Deterioration Mechanisms of Building Materials of Jiaohe Ruins in China

Journal of Cultural Heritage 14 (2013) 38–44

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Original article

Deterioration mechanisms of building materials of Jiaohe ruins in China

a,∗,b b a a c

Mingshen Shao ,Li Li , Sijing Wang , Enzhi Wang , Zuixiong Li

State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China

Chinese Academy of Cultural Heritage, Beijing 100029, China

Conservation Institute, Dunhuang Academy, Dunhuang, Gansu 736200, China

a r t i c l e i n f o

a b s t r a c t

Article history: Almost all defects of earthen buildings such as roughening, erosion, volume reduction, cracking as well

Received 6 May 2011

as crazing, etc., have been witnessed in the ancient city of Jiaohe, an earthen architectural heritage in

Accepted 19 March 2012

northwest China. In this paper, their long-term durability and deterioration due to prolonged exposure

Available online 19 April 2012

to environmental factors were studied, based on the basis of ﬁeld investigation and laboratory analysis.

The results indicated that the deterioration of building materials should be attributed to their basic

Keywords:

properties, including density, particle size distribution, soluble salts, mineral, mechanical strength, etc.,

Jiaohe

and interaction with environmental factors. And then, four main deterioration modes can be identiﬁed,

Material properties

namely: wind-related deterioration, water-related deterioration, temperature-related deterioration and

Deterioration mechanisms

Durability chemical related deterioration. It can be concluded that the greatest deterioration was wind-related

Reinforcing measures deterioration on west-north facing fac¸ ades, and chemical related deterioration on the surface of building

materials.

1. Introduction natural factors and human factors, some buildings were destroyed

completely. Fortunately, from the middle and late parts of 1990s,

The ancient city of Jiaohe, located in the Xinjiang Uygur some research workers started to pay attention to these problems

Autonomous Region of China, is considered to be the largest, old- and to take effective measures to solve them. Z.X. Li analysed the

est and best-preserved earthen city in the world (Fig. 1). The city destroying mechanism of the cliff mass and the classiﬁcation of

was built on a loess plateau atop a cliff of over 30 meters as the deteriorations [1]. Experts from the Nara National Cultural Proper-

capital of the state of South Cheshi, one of the kingdoms of the ties Research Institute and the Xinjiang Cultural Relies Bureau have

Han dynasty (206 BC–AD 220), and deserted in the Yuan dynasty jointly undertaken archaeological surveys and reinforced protec-

(AD 1271–1368). During this period it was of great military signif- tive measures for the site [2]. Since June 2006, Dunhuang Academy

icance and played a pivotal role in the economic development of began to take emergency reinforcement measures, such as environ-

Western and Eastern countries. After having experienced all kinds ment treatment, anti-weathering consolidants for buildings and

of changes and erosion by wind and rain for more than 2000 years, consolidation of the cliffs, with rather satisfactory results, which

the main structure of the architecture layout of the Jiaohe Ruins may be regarded as references for similar projects [3]. Based on

is still well preserved till the present day, and most architecture fundamental property of the materials themself, this paper aims

are clear and distinguishable such as temple, sites of government, to study their mechanisms of degradation exposed to the arid and

courtyards, streets, handicraft workshops, watchtowers and so on. semi-arid environment in west of China, to supply the theory sup-

The current scale of the Jiaohe Ruins is mainly the architecture of port for preservation of the ancient city of Jiaohe.

the Tang Dynasty and the architectures are mainly located in the

area of about 1000 meters in the southeast part of the platform.

2. Material properties

As one of the world’s architectural wonders, most buildings of

Jiaohe city were dug from earth, partially built with one, or a combi-

2.1. Preparation for soils

nation of man-made materials including rammed earth, mud brick

and cob. However, due to vulnerability of the materials themselves,

As one of the world’s architectural wonders, most buildings of

they have steadily deteriorated under the combined action of

Jiaohe city were dug from earth, partially built with one, or a combi-

nation of man-made materials including rammed earth, mud brick

and cob (Fig. 2). The three soil samples were taken from Northeast

∗

temple, Great buddhist temple, Pagodas, Government ofﬁce and

Corresponding author.

E-mail address: [email protected] (M. Shao). other sites, whose relative positions are shown in Fig. 1.

