ORIGINAL ARTICLE

Site monitoring of suction and temporary pore water pressurein an ancient landslide in the Three Gorges reservoir area, China

Minggao Tang • Qiang Xu • Runqiu Huang

Received: 23 December 2013 / Accepted: 17 October 2014 / Published online: 1 November 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Rainfall is the main factor influencing the sta-

bility of landslides, but the precise mechanisms and pro-

cesses are still poorly known. Unsaturated soil mechanics

can be used to study how suction and temporary pore water

pressure affect landslide stability. During rainfall, the

groundwater level in the landslide will change, and the

suction and temporary pore water pressure will also change

with time and rainfall. Construction of a field observation

well is an effective method of studying changes in suction.

An observation well was constructed in an ancient land-

slide in the Three Gorges reservoir area in China. The well

has a depth of 20 m, a diameter of 2 m, and is the largest

observation well for monitoring of suction and temporary

water pressure in the world. Suction, temporary water

pressure, and rainfall were measured between October

2002 and December 2004; this is the longest observation

period for this type of study. Analysis of the results shows

that the distribution of suction is determined by the depth,

soil materials, gradation, and structure of the landslide. The

changes in suction are related to the intensity and duration

time of rainfall, the season, and the climate. If the suction

does not change promptly after rain, there is a time dif-

ference between the timing of rainfall and of suction

change. The suction varied between 0 and 15 kPa. These

conclusions form a basis for further scientific studies on

how rainfall affects landslides.

Keywords Ancient landslide � Rainfall � Suction � Porewater pressure � Site monitoring � Climate change

Introduction

Rainfall is one of the main causes of slope failure (John

and Douglas 2012). Landslides often occur during the

rainy seasons, generally happening in the natural slope

(Brand et al. 1984; Canuti et al. 1985; Finlay et al. 1997;

Polemio and Sdao 1999), slope of highway and hydro-

power project (Mark and Richard 1998; Huang 2007), and

opencast mines slope (Vishal et al. 2010; Pradhan et al.

2011; Singh et al. 2013), as well as underground mining

slope (Yin et al. 2013). Rainfall will affect the hydroge-

ology of the inner parts of landslides when viewed

macroscopically, and will also change the physicochemi-

cal characteristics of landslide materials at the micro-

scopic scale. These are regarded as very complex random

processes. However, the soil in all landslides changes

from unsaturated to saturated, or from saturated to

unsaturated during rain. Because of these factors, it is

necessary to use unsaturated soil mechanics theory to

analyze and discuss the suction changes in landslides that

occur under different levels of rainfall. There are a

number of in situ and laboratory devices for measurement

soil suction. Site monitoring is more direct than labora-

tory measurements and plays a key role in unsaturated

soil mechanics in engineering practice. This study

addresses groundwater migration and seepage flow during

rainfall, and attempts to analyze landslide stability on the

basis of site monitoring of suction and groundwater

pressure in an ancient landslide. Site monitoring of suc-

tion in a typical ancient landslide is helpful and necessary

for understanding and monitoring of the stability of

thousands of ancient landslides in the Three Gorges res-

ervoir area, China.

Similar experiments have been carried out previously.

Sweeney (1982) built two wells of less than 10 m depth in

M. Tang (&) � Q. Xu � R. HuangState Key Laboratory of Geohazard Prevention and

Geoenvironment Protection, Chengdu University of Technology,

No.1 Erxianqiao Rd, Chengdu 610059, China

e-mail: tomyr2008@163.com

123

Environ Earth Sci (2015) 73:5601–5609

DOI 10.1007/s12665-014-3814-4

residual soil slopes in Hong Kong, and observed them for

1 year. Fredlund et al. (1995) monitored matric suction and

deformation at a depth of about 5 m in an expansive soil in

southern China. Lee et al. (1996), Leong et al. (1998), and

Rahardjo et al. (2005) analyzed matric suction in a residual

soil slope at depths of 0.5, 1.0, and 1.5 m. Gong et al.

(1999), Wang et al. (2001), and Ng et al. (2003) monitored

matric suction at a distance of 3 m from the slope surface

in an expansive soil slope. They monitored matric suction

in expansive soil and residual soil slopes and accumulated

a large amount of observational data.

