Triggering and prevention of landslide disasters to cave dwellings in loess region of China.
In China, currently over 40 million residents live in cave dwellings which distribute over areas of around 630,000 [km.sup.2] surrounding drainage basin of Huanghe river in the north and north-west China (Hou et al. 1994, Wang et al. 2001 and 2004). Cave dwelling, as a special and long-standing style of dwelling, was initiated 2000 years ago by Chinese ancestors who lived in loess physiognomy, where limited living lands and scarce construction resources enforce residents to resolve their living concerns using local featured geological resources: loess. Self-standing behavior of loess deposits prompts the concept of digging cave dwellings in loess deposit, where the inner of dwellings is kept warm in winter and cool in summer. More advantages of constructing cave dwellings include reduced construction budget and occupation, simplified construction procedures, natural resource and energy reservation, orientation to local geological physiognomy and harmonious living environments (Liao et al. 2000).
In recent years, disasters to cave dwellings, particularly caused by landslides, became more frequent and disastrous in China. One-third landslides occurred in loess regions (Qu et al. 1999). In loess regions, loess landslide accounts for over 70%. In addition, many large-scale and extremely destructive disasters, such as Bailu plateau landslide, Haiyuan earthquake landslide, and Jiangliu landslide, also happened in loess regions. As a result of loess landslide disasters, numerous casualties and property damages were reported. According to statistical documents, around 1000 persons per year dies from landslide disasters to cave dwelling, which occupy 80% of total disaster to cave dwellings. From 1949 to 1990, at least 1025 people directly died from landslide disasters merely in part areas of three provinces of Shanxi, Gansu, and Qinhai where loess widely exists. Not only residents living in cave dwellings risk their lives and properties each moment, but also the landslide disaster becomes a major obstacle hindering the social and economic development of loess regions. Utilization of abundant resources of mineral, water, electricity, land, light, and heat are delayed, and considerable developing potential of loess regions is depressed. How to prevent and mitigate landslide disasters to cave residents, dwellings, and related facilities is a pressing research topic facing geotechnical engineers and related professionals. A state-of-the-art study is launched to address this issue. Work scope covers three aspects: geological requirements of constructing cave dwellings, prerequisites and triggering factors of loess landslide disasters, warning and preventing scenarios of loess landslide to mitigate disasters.
GEOLOGICAL REQUIREMENTS OF CAVE DWELLINGS
Surrounding drainage basin of Huanghe river in north China, loess deposit is greatly developed, where extensive and consistent loess layers of up to 100 to 200 m thick are uniformly distributed over ground surface (Fig. 1). In this region, the local climate is featured by being few raining and arid most of the seasons, numerous well developed loess precipices. Loess particles have characteristics of being dry, calcic and porous. The shear strength and compressive strength of loess mass are high. Physical properties of two typical loess samples are presented in Table 1. Main ingredients of loess particles consist of clayey and sandy particles. Characteristics of loess particles contribute to its engineering behaviors of self-standing, highly-developed vertical joints, easy excavation. It was reported (Liao et al. 2000) that a loess precipice is able to stand with its height up to 10 to 20 m without external supporting structures. Accordingly, local residents call loess deposits as "standing soil" or "lying soil". These geological conditions become technically constructive advantages for digging durable and large-scale cave dwellings in loess deposit. The life time of cave dwellings is generally 40-50 years, with a few cave dwellings still in service aging up to 100-200 years (Chen et al. 1995).
[FIGURE 1 OMITTED]
Cave dwellings built into or against loess deposits are broadly classified into three types depending upon their site selection and embedding manners: precipice cave dwellings, courtyard cave dwellings, and pit cave dwellings. Cave dwellings are popularly built along the slopes of various scales of loess precipices, which are called precipice cave dwellings. This cave dwelling is horizontally aligned against loess slope, mostly facing south to absorb sunshine during daytime. In circumstances where loess plateaus dominate, cave dwellings are frequently subsided vertically into loess plateau to form a courtyard, which is called courtyard cave dwelling. Obviously, the disadvantages of courtyard cave dwellings are less sunshine absorption and worse drainage system. Some cave dwellings, which are similar to precipice cave dwellings, are partially dug into loess deposit vertically or horizontally and covered by extra loess mass. This dwelling is called pit cave dwellings. Normally, 3 to 5 cave dwellings cluster together to form a living community for a family. The dimension of a cave dwelling is generally 7 to 10 meters in depth, 3 to 4 meters in width, and around 3.3 m in height, which is competitive against a conventional style of dwelling. An arch style ceiling is adopted to conform to theory of structure mechanics.
