10th Asian Regional Conference of IAEG (2015)

Rapid Weathering and Salt Water Migration Processes near a Slope Surface in Plio-Pleistocene
Mudstone Badlands in Southwest Taiwan

Kohei HIGUCHI (1), Masahiro CHIGIRA(2), Der-Her LEE(3), and Jian-Hong WU(3)(1) Graduate School of Frontier Sciences, the University of Tokyo, Japan
E-mail:khiguchi@geoenv.k.u-tokyo.ac.jp(2) Disaster Prevention Research Institute, Kyoto University, Japan, (3) Department of Civil Engineering,
National Cheng Kung University, Taiwan
Abstract
High rates of surface wash erosion and rapid weathering of impermeable mudstone form
barren badlands consisting of sharp ridges and dense gully networks on steep slope surfaces
under the humid subtropical climate with distinct dry and rainy seasons in southwest
Taiwan. We analyzed pore water chemistry, swelling of the mudstone, and monitored watercontents and salt contents near the slope surfaces, and revealed the weathering and erosion
processes of the mudstone, as follows. The mudstone, which contains salty interstitial waterand has low permeability, is dried with developed desiccation cracks during dry seasons.
The salty interstitial water migrates from the depth to the surface and condensed with salt,
which precipitate near the slope surfaces. Water from occasional rainfall immediately
infiltrates down to the depth along open fractures, dissolving precipitated salts. During rainyseasons, on the other hand, interstitial water of rock increases in amount and is diluted,
increasing the osmotic pressure, which expands bulk volumes, and closes desiccation cracks.
Diluting the interstitial water by meteoric water finally disperses the rock-forming grains
due to chemical osmosis. This dispersed surface material is rapidly removed by surface
wash during subsequent rainfalls in the rainy season, and next cycle of weathering starts
again.
Keywords: erosion, weathering, Plio−Pleistocene mudstone, badlands, in-situ monitoring
1.

Introduction

cracks near the slope surface and essentiallyassociates with rapid weathering processes inbadlands (Howard 1994; Faulkner et al. 2004;
Higuchi et al. 2014). However, the detailed waterand salt content migrations during each rainfall
event and mechanical volume changes of the
mudstone have not been revealed. Therefore, we
here discuss migration of water and salt contents ateach rainfall event, and the rock swelling, andsummarize the weathering and erosion processes.

High rates of surface wash erosion and rapid
weathering of the mudstone predominantly occur
and develop sharp ridges and v-shaped gullies on
steep slope surface in badlands in Taiwan (Fig. 1,
Lee et al 2007; Nakata and Chigira 2009; Higuchi
et al. 2013, 2014). Since the mudstone has lower
permeability and low resistivity to the erosion, therainwater cannot penetrate into deeper depth
immediately,
andsurfacewasherosionpredominantly occurs during heavy rain. Such highrates of erosion and sediment yield cause
environmental problems as well as poor use of land.
During the rapid weathering, volumetric changeof the mudstone causes initiation and closure of2.

Study area

The barren badland landforms widely distributein the middle of the Erhjen River basin insouthwestern foothill of Taiwan, where is1

10th Asian Regional Conference of IAEG (2015)of Tian-liao county, 20 km east of Tainan insouthern Taiwan (Fig. 2, Higuchi et al. 2014),
where the climate is humid sub-tropical with arainy season from May to August and a dry seasonfrom September to April. The slope surface wascovered by friable crust that was a few centimeters
thick and that became desiccated with open
fractures in the dry season. Beneath the crust, someof the desiccation cracks penetrated to depths of
20−30 cm but no cracks extended to 40 cm deep.
The bedrock comprises impermeable, dense
mudstone (siltstone) as low as <0.2−0.3 of void
ratios, and whose properties are reported in Higuchi
et al. (2013, 2014) as follows. In dry season, theslope surface is dried and precipitates salts along
the opened fracture surfaces with decreasing watercontent and with increasing rock hardness. After thedry season, hard rainfall moistens the rock with
dissolving the precipitated salts and decreases therock hardness significantly with developing lower
bulk density and higher porosity near the slope
surface in the early rainy season. This deteriorated
surface is eroded as high as 9 cm/y in average in thesubsequent late rainy season.