To observe matric suction and temporary pore water

pressure in ancient landslides, site monitoring and experi-

ments were carried out. An observation well was con-

structed in the Xietan ancient landslide of the Three Gorges

of the Changjiang (Yangtze) River in May 2002. The well

has a depth of 20 m, a diameter of 2 m, and is the largest

well for suction observation in the world, deeper by 10 m

than of Sweeney (1982). The well was activated and

measurements of suction, temporary pore water pressure,

and rainfall quantity were carried out from October 2002

until the end of 2004. This is the first instance of such

large-scale observations being conducted over such a long

period of time. This paper shows the results of site moni-

toring of suction and temporary pore water pressure and

discusses their changes with rainfall and time.

Unsaturated soil mechanics

The effective stress principle in unsaturated soil

effective stress principle

Unsaturated soil is composed of solids, water, air, and a

contractile skin. The contractile skin is a fourth indepen-

dent phase called the air–water interface and acts as a thin

membrane interwoven throughout the voids of the soil;

thus, the mechanics of unsaturated soil are more compli-

cated than those of saturated soil. Additionally, it is nec-

essary to consider the contractile skin during stress analysis

(Fredlund and Rahardjo 1993, 1997). The most widely used

unsaturated soil effective stress principle is Bishop’s

effective stress equation (Bishop 1959; Bishop and Blight

1963):

r0 ¼ ðr� uaÞ þ xðua � uwÞ; ð1Þ

where r0is the effective stress; r is the normal stress on the

failure plane; ua is the pore air pressure; uw is the pore

water pressure; ua � uwð Þ is the matric suction in the soil; x

is a soil parameter related to the degree of water saturation,

ranging from 0 to 1; ua is the relative air pressure in the soil

pore spaces; and uw is the relative water pressure in the soil

pore spaces.

The difference between the pore water pressure and the

pore air pressure in the soil ua � uwð Þ is called the suction,

and represents the hydrophilic ability of the soil grains.

Shear strength formula for unsaturated soil

The shear strength formula for unsaturated soil can be

expressed using the independent stress condition variable

(Fredlund et al. 1978). Two of the three stress condition

variables in the shear strength formula are used. It has been

shown that the stress condition variables r� uað Þ and

ua � uwð Þ are the most favorable association. The shear

strength formula for unsaturated soil including these vari-

ables is as follows:

s ¼ c0 þ ðr� uaÞf tg/0 þ ðua � uwÞf tg/

b; ð2Þ

where c0 is the intercept of the Mohr–Coulomb failure

envelope and the shear stress axes; r is the total normal

stress when the surface breaks; ua is the pore air pressure

on the broken surface, which will often be standard

atmospheric pressure; /0 is the internal friction angle,

which is related to the net normal stress condition variable

r� uað Þ ; uw is the pore water pressure when the surface

breaks; and /b represents the friction angle, which changes

with the shear strength as the suction alters.

Dc0 ¼ ðua � uwÞf tg/b ð3Þ

Dc0is the increment of effective cohesive strength when

ðua � uwÞf changes; ðr� uaÞf is the net normal stress

condition when the surface breaks; ðua � uwÞf and is the

suction on the failure plane.

The effective stress principle and shear strength formula

of unsaturated soil are both extensions of those of saturated

soil. Bishop and Blight (1963) suggests that the relation

between x, tg/b and tg/t can be obtained using the formula

for the effective stress of unsaturated soil, and can be

shown to be:

x ¼ tg/b�tg/t: ð4Þ

These theories about unsaturated soil are not entirely

perfect, but do provide a means of carrying out further

studies on unsaturated soil.

Purpose, materials and methods of site observations

The purpose of field observations

From the analysis and equations given above, studies of

landslide stability and the influence of deformation must

involve a fundamental parameter: suction. Suction also

plays a key role in unsaturated soil theory and saturated soil

5602 Environ Earth Sci (2015) 73:5601–5609

123

theory. Therefore, studies of and experiments on suction

are necessary.

Surveys should be conducted taking the original suction

of the landslide into account, and the rules governing

changes in suction should be determined on the basis of

robust experiments. During the observation process, the

main priority is to obtain the suction at different depths in

the landslide body, the pore water pressure, the amount of

rainfall in the landslide region, and the changes in these

measurements with time, season, and depth. The aims of

this site observation were to determine how the matric

suction changed with changes in depth, time, season,

rainfall, and the processes of moisture absorption and loss

in the unsaturated zone of the Xietan ancient landslide.

The location of the field observation well

The field observation well was constructed in the Xietan

ancient landslide, which is located in the Three Gorges area

of Hubei province, China (Fig. 1). The well was positioned

in the middle and slightly to the right of the landslide body,

as shown in Fig. 2. The elevation of the well mouth is

?100 m. The well’s diameter was 2 m, its depth 20 m, and

its base is at the sliding belt. A brick wall was constructed,

and a small gap was left for the vadose zone of the land-

slide. The wall thickness of the well is 0.24 m (Fig. 3).