M. Shao et al. / Journal of Cultural Heritage 14 (2013) 38–44 39

100

80 Raw soil Ramm ed earth Cob

60 ed percentage /% percentage ed

20 Accumulated retain

1000 100 10 1

Par ticle size /µm

Fig. 3. Particle size distributions of three soils from Jiaohe city.

water content, dry density, porosity, liquid limit, plastic limit, etc.

are showed in Table 1.

2.2.1. Particle size distribution

The particle size distribution test results conﬁrmed that soil was

Fig. 1. The ancient city of Jiaohe.

composed of all the four main soil fractions: ﬁne gravel, sand, silt,

and clay, and the soil type was predominantly silt (Fig. 3). The soil

has sufﬁcient proportions of ﬁnes (silt and clay), as well as a propor-

tion of coarse fraction (ﬁne gravel and sand) for the skeletal frame of

the building. Compared to the raw soil and cob, rammed earth has

lower amount of silt, about 25%. This is because that rammed earth

is usually used to play the part of ﬂattening materials between raw

soil and cob, or mud brick, where is subjected to the weathering of

water and wind, could not hold on lower adhesion silt.

2.2.2. Plasticity

Soil plasticity, the ability of a soil to undergo irreversible defor-

mation while still resisting an increase in loading, is indicated by

the plasticity index. Plasticity index is the numerical difference

between liquid and plastic limits. The plasticity index is an indi-

cation of the clay content and characteristics of the soil. The test

resulted that the plasticity index of raw soil was 10.4, cob 11.8 and

rammed earth 11.7, respectively, which indicates that the soils have

active clay mineral and that higher shrinkage would occur when the

soils dries.

Fig. 2. Wall made up of three materials.

2.2.3. Soluble salts

The content of soluble salts in these soils is usually high (Table 2).

The average degree is the highest for raw soil and the lowest for cob,

2.2. Soil speciﬁcation and rammed earth and it distributes more concentrated in wall

footing, where salts are susceptible to environmental humidity.

2+ + + 2−

The basic physical properties of the soils have been well estab- Cation in the soluble salts is Ca , Na and K , while anion is SO4 ,

− − 2−

lished over the past 6 years of intensive research. The values for Cl and NO3 , almost without CO3 ; some of them are shown in

Table 1

Basic physical properties of building materials.

Soil Water Density Speciﬁc Porosity Liquid limit Plastic Plasticity Slaking speed

content (%) (g/cm ) gravity (%) (%) limit (%) index (g/min)

Rammed earth 1.76 1.60 2.70 40.74 27.50 18.42 9.08 15.0

2.41 1.56 2.72 42.65 29.00 16.00 13.00 20.6

Cob 1.38 1.55 2.72 43.01 30.60 17.20 13.40 10.5

2.14 1.66 2.72 38.97 31.58 18.15 13.43 11.5

Raw soil 2.65 1.72 2.70 36.30 27.60 17.70 9.90 11.7

3.03 1.68 2.70 37.78 28.00 18.75 9.25 12.4

40 M. Shao et al. / Journal of Cultural Heritage 14 (2013) 38–44

Table 2

Soluble salts in soils from Jiaohe city.

Soil Depth (cm) PH Anion (mg/kg) Cation (mg/kg) Amount of soluble

salts (mg/kg) − 2− − 2− − 2+ 2+ + + −

NO3 CO3 HCO3 SO4 Cl Ca Mg Na +K NH3 N

Rammed earth 0 7.37 95 0 227 5627 619 2041 64 733 12 9381

0 7.58 1212 0 228 6579 7656 2892 31 5270 1 24,066

Cob 0 8.05 142 15 192 3787 812 1250 63 921 1 7183

0 7.90 755 0 226 4381 2892 1825 95 2063 1 12,226

Raw soil 0 7.62 1454 0 193 7583 14,094 6243 382 5444 34 35,514

10 7.49 1593 0 229 6015 12,798 7173 352 2861 62 31,106

0 7.80 271 0 193 7197 1454 2201 95 1837 11 13,320

10 7.82 335 0 161 3796 2262 1201 31 2030 1 9832

Table 2. The abundant sulphates of Na, Mg and Ca, as Houben and reduced by 12.39%, mud resistance to wind erosion and rain ero-

Guillaud [4] reported, are harmful to soils because they crystallize sion was obviously worse, especially rammed earth, as shown in

and make the soils fragile. Fig. 4.