FACTORS TRIGGERING LOESS LANDSLIDE DISASTERS
Most cave dwellings are built along the slopes of various scales of loess precipices or plateaus, which means that safety of cave dwellings is closely correlated to the slope stability of loess masses. Once loess slopes fail to stand so as to cause landslides, the cave dwellings in the vicinity of landslides may possibly be buried, overturned, or destroyed. Multiple factors are relevant to triggering landslides, mainly including precipitation, earthquake, geological stratum, freezing-and-thawing cycles, and human activities.
About 90 percent of landslides in loess region are related to precipitation and earthquake in the aspect of external triggering factors. In the other aspect of internal factors, occurrence of loess landslides is closely relevant to collapsibility and liquidation of loess mass. External and internal factors interact during loess landslides. Loess mass has engineering behaviors of collapsibility and liquidation if exposed to infiltration of external water and shaken force incurred by earthquakes. Due to the effect of gravitation and existence of huge pores, loess particles rapidly collapse and subside when wet or soaked by water. If an additional external shaken force, such as an earthquake, is applied, the loess deposit would further collapse. If the water content is high enough to reach a point greater than its plastic limit, the pore water pressure increases significantly with the effect of shaken force. Residual strain of loess deposit under shaken force increases rapidly and loess liquidation occurs which is closely similar to sandy soil liquidation. Besides infiltration and earthquake, Qu et al. (2001) found that loess collapsibility is controlled by the content of clay. Over areas where clay content is less than 10%, collapsibility occurs frequently. Over areas with clay content between 10% and 20%, collapsibility occurs sparsely. Over areas with clay content more than 20%, no collapsibility is observed.
Supplemental evidences supporting precipitation as a loess landslide triggering factor is from statistical investigations by Wang et al. (2004) that landslides occur sparsely in the northern loess plateau with less than 400 mm of annual precipitation, but frequently in the other area with more than 400 mm of annual precipitation. Loess landslides mostly happen in rainy season of July, August and September, particularly during moderate and heavy raining periods, but relatively less in other rarely raining seasons. It is suggested that loess mass is soaked during raining and post-raining period, which not only leads to the increase of loess slope weight but also reduce the coherent strength of loess mass. When a great number of rainfalls reach confining layer to form phreatic water layer, the cohesive force of soil body and frictional force of interlayer greatly decrease, and thus is favorable to landslide occurrence. Because the loess slopes can be cut down and eroded continuously by the surface running water, the slopes become steeper and unstable, and the landslides may occur readily. The existence and variation of ground water flow can directly affect gravitational condition of slopes and mechanical characteristic of loess mass, which are main factors determining slope stability.
Affected by earthquake, loess landslides are developed densely along earthquake strips. According to relevant data, every more than 6-magnitude earthquake may incur loess landslides, and higher the magnitude, more destructive the landslide disasters. For example, during the periods of Tianshui earthquake of 7.5-magnitude in 1654, Tongwei earthquake of 7.5-magnitude in 1920 and Haiyuan earthquake of 8.5-magnitude in 1920, landslides occurred in all the earthquake areas mentioned. In recent years, with social and economic development, large-scale cities have been emerged over loess areas where earthquakes are relatively active. From the seismic ground motion parameter zonation of China with 10% probability of exceedance in 50 years, around a half of loess region has seismic acceleration greater than 0.2 g, up to 80% has seismic acceleration greater than or equal to 0.15 g. Four provincial capitals and six middle-level cities are surrounded by regions of 0.2 g seismic acceleration. Three provincial capitals and two middle-level cities are surrounded by regions of 0.15 g seismic acceleration. And it is acknowledged that a seismic activity with seismic acceleration greater than 0.15 g is able to cause loess disasters, including seismic landslides, seismic collapsibility, and liquidation of saturated loess mass. Hence, loess landslide disasters incurred by earthquakes are a pressing concern.