Fig. 1 Photograph of badlands around study area.
The National express way in the center of thephoto goes through the mudstone area.
underlined by Plio−Pleistocene mudstone of theLower Guting-keng Formation. In this area,multiple erosional river terraces of middle−late
Holocene paleo-floodplain,
whichforming
calcareous coarser sandstone surface of a few toseveral tens meters thick, cover the weakly resistantmudrock and are covered by vegetation due tohigher resistivity to the surface erosion. Tropicaltyphoons commonly bring disastrous floods to thebasin where is undergoing extremely rapid tectonic
uplift (Hsieh and Knuepfer 2001). After theabandonment of the paleo-floodplain of around 2 ka,river bottom incision due to the uplift andsubsequent lateral surface erosion have extended
wide badlands (Hsieh and Knuepfer 2001).
The studied slope is located in typical badlands

3.

Methodology

Swelling tests were performed for the mudstone
by initially cutting a fresh mudstone disc (5 cm indiameter, 2 cm in height) from a block by drillingand dry saw. The disc was placed in a consolidation
test machine and loaded to a stress of 50 kPa before
immersion in 8% salt water or pure water. Strain
was monitored for 50 hours.
From November 2009 to March 2011,
temperature, water content, and pore water EC(Electrical Conductivity) were monitored at andbelow the slope surface with simultaneous
measurementsofrainfallandair
humidity/temperature in front of the slope (Fig. 2,
Higuchi et al. 2014). The slope, which had been
artificially cut several years prior to themeasurements, faced southeast, was inclined at 50○,and lacked vegetation. The detailed technique for
installing the sensors and monitoring instruments
were described in Higuchi et al. (2014).
The 5TE sensor that we used for themonitoring cannot measure pore water EC in

Fig. 2 Photograph of monitoring site and schematic
sketch of setting of sensors.

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10th Asian Regional Conference of IAEG (2015)

Fig. 3 Piper diagram of extracted pore water of the mudstone at each loading pressure.
(761 compact IC; Metrohm Corporation).

extremely dry soil because the derived equationreferred from Hilhorst (2000) is not available when
the water is bound to the soil rather than being free.
We calculated the minimum water content for theapplication of the Dirksen and Dasberg (1993)
equation, which gives the relationship between theminimum water content and the specific surface
area of soil. We measured the specific surface areaof the mudstone as 19.6 m2/g using the BET(Brunauer-Emmett-Teller) method and obtained aminimum water content of 0.006, which was much
lower than the water contents observed during field
monitoring.
The extraction of interstitial water was madeusing a uniaxial compression water-extraction
apparatus developed by Kiho et al. (1999).
Extracted water was filtered with a 0.45 μm filter
before analysis using an EC meter (B-173; Horiba
Corporation), a pH meter (B-212; Horiba
Corporation), and an ion chromatography device

4.

Results

3.1 Chemistry of the Extracted Pore Water andSwelling
Extracted water volumes from the samples
ranged from 0.3 to 4.8 ml at each pressure
increment, bringing the total volume to 17−25 ml
for each sample. Pore waters contained highamounts of sodium and chorine ions, and had
similar compositions to that of sea water (Fig. 3).
The chemical composition changed with increasingof the loading pressure from 200−300 MPa, in
especially sample No. 2, 3, 4, which have higher
SO4 contents. At the higher loading, the pore water
decreased Na+K and increased Ca+Mg.
The swelling test showed that the mudstoneswelled 5% in pure water and 4% in salt water (Fig.
4). The larger swelling in pure water may reflect
chemical osmosis, as discussed below. The normal3