2.2.4. Mineral 3. Deterioration mechanisms in materials

The results of X-ray diffraction (XRD) test show that the mineral

components are quartz, feldspar, calcite dominantly, with minor As is the case with most other building materials, deterioration

dolomite, hematite, magnetite, pyrite, gypsum and other clay min- mechanisms in Jiaohe soils are varied and complex. Performance

erals including montmorillonite, kaolinite and illite. deterioration of building materials is a multifactor disorder that

is thought to result from an interaction among environmental

factors such as wind, rainwater, high temperatures and rela-

2.3. Mechanical strength

tive humidity. Four main deterioration modes can be identiﬁed,

namely: wind-related deterioration, water-related deterioration,

Raw soil, as any other form of earth construction such as

temperature-related deterioration, chemical related deterioration.

rammed earth, cob and mud brick, has relatively good strength in

compression but generally poor strength in shear and tension, espe-

cially when moist. The results in Table 3 show that their mean value 3.1. Wind-related deterioration

of dry compression strength is 2.77 MPa; shear strength, 0.24 MPa;

tensile strength, 0.57 MPa; meanwhile, raw soil has greater com- Wind erosion is one of the common deterioration modes for

pression strength and tensile strength, nearly the same shear most earthen materials in arid and semi-arid area of Northwest

strength compared with cob and rammed earth. This may be used China, and it is facilitated by natural environment of extremely

to illustrate why the well-preserved city is not Gaochang (rammed drought, strong wind, sufﬁcient sources of sand and unsheltered

earth building) but Jiaohe although they are restricted to the same ﬁeld, which have crucial effect on extent and degree of erosion dam-

particular condition. age. Damage caused by wind erosion action can be seen everywhere

in Jiaohe city (Fig. 5).

However, it cannot be neglected that particle size distribu-

2.4. Durability

tion accelerates the deterioration of building materials. Research

has shown that the mass of particles (0.075 mm < d < 0.45 mm)is

Wind and rain are the most destructive natural actions causing

the most susceptible to wind erosion, but those (d < 0.05 mm and

erosion and deterioration of earthen elements. Accidental freeze-

d > 0.84 mm,) refuse to leave their reference positions, attributed

thaw is also a signiﬁcant agent of deterioration. The ability of the

to stronger binding force between ﬁne grains or their relatively

structure and the elements of the building materials to withstand

great weight and mass[6–8]. And it is supported by wind tunnel

the destruction continuously degenerates under the combined

experiments, as showed in Fig. 6. Five soils, whose proportion of

actions of wind, rain and other weathering behaviors. A large num-

susceptible particles were 11%, 14.37%, 20.46%, 43.84% and 48.93%

ber of ﬁeld investigations indicates that the primary damages of

respectively, were taken from the edge of Jiaohe cliff, and put in

the ruins of Jiaohe have surface erosion, cranny and collapse. At

sand ﬂow in Wind Tunnel for 15 minutes. Then wind erosion mod-

the same time, some speciﬁc test procedures have been devel-

ulus was obtained from the equation:

oped to measure the relative erosion resistance of earth elements,

namely slaking tests, wind erosion tests and water spray tests. A

E = Q/TS

fourth test procedure, devoted to wetting and drying cycles [5],

was also carried out to estimate the durability of the natural earth. where Q is amount of soil erosion, T is erosion time, S is erosion

The test results proved to be unsatisfactory, slaking rate of raw area.

soil was 12.9 g/min, rammed earth 15.6 g/min, cob 12.0 g/min, the The result of the wind erosion test indicated that the erosion

compressive strengths of samples, after 50 wet and dry cycles, occurred at the susceptible particles ration beyond 15% and the

Table 3

Mechanical strength values of three samples (Unit/MPa).

Soil No. Uniaxial compressive strength Direct shear strength Tensile strength

Vertical Parallel Parallel (o) Vertical Parallel

Raw soil J-27 4.25 3.67 0.24 61.90 0.76 0.41

Raw soil J-28 2.39 1.66 0.25 28.90 0.49 0.39

Cob J-12 1.49 0.76 0.27 42.50 0.42 0.40

Mean 2.77 2.18 0.24 44.28 0.57 0.44

Rammed earth Gaochang 1.57 1.42

M. Shao et al. / Journal of Cultural Heritage 14 (2013) 38–44 41

Fig. 4. Simulated tests of wind erosion and rain erosion in Jiaohe city.

higher the percentage, the more severe the erosion occurred. So far rammed earth is more vulnerable to wind rather than raw soil and

as the Jiaohe city is concerned, the proportion of susceptible par- cob.

ticles is widely different among the three materials; respectively,

rammed earth, raw soil and cob are 44.7%, 9.1%, 4.2%. That is why

3.2. Water-related deterioration

Water, is not only related to most of the observed deteriora-

tion defects in Jiaohe city, it also serves as a common denominator

for other deterioration mechanisms such as chemical activity. The

main sources of water linked to such deterioration mechanisms are

rainwater, rising damp and condensation. The action of water in

causing deterioration in soils can likely occur in two ways: solvent

action and erosion action.

