',~ i
-
--"
-:. :
-
"
"0
l")
~ 0-,L
& 1
a
..ilJi
.
i;51
..-
Sa n dstone
[,,., 10-3 . 10 ~
, 102
10'
103
10'
~. .. '..10 s 10 ~
..
- " -. " '
."
..
". . " -
."..." . ." ,
.:"
.10'
Mudstone 102
103
Weathering time, t, (years)
1 O"
105
Weathering time, t (years)
Fig. 13. Relationship between Z,, the depth of the front of weathered zones with different Rs, and the weathering time t. Thick lines show the regression lines for the measured range and the dotted lines are those interpolated and extrapolated by using Eq. (8).
E E
I
1.5
Sandstone
"13
'"", c0 N "~
Mudstone
,
1.0
c'-
0 or)
c ._o
"'....
0.5
r
"-_
0
._= 0 or}
n"
0
....
101
102 Weathering
103 time,
t (years)
10"10'
102 Weathering
103
10'
time, t (years)
Fig. 14. Relationships between the rates of increase in thickness of weathered zones d Z , / d t and the weathering time t. Thick lines show the regression lines for the measured range and the dotted lines are those interpolated and extrapolated by using Eq. (9). The symbols are the same as those in Fig. 13.
4.2. Rates of deepening of discoloredfront The weathering profile of sedimentary soft rock is often divided into three zones, i.e. surface oxi-
dized zone, oxidized zone and dissolved zone in the order from the bedrock surface to deeper parts. The oxidized zone is generally characterized by discoloring from dark gray to yellowish brown
182
with FeO decreasing and Fe203 increasing (e.g. Chigira, 1990; Chigira and Sone, 1991). In the study area, the lower limit of the discolored zone of the bedrock is clear and sharp, especially in sandstone, but rock properties such as Rs and PSD still change even below the discolored zone. Accordingly, the lower limit of the discolored zone seems to be the oxidation front of iron in the present study, although XRF cannot distinguish FeO from Fe203. The relationship between the depth of the discolored front Zdc and the weathering time t for both lithologies is plotted in Fig. 15, in which the relationship between the residual strength ratio and time (Fig. 13) is added with broken lines. Fig. 15 shows that mudstone starts to discolor earlier than sandstone, and the discolored front reaches the depth where Rs ranges from 40 to 80% for sandstone and over 90% for mudstone during the measured range in the weathering time. The regression lines in Fig. 15 are expressed to a first approximation as
Zdc=et I,
f = 5.3 x 10-1 for mudstone. Differentiation of Eq. (10) gives the rates of deepening of the discolored front (dZdc/dt) as
dZac/dt= gt h,
( 11 )
where g and h are constants: g = 1.5 x 10 .3 and h = - 3.4 x 10-1 for sandstone and g = 4.0 x 10- 3 and h = 4 . 7 x 10-1 for mudstone. As demonstrated in Fig. 16, the rate of deepening of the discolored front of mudstone is faster than that of sandstone before about 2000 years in the elapsed time, but the former is expected to be exceeded by the latter after that time.
4.3. Rates of decrease in the bedrock strength due to weathering at arbitrary depths For practical purposes, we need the rates at which the bedrock strength at arbitrary depths decrease with time due to weathering. Based on Eq. (3), temporal decreases in the bedrock strength at four different depths Z = 0 . 0 1 , 0.03, 0.10 and 0.30 m are demonstrated in Fig. 17. The strength of sandstone at the 0.10 m depth, for example, remains unchanged for about 200years in the elapsed time, and after that the strength will begin
(10)
where e and f are constants: e = 2 . 3 x 10 -3 and f = 6.6 x 10-1 for sandstone and e = 7.6 x 10-3 and
10' L
90 %
.. 1 0 ~
.'
-"
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.-.,
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I
-"'"
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.."
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./
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,
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.
.
.
.
.
.
.
.
.
.
.
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.
.
10 3
.
.
Mudstone J
.
10' 10'
time, t (years)
10 2
iO 3
10 ~
Weathering time, t (years)
Fig. 15. Relationship between the depth of the front of discolored zones Zdc and the weathering time t of the measured range for sandstone and mudstone. The broken thin lines are the regression lines for Rs shown in Fig. 13.
183
10 4
t
assumed to be constant, differentiation of R~ with respect to t in Eq. (3) gives the rates as follows:
:-A
dZdo/dt -
dRs dt
= e -zk {Z[Rsf- R~(o)]k'+ R~(o)}.
.
..
.......
Mudstone
-...
The rates d R s / d t thus defined are the weathering rates at which the strengths of weathered materials at an arbitrary depth decrease with time or the weathering rates of the aforementioned second definition.
Sa 10 2 ! -..
7
r 101 t 10 2
10 ~
10 3
Weathering
5. Discussion and conclusions
10 4
time,
t
(years)
Fig. 16. Relationship between the rates of increase in the depth of discolored front dZdc/dt and the weathering time t for sandstone and mudstone. Thick lines show the regression lines for the measured range and the dotted lines are those interpolated and extrapolated by using Eq. (11 ).
I00,
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.
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.
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.
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,.
.
.
.
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103
10'
105
Weathering time, t (years)
Fig. 17. Decreases in the residual strength ratio Rs with weathering time t at four different depths of sandstone and mudstone.
to decrease due to the start of weathering at this depth. Then, the strength decreases with time and will become 30% of the initial strength after 2000 years. When the depth from the bedrock surface Z is
5.1. C o m p a r i s o n o f w e a t h e r i n g rates between sandstone and mudstone
To compare the modes of decrease in the strength of sandstone and mudstone, the curves showing the temporal decreases in Rs for sandstone and mudstone in Fig. 17 are superposed for each of the four different depths (Fig. 18). The strength Rs decreases with time t, but the weathering rates d R / d t are different between sandstone and mudstone at every depth and at every weathering time except in the case of about 70 years at 0.01 m depth and 0.03 m depth. At shallower zones, where the depth is less than 0.03 m, mudstone begins to weather earlier than sandstone, but after a certain length of time from the beginning of weathering, i.e. about 70 years, mudstone will be exceeded by sandstone in the rates of decrease in Rs [Fig. 18(A)]. However, at zones deeper than about 0.03 m, sandstone starts to weather earlier and faster than mudstone [ Fig. 18 (B)-(D)]. The reason for the earlier and faster weathering of mudstone at shallower zones mentioned above is explained as follows. In the study area, the bedrock of erosional marine terraces is considered to begin weathering just after the emergence of wave-cut platforms due to the abrupt uplift associated with great earthquakes because of the slaking at shallower parts of bedrock. Generally, the slaking rates of sedimentary rock are closely related to both the content of montmorillonite and the volume of smaller pores (Matsukura and Yatsu, 1982). Field observations in the study area show that mudstone is higher than sandstone in terms
184
100 . . . . . .