Geological stratums of loess deposits may influence occurrence of loess landslides. Loess deposits have been accumulated since Quaternary, and are further divided into ancient loess formed in early Pleistocene (Q1), old loess formed in mid-Pleistocene (Q2), new loess formed in late Pleistocene (Q3) and recent loess formed in Holocene (Q4) according to its geologic depositing time. Owing to long time depositing, Q1 loess and Q2 loess have characteristics of low void ratio, few large pores, and high strength, and accordingly are not readily to incur earthquake landslides. Q3 loess and Q4 loess have abundant large pores, loose structures, developed vertical joints and strong collapsibility, and thus are relatively easy to incur earthquake landslides. Besides the identification of loess stratums, boundaries between loess mass and underlying bedrocks influence loess landslides as well. Sliding failures relatively readily follow those existed structure surfaces of poor shear strength.
Freezing-and-thawing cycles due to climate alternation may induce loess landslides. In the northwest China, during the alternation from winter to spring, freezing-and thawing cycles of loess masses occur. As a result, frozen loess mass may be intenerated or even liquefied. Strength of loess deposit decreases. Stability of loess slopes reduces correspondingly. Landslide disasters to cave dwellings are readily triggered. Thus, cave residents should be aware of landslide disasters during the alternation from winter to spring, or an ice-melting period from March to May. Typical landslides of this type include Saleshan landslide, New Saleshan landslide, and Wolongshi landslide.
In the catastrophic loess landslides happened in recent years, a majority of landslides are related to human activities. In recent twenty years, population in loess regions increases so rapidly that it has reached 81.49 million, accounting for 7.8% of the national population. With the rapid increase of population and the development of capital construction, irrational human activities, such as excavating foothill, filling hillsides and over-cultivating, frequency and intensity of loess landslide disasters are raised. One of the changes related to human activities is the change of loess slope. Slopes of loess deposits become steeper as a result of human activities, such as cultivation, engineering exploitation and excavation, and other abused uses of loess deposit or related natural environments. After removal of original soil portions, stable stress distribution is broken. With the increase of slope angle, the angle of possible failure surface increases as well, which accordingly results in the reduction of slope stability. The worst case, which would finally approach due to human activities on natural environments, is the loss of adequate anti-sliding resistance and occurrence of landslides. From the GIS analysis performed by Wang et al. (2004), over loess regions in China, slopes of most loess deposits have angles of 20-30 [degrees]. Some loess slopes have angles greater than 40 [degrees], which may readily incur loess landslides.
Landslides in loess regions, affected by geological stratums, climate, earthquake activities and human activities, are distributed with apparent rules of time and space. Loess landslides occur relatively frequently over areas of loose and recent depositing loess stratums including Q3 and Q4 loess deposits, over areas and during seasons of higher precipitation and freezing-thawing cycles, over areas and during periods of active earthquakes, and over locations with dense human activities.
MEASURES AGAINST LOESS LANDSLIDE DISASTERS
Measures preventing loess landslide disasters to cave dwellings are established taking triggering factors of loess landslides into account. First of all, construction site selection of cave dwelling is a vital step preventing potential landslides. It is decisive to identify solid and durable Q1 and Q2 loess stratums which are preferred sites to accommodate or support cave dwellings. Newly formed Q3 and Q4 loess deposits and sites are not suitable for cave dwellings. Cave dwellings are better selected along stable loess slopes or precipices where potential of torrents or running streams rarely occur. The worst case is the site close to or against steep precipices which possesses rather poor stability. Cave dwellings should also avoid sites of exploiting areas, mining areas, water reservoirs, and irrigation facilities.
