10th Asian Regional Conference of IAEG (2015)

A Case Study: Characteristics of Unsaturated Loess (Q3) Sampling on Loess Plateau, NW
China
Li FENG(1, 2), Genlong WANG*(1), Maosheng ZHANG(1) and Wanli XIE(2)(1) Key Laboratory of Geo-hazards in Loess area, Xi’an Center of Geological Survey, Xi’an, China
E-mail:wang2006@mail.iggcas.ac.cn(2) State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China
Abstract
The late Pleistocene loess (Q3) widely distributes in the northwest area of China. As
for loess landslides, it has great importance to research the physical and mechanicalcharacteristics of unsaturated loess (Q3). Therefore, to determine the Soil Water Retention
Curve (SWRC), Hydraulic Conductivity Function (HCF) and Suction-Stress Characteristic
Curve (SSCC) play a key role in analyzing the mechanical behavior of unsaturated loess. In
this paper, the SWRC, HCF and SSCC of loess (Q3) sampling on Chinese loess plateaus
have been measured by the TRIM apparatus. The results show that the parameters haveobvious difference under drying and wetting state, and the difference reflects the hysteresis
characteristics of SWRC, HCF and SSCC. In fact, “the hysteresis effect” will control therainfall-induced loess landslides. Finally, the research indicated that the suction stress ismore able to reflect the unsaturated hydraulic properties of loess than the matric suction.
Keywords: Unsaturated loess, Suction stress, TRIM test, Loess landslide
1.

Introduction

and limited measuring range (Hilf, 1956; Spanner,
1951; Gee et al., 1992; Houston et al., 1994; Likos
and Lu, 2003). In the research, the Transient Release
and Imbibition Methods (TRIM) has been used tomeasure the Soil-Water Retention Curve (SWRC),
Hydraulic Conductivity Function (HCF) and Suction
Stress Characteristic Curve (SSCC) of loess within arange of suction (0-106 kPa) under both drying andwetting conditions. The research can be used toestablish the irrigation or rainfall warning criteria ofloess landslide hazards.

According to unsaturated soil theory, the shear
strength, stress-strain relationship, consolidation andgroundwater seepage are closely related to the matricsuction. Moreover, the soil water characteristic curve(SWCC) reveals the relationship between matricsuction (ua-uw) of unsaturated soil and humidity (e.g.,
volumetric water content, water content or saturation)(Fredlund and Rahardjo, 1993; Miller et al., 2002;
Mbonimap et al., 2006). So far, the soil water
characteristic curve (SWCC) has been widely used inunsaturated soil theory (Gardner, 1958; Li, 2005;
Pham et al., 2005; Nuth and Laloui, 2008; Mualem
and Beriozkin, 2009; Min and Huy., 2010; Vanapalli
et al., 1999; Romero et al., 2001; Imbert et al., 2005).
In fact, the water-soil coupling effect of unsaturated
soil is one of the research hotspots. In recent years,
the concept of suction stress has been proposed by Lu
et al. (2004, 2006a, 2006b, 2010), and the suction
stress characteristic curve (SSCC) has become analternative method to study the hydraulic property ofunsaturated soil.
Generally speaking, to measure unsaturated
hydraulic properties includes different experimental
tests, such as tensiometer, psychrometer and chilled
mirror hygrometer. However, these methods havesome disadvantages of complexity, time-consuming

2.

Study Area

The Heifangtai terrace is located 42 km west ofLanzhou city, Gansu Province, NW China. The study
area is on the fourth terrace of the Yellow River, andis about 100 - 133 m higher than the water table. The
terrace has been cut by the Yellow River, Huangshui
River and other small valleys, and formed deep
valleys and slopes with severe water and soil losses.
The outcropped layers are the Upper Pleistocene
wind-blown loess underlying sandy gravel layer. The
bottom of layers is the Cretaceous mudstone andsandstone, presently forming steep, high cliffs along
the river corridor. The upper layer of loess iscomposed of silty particles, and is loose with big
voids and vertical joints.

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10th Asian Regional Conference of IAEG (2015)

In the past decades, agricultural irrigation in the
study area has greatly changed the groundwater
through infiltration of irrigation water. Irrigation
penetration increased the recharge of the groundwaterand manually broke the balance of the groundwatercondition. In recent years, the average yearly
irrigation volume is about 590.91×104 m3/a and thegroundwater recharge has gone up to 362.57×104
m3/a. As a result, the water table has risen several
meters, and the unsaturated zone of loess has
increased gradually, resulting in many loesslandslides. According to statistics, there are 14 loesslandslides in the 2.8 km distance of Jiaojiaya slopes.
Therefore, the research on hydro-mechanicalproperties of loess in the study is very important.
3.