~,
90L-
>\ ................ ",
.... Z " = o.o1m ~
\
8ol- Mudstone",'-\ \ Sandstone ,
7o~
"-~
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",\
so,L 4oi A 30~
, ............. 10'
10 ~
100 . . . . . . . . . . . .
X-. I0 z
,~ >-,k..... --
1
' I'~'O'
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~ .............................
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Sandstone
94.o --
",
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c
10 ~
90~,
:::3
8G-
Mudstone
................... 10'
\ "',, "~ 102
.~..., 10 ~
70~ t.~
................. 10"
""
!
10 s
J
60L
"",,,..,
so~40
'
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_.m r
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C
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, .................
.~
.
..
' .... i 0 ' " . . . . . .
i'0 s
.>,...,,.Z,,,: o.30, ~m.,
i
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"",, Mudstonej
50i
i
",
40~ D 301 10 ~
",,
.......... . . . . . . . 101
I 0z
1 03 W e a t h e r i n g time,
i
...~i",';": 10 ~
5.2. Predictive model for decreases of bedrock strength due to weathering To predict the bedrock strength at a given depth and at a given elapsed time after the beginning of weathering, a model is proposed below. Fig. 19 shows the relationships between the residual strength ratio Rs and the depth from the bedrock surface for a given weathering time t at intervals of 500 years after the beginning of weathering. By using Fig. 19, it is possible to predict the strength in the form of Rs at a given depth and at a given time for the same sandstone and mudstone and under the same weathering environment as those in the study area. Fig. 19 also implies the temporal changes in both the rates of increase in the thickness of weathered zone with different strengths, i.e. that of the first definition of weathering rates mentioned above, and the rates of decrease in the bedrock strength at an arbitrary depth, i.e. that of the second definition of weathering rates. The model proposed here cannot be applied to such cases as man-made high cut-slopes and natural hill slopes, which are different from the study area. However, since the needle penetration hardness has a good correlation with the modulus of deformation and elasticity (Ichikawa et al., 1988),
10 s
t (years) 0[
i
,
i
Fig. 18. Difference of temporal changes in the residual strength ratio R, with weathering time t between sandstone and mudstone at four different depths Z. o t~
of slaking susceptibility. The reasons for this are that first, mudstone has about twice the volume of smaller pores as does sandstone (i.e. Vy and V~), as shown in Fig. 4, and second, because mudstone has about half the Np of the fresh part compared with sandstone, as shown in Figs. 2 and 3. Accordingly, at zones shallower than 0.03 m from the bedrock surface, mudstone responds quickly and sensitively to slaking. However, there are no data useful to explain the fact that the weathering rates of mudstone are exceeded by that of sandstone after a certain time length at a specific depth.
o0 x.. 73 (D XD
%
0.5
E 0
Sands
" Mudstone
30 40 50 60 70 80 90 100
30 40 50 60 70 80 90 100
Residual strength ratio, Rs(% )
Residual strength ratio, Rs(% )
Fig. 19. Predictive decreases in the residual strength ratio R, due to weathering at arbitrary depths Z and at weathering time t given at intervals of 500 years for sandstone and mudstone.
185
the m o d e l m a y give a conceptual basis to establish an empirical equation to predict the bedrock strength under various environments for weathering in a m o r e practically and useful f o r m for engineering purposes in the future.
Acknowledgements The authors would like to t h a n k the students majoring in Engineering G e o l o g y at C h u o University for their help with drilling and measurements of rock properties in the field and in laboratories. The authors also express their appreciation to D r T. H a t t a of J a p a n I n t e r n a t i o n a l Research Center for Agriculture Sciences, Ministry of Agriculture, to permit the use of his l a b o r a t o r y and for his helpful suggestions. T h a n k s should also be extended to D r C.T. Oguchi and Professor Y. M a t s u k u r a of the University of T s u k u b a for their help with X R D and X R F tests, and also to D r H. Kiya, D r T. O h t a and D r S. Y o k o y a m a of the Railway Technical Research Institute for their support and e n c o u r a g e m e n t . This study was partly supported by the G r a n t - i n - A i d for Scientific Research (contract no. 07640609 in 1995-1996 for T. Suzuki) and the G r a n t - i n - A i d for JSPS Fellows (in 1996-1998 for S. H a c h i n o h e ) of the Ministry of Education, Science, Sports and Culture of Japan.
References Chigira, M., 1990. A mechanism of chemical weathering of mudstone in a mountainous area. Engineering Geology 29, 119-138. Chigira, M., Sone, K., 1991. Chemical weathering mechanisms and their effects on engineering properties in soft sandstone and conglomerate cemented by zeolite in a mountainous area. Engineering Geology 30, 195-219.
