Stability analysis of a high excavated slope.

Hai-Jia, Wen ; Jia-Lan, Zhang ; Yong-Xin, Zhang 等

This work aimed to study on stability-assessment way of a high excavated-slope. A stable slope was the key to insure the security of building site adjacent to the high slope, which frequently encountered in town construction at mountain areas by terrain limit. Some crucial problems were summarized for stability assessment of an excavated high slope based on several typical engineering cases in Chongqing. A synthetical-analysis way was explored to assess stability of high slope from its natural geological conditions and its man-destroyed degree, engineering environment, potential failure pattern of a high slope, calculation parameters and analysis methods. As a result, it is indicated that the conclusion of stability assessment can be determined according to the aspects abovementioned, and the stability assessment is one of the fundamental data to insure the safety of the related construction, site and buildings.

Key words: high slope; geological environment; synthetical-analysis; stability assessment

INTRODUCTION

Mountains and highlands cover about one quarter of the world's landmass and are home to about 10% of the world's population. Building development of mountain areas, has received international attention, and the 2002-year was the United Nations International Year of the Mountains (U.N. IYM 2002).

In China, mountains and highlands cover 69% of the country's land surface and are home to 56% of the Chinese population. So the 'All-out Development Strategy for Western Regions', brought into effect by the central government of P.R. China in 2000, is also a great movement towards developing mountainous areas in western China.

Increased development of mountain areas has appeared specific engineering problems associated with hill-side construction. Typically, the safety problem of building sites adjacent to high excavated slopes, widely encountered in west China, is an issue.

This problem is widespread in Chongqing, a city representative of mountainous regions in west China. To ensure the safety of buildings and construction sites, an assessment of stability of the excavated slope is required to determine whether advance reinforcement is necessary. Unstable excavated slopes may lead to landslides, such as the incident at Wulong, Chongqing, (2001), shown in Figure 1, where a 9 storey building collapsed and 86 people were killed/injured.

Therefore, stability assessment is very important for building sites adjacent to excavated high slopes. However, it is difficult to obtain accurate assessments because of the many complicated factors determining the in-situ slope stability. Crucial factors include: natural geological conditions, degree of anthropogenic disturbance, engineering environment, potential failure pattern, calculation parameters and analysis methods.

The natural geological condition is essential in determining the stability of high excavated slopes. The main factors of influence are: the properties of rock and soil mass, stratum attitude, associated dip situation of the stratum and excavated slope, geological structure (such as the growth degree of fissures, faults and groundwater regime). Building site excavation can also disturb geological conditions with deletesions effects.

Combinations of weak rock and/or soil, widely encountered in slope engineering, usually leads to depressed slope stability. In Chongqing, strata including incompact soil, sandstone, and mudstone are widespread. In the Wulong landslide example (Figure 1.), the main strata are [Q.sup.4] soil, [T.sub.3xj] sandstone and mudstone weak interbed. The combined strata of [Q.sup.4] soil, [J.sub.2][S.sup.2] sandstone and mudstone is typical at many building sites in the Chongqing urban district, as shown in Figure 2.

[FIGURES 1-2 OMITTED]

The slope may be stable state if the stratum tilt direction is opposite to the excavated slope's dip. However, many excavated slopes are unstable because of the identical tilt direction of the stratum and slope, following a disadvantageous excavation, without retaining structures, as shown in Figure 2. and Figure 3. For instance, during 319 state railway and building construction, at the site of the Wulong landslide, the foot of the slope has been badly excavated since 1989.

[FIGURE 3 OMITTED]

Generally, the integrity of natural rock and soil is destroyed and the mechanics properties are weakened by growing geological structures such as fissures. The Wulong landslide had a slight tilt angle stratum, however the rock mass was split into fractional clump layers mainly by two joint set parallel and vertical to slope surface respectively. This rock slope was unstable because the fractional clump was split by fissures.

According to surveys, over 90% of high slope landslides are attributed to groundwater. Groundwater may increase the unit weight and decrease the mechanical strength of the rock-soil mass, causes static and dynamic water pressure, scours or corrodes the slope foot and results in flotage of rock-soil. During the rainy season, slope stability is greatly reduced due to the groundwater, and additionally, soil moisture and surface water. For instance, the Wulong landslide was preceded by a heavy rainfall.

ENGINEERING ENVIRONMENT

Engineering environment refers to construction-engineering movements at the building site adjacent to the high excavated slope. Generally, natural slopes are stable, but slopes become unstable once the equilibrium condition has been disturbed by anthropogenic engineering movement. The Wulong landslide is a typical example.

Other engineering factors of slope stability include unsuitable landfill materials, construction materials, deposited at the top of the slope, engineering blasts construction, traffic vibration and seepage from an engineering service pipe or drainpipe. Such factors are typically observed in Chongqing city, as shown in Figure 4. and Figure 5.: a drainpipe crossing the upper side of an high excavated slope and heavy traffic at the top of an high excavated slope respectively.

[FIGURES 4-5 OMITTED]

STABILITY ASSESSMENT

A stability assessment will provide important data for the design and construction for slope retaining structures. An accurate and precise assessment of the excavated slope stability is essential to ensure the safety of the site and buildings. In Chongqing, it is compulsory to assess the following classes of slope prior to construction: classification slope and its height over 25 m, classification and H[greater than or equal to]15m, classification and H[greater than or equal to]8m.