In particular, the loess and the ceramic stone mustbe saturated before the TRIM test; otherwise theexperimental data cannot be used as the objectivefunction of inversion. After several tests andimprovements, we used the saturated method ofcontinuous pumping air. The detailed operation
includes the following steps of: (1) Put the loess andthe ceramic stone into the vacuum apparatus for twohours, and then open the valve to make theapneumatic water into the vacuum apparatus; (2)Close the valve when the water rises to half height of
the loess and the ceramic stone, and then closes thevalve; (3) Pump continuously for more than an hour,
and then open the valve when the water completely
submerged the loess and the ceramic stone; (4) Closethe valve, and then pump more than an hour; (5)Finally, make the loess and the ceramic stone
immerse in the water for twenty-four hours. Usually,
according to the improved method, the degree ofsaturation can reach 100%.

Experimental method

3.1 Apparatus and method
The apparatus of TRIM tests introduced from
Colorado School of Mines in America showed in
Figure 1. The TRIM System is a simple-to-use
laboratory device for measuring SWRC and HCF of
unsaturated soils. The SWRC and HCF may bemeasured along either wetting or drying paths in as
little as 5-7 days using either disturbed or undisturbed
samples. The system may be operated in either
steady-state mode or transient mode and is applicablefor all major soil types, including sand, silts, andclays. Some features of the TRIM System as follows.
 Precision
flow
cell
with
integrated
high-air-entry base (300 kPa); Capability to extrapolate suction range toseveral thousand kPa;
 Control panel for dual-range air-pressure (< or> 15 kPa) control; Undisturbed or remolded specimen for
dimensions of 5-14 cm (diameter) x 5-10 cm(height); Completion of both wetting and drying SWCC
and HCF shown above in one week.
3.2 Sampling and parameters
In the study, the undisturbed loess samples werecollected from the Heifangtai platform in Gansu
Province of Northwest China. The size of thesesamples is H2.54 cm×Φ6.18 cm. The basic physicalparameters of the test completed in the Key
Laboratory of Geo-hazards in Loess area, Xi’an
Center of Geological Survey. All the parameters wereshown in Table 1.

Fig. 1 (a) TRIM system; (b) The components ofthe flow cell
4.

Results and Discussion

4.1 SWRC and HCF
Based on the theory of unsaturated soil, thesoil-water retention curve (SWRC) and hydraulicconductivity function (HCF) can be used to predict

Table 1 Physical parameters of undisturbed loessSampling
area

Density(g/cm3)Water
Dry density
content(g/cm3)(%)

Heifangtai

1.452

7.7

Porosity
Ratio

Specific
Gravity(kN/m3)LiquidLimits(%)

Plastic
Limits(%)

Saturated
Infiltration
Coefficient

1.01

2.71

26.55

15.92

5.07E-4

1.348

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10th Asian Regional Conference of IAEG (2015)

the effective stress and the change rules of the shear
strength. By means of Hydrus-1D code, the SWRC
and HCF of the loess samples were obtained underboth drying and wetting conditions, and suction of
0-106 kPa. The results and the model parameters asshown in Figure 2.
For a given water content, the value of matricsuction under wetting conditions is lower than thevalue of matric suction under drying condition.
Moreover, the energy of pore water along the wetting
path is higher than the drying path. The results reflect
the soil’s hysteresis, which generally results from
tortuous soil pore structure and /or difference inaffinity between soil grains and water under different
wetting histories. Loess landslides usually occur due
to heavy rainfall, so the wetting path is generally
more relevant in describing the failure processes thanthe drying path. Specifically, although both thedrying curve and the wetting curve can be used tobuild rainfall warning criteria for rainfall-induced
landslide, the wetting curves of SWCC are the mostsignificant hysteresis model for unsaturated soil slope
seepage.

to trigger loess landslides in the study area.
Usually, the suction stress can supply tensile
forces between the soil particles. Therefore, it canincrease the effective stress of soils. As we all known,
the effective stress may be very low (0-20 kPa) in thelayer of shallow soil, so the variation of suction stressis very sensitive. This is why the loess hill slopes
slide frequently under irrigation and rainfall
conditions.

Fig. 3 SWRC and SSCC of undisturbed loess underwetting and drying conditions

Fig. 2 SWRC and HCF of undisturbed loessunder wetting and drying conditions
4.2 SSCC
The suction stress characteristic curve (SSCC)
can be quantified from SWRC. The Figure 3 and 4
show the SWRC and SSCC for Q3 loess under bothwetting and drying conditions. The maximum suction
stress under the drying conditions is -684 kPa
corresponding to a water content of 9.08%. However,
the maximum suction stress under the wetting
conditions is -177 kPa corresponding to a watercontent of 8.38%. Meanwhile, the air-entry pressure(1/α) under the drying path is 52.63 kPa，and the
air-entry pressure (1/α) under the wetting path is
15.15 kPa. The difference of suction stress due tohysteresis for the loess sample can be up to 37 kPa.
This difference of suction (effective stress) is enoughFig. 4 SSCC, as a function of matric suction
under wetting and drying conditions
5.

Conclusions

Firstly, the hysteresis behaviors for the SWRC,HCF and SSCC of the loess are quite different under
the wetting and drying conditions. Secondly, loesslandslides usually occur due to heavy rainfall or
irrigation, so the wetting path is generally more
relevant in describing the failure processes than thedrying path. Thirdly, the difference of suction stressdue to hysteresis for the loess sample can be up to 37
kPa, and the difference of suction (effective stress) is3

10th Asian Regional Conference of IAEG (2015)

enough to trigger loess landslides in the study area.
Finally, suction stress is more able to reflect theunsaturated hydraulic properties of loess than thematric suction.