Hirano, M., 1990. Mass diffusion models in Geomorphology and the related problems, Transactions, Japanese Geomorphological Union, 11, 191-215. In Japanese with English abstract. Ichikawa, K., Hirano, I., Sasaki, Y., Jinbo, S., 1988. Investigation of classification of soft bedrock for dam foundation (part 2) - - a case study of Miocene sandstone. Technical memorandum of Public Works Research Institute, Ministry of Construction, 2545, pp. 1-82 (in Japanese). Kojima, K., 1992. Treatment of weathering and decomposition in engineering works; 1. Introduction, Japanese Society of Soil Mechanics and Foundation Engineering, 40 (5), 65-66. (in Japanese). Kotake, N., 1988. Upper Cenozoic marine sediments in southern part of the Boso Peninsula, central Japan. Journal of the Geological Society of Japan, 94, 187-206. In Japanese with English abstract. Kukal, Z., 1990. The rates of weathering. In:The Rates of Geological Processes. Earth-Science Reviews 28, 73-82. Matsukura, Y., 1994. A review of the studies on rock control in weathering processes, Transactions, Japanese Geomorphological Union 15, 203-222. In Japanese with English abstract. Matsukura, Y., Yatsu, E., 1982. Wet-dry slaking of tertiary shale and tuff. Transactions, Japanese Geomorphological Union 3, 25-39. (in English). Nakata, T., Koba, M., Imaizumi, T., Jo, W.R., Matsumoto, H., Suganuma, T., 1980. Holocene marine terraces and seismic crustal movements in the southern part of Boso Peninsula, Kanto, Japan, Geographical Review of Japan 53, 29-44. In Japanese with English abstract. Oilier, C.D., 1969. Weathering. Oliver & Boyd. Selby, M.J., 1993. Hillslope Materials and Processes. 2nd edition, Oxford University Press. Suzuki, T., Hachinohe, S., 1995. Weathering rates of bedrock forming marine terraces in Boso Peninsula, Japan. Transactions, Japanese Geomorphological Union 16, 93-113. (in English). Suzuki, T., Matsukura, Y., 1992. Pore-size distribution of loess from the Loess Plateau, China. Transactions, Japanese Geomorphological Union 13, 169-184. (in English). Yamashita, S., Suzuki, T., 1986. Change in pore-size distribution of sedimentary rocks due to weathering and the resultant decrease in their strength, Transactions, Japanese Geomorphological Union 7, 257-273. In Japanese with English abstract. Yokota, S., 1992. Mathematical models of weathering process in jointed rock masses, Journal of the Geological Society of Japan 98, 155-163. In Japanese with English abstract.
Rates of weathering and temporal changes in strength of bedrock of marine terraces in Boso Peninsula, Japan Shoichi Hachinohe ~, Nobuaki Hiraki
b Takasuke
Suzuki r
Railway Technical Research Institute, 2-8-38 Hikari-cho, Kokubunji, Tokyo 185-8540, Japan b Sanko Consultant Co. Ltd, Tokyo, Japan Institute of Geosciences, Chuo University, Bukyo-ku, Tokyo 112-8551, Japan
Accepted for publication 24 May 1999
Abstract
The weathering rates of bedrock of the dated erosional marine terraces were examined for two definitions: (1) the rates at which the thicknesses of weathered zones with different strengths increase with weathering time and (2) the rates at which the strength of weathered materials at an arbitrary depth decrease with time. The bedrock of terraces in the study area consists mainly of Tertiary sandstone and mudstone. The weathering profiles were observed from drill cores on three levels of terrace surfaces, i.e. 2850, 290 and 70 years B.P. in the emergence age of terraces, which is regarded as the weathering time of bedrock. The degree of weathering grade was described by the residual strength ratio, which is defined as the ratio of the needle penetration hardness of weathered part to that of fresh part of bedrock. The weathering rates of the first definition do not decrease monotonically, but decelerate logarithmically with time. Zones with lower weathering grades have faster weathering rates than those with higher weathering grades. With respect to the weathering rates of the second definition, mudstone starts to weather earlier than sandstone, but is exceeded by the latter after a certain time has elapsed, because mudstone has a higher slaking susceptibility. 9 1999 Elsevier Science B.V. All rights reserved. Keywords: Needle penetration hardness; Marine terrace; Soft rock; Tertiary sedimentary rock; Weathering rates
1. I n t r o d u c t i o n
Prediction of long-term changes in the properties of bedrock due to weathering has recently been a concern of civil engineers in relation to the stability of large-scale structures such as high cutslopes, huge bridge-anchors, underground power stations and radioactive waste repositories (e.g. Kojima, 1992). Most previous papers dealing with weathering rates, however, have not necessarily been relevant to or useful for engineering purposes, * Corresponding author. Fax: + 81-3-3817-1880.
E-mail address: [email protected] (T. Suzuki)
because most of the data reported have not been based on the direct measurements of the thickness of weathered materials of bedrock in the field or the exact determination of the duration of time for weathering, as reviewed for example by Ollier (1969), Kukal (1990, pp. 73-82), Selby (1993, pp. 155-156) and Matsukura (1994). Such a gap in the study of weathering rates stems from the difficulty in determining both the thickness of weathered materials and the weathering time. The main reason for the difficulty is that weathered materials are easily eroded away on hill slopes in particular, and hence the weathering time is hardly determined.
172
In order to overcome the above-mentioned difficulty, the weathering rates of bedrock under the dated marine-erosional terrace surfaces were investigated in the present study. This is because the thickness of the weathered part of terrace bedrock can be regarded as the whole thickness of materials that have been weathered since the emergence of terrace, and the age of the emergence is assumed to be equal to the duration of weathering.
2. Method
2.1. Studyarea Data were obtained for the bedrock that forms erosional marine terraces in the Boso Peninsula, south of Tokyo, Japan (Fig. 1). The terraces had been uplifted abruptly and repeatedly in association with great earthquakes. They are divided into five levels named the Numa I, Numa II, Numa III, Numa IV (Genroku) and Taisho surfaces, and their ages of emergence date back to around 6150 years, 4350 years, 2850 years, 290 years (1703 A.D.) and 70 years (1923 A.D.) B.P. respectively (Nakata et al., 1980). These ages were regarded as the duration of time for the weathering of
bedrock. The heights of former shorelines of these terraces in the study area are about 23 m, 17.5 m, 12.5 m, 5.5 m, and 1.5 m respectively. The bedrock of the terraces is the Pliocene marine sedimentary rock, which is divided into the Shirahama Formation, Shiramazu Formation and Mera Formation (Kotake, 1988). The Shirahama Formation consists mainly of tuffaceous sandstone interbedded with occasional thin beds of mudstone and conglomerate, whereas the Shiramazu Formation and the Mera Formation consist mainly of mudstone with thin beds of unconsolidated sandstone. Each bed varies from 5 cm to 3 m in thickness. These strata strike east-west in general and vary from 18 to 70 ~ in dip. In the present paper, the weathering rates were examined for the sandstone in the Shirahama Formation, and the mudstone in the Shiramazu Formation and Mera Formation. The bedrock of the terraces is covered with a veneer of alluvium (a sand bed including gravel) ranging from 0.1 to 1.6 m in thickness.