The solvent action of water is one of the most common deteri-

oration mechanisms occurring in many materials of earthen sites.

This also affects Jiaohe soils, even more seriously, for their sufﬁ-

cient proportions of ﬁnes (silt and clay), high porosity and slaking

speed, as showed in Table 1. The better ability of the ﬁne surface

to easily absorb and retain water for a long time, and the worse

capacity of the soil against disintegration are two properties likely

to leave the material vulnerable to the solvent action of water. As

water permeates the block, any unstabilised soil fraction present,

together with the freed calcium hydroxide from the hydration reac-

Fig. 5. Damage caused by wind erosion action.

tion, can be expected to dissolve and subsequently leach out. The

2.0 repeated action over the years can lead to overall weaken and alter

the fabric of a building, which make the building more vulnerable to

other forms of deterioration such as the erosive action of rainwater

V=40m/s

h) • droplets. 2 V=30m/s

1.5

V=20 m/s Surface erosion is usually associated with frequent and intense

kg/m

( V=18 m/s rainfall. Unfortunately, short duration and heavy rainfall are con-

sidered to be the prominent rainfall characteristics in Turpan.

In recent 50 years, it has happened several times, for example,

1.0

44.9 mm on 26 June 1969, 30∼36 mm on 21 June 1984, and others

in 1958, 1972, 1977, 1978, 1983, 1987, 1992, 2002.

When rainwater strikes an exposed building surface, it will

0.5 directly impact on soil particles. It has been estimated that up to

75% of the energy of a raindrop is dissipated on impact [9,10]. The

Wind erosion modulus

erosivity of raindrops depend on the state of bonding of the building

surface, and on the characteristics of the rain such as the drop size,

0.0 its distribution, fall velocity and impact kinetic energy [11]. Obvi-

10 20 30 40 50 ously, the higher the impact velocity of a raindrop and the weaker

Quality percentage of susceptible particles /% the state of bonding at the soil building surface, the greater would

be the effect of surface erosion. While the impact can be likened to

Fig. 6. Relationships between susceptible particles and wind erosion modulus. the removal of loose particles, particles can then be easily removed

42 M. Shao et al. / Journal of Cultural Heritage 14 (2013) 38–44

1 2 3

1-Crusted layer 2-Loose layer

3-Host soil

Fig. 7. Soil crust sculpted by raindrop erosion.

Fig. 9. Cross section of the host wall covered by surface crust.

by the resulting wall surface ﬂow (Fig. 7). The consequences linked

chemical properties of the materials. These changes are likely to

to this process include mass loss, lowering of surface hardness,

inﬂuence the durability of buildings.

compressive strength and density, and increase in permeability.

At high temperatures, soil can easily expand and induce signiﬁ-

Further consequences can be expected, exposure of the interior

cant internal stresses (compressive and tensile). Since soil is weaker

fabric can lead to the speeding up of the rate of deterioration of

in tension as showed in Table 2, such stresses can be expected to

whole building materials.

be more harmful to its fabric. Further, as stress and strain tend to

occur together, any restraint of movement for the expanding soil

3.3. Temperature-related deterioration introduces a stress corresponding to the restrained strain [12–14]. If

this stress and the corresponding restrained strain within a clod are

The environment characteristic of high temperature and large allowed to develop to such an extent that they signiﬁcantly exceed

temperature variations is shared by Jiaohe city and other earthen the bulk strength or its strain capacity, then interfacial bonds that

architectural heritages in northwest of China. The surface tem- bind and hold the soil particles within its fabric together can be

perature of Vajrasana Pagoda in Jiaohe city has been measured weakened. In more extreme conditions, they might even be severed

by researchers, from Lanzhou University in china. The results of apart altogether.