The observation equipment

Thirty vacuum tubes were used to measure suction, and 20

sensors (vibrating string uplift water pressure gages) were

used to measure the temporary pore water pressure in the

well. The sensors were positioned 1.5 m from the side of

well. The vacuum tubes were installed in the well mostly in

the direction S 75�E, and the sensors were parallel to the

vacuum tubes. The vacuum tubes and sensors were distrib-

uted from the mouth to the base of the well. A ladder was

placed in the well to allow access to read the data (Fig. 4).

Geological structure of the Xietan ancient landslide

The Xietan landslide is divided into three layers from the

surface to the base: the landslide body, the slide zone, and

bedrock, as shown in Fig. 5. The body of the landslide is

divided into five sub-layers according to their composition.

I: rock fragments and soil, size of rock fragments

0.5–20 cm, brown, volume ratio of soil to grains

9:1.

II: rock fragments and soil, size of rock fragments

10–20 cm, gray-green, volume ratio of soil to

grains 4:1.

III: rock fragments and soil, size of rock fragments

20–60 cm, gray-green or purple, volume ratio of

soil to grains 5:3.

Fig. 1 Map showing the location of the Xietan landslide in the Three

Gorges reservoir area, China

Fig. 2 Xietan ancient landslide and the site of the observation well

Fig. 3 Observation well at the surface

Environ Earth Sci (2015) 73:5601–5609 5603

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IV: rock fragments and soil, size of rock fragments

10–20 cm, purple, volume ratio of soil to grains

3:1.

V: rock fragments and soil, size of rock fragments

50–120 cm, gray-green, volume ratio of soil to

grains 1:7.

Results

Distribution patterns of suction and temporary pore

water pressure

The measurement of temporary pore water pressure used

sensors that are also called vibrating string uplift pressure

gages. These devices are fast and sensitive, but have a

comparatively high cost. As the instruments were located

in the unsaturated zone, most of the pressures were below

zero, expressed as negative pore water pressures. Part of

the landslide body was found to have been temporarily

saturated as a result of rainfall, thus some results are

greater than zero. Figure 6 (left) shows the results of

observations made on three separate days at two-month

intervals. These results reflect the distribution of pore water

pressure with depth and in different parts of the rock and

soil mass.

The values for suction were obtained from the dials of

the vacuum tubes, providing us with a series of data on

suction. For reasons of space, data for only three days at

monthly intervals are illustrated (Fig. 6, right). These

reflect the different distribution patterns of suction from the

landslide surface to the lower bedrock.

The distribution patterns of suction and pore water

pressure are described below.

1. The Xietan landslide body is mainly composed of

crushed rock, rock fragments, and soil. As a result of

gravitational sorting, the sub-layers in the landslide

body are sorted by material type. The suction and

temporary pore water pressure of the Xietan ancient

landslide are closely related to the material and

composition of the landslide body. Figure 6 shows

the suction and pore water pressure values through the

landslide section: both measurements have a zoned

distribution. The values and spatial distributions of

suction and pore water pressure are different because

of different percentage compositions of soil, rock

fragments, and moisture. This conclusion has not been

mentioned in previous studies.

2. The landslide body can be divided into four zones, �,

`, ´, and ˆ, according to the changes in suction.

The proportion of rock fragments increases and that

Fig. 4 Sketches showing

equipment positioning

(a external view on ground

surface, b vertical section)

Fig. 5 Cross-section through the Xietan ancient landslide

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of soil decreases from zone � to zone ˆ, and the

percentage water content changes from low to high.

The suction value of zone � is higher than that of

zone ` because the grain composition of zone � is

finer than that of zone `, but the water content of

zone ` is higher than that of zone �. The interfaces

of the different layers hinder water migration. The

water content is higher near the interface of zones �

and ` or zones ` and ´, where the suction is much

lower and the pore water pressure is comparatively

high. The main slide zone and the secondary slide

zone have formed a layer that is relatively imperme-

able. In the rainy season, water cannot penetrate this,

so the soil near this layer becomes saturated, with a

suction close to zero.

3. The temporary pore water pressure in unsaturated areas

is mostly less than zero, but some parts of the landslide

will temporarily show saturated water pressure. The

larger the pressure value, the more moisturized the

landslide body. However, the moisture absorption

ability is comparatively weak.