10th Asian Regional Conference of IAEG (2015)

a−e in Fig. 5). Water contents before the rainfall
were 0.02−0.1 with the lowest value at 2.5 cm
depth. During precipitation, water contents
increased from 0.02 to 0.1 at 2.5 cm depth withbreaks corresponding to steps a, b, and c (Fig. 5),
from 0.05 to 0.07 at 5 cm depth with a breakcorresponding to step c, from 0.07 to 0.12 at 15 cm
depth with breaks at steps c and d (e), and from 0.1
to 0.14 at 40 cm depth with breaks at steps c and d.
The water content breaks thus moved downwards
through the material. Pore water EC before therainfall was 2−3 dS/m at depths of 2.5−15 cm, andwas 10 dS/m at 40 cm depth. During the
precipitation, EC at 2.5 cm depth gradually
increased from 2 to 10 dS/m, and at 5 cm depth
increased from 3 to 8 dS/m with two peaks at b andc. At 15 cm depth, EC increased from 2 to 9 dS/mwith a very small peak just after step e. In contrast,
at 40 cm depth, EC peaked immediately after step c,then decreased and recovered reach >10 dS/m.
Between 10 and 20 June 2010 in the early rainyseason, 200 mm of precipitation occurred with amaximum intensity of 17 mm per 10 min (Fig. 6).
During this rainfall event, intense precipitation (>5mm per 10 min) continued, and the total was 10times larger than the dry season’s total precipitation.
Water contents were 0.1−0.16 before the rainfall(Fig. 6). During the precipitation, the watercontents increased from 0.1 to 0.28 at 2.5 cm depth
and from 0.15 to 0.29 at 5 cm depth. In contrast,

Fig. 4 Volumetric changes in mudstone by using
pure and salty water during the swelling test
under a normal stress of 0.5 kg/cm2.
pressure of 0.5 kg/cm2 corresponds to the pressure
at 2.3 m depth when the density of rock is 2.2 g/cm3,
as obtained by Higuchi et al. (2013).
3.2 Monitoring of water contents, pore water EC,
and precipitation
The seasonal changes in water content and porewater EC were described in Higuchi et al. 2014, thedetailed responses of these properties to rainfall
events from dry season to the rainy season were as
follows. On 23−24 April 2010 in the dry season, 30
mm of rainfall occurred with three steps in the
cumulative curve of rainfall amounts (marked as

Fig. 6 Changes in water content and porewater EC for the rainfall event of 10−20 June
2010.
The vertical lines indicate breaks in valuesof cumulative rainfall.

Fig. 5 Changes in water content and pore water
EC for the rainfall event of 23 April 2010.
The vertical lines indicate breaks in values ofcumulative rainfall.

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10th Asian Regional Conference of IAEG (2015)

The water contents at 15 cm depth increased in
response to the rainfall event, which had amaximum intensity of 5 mm per 10 min, equivalent
to 8 × 10-4 cm/s, much higher than the permeabilityof rock matrix (10-8 cm/s; Lee et al. 2007).
Rainwater probably traveled along open cracks
further than 40 cm depth, as supported by watercontent changes during the rainfall in April (Fig. 5).
The vertical shifting of changes in water content
suggests that first the surface portion became
saturated then water infiltrated through open cracks
to reach the deeper parts of slope material. Voidratio of the fresh mudstone from a nearby slope
measure 0.2 (Higuchi et al. 2013), which is equal toa mass-basis saturated water content of 0.08. The
mudstone at 40 cm depth might have slightly higher
water contents than the assumed saturated watercontent.
In the early rainy season, the mudstone near aslope surface increases in water content muchhigher than saturation (Figs 5). Subsequently, after
the early rainy season, the increase in watercontents propagated to 40 cm depth (Higuchi et al.
2014), where water contents measured 0.16−0.18
after June and when water contents at 15 and 40 cm
depths scarcely responded to rainfall (Figs 6 and 7).
The very weak response at depth to rainfall events
may be a result of the closure of desiccation cracks
near the slope surface by the swelling of rocks. The
void ratio of rocks from depths of 20−30 cm on a
nearby slope measure 0.2 (Higuchi et al. 2013),
which suggests that the mass-basis water contents
at saturation at these depths is 0.08. In contrast, themonitored water contents at depths of 20−40 cm
were about twice that value, which suggests that therock volume at 40 cm depth might have increased
by absorbing more dilute water than pore water inthe dry season.
Abrupt increase in pore water EC near the slope
surface (2.5 and 5 cm) occurred during intermittent
rainfall events like those in Dry season (April) (Fig.
5), when EC increased sharply and then decreased
gradually. The sharp increase in EC may be due tothe dissolution of precipitated salts on fracturesurfaces (Higuchi et al. 2014). While after therainfall events, EC decreased gradually at thesurface (Fig. 5), when evaporation and salt
precipitation most likely occurred. But that salt