Lu, N., Wayllace, A., Carrera, J. and Likos, W. J.
(2006b): Constant Flow Method for Concurrently
Measuring Soil Water Characteristic Curve andHydraulic Conductivity Function, Geotechnical
Testing Journal, Vol. 29, No. 3, pp. 256-266.
Lu, N., Godt J.W. and Wu, D.T. (2010): A
closed-form equation for effective stress inunsaturated soil, Water Resources Research, Vol.
46, No.5, W05515.
Mbonimap, M., Aubertin, M., Maqusoud, A., et al.
(2006): Predictive model for the water retention
curve of deformable clayey soils, Journal of
Geotechnical and Geoenvironmental Engineering,Vol. 132, No. 9, pp. 1121-1132.
Miller, C.J., Yesiller, N., Yaldo, K., et al. (2002):
Impact of soil type and compaction conditions on
soil water characteristic, Journal of Geotechnicaland Geoenvironmental Engineering, Vol. 128, No.
9, pp. 733-742.
Min, T.K. and Huy, P.T. (2010): A soil-water
hysteresis model for unsaturated sands based on
fuzzy set plasticity theory, KSCE Journal of Civil
Engineering, Vol. 14, No. 2, pp. 165-172.
Mualem, Y. and Beriozkin, A. (2009): Generalscaling rules of the hysteretic water retentionfunction based on Mualem`s domain theory,
European Journal of Soil Science, Vol. 60, No. 4,
pp.652-661.
Nuth, M. and Laloui, L. (2008): Advances inmodeling hysteretic water retention curve in
deformable soils. Computers and Geotechnics,
Vol. 35, No. 6, pp. 835-844.
Pham, H.Q., Fredlund, D.G. and Barbour, S.L.
(2005): A study of hysteresis models forsoil-water characteristic curves. Canadian
Geotechnical Journal, Vol. 42, No. 6, pp.
1548-1568.
Romero, E., Gens, A. and Lloret, A. (2001):
Temperature effects on the hydraulic behavior ofan unsaturated clay, Geo-technical andGeological Engineering, Vol. 19, No. 3-4, pp.
311-332.
Spanner, D.C. (1951): The Peltier Effect and Its Use
in the Measurement of Suction Pressure, Journalof Experimental Botany, Vol. 11, No. 2, pp.
145-168.
Vanapalli, S.K., Pufahl, D.E. and Fredlund, D.G.
(1999): The influence of soil structure and stresshistory on the soil-water characteristics of acompacted till, Geo-technique, Vol. 49, No. 2, pp.
143-159.

Acknowledgements
This work was funded by the Key Laboratory ofGeo-hazards in the Loess Area, and supported by the
Science Innovation Projects of Shaanxi Province (no.
2011KTZB03-02-01).
References
Fredlund, D.G. and Rahardjo, H. (1993): Soilmechanics for unsaturated soils, New York, John
Wiley & Sons.
Gardner, W.R. (1958): Some steady-state solutions of
the unsaturated moisture flow equation withapplication to evaporation from a water table,
Soil Science, Vol. 85, No. 4, pp. 228-232.
Gee, G.W., Campbell, M.D., Campbell, G.S., andCampbell, J.H. (1992): Rapid Measurement of
Low Soil-Water Potentials Using a Water
Activity Meter, Soil Science Society of America
Journal, Vol. 56, No. 4, pp. 1068-1070.
Hilf, J.W. (1956): An Investigation of Pore Water
Pressure in Compacted Cohesive Soils, Technical
Memorandum No.654,U.S. Department of theInterior, Bureau of Reclamation, Design andConstruction Division, Denver, Colo.
Houston, S.L., Houston, W.N. and Wagner, A.
(1994): Laboratory Filter Paper Suction
Measurements, Geotechnical Testing Journal, Vol.
17, No. 2, pp. 185-194.
Imbert, C., Olchitzky, E., Lassabatère, T., et al.
(2005): Evaluation of a thermal criterion for anengineered barrier system, Engineering Geology,
Vol. 81, No. 3, pp. 269-283.
Li, X.S. (2005): Modeling of hysteresis response forarbitrary wetting/drying paths, Computers andGeotechnics, Vol. 32, No. 2, pp. 133-137.
Likos, W. J. and Lu, N. (2003): An Automated
Humidity System for Measuring Total Suction
Characteristics of Clays, Geotechnical Testing
Journal, Vol. 28, No. 2, pp.178-189.
Lu, N. and Likos, W. J. (2004): Unsaturated Soil
Mechanics, John Wiley & Sons.
Lu, N. and Likos, W.J. (2006a): Suction stresscharacteristic curve for unsaturated soil, Journalof
GeotechnicalandGeoenvironmental
Engineering, Vol. 132, No. 2, pp. 131-142.

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