Change in suction and temporary pore water pressure

with time and rainfall

The rainfall, matric suction, and temporary pore water

pressure were observed at depths of 1.0, 6.8, and 18.0 m in

the landslide. Figure 7 shows the variation in suction with

rainfall and time at these depths. The matric suction will

change with the alteration of seasons and changes in cli-

matic conditions. Figure 8 shows variations in temporary

pore water pressure with rainfall and time, again at depths

of 1.0, 6.8, and 18.0 m. The quantity of rainfall will

directly raise the moisture content in the landslide body, so

the pore water pressure in the landslide will show different

change tendencies with rainfall and the alteration of the

seasons.

The monitoring sensors were installed on October 6,

2002, but data collection began on October 15, 2002, after

the ground water in the landslide had returned to the nor-

mal (pre-installation) conditions. Some information on the

changes in suction and temporary pore water pressure in

the main body of the Xietan ancient landslide with rainfall

and time can be obtained from detailed analysis of the

observational data and monitoring curves.

1. The fluctuations in the monitoring data indicate that

the monitoring curve is sensitive to changes in time

and weather. When the quantity of rainfall reaches

10 mm/day, 80 % of the suction data readings will

change, with an apparent declining tendency and

100 % of the water pressure data readings will change

with an apparent rising tendency, because the sensor

is sensitive to minute changes in water pressure and

temperature.

Fig. 6 Curve of suction and

temporary pore water pressure

change with depth

Environ Earth Sci (2015) 73:5601–5609 5605

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2. Over 1 year, rainfall has an obvious Gaussian distri-

bution, centered in the period May to October. The

amount of water in the landslide body will change

cyclically with rainfall, but with a time lag. The

reason for this is that it takes time for the body of the

landslide to absorb moisture after rainfall: this time is

short in dry seasons but longer in rainy seasons. The

average value of suction in the observation period

between October and May is larger than the average

value for the period between May and October.

3. The weather in autumn and winter is always dry, and

the quantity and duration of rainfall is reduced.

Rainfall is greater during spring and summer, so the

suction is generally small and the pore water pressure

is higher during this time.

4. The changes in rainfall and weather that are reflected

in the curves will show a delay time related to the

composition of the geological bodies and the charac-

teristics of the rock-soil mass. When rain falls on the

ground surface, the suction at a depth of 1.0 m in the

landslide will change within a day, and the suction at

a depth of 6.8 m in the landslide will change after

1–2 days.

5. When rainfall is greater than 8 mm/day, the suction

measurements in the field will show a decline over

several days, but the temporary water pressure will

rise. This indicates that the suction will decrease

while the water pressure will increase during the

period in which the amount of water in the landslide

body increases. When rainfall is less than 8 mm/day,

this phenomenon is not observed.

6. When the weather is fine for a whole week, the

monitoring curve for suction will show an obvious

rising tendency, and the water pressure measured by

the sensor will decline. This indicates that the suction

will become large, while the water pressure will

decrease during the period in which the amount of

water in the landslide body decreases.

Fig. 7 Changes in suction and

daily rainfall through time

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7. The rainfall strength and the quantity of rainfall will

directly affect the suction and the pore water pressure.

When rainfall is greater than 15 mm/day, the suction

readings from the vacuum tubes suction will decrease.

For example, during three days of rainfall from

November 14, 2002, 80 % of the vacuum tubes suction

data readings reduced, even decreasing to zero; at the

same time, most sensor data changed significantly.

When rainfall is between 15 and 10 mm/day, some of

the suction readings will decrease, some of the sensor

data will change significantly, and the pore water

pressure will markedly rise.

8. The changes in the suction readings from the vacuum

tubes are closely related to the rainfall permeation

process, i.e., the closer a vacuum tube is to the earth’s

surface (h\ 3 m), the shorter the lag time will be;

conversely, the suction readings at the bottom of the

well change relatively slowly with changing rainfall.

The suction reading of the vacuum tube closer to the

surface (less than 3 m) falls on the day of rainfall

(within 12 h), whereas the suction readings of the

vacuum tubes that are furthest from the surface (more

than 5 m) show a time lag of 18–24 h. During

observation, the temporary pressure of pore water was

normally between -10 and 2 kPa.