Fig. 7 Changes in water content and pore water
EC for the rainfall event of 27−29 August 2010.
The dashed line indicates the timing of thesurface erosion identified by the abrupt change ofthe values at 2.5 cm depth.
water contents at 15 and 40 cm depths increased byonly 0.01 after rainfall ceased. EC values varied
from 7 to 12 dS/m before the rainfall, anddecreased from 8 to 5 dS/m at 2.5 cm depth andfrom 12 to 9 dS/m at 5 cm depth during the
precipitation. In contrast, at 15 cm and 40 cm
depths, the values of EC were 7 and 9 dS/m,
respectively, and increased by only a small amountduring the rainfall.
On 27−28 August 2010, 150 mm ofprecipitation occurred with a maximum of 9 mm
per 10 min (Fig. 7). Water contents before the event
were 0.09−0.15, with the lowest occurring at 2.5
cm depth. During the precipitation, only watercontent at 2.5 cm depth increased, from 0.09 to
0.12; the water contents at other depths did not
respond to the rainfall. During the precipitation, EC
at 2.5 cm depth increased to 6 dS/m then decreased
to 2 dS/m. In contrast, at 5, 15, and 40 cm depths
the EC was almost constant at 7−10 dS/m. At 2.5
cm depth, the measured values of both watercontent and EC showed radical change, whichstarted 00:30−10:30 on August 28, which suggests
the timing of sensor disturbance and displacement
due to surface erosion.
5.

Discussion

In dry season in the several weak rainfall events,
during which there was a dramatic response ofwater content and pore water EC values (in April).

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10th Asian Regional Conference of IAEG (2015)

Fig. 8 Schematic sketch of the changing physical property profiles of the mudstone beneath the slope surface
during the dry and rainy seasons.
t1: dry season without precipitation; t2: after sparse precipitation during the dry season; t3: late dry season
after wetting and drying; t4: beginning of the rainy season; t5: after precipitation in the early rainy season (justbefore erosion). EC: Electrical Conductivity; WC: Water Content. The depths 5, 10, and 20 cm are indicative
averages based on the monitoring data and on the results of Higuchi et al. (2013, 2014).

during immersion because of the narrower pore
openings. That can inhibit passage of ions, thereby
the rock acts as semi-permeable membrane (Marine
and Fritz 1981). The salty interstitial water of the
mudstone would presumably cause larger
infiltration of surface fresh water during rainfall
due to unequal ionic concentration like osmotic
membrane. Therefore, the mudstone most likely
disperses (dissociates) mudstone-forming grains
and subsequently swells the volume at the slope
surface where salts concentrated during heavy rainin the rainy season.
On the basis of the above discussion of thechanges in both water content and EC, we here
discuss the overall scheme of weathering anderosion processes as seasonal phenomena (Fig. 8).
In the early dry season, mudstone is driedsignificantly near the slope surface (labeled t1 in Fig.
8). Desiccation cracks form near the surface, andare particularly abundant to a few centimeters in
depth. Water potential gradient brings salt waterfrom depth to the surface and the evaporation of thewater deposits salt on fracture surfaces (Higuchi etcrystallized on fracture surfaces and salt in the porewater of the rock matrix may have migrated to thesurface and therefore have decreased (Higuchi et al.
2014). In the early rainy season, EC at 2.5 and 5 cm
depths increased to their highest recorded levels
and thereafter decreased. During the rainy season,
the immediate responses of pore water EC torainfall events were limited to the shallow parts ofthe slope material (Figs 6 and 7). The
aforementioned changes in both water content andEC strongly suggest that open desiccation cracks,
which formed during the dry season, were closed at
the beginning of the rainy season. This crackclosure was observed in the field and could becaused by the swelling of rocks by water adsorption.
As shown by swelling experiments (Fig. 4), themudstone swelling occurs without expandable clay
minerals and the larger swelling occurs withimmersion of pure water, suggesting that thechemical osmosis-induced swelling occurs. The
mudstone has compacted fabric (0.2< void ratio)
and soluble salts, that can form overlapping of ionicdouble layer on negatively charged particle surface