An accurate assessment of a high excavated slope may be based on the clear investigation of the afore mentioned natural geological conditions, and the degree of anthropogenic/engineering disturbance to the building site. To obtain an exact assessment conclusion, the following aspects also need to be thoroughly studied, in terms of the related geological survey datum and technical standard. Potential failure pattern

The potential failure pattern of high excavated slopes may be determined during the stability assessment.

The potential failure pattern of the case shown in Figure 3 may be linear shear failure with a single slip surface. Rotary failure with an are slip surface can occur in homogeneous soil slopes, landfill slopes and rock slopes with a prosperous fissure. Shear failure with multi-replicate slip surfaces may occur in rock masses with identical dips multi-discontinuity or landfill on the replicate surface of bedrock. Excavated slopes with steep gradients and ground fissuration at the top of slope are prone to toppling collapse. Fractionary collapse with complex slip surface may occur at slopes with slight dip angle Figure 5. The heavily trafficked road at the top of a high excavated slope stratum, layer on layer with crashed part by joint set, as observed at the Wulong landslide, Figure 6.

[FIGURE 6 OMITTED]

Assessment method

The potential failure pattern of high-excavated slopes is an important criterion in the selection of stability assessment method.

The ordinary method is appropriate for linear shear failure with single slip surface. Integrative C value method is usually adopted for rotary failure with are slip surface. The transfer coefficient method may be suitable for shear failure with multi-replicate slip surfaces. The SARMA method is generally used for arbitrary boundary of slip surface and the rock mass separated by distinct discontinuity. Graphic methods include stereographic projection and sphere projection method. The former is suitable for rock masses with less distinct discontinuity and the latter is used for failure controlled by a large dimension wedge. More complex failures need to be analyzed by numeric methods such as FEM (finite element method).

It is difficult to obtain a convincing assessment conclusion using a single assessment method, therefore the local technical standard in Chongqing recommends that: "... engineering geological analogy and limit equilibrium method should be comprehensively used.... Numeric method ought to be combined to analyze the stability for the slope with complex deformation and failure mechanism."

Calculation parameter

An accurate calculation parameter for rock and soil mass, specific to the site, must to be used or the stability assessment will be inaccurate, regardless of the good computational method used.

The calculation parameter of rock-soil mass can be determined from the geotechnical investigation report of the specific slope plus the related technical standard. The parameters described by the site investigation report may not accurately represent the rock-soil mass in-situ because the experiment conditions to test these parameters are often simplified. Therefore Technical Standards recommend that the final calculation parameter should additionally consider factors such as service condition of high slope with the length of building service. In the absence of experimental values, the related technical standard may be used to offer these calculation parameters.

Engineering significance

The conclusions of stability assessment are important to help ensure slope stability and therefore building safety. Moreover, it is an important factor in determining the need for structures and the methods of excavation.

The excavation of natural rock-soil is disadvantageous to the stability of high slopes due to the intrinsic destruction of the equilibrium and the mechanical properties of rock mass, which becomes weakened by weathering over time. Retaining structures should be used when the stability assessment shows that the excavation results in slope failure. Specific measures are necessary to deal with the sensitive factors forward to reduce high slope stability. The disjunction and topdown construction method ought to be used in slope excavation construction if it is shown that a large degree of distortion would occur if a "one-off" excavation procedure was adopted. Some other aspects, including the use of engineering blast methods and the quantity of explosive used in excavation construction, may be decided according to the stability dynamic assessment.

CONCLUSIONS

1) Accurate stability assessment of high excavated slopes is fundamental to ensure the safety of the related excavation construction site and buildings.

2) The stability assessment is influenced by several crucial factors: the natural geological conditions and anthropogenic disturbance of these conditions, the engineering environment, potential slope failure pattern, assessment calculation parameters and analysis method.

3) The conclusion of the stability assessment helps estimate the safety of building sites adjacent to high slope and also provides important data used to determine the strategy for excavated slope-retaining structures.

REFERENCES

Wen, H.J. (2001). "Stability Assessment Report of Building Site adjacent to J2&K2 Commercial Building of Chongqing Linjiang Plaza", 10~12

Wen, H.J. (2001). "Stability Assessment Report of Building Site adjacent to Excavated High Slope, Yaoyong Shock Absorber Workshop", 10~11

Chongqing Nanjiang Eng. Geo. Inv. Co.. (2001). "Engineering Geological Investigation Report of Bayangshan Landslide, Wulong County Urban. Chongqing", 11~15

Wen, H.J (2002). "Crucial problems on security assessment of a building site adjacent to an excavated high slope". J of Chongqing University, Vol.1 No.2, 38~41

Construction Supervision Bureau. (1998). "Chongqing Local Technique Standard. Code for Engineering Geological Investigation (DB50/5005-1998)", 31~37.

Construction Supervision Bureau. (2001). "Chongqing Local Technique Standard. Technical Specification for Retaining Structure in Excavation (DB50/5018-2001)", 7~20

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WEN HAI-JIA, ZHANG JIA-LAN, ZHANG YONG-XIN

Faculty of Civil Engineering, Chongqing University, Chongqing 400045, P.R. China