Aug. 14–15, 2007 (Fig. 8) show that the daily temperature varia- Detachment of surface crust is a very important phenomenon,

◦

tion can therefore exceed 50 C. Such extremes provide contrasting which exists extensively in Jiaohe city (Figs. 7 and 9) [15]. Sur-

settings for temperature-related deterioration to occur in building face crusts are characterized by ﬁner particles and lower soluble

materials. salts contents compared with the host soil. Temperature monitor-

As repeated exposure to adverse circumstances, the exterior ing by IR thermography camera and thermal sensors shows that

part of buildings will inevitably experience regular temperature thin crusted layers response greatly and quickly to the environmen-

variations. Temperature variations of such magnitude can cause tal temperature change (Fig. 10) [15]. It is concluded that the crust

both reversible and irreversible changes in the physical and will be subsequently detached by wind erosion and expansion-

contraction induced by the change of temperature. Surface crusted

60 32

East wall 50 South wall 28

West wall

North wall Surface crust

40 24

Atmosphere Wall surface (no crust ) 20 30 16

20 Temperature/ºC 12 Temperature/ºC

10 8

0 510152025

Time/h 0 4 812162024

Time/h

Fig. 8. Air and wall surface temperature of Vajrasana Pagoda between Aug 14,

6:00 AM and Aug 15, 8:00 AM. Fig. 10. Temperatures monitored at the surface of natural earth wall (Nov 17, 2007).

M. Shao et al. / Journal of Cultural Heritage 14 (2013) 38–44 43

layer prevents the wall from weathering to some degree at its ear-

lier formation stage but tend to accelerate the deterioration of the

earthen sites in the long term considering that Jiaohe city suffer

from strong sandstorm and strict weather condition.

In addition, temperature is an important accomplice to

other deterioration behaviors, such as chemical deterioration,

ice-splitting action, etc. In summary, temperature-related deteri-

oration in soil materials is likely to affect the following material

properties: shape, dimensions, strength, surface hardness, rigidity,

permeability, brittleness and appearance.

3.4. Chemistry-related deterioration

Soil salinization is the major form of chemistry-related deterio-

ration in Jiaohe city. Just as shown in Table 2, there are rich sources

of soluble substances in these walls of Jiaohe, which could cause

severely damages of earthen materials, especially sodium sulfates

Fig. 11. Deterioration caused by interaction of material properties and existence

and sodium chloride. conditions.

In most cases, these chemical substances can remain dormant

and stable when not in active contact with environmental ele-

to 314% [17,18]. But any further increase must be resisted by the

ments (rainwater, high or low temperatures, relative humidity and

rigid soil fabric. This leads to the creation of signiﬁcant stresses

gasses), since the precondition for chemical reaction to start in most

within the pores. The induced stresses can cause cracking and dis-

materials is the presence of moisture. Due to severe moisture varia-

integration at the surface of the clod. Progressive deterioration of

tions from heavy rainstorms with a long interval in Turfan, chemical

the clod surface can then take place as moisture and temperature

reactions can be unavoidable to occur during its service lifetime.

variations occur during the service lifetime. The outcome is that the

Moreover, good properties of Jiaohe soils such as water absorp-

salt-affected soils gradually shed off in powder form.

tion and permeability are likely to ensure that adequate moisture

is absorbed and circulated within a wall. It may simply be washed

4. Main reinforcing measures for Jiaohe city

out of a wall through surface ﬂow on saturation during rainstorms,

or it may be expelled onto the wall surface by evaporation due to

In fact, the damage of Jiaohe city is the result of platform losing

high temperatures.

stability and deterioration of materials. In details, it can be showed

Leaching that involves the migration, concentration and wash-

in material properties, engineering geology and site environment

ing out of soluble substances from a material appears mainly on

(Fig. 11). Consequently, the main reinforcing measures for Jiaohe

upper portion of a wall. Kaolinite clays and montmorillonite clays

city should involve two procedures:

have been found in soil of Jiaohe city. Owing to its ﬁneness and

high speciﬁc surface area, not only can these highly hydrophilic

◦

stability of platform and buildings;

clays compete for the mix-water required for the hydration, but

◦

surface weathering-resistant consolidation.

they are also likely to obstruct the stabilisation process by inhibit-

ing the binding effect on soil particles. Dispersed clay in an earthen

The stability measures mainly consist of retaining and protec-

fabric can easily be washed out as moisture permeates and cir-

tion with adobe, anchoring, granny grouting, and conservation of

culates within it. Obviously the long-term leaching increases the

ceiling of cave, etc. Surface weathering-resistant consolidation is

porosity of a wall, which can cause a building to become progres-

needed to lower degradation loss of the material while it is exposed

sively weaker, and more permeable. A weakened building surface

to adverse conditions in its whole service life. Up to now, some

is more vulnerable to the direct abrasive action associated with

chemicals, such as alkaline lime, natural ﬁbre and high-modulus

driving rains.