2.2. Definition of weathering rates The weathering rates are generally classified into the following three categories (e.g. Matsukura, 1994). The first definition is the rate dZ/dt at
Fig. 1. Distribution of marine terraces in the study area, south of Tokyo, Japan (Nakata et al., 1980). Drilling sites are indicated by open triangles for sandstone (S1-S 13) and open squares for mudstone (M 1-M 13).
173
which the thickness Z of weathered materials of bedrock increases with time t; the second is the rate dP/dt at which the rock properties P change with time t at an arbitrary depth; and the third is the rate of chemical denudation. Concerning the first definition, Suzuki and Hachinohe (1995) recently proposed empirical equations to express the rates of increase in thickness of four zones with different weathering grades; these were divided according to the visual observation of cut-slopes and drill cores and measurements of rock properties, including the pore-size distribution (PSD) for the bedrock of four levels of terraces in the same study area as the present one. However, the equations of Suzuki and Hachinohe (1995) were based on undifferentiated data for various kinds of lithology, such as sandstone, mudstone, conglomerate, alternated mudstone and sandstone, etc. In the present study, the weathering rates of both the first and second definitions were further re-examined on the basis of the changes in rock strength, PSD, mineralogical and chemical characteristics in the same study area as, but in a different way from, those by Suzuki and Hachinohe (1995). The weathering profiles were examined for drill cores obtained continuously from the top surface to a fresh part of the bedrock at 26 sites (13 sites for sandstone and 13 sites for mudstone) on the younger three levels of terrace surfaces, i.e. Numa III, Numa IV and Taisho (Fig. 1).
3. Occurrence of weathered materials and rock properties
3.1. Needlepenetration hardness In order to examine the changes in mechanical properties of bedrock due to weathering, it is preferable to test the weathering profiles continuously from the weathered part to the unweathered part for one parameter. The needle penetrometer (Model SH-70, manufactured by Maruto Co., Ltd, Tokyo) used in the present study can measure penetration loads Lp (kgf) and penetration depths Op (mm) simultaneously. The needle penetration
hardness Np (kgf/mm) is defined as
Np =Lp/Dp,
(1)
and Np is closely related to mechanical properties of rock, e.g. uniaxial compressive strength for a range from 0.3 to 40 MPa according to the test result by Maruto Co., Ltd. The uniaxial compressive strength of the fresh part of bedrock in the study area ranges from about 10 to 35 MPa for sandstone and from about 10 to 22 MPa for mudstone. Therefore, Np was adopted as an effective parameter to test the strength of bedrock from the weakest part to the hardest part in the study area. The needle penetration test was performed three times at the same depth at intervals of about 5 cm for all drill cores. Figs. 2 and 3 show examples of data of the needle penetration test for sandstone and mudstone respectively. For each of the weathered profiles, Np shows an almost constant value at the unweathered part, but decreases gradually as weathering proceeds. The depth where Np begins to decrease appears under the discolored zone of the weathering profile in general.
3.2. Pore-sizedistribution The PSD was measured by a mercury intrusion porosimetry, AUTO-PORE #9200 manufactured by Micromeritics Co., USA, using the same method as that by Suzuki and Matsukura (1992). The 'significant range' of PSD reading is from 0.0046 to 316 mm. The PSD was tested at eight sites for sandstone and five sites for mudstone (Table 1). Pores were tentatively divided into four grades according to the pore diameter D: large ( 101"5> D~ > 100.5 ~tm), medium ( 100.5 > D~ > 10-~ ~tm), small ( 10 -0.5 > D r > 10-1.5 ~tm) and very small ( 1 0 - 1 5 > D ~ > 4 . 6 • 10 -3 ~tm). The pore volume per unit weight of rock specimen is denoted as V~, V~, V~ and V~ (cm3/g) for the four grades respectively. In general, the highest peak of the PSD for the unweathered part of sandstone appears in the D~ zone, then, as weathering proceeds, the value in the D~ zone becomes smaller and the clear peak migrates toward the D~ zone (Fig. 4). Similarly, a clear peak in the case of mudstone migrates from
174
PSD
NPH v. E 0 KI
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0.10
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10
40
Weight
percent
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50
60
70
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Fig. 2. Changes in rock properties due to weathering of sandstone (an example, at $4 site). NPH: needle penetration hardness, where open triangles show the data of discolored zones and filled triangles are those of undiscolored zones. PSD: pore-size distribution (for the ranges of V=, V~, Fv and F~ refer to the text). XRF: X-ray fluorescence analysis.
PSD
NPH v,
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I
v~
XRF Vx
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i
o
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!
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2
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2050
60
70
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Fig. 3. Changes in rock properties due to weathering of mudstone (an example, at M8 site). NPH: needle penetration hardness, where open circles show the data of discolored zones and filled circles are those of undiscolored zones. PSD: pore-size distribution (for the ranges of V=, V~, Fv and V~ refer to the text). XRF: X-ray fluorescence analysis.
175
Table 1 Summary of the depths from the bedrock surface to the thickness to the discolored front Zac, the mean needle penetration hardness of unweathered part Npf and the analysis method: NPH is the needle penetration hardness; PSD is the pore-size distribution; X R D is X-ray diffraction analysis; XRF is X-ray fluorescence analysis. Open circles show those measured and crosses are those not measured for any drill core Lithology
Terrace surface
Sandstone
Taisho
Numa IV
Numa III
Mudstone
Taisho
Numa IV
N u m a III
Loc. no.
Zac (m)
Npf (kgf/mm)
weathering time (years B.P.) 70
290
2850
70
290
2850
Analysis method NPH
PSD
XRD
XRF
S1 $2
0.06 0.04
3.25 8.04
O O
O O
O Q
O O
$3
0.04
5.28
O
O
•
•
$8 Sll
0.05 0.09
5.03 4.94
O Q
x x
x 9
x x
S12
0.09
5.92
O
x
x
x
S13
-
2.56
O
O
x
O
$4 $5
0.70 0.78
8.47 8.94
O O
O O
O x
O x
$6
0.30
-
O
x
x
x
$7
0.50
-
$9
0.28
8.62
0 Q
0 0
x 0
0 0
S10
0.30
8.62
Q
x
x
x
M5 M7
0.02
3.45 3.83
O
x
x
x
M8 M9
0.05 0.12
4.95 3.27
9 9
9 9
9 9
9 9
O
x
x
x
M6 M12
0.13
3.78 2.71
O O
x x
x x
x x
M13
0.14
2.56
O
x
x
x
M1 M2
0.56 0.65
2.68 3.01
O O
x x
x x
x x
M3 M4
0.45 0.46
2.74 2.85
9 9
9 x
x x
x x
M10
0.55
5.18
Mll
0.85
5.26
9 9
9 0
9 9
9 9
smaller pore-diameters toward larger ones, as weathering proceeds (Fig. 4).