9. When discontinuities are large, or, as in the lower

zone, there is an impermeable layer with a high clay

content, the suction may be high and it will not be

easy for water to seep through (e.g., the 3.2–7.5 m

section). The suction reading of the vacuum tubes will

be relatively small, as will the material suction. When

the sensor measuring pressure is located/installed in

an impermeable stratum or a suction stratum, the

pressure reading will decrease. However, in thick clay

strata and sliding belts, it is difficult for water to

evaporate in the vacuum tube, so the reading might

not change noticeably. Therefore, cases are known in

which a cloudburst after a long-term drought caused

the soil suction in the sliding zone to decrease

suddenly, which consequently resulted in a landslide.

Fig. 8 Pore water pressure at

three different depths and daily

rainfall plotted against time

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10. When the weather is fine, water in the non-saturated

region drains off mainly by gravity, as well as by

evaporation. The observation records show that the

suction readings from the vacuum tubes at the bottom

of the observation well increase first.

11. The effective stresses rise with an increase in matric

suction and a decrease in temporary pore water

pressure, according to the effective stress principle of

unsaturated soil.

Discussion and conclusions

The matric suction in unsaturated soil is affected by the soil

texture, the distribution of suction, and the interaction of

rainfall and soil. Thus, its influence on landslide stability is

very complicated. The application of the theory of unsat-

urated soil strength to solve problems in actual projects is

largely restricted by the strength theory of unsaturated soil

and the technology available for parameter testing. So, the

relationships between suction, temporary pore water pres-

sure, soil shear strength, and landslide stability require

further investigation. This research and site monitoring of

suction will greatly improve the understanding of the

complexity of the influence of rainfall on the landslide

stability and of the processes of storage, migration, and

seepage of groundwater in geologic bodies, and quantify

the influence of rainfall on landslide stability. Therefore,

this research has significant practical value for the geo-

technical engineers to improve their designs for economi-

cal and effective control of landslides.

Rainfall and the rainwater permeation process will

directly influence the suction and temporary pore water

pressure in the body of the landslide. On the basis of two-

year site monitoring and observation of soil matric suction

in the Xietan ancient landslide in the Three Gorges reser-

voir area, the following conclusions may be drawn;

1. The distribution characteristics of matric suction and

pore water pressure in the Xietan ancient landslide are

related to the depth, soil materials, gradation, and

structure of the landslide body. The distribution

characteristics of matric suction with depth in the

Xietan ancient landslide is similar to the Hong

Kong’residual soil slopes (Sweeney 1982).

2. The changes in matric suction and pore water pressure

in the Xietan ancient landslide are affected by the

intensity and duration of rainfall. Both matric suction

and pore water pressure show a time lag with respect to

rainfall time.

3. The matric suction and pore water pressure in Xietan

ancient landslide show changes with season and

climate, and are also closely related to humidity

changes over the whole region. The matric suction in

the dry season is greater than in the rainy season, but

the pore water pressure shows the opposite pattern.

4. The average value of matric suction in Xietan ancient

landslide is less than the average value in expansive

soil and residual soil slopes because the grain size in

the landslide is greater than that in expansive clay soil.

5. The maximum value of matric suction in Xietan

ancient landslide is about 15 kPa, so its contribution to

improving the stability of the Xietan ancient landslide

is very small.

6. Previous studies on suction and temporary pore water

pressure in the unsaturated regions of landslide have

been mostly theoretical and lacked verification using

practical information. In this study, large-scale mon-

itoring of suction has been conducted on a previously

unknown scale. The results reveal the rules governing

changes in matric suction and moisture content inside

the landslide. The results are based on large amounts of

site monitoring data and information, and are thus

practical and reliable.

7. After the completion of the Three Gorges Project, the

first water impoundment and the repeated rise and fall

of reservoirs might cause a decrease in landslide

stability. At the same time, if heavy rainfall occurs,

stability will be reduced further because of the

decrease in suction and increase in water pressure.

Therefore, this research is also important for geolog-

ical hazard prevention in the Three Gorges reservoir

area or in other reservoirs that may suffer from

landslides influenced by pore water pressure.

Acknowledgments This study was financially supported by the

National Basic Research Program of China (973 Program, Grant No.

2013CB733202), the National Natural Science Foundation of China

(Grant No. 41002111), and the State Key Laboratory of Geohazard

Prevention and Geoenvironment Protection (SKLGP2009Z016). The

authors are grateful to Dr. Niek Rengers (International Institute of

Geo-information Science and Earth Observation, ITC) for his valu-

able comments on the manuscript and his careful review of this

paper, Dr. Qi Guoqing for his generous help in site observation and

sincerely thank the reviewers of this paper for their valuable com-

ments and suggestions.

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