potassium silicate, have been employed to strengthen the walls in

As a necessity, moisture ﬁlters through the weaker soil and

order to protect its bulk from exposure. High-modulus potassium

into the internal earth more easily. The common type of sulfates

silicate, used in Jiaohe, proves to be very effective by spraying the

in earthen soils such as Jiaohe and Gaochang are calcium and

surface of walls. In spite of that, these are not enough for the con-

sodium sulfates (Table 1). In the presence of sufﬁcient amounts

servation of earthen architectural heritages, and further research

of moisture, these sulfates can readily dissolve in water and react

work is quite necessary and urgent.

with other hydrated products namely, calcium hydroxide and cal-

cium aluminate, then form calcium sulfate (gypsum), and calcium

sulphoaluminates compounds (ettringite) respectively. The vol- 5. Conclusions

ume of these two by-products is much greater than that of the

original substrates in the clods. As these products expand in order The results obtained in this study show that the building mate-

to occupy more space within a clod, and when this expansion is rials of Jiaohe city consist of rammed earth, mud brick and cob.

restrained by adjacent particles and phases within the core of the Moreover, as an interface between raw soil and cob or mud brick,

clod, signiﬁcant internal stresses are generated [16]. The gener- rammed earth has even more potential threat to their safe because

ated stresses are capable of disrupting bonding within the building of its vulnerability. This paper also suggests that durability of the

materials and further weakening their strength. building materials is determined by their own properties including

When sulfates are present in solution, they are likely to perme- density, particle size distribution, soluble salts, mineral, mechani-

ate into capillary pores. Due to high temperatures leading to evap- cal strength, etc. and it is intensely inﬂuenced by the environment

oration, moisture is driven off from the solution, which causes the change. Deterioration mechanisms of the building materials of

salts to crystallize within the pores and void spaces of the clod. The Jiaohe city should be attributed to an interaction between building

volumes of the crystals increase as the pore spaces get ﬁlled. Take materials and environmental factors such as wind, water, temper-

sodium sulphate for instance, the volume of the crystal can increase ature and relative humidity. Four main deterioration modes can be

44 M. Shao et al. / Journal of Cultural Heritage 14 (2013) 38–44

identiﬁed, namely: wind-related deterioration, water-related dete- [5] M.S. Shao, L. Li, Q.Q. Pei, et al., Laboratory soil test on impact of environmental

factors to earthen ruins reinforced with PS, J. Eng. Geol. 18 (03) (2010) 371–375

rioration, temperature-related deterioration and chemical related

(in Chinese).

deterioration. So, the conservation of building materials requires

[6] W.S. Chepil, Properties of soil which inﬂuence wind erosion (I-V) [J], Soil Sci.

more comprehensive measures and further techniques. (1950–1951) 69–70.

[7] Z.B. Dong, Wind Erodibility of Aeolian Sand as Inﬂuenced by Grain-size Param-

eters, J. Soil Erosion Soil Water Conserv. 4 (4) (1998) 1–6 (in Chinese).

Acknowledgement

[8] K. Cui, Study on effects of different wind erosion form multivariate layered

steep soil slop – A case of Jiaohe ancient city’s slope [D], Lanzhou University,

Lanzhou, 2009, p. 57–65 (in Chinese).

The work is supported by the Chinese “11th Five-Year Plan”

[9] W.D. Ellison, Studies of Raindrop Erosion, J. Agric. Eng. 25 (1944) 131–136.

to support science and technology project (No. 2010BAK67B16)

[10] F.B. Goldsmith, Tropical Rain Forests –AWider Perspective, Chapman and Hall

and the National Natural Science Foundation (No. 40872152). The Ltd, London, UK, 1998, pp. 54–131.

[11] Engineering Hydrology, 4th ed, Macmillan Press Ltd, Kent, England, 1990, pp.

authors want to warmly thank Dunhuang Academe for their sup-

12–42.

port and helpfulness in this research work, Lanzhou University for

[12] A.M. Neville, Properties of Concrete, 4th ed, Addison Wesley Longman Ltd,

ﬁeld investigation and laboratory test. Essex, England, 1995, pp. 213–256.

[13] J. Case, L. Chilver, C. Ross, Strength of Materials and Structures, 4th ed, Arnold,

New York, 1998, pp. 112–175.

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