3.3. Mineralogical and chemical properties Mineral analysis was done by the X-ray diffraction method (XRD) for bulk powder samples using a Rigaku RAD-C. Chemical analysis was carried out for SiO2, TiO2, A1203, FeO" (FeO+Fe203), MnO, MgO, Ca9 Na20, K20 and P 2 0 5 by the X-ray fluorescence (XRF) analysis method using a Rigaku 3270. Observation under a polarization-microscope and scanning electron microscope (SEM) clarified changes in
the shape of minerals due to weathering. Based on this investigation, the'general characteristics of the mineralogical and chemical properties of sandstone and mudstone in the study area are summarized as follows. 3.3.1. Sandstone
Sandstone contains quartz, feldspar, smectite, kaolinite and zeolite. As weathering proceeds, Na20 starts to decrease (e.g. at around 2 m deep from the bedrock surface, Fig. 2), whereas other chemical components show no systematic changes. The diffraction peaks of smectite for the glyceroltreated specimens have a lower peak in shallower
176
Sandstone 0.03 . . . . . . . .
o.o -
I...... ~ '
_2_
!. . . . . . .
5(0.65)[
~
I. . . . . . .
Mudsto n e w. . . . . . .
7
!
i
i
IJII
103
i
i
_i
l
II
102
101
l
i
, i
0
I..... ;' ' '
10 ~
10 1
l
10 .2
Pore diameter, D ( F m)
I
----
10 .3 103
102
101
10 ~
10 1
10 .2
10 "3
Pore diameter, D ( ,u rn)
Fig. 4. E x a m p l e s o f the c h a n g e s in the differential p o r e - v o l u m e d i s t r i b u t i o n d u e to w e a t h e r i n g o f s a n d s t o n e (at $4 site in Fig. 2) a n d m u d s t o n e (at M 8 site in Fig. 3). F o r the r a n g e s o f V~, Va, V~ a n d V~ refer to the text.
parts than in the deeper parts (Fig. 5). This means that the crystallinity of smectite becomes lower due to weathering. Surface features of feldspar in the weathered part change, compared with the fresh part, to have a number of large cracks around 1 lam in width (Figs. 6 and 7). 3.3.2. Mudstone
Mudstone contains quartz, feldspar, smectite, kaolinite, illite and calcite. The peaks of calcite and feldspar at the top of the weathered part are markedly lower than those in the deeper part of the weathered zone (Fig. 8). This trend was seen with all drill cores of mudstone. C a d tends to decrease in the weathered part (e.g. at 0.08 m deep, Fig. 3), whereas other chemical components show no systematic changes. A lot of calcite is contained in both the unweathered part and the highly weathered part, but some grains of calcite are dissolved in the weathered part. In addition, pyrite was not specified by the XRD method for sandstone and mudstone.
3.4. Mechanisms of decrease in rock strength
Based on the above-mentioned occurrence and properties of weathered materials, decreases in rock strength are considered to result mainly from the increases in the pore volume of larger pores due to weathering (Fig. 4), because the larger the pore volume of larger pores in the weathered bedrock is, generally the smaller the strength becomes (Yamashita and Suzuki, 1986). In the case of sandstone, increases in pore volume of medium-size pores (V~) ma3? be mainly due to the formation of large cracks on the feldspar surface, as supported by Figs. 6 and 7. In the case of mudstone, on the other hand, increases in pore volume of medium- and small-size pores (V~ and V~) may have resulted from the dissolution of calcite as well as the formation of large cracks on feldspar surfaces due to weathering; hence the initial size and content of calcite may be the main variables controlling changes in the PSD due to weathering, because the dissolution of calcite and
177
Fig. 5. Changes in X-ray diffractograms of oriented and glycerol-treated bulk samples (G.t.) due to weathering of sandstone at the S1 site: Sm, smectite; Z, zeolite; A, the front of discoloration; B, the depth where R~ becomes almost constant. Figures on the right-hand side show the depth (meters) from the bedrock surface.
Fig. 6. Representative photographs of feldspar in sandstone (at the S1 site) under an optical microscope: (a) weathered (discolored) part; (b) fresh part.
decreases in CaO are recognized during the early stage of weathering of mudstone in the study area.
4. Weathering rates
4.1. Rates of increase in the thickness of each weathered zone with different strengths The needle penetration hardness Npf of the initial bedrock or the flesh part differs in different beds, ranging from 2.56 to 8.94 kgf/mm for sandstone and from 2.56 to 5.26 kgf/mm for mudstone (Table 1 ). In order to compare the weathering grades for all drill cores, therefore, the effect of the difference in Npf on the changes in the mea-
sured Np of the weathered part was corrected by converting the measured Np into the residual strength ratio R~ (%), which is defined as
R~=(N./N.O •
100,
(2)
where Np is the measured value and Npf is an averaged value for the fresh part of each drill core (Table 1). The relationship between the residual strength ratio Rs and the depth from the bedrock surface Z (Fig. 9) shows that R~ decreases with increases in the grade of weathering and that the longer the weathering time is, the larger the decrease in Rs is. The dispersion of data in Fig. 9 may be mainly
178
Fig. 7. Secondary electron image of feldspar of sandstone in the weathered (discolored) part at the S1 site.
due to the variety in the microtexture of the rock even in the fresh part. Regarding the temporal change in strength due to weathering as a mass diffusion process from the bedrock surface, therefore, the relationship shown in Fig. 9 is redrawn schematically in Fig. 10 and expressed by a function similar to that proposed by Hirano (1990) and Yokota ( 1992): R~ = R~o)
+ (Rsf Rs(o) ) ( 1 --
e - kZ ) ,
(3)
where Rs(o) is R~ at the top of the bedrock surface, Z is the depth from the bedrock surface, and k and R~f (= 100%) are constants. Eq. (3) leads to
, In [ 1 - Rs Rs,o,].