In some circumstances, the steep slopes close to cave dwellings are flatted moderately by excavating sliding, collapsing or foreign parts. The excavation should start from top, then gradually towards bottom. Vegetable growing is proposed over the slopes at the back of cave dwellings. To prevent loess mass from being intenerated by water infiltration and thus shear strength decrease along potential sliding surface, drainage system is suggested to channel water penetrating into loess mass, particularly for courtyard cave dwellings where raining waters are readily accumulated. Drainage system mainly includes two parts: hidden cutting channels in the ground to cut underground infiltration into potential landslide areas, and hidden drainage channels to discharge penetrated water in potential landslide areas. Sometimes, surface drainage system is applied to channel or prevent surface water approaching potential sliding areas.
Geotechnical works and soil treatment techniques are sometimes applied to improve stability of potential sliding mass. Mostly used treatments include anti-sliding piles, retaining walls, baking of soils, blasting and grouting. It is common and effective to combine multiple measures to prevent loess landslide disasters, such as combination of drainage system, retaining walls, and grouting.
Monitoring is a popular measure to forecast landslide, which includes displacement monitoring of ground surface and dwellings, crack monitoring of ground surface and dwellings, deep displacement monitoring, dynamic variation monitoring of groundwater and stress monitoring of loess mass. In theory, forecast of loess landslide includes long-term forecast, short-term forecast, and urgent forecast. Creep theory is usually used to obtain the prediction of landslides.
However, due to limited availability of monitoring instruments for most possible loess landslides, professional monitoring techniques and computing theory cannot be used. Hence, forecast based on signals and experiences are favorably adopted. For instance, before, during or even immediately after raining, it is worthwhile and urgent to exam potentially sliding regions. Possible negative signals indicating landslides include water seepage in the back of cave dwellings, heave or swell of ground surface, sliding of dwellings, cracks on sidewalls of dwellings, fallings of soil masses, deformed doors and windows and thus hard to open. A large-scale of steep slope over cave dwellings is also a negative signal indicting a landslide disaster. It was suggested that the height of a slope where a cave dwelling locates is not expected to exceed 8 to 10 m, and the angle of the slope is properly less than 75 to 80 [degrees] (Mou 1995). When the slope is rather high, cutting slope into a ladder style is applied. To train residents living in cave dwellings about how to identify disaster signals and handle urgent events helps mitigate loess landslide disasters. They should have the concept in their mind that raining infiltration may cause rather adverse consequences to their dwellings, and that being aware of any negative signal, such as slope activity, is greatly helpful for their life-savings.
Loess landslide disasters are identified as vast threats to cave residents living in loess regions due to the special behaviors of loess mass: collapsibility and liquidation given external effects, such as water infiltration and earthquake shaking force. Occurrence of loess landsides is spatio-temporal. Loess landslides occur relatively frequently over areas of loose and recent depositing loess stratums including Q3 and Q4 loess deposits, over areas and during seasons of high precipitation and freezing-thawing cycles, over areas and during periods of active earthquakes, and over locations with dense human activities. Prevention of loess landslide disaster is mainly accomplished by eliminating its triggering factors, such as construction site selection, human impact reduction, supplemental slope treatment scenarios, etc. Due to the unavailability of advanced instruments, judgment on negative signals and experiences help forecast occurrence of loess landslides
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Geotechnical Research Institute, Hohai University, 1 Xikang Rd Nanjing 210098, China
Table 1. Physical properties of two loess samples (Guo et al. 2003.) Unit Water weight, kN/ Specific Sample content, % [m.sup.3] gravity I 7.0 13.92 2.74 II 3.1 12.94 2.70 Vertical Void Saturation permeability, Sample ratio degree, % cm/sec I 1.065 18.5 7.7 x [10.sup.-5] II 1.109 7.5 7.2 x [10.sup.-5] Internal Sample Compression Cohesion, friction modus, MPa MPa angle, [degrees] I 41.61 0.091 24 II 31.92 -- -- Liquid Plastic Sample limit, % limit, % Plastic index I 26.6 17.0 9.6 II 26.3 17.3 9.0
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|Publication:||Geotechnical Engineering for Disaster Mitigation and Rehabilitation|
|Date:||Jan 1, 2005|
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