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10th Asian Regional Conference of IAEG (2015)

mudstone swelling and time series fluctuations ofwater content and pore water EC near the slope
surface in badlands in Taiwan, and revealed the
mudstone mechanical properties and weathering
processes near the slope surface, as follows.
In the dry season, near the mudstone slope
surface, the mudstone of low water content
develops desiccation cracks. Rain water fromoccasional rainfalls immediately moves down to
the deep along open fractures, dissolving
precipitated salts near the slope surface. The
mudstone is essentially impermeable so that the
rainwater cannot infiltrate immediately into therock matrix. After the dry season, water content of
the mudstone increases and closes opened cracks
due to swelling near slope surface during heavy
rain in the early rainy season. This crack closureprohibits penetration of water to the deeper depth,
and consequently, water concentrates at the slope
surface with dilution during heavy rain. In theheavy rain, dilution of salty interstitial water during
the infiltration of surface fresh water disperses
rock-forming grains due to chemical osmosis. This
deteriorated part would be removed during surface
wash in the late rainy season. After the rainyseason, stripped surface is dried and starts to be
weathered again.

al. 2014). During the following late dry season,water potential is greatest at 15 cm depth, andtherefore water movement follows the potential
gradients at depth greater than 15 cm depth. At
depths shallower than 15 cm, water ascends
according to the potential gradients but water vapor
may return to 20−40 cm depth and condense under
the cooler conditions there. Water from intermittent
rainfall in the late dry season flowing down open
cracks may be another source of water. Salt
precipitation occurs as a result of the migration ofsalt from the inner matrix to cracked surfaces when
the slope surface material is dried. Rainfall in thedry season increases moisture content in surface
cracks and dissolves the precipitated salt (labeled t2
and t3 in Fig. 8). After rainfall, water content
decreases because of drying and the migration ofdissolved salt to surface cracks. In the early rainyseason, rainfall causes water contents near the slope
surface to increase up to much higher thansaturation (Higuchi et al. 2014) (labeled t4 to t5 in
Fig. 8). This increase in water content dilutes porewater and probably increases the chemical osmoticpressure and affects pore size enlargement. The
surface fresh water during rainfall most likely
disperses (dissociates) the rock-forming grains due
to unequal of ionic concentration with theinterstitial salty water and subsequently swells thevolume. The enlargement of pore sizes is morecommon near the surface, as identified in pore size
distribution curves for drilled cores (Higuchi et al.
2013). Peak pore sizes ranged from 0.04 to 0.06 μm
in rock deeper than 20 cm and measured 0.1 μm
with an additional peak at 0.3 μm at depths
shallower than 10 cm. The high water contents andpore size enlargement act to weaken rock near theslope surface, as indicated by tests of needle
penetration resistance in drill core material(Higuchi et al. 2013). High amounts of rainfall in
the rainy season precede increases in water content,
salt water dilution, pore size enlargement, anddeterioration in the surface layer (after t5 in Fig. 8),with the surface layer finally being removed by
predominant surface wash erosion.
6.

Acknowledgements
Eiji Nakata of Central Research Institute ofElectric Power Industry (CRIEPI) is acknowledged
for his help of measuring the pore water chemistry
and for his advice in the field and laboratory. Prof.
Yuki Matsushi of the Disaster Prevention ResearchInstitute of Kyoto University is acknowledged for
his advice in the field and laboratory. Prof.
Yen-Hua Chen of the National Cheng-Kung
University is acknowledged for her help of
measuring the BET specific surface area. Prof.
Hun-Ming Lin of the National Cheng-Kung
University is acknowledged for his help in the field.
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