k=
Z
(4)
Rsf - Rs(o)
According to the data of Fig. 9, the constant k for both lithologies decreases with the weathering time t. The relationship between k and t is shown in Fig. 11 and is expressed as k=at
(5)
b,
where a and b are constants: a = 6 . 5 x 10 2, b = - 7 . 6 x 10 -a for sandstone and a=4.3 x 102, b = - 5.7 x 10-1 for mudstone. According to R~ for the bedrock surface, the relationship between Rsm) and t is shown in Fig. 12 and is expressed as (6)
R~
where
Fig. 8. Changes in X-ray diffractograms of oriented samples due to weathering of mudstone at the M8 site: Q, quartz; F, feldspar; C, calcite; A, the front of discoloration; B, the depth where Rs becomes almost constant. Figures on the right-hand side show the depth (meters) from the top surface of the bedrock.
c and
d are
constants:
c = 8 . 8 x 10 ~,
d = - 2.7 x 10-1 for d= - 6.7 x 10- 2 for Eqs. (5) and (6), R0 ing time t. By using bedrock surface to expressed by Z
sandstone and c = 2.4 x 101, mudstone. As indicated by and k decrease with weatherEq. (3), the depth from the the weathered zone Z is
1 ln[J~f~R~m) ] ~
~
k
,
(7)
!_ R s f - Rs
The depth Z differs depending on the weathering grade indicated by Rs. By using Rs as a parameter, the weathering profiles were tentatively divided into seven weathered zones, of which the bases or weathering fronts are from 30 to 90% at intervals
179
Numa A)
111
Numa
B)
t = 2,850 yrs.
Taisho
IV C)
t = 290 yrs.
E N
9
~x.
9 ,-
1
CO
-;~
-
0.1
i
----~
...
0.2
1.5
]
0.05
l
0.15
I
~e
o O
,
0.05 0.5
o ,'-
.~,~
t = 70 yrs.
0
o
9
0.1
0.25 rn
2~--
E =
0.3
2.51
0.35 !
041 s.n,,,one t
I Sandston e
c~ 3
100
10
10 Strength
Numa
D)
Remaining
111
0.2 10
100 Ratio,
Rs
100
(%) Taisho
Numa E)
t = 2,850 yrs.
F)
t = 290 yrs.
t = 70 yrs.
0
0:
ol
A
E N
o
0.05 O.5
..
d
~
L :~
~ '
0.05
0.1 -
0.15
-
-}
r e t
o 0
1.5
m m E o ,-
~ 2L ~ t 2.5!
L
L
a
.o-~-
~
-,-!"
'
1 e
i
0.2
~ r
1
100
.
.
.
. .
.
. .
. .
.
o4 .
9
-
.e
J
0.15
0.3
. . . . . . . . . . . . .
0.35
3' 10
. ...
~.. -e.
.
18-
iq
L
~ Mudstone
....
0.25
.-i~ . . . .
0.1
Mudstone
0.4
0.2
100
10 Strength
Remaining
Ratio,
10
o-
9
. . . . =
I J
100
R s (%)
Fig. 9. Changes in the residual strength ratio Rs at different depths Z of sandstone and mudstone at three different levels of terraces, i.e. Numa III (2850 years B.P.), Numa IV (290 years B.P.) and Taisho (70 years B.P.). All data obtained from the drill cores on each level of terraces are plotted together on each diagram, excluding exceptional data such as those along major joints where bedrock is markedly weathered. Data for sandstone from: (A) $4, $5, $9, S10; (B) $8, S11, S12, S13; (C) S1, $2, $3. Data for mudstone from: (D) M1, M2, M3, M4; (E) M6, M12, M13; (F) M5, M7, M8, M9. Open symbols show the data of discolored zones and filled symbols show those of undiscolored zones. The bedrock surface is the same as the bottom of the veneer of alluvium.
180
Fig. 12. Relationship between Rs(o~ in Eq. (6) and weathering time t for sandstone and mudstone. Data for the cores of which the top parts were broken with drilling were eliminated. Fig. 10. A schematic diagram of the relationship between the residual strength ratio Rs and the depth from bedrock surface Z. The suffix (0, t,) of Rs means R~ at the bedrock surface (Z = 0) at a given weathering time t.
of n = Rs into Eq. (7) gives 1 ln~Rsf--R~~ ]
Z.:~ L
Rsf--/'/
(8) "
By using Eq. (8), the weathering fronts indicated by Z, are plotted against the weathering time t in Fig. 13 separately for sandstone and mudstone. When the weathering grade indicated by n is constant, differentiation of Eq. (8) gives the rates of increase in the thickness of Z, as = dt
,{ iRsf-Rso,l -In
k2
k'+
Rsf - n
Rs(o) - Rsf
t
. (9)
Fig. 11. Relationship between coefficient k in Eq. (5) and weathering time t for sandstone and mudstone.
of 10% in Rs. The depth from the bedrock surface to each of the weathered zones thus divided is designated as Z,, in which the suffix n is Rs. Defining the weathering grade as n, substitution
The values of dZ,/dt thus defined are the weathering rates at which the thickness of weathered zones with different strengths increases with time or the weathering rates of the aforementioned first definition. It shows that the weathering rates do not decrease monotonically, but exponentially with time (Fig. 14), and that the zones with lower weathering grades have faster weathering rates than those with higher weathering grades.
181
.'..'."
10'
"
90%
N=
~
80%
0
'~
60%
~z
9 5o%
"" :" .,," --:--".".
..-.~....
"
90%
~
80%
9
70%
-"""
. .
.
/////.-..
:~:
o
40%
<>""
o
40%
"
*
30%
"
9
30%
o'-',~ i
-
--"
-:. :
-
"
"0
l")
~ 0-,L
& 1
a
..ilJi
.
i;51
..-
Sa n dstone
[,,., 10-3 . 10 ~
, 102
10'
103
10'
~. .. '..10 s 10 ~
..
- " -. " '
."
..
". . " -
."..." . ." ,
.:"
.10'
Mudstone 102
103
Weathering time, t, (years)
1 O"
105
Weathering time, t (years)
Fig. 13. Relationship between Z,, the depth of the front of weathered zones with different Rs, and the weathering time t. Thick lines show the regression lines for the measured range and the dotted lines are those interpolated and extrapolated by using Eq. (8).
E E
I
1.5
Sandstone
"13
'"", c0 N "~
Mudstone
,
1.0
c'-
0 or)
c ._o
"'....
0.5
r
"-_
0
._= 0 or}
n"
0
....
101
102 Weathering
103 time,
t (years)
10"10'
102 Weathering
103
10'
time, t (years)
Fig. 14. Relationships between the rates of increase in thickness of weathered zones d Z , / d t and the weathering time t. Thick lines show the regression lines for the measured range and the dotted lines are those interpolated and extrapolated by using Eq. (9). The symbols are the same as those in Fig. 13.
4.2. Rates of deepening of discoloredfront The weathering profile of sedimentary soft rock is often divided into three zones, i.e. surface oxi-
dized zone, oxidized zone and dissolved zone in the order from the bedrock surface to deeper parts. The oxidized zone is generally characterized by discoloring from dark gray to yellowish brown
182
with FeO decreasing and Fe203 increasing (e.g. Chigira, 1990; Chigira and Sone, 1991). In the study area, the lower limit of the discolored zone of the bedrock is clear and sharp, especially in sandstone, but rock properties such as Rs and PSD still change even below the discolored zone. Accordingly, the lower limit of the discolored zone seems to be the oxidation front of iron in the present study, although XRF cannot distinguish FeO from Fe203. The relationship between the depth of the discolored front Zdc and the weathering time t for both lithologies is plotted in Fig. 15, in which the relationship between the residual strength ratio and time (Fig. 13) is added with broken lines. Fig. 15 shows that mudstone starts to discolor earlier than sandstone, and the discolored front reaches the depth where Rs ranges from 40 to 80% for sandstone and over 90% for mudstone during the measured range in the weathering time. The regression lines in Fig. 15 are expressed to a first approximation as
Zdc=et I,
f = 5.3 x 10-1 for mudstone. Differentiation of Eq. (10) gives the rates of deepening of the discolored front (dZdc/dt) as
dZac/dt= gt h,
( 11 )
where g and h are constants: g = 1.5 x 10 .3 and h = - 3.4 x 10-1 for sandstone and g = 4.0 x 10- 3 and h = 4 . 7 x 10-1 for mudstone. As demonstrated in Fig. 16, the rate of deepening of the discolored front of mudstone is faster than that of sandstone before about 2000 years in the elapsed time, but the former is expected to be exceeded by the latter after that time.
4.3. Rates of decrease in the bedrock strength due to weathering at arbitrary depths For practical purposes, we need the rates at which the bedrock strength at arbitrary depths decrease with time due to weathering. Based on Eq. (3), temporal decreases in the bedrock strength at four different depths Z = 0 . 0 1 , 0.03, 0.10 and 0.30 m are demonstrated in Fig. 17. The strength of sandstone at the 0.10 m depth, for example, remains unchanged for about 200years in the elapsed time, and after that the strength will begin
(10)
where e and f are constants: e = 2 . 3 x 10 -3 and f = 6.6 x 10-1 for sandstone and e = 7.6 x 10-3 and
10' L
90 %
.. 1 0 ~
.'
-"
0
'1:::::
o 10 -'k
.-.,
"0
t-
"Q
I
-"'"
E
k
.."
0
0.2=
I
,.~.-
.- . -
"'" -" -" ~" ," ." ,."
_
60
,
90% 80 70 60 50 40
so
. ..-...... ." . ' , - ' . - ' . - " ," ." " , " , " . . "
tat)
I,,-
8o
140 30
.-
./
30
"
,
-
Q.
o
Sandstone
I 103: 10 ~
.
.
.
.
.
.
.
.
.
.
.
10 2 Weathering
.
.
10 3
.
.
Mudstone J
.
10' 10'
time, t (years)
10 2
iO 3
10 ~
Weathering time, t (years)
Fig. 15. Relationship between the depth of the front of discolored zones Zdc and the weathering time t of the measured range for sandstone and mudstone. The broken thin lines are the regression lines for Rs shown in Fig. 13.
183
10 4
t
assumed to be constant, differentiation of R~ with respect to t in Eq. (3) gives the rates as follows:
:-A
dZdo/dt -
dRs dt
= e -zk {Z[Rsf- R~(o)]k'+ R~(o)}.
.
..
.......
Mudstone
-...
The rates d R s / d t thus defined are the weathering rates at which the strengths of weathered materials at an arbitrary depth decrease with time or the weathering rates of the aforementioned second definition.
Sa 10 2 ! -..
7
r 101 t 10 2
10 ~
10 3
Weathering
5. Discussion and conclusions
10 4
time,
t
(years)
Fig. 16. Relationship between the rates of increase in the depth of discolored front dZdc/dt and the weathering time t for sandstone and mudstone. Thick lines show the regression lines for the measured range and the dotted lines are those interpolated and extrapolated by using Eq. (11 ).
I00,
I
o~
.
" i"I
70
.
60:.
.
40 F
--~
0100
3
0
.
.
.
" , ....
. :.
...... "
.
0 30m
\ ~k, ~,
\ i
'
I
I
[I:
102
% --0.01
m----O.03
l'~l
.
"
m,
O . l O m
IT"
5 0 !,...... " . . . . . .
..... :
~.. M u d s t o n e
--
I I
" l X
' I]
I
I
I
'
I. . . .
I O"
0.30m
.....
" ~i
I0 s
---"
:. \:.i-.i ::-- ,: :..i
m
,,
..::..., .
10 ~
I
::.. lI
103
so,~...:-.-.-:-..--i :.:!i---....k.-.-.i.-.:.-i.,..-:-.;..\... -,....,~:-..-.-
~i. . . .
"
:-
n e .:..,.., ~ . . . . . . . . . . . . . . . . .
-a
10 ~
--
,
"
I O'
t~. . . . .
~
. . . . . . . . . . . .
Sands . . . . . . . . . . . . .
, "-~
"~
O.Olm:O.O3mO.lOm
!
9.i-.,, co
i-'--.
: "I' " I'I~:I
SO0 "-~
,.
.
.
.
102
103
10'
105
Weathering time, t (years)
Fig. 17. Decreases in the residual strength ratio Rs with weathering time t at four different depths of sandstone and mudstone.
to decrease due to the start of weathering at this depth. Then, the strength decreases with time and will become 30% of the initial strength after 2000 years. When the depth from the bedrock surface Z is
5.1. C o m p a r i s o n o f w e a t h e r i n g rates between sandstone and mudstone
To compare the modes of decrease in the strength of sandstone and mudstone, the curves showing the temporal decreases in Rs for sandstone and mudstone in Fig. 17 are superposed for each of the four different depths (Fig. 18). The strength Rs decreases with time t, but the weathering rates d R / d t are different between sandstone and mudstone at every depth and at every weathering time except in the case of about 70 years at 0.01 m depth and 0.03 m depth. At shallower zones, where the depth is less than 0.03 m, mudstone begins to weather earlier than sandstone, but after a certain length of time from the beginning of weathering, i.e. about 70 years, mudstone will be exceeded by sandstone in the rates of decrease in Rs [Fig. 18(A)]. However, at zones deeper than about 0.03 m, sandstone starts to weather earlier and faster than mudstone [ Fig. 18 (B)-(D)]. The reason for the earlier and faster weathering of mudstone at shallower zones mentioned above is explained as follows. In the study area, the bedrock of erosional marine terraces is considered to begin weathering just after the emergence of wave-cut platforms due to the abrupt uplift associated with great earthquakes because of the slaking at shallower parts of bedrock. Generally, the slaking rates of sedimentary rock are closely related to both the content of montmorillonite and the volume of smaller pores (Matsukura and Yatsu, 1982). Field observations in the study area show that mudstone is higher than sandstone in terms
184
100 . . . . . .
~,
90L-
>\ ................ ",
.... Z " = o.o1m ~
\
8ol- Mudstone",'-\ \ Sandstone ,
7o~
"-~
Go!
",\
so,L 4oi A 30~
, ............. 10'
10 ~
100 . . . . . . . . . . . .
X-. I0 z
,~ >-,k..... --
1
' I'~'O'
~O]
~ .............................
z _-o.o3m i "'
80L
Sandstone
94.o --
",
S0
c
10 ~
90~,
:::3
8G-
Mudstone
................... 10'
\ "',, "~ 102
.~..., 10 ~
70~ t.~
................. 10"
""
!
10 s
J
60L
"",,,..,
so~40
'
",
4~ 3ol
_.m r
"
C
37()~ 100~
90" 8~ ~;
"10'
' ..... i0'
' .... i'[)'
, .................
.~
.
..
' .... i 0 ' " . . . . . .
i'0 s
.>,...,,.Z,,,: o.30, ~m.,
i
Sandstohe\ . ~
"",, Mudstonej
50i
i
",
40~ D 301 10 ~
",,
.......... . . . . . . . 101
I 0z
1 03 W e a t h e r i n g time,
i
...~i",';": 10 ~
5.2. Predictive model for decreases of bedrock strength due to weathering To predict the bedrock strength at a given depth and at a given elapsed time after the beginning of weathering, a model is proposed below. Fig. 19 shows the relationships between the residual strength ratio Rs and the depth from the bedrock surface for a given weathering time t at intervals of 500 years after the beginning of weathering. By using Fig. 19, it is possible to predict the strength in the form of Rs at a given depth and at a given time for the same sandstone and mudstone and under the same weathering environment as those in the study area. Fig. 19 also implies the temporal changes in both the rates of increase in the thickness of weathered zone with different strengths, i.e. that of the first definition of weathering rates mentioned above, and the rates of decrease in the bedrock strength at an arbitrary depth, i.e. that of the second definition of weathering rates. The model proposed here cannot be applied to such cases as man-made high cut-slopes and natural hill slopes, which are different from the study area. However, since the needle penetration hardness has a good correlation with the modulus of deformation and elasticity (Ichikawa et al., 1988),
10 s
t (years) 0[
i
,
i
Fig. 18. Difference of temporal changes in the residual strength ratio R, with weathering time t between sandstone and mudstone at four different depths Z. o t~
of slaking susceptibility. The reasons for this are that first, mudstone has about twice the volume of smaller pores as does sandstone (i.e. Vy and V~), as shown in Fig. 4, and second, because mudstone has about half the Np of the fresh part compared with sandstone, as shown in Figs. 2 and 3. Accordingly, at zones shallower than 0.03 m from the bedrock surface, mudstone responds quickly and sensitively to slaking. However, there are no data useful to explain the fact that the weathering rates of mudstone are exceeded by that of sandstone after a certain time length at a specific depth.
o0 x.. 73 (D XD
%
0.5
E 0
Sands
" Mudstone
30 40 50 60 70 80 90 100
30 40 50 60 70 80 90 100
Residual strength ratio, Rs(% )
Residual strength ratio, Rs(% )
Fig. 19. Predictive decreases in the residual strength ratio R, due to weathering at arbitrary depths Z and at weathering time t given at intervals of 500 years for sandstone and mudstone.
185
the m o d e l m a y give a conceptual basis to establish an empirical equation to predict the bedrock strength under various environments for weathering in a m o r e practically and useful f o r m for engineering purposes in the future.
Acknowledgements The authors would like to t h a n k the students majoring in Engineering G e o l o g y at C h u o University for their help with drilling and measurements of rock properties in the field and in laboratories. The authors also express their appreciation to D r T. H a t t a of J a p a n I n t e r n a t i o n a l Research Center for Agriculture Sciences, Ministry of Agriculture, to permit the use of his l a b o r a t o r y and for his helpful suggestions. T h a n k s should also be extended to D r C.T. Oguchi and Professor Y. M a t s u k u r a of the University of T s u k u b a for their help with X R D and X R F tests, and also to D r H. Kiya, D r T. O h t a and D r S. Y o k o y a m a of the Railway Technical Research Institute for their support and e n c o u r a g e m e n t . This study was partly supported by the G r a n t - i n - A i d for Scientific Research (contract no. 07640609 in 1995-1996 for T. Suzuki) and the G r a n t - i n - A i d for JSPS Fellows (in 1996-1998 for S. H a c h i n o h e ) of the Ministry of Education, Science, Sports and Culture of Japan.
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