Engineering Geological Mapping and Land-Capability Analysis in Tangshan City

ENGINEERING GEOLOGICAL MAPPING IN TANGSHAN CITY

There are three major geo-hazards in Tangshan city: earthquake, ground collapse caused by karst and coal mining activities, and special foundation soils (young sediments, soft soil, organic soil, swell soil and artificial back filled soil). The study of these geo-hazards created the following geological maps: (1) map of topography in Tangshan City; (2) map of bedrock geology in Tangshan City; (3) map of Quaternary thickness contour in Tangshan City; (4) map of shallow underground water level contour in Tang-shan City; (5) map of distribution of site seismic liquefaction in Tangshan City; (6) map of underground collapse and under-mining area in Tangshan City; (7) map of seismic effect of active belts in Tangshan City; (8) map of site classification in Tangshan City.

METHOD FOR QUANTITATIVE ANALYSIS OF LAND-USE CAPABILITY

Quantitative analysis of land-use capability is the most important part of land-use project. Land-use capability is a certain economic value when land is developed for some purpose. Land-use is closely related not only to politics, science and economy, but also to geological environments. Evaluation of land-use capability must include all natural features and functions of land. The quality of land-use capability depends on its own geological environment conditions and land-use types (Li and Fang, 1996; Li, 1991). The method for the quantitative analysis of land-capability includes the following context and application programs: (1) studying the policy of land-use and general urban planning, deciding the characteristics of urban development, land-use types and their natural characters, and estimating the value of buildings and the corresponding properties; (2) studying the basic geoscientific information, deciding the themes of geological environment which are constraints for each land use concerned; (3) compiling each geo-hazard evaluation map which can be used to appraise geological environment's themes; (4) making social costs evaluation for each development type and theme of geological environment, where the social costs refer to those caused by one geological constraint, regardless of who pays them. There are three kinds of costs: ① basic costs. Costs for project surveying, design, and engineering measures; ② risk costs. Costs which are caused by potential geological disaster and whose value is a possibility on quantity and time; ③ opportunity costs. Annual benefits when land is developed for some purpose (generally speaking, the present value of its future expected cost is calculated using benefit and discount rate); ④ accumulating the total expected costs caused by geological environment themes for each land-use type, which is used as an index to evaluate land-use capability; ⑤ applying the land-capability to decision-making.

PRINCIPLE OF COST CALCULATION

It is assumed that the probability rate of occurrence of several different geologic events, for example, that of earthquake is constant in time. That is, the probability that n events occur in a given time interval, at Δt, is governed by the Poisson probability distribution.

The probability that n events occur in a time interval is shown below

where λ is the instantaneous rate of occurrence. It can be shown that if the number of events on time intervalt is governed by the Poisson distribution, then the probability that the next event will occur in t units of time from now is determined by the exponential distribution, as shown below

The term "recurrence interval" is often used. The expected time for the next event is one interpretation that can be made of initiative term recurrence interval

Thus, an estimate of the recurrence interval can be used to estimate the parameter λ. In all the expected cost calculations, it is assumed that the cost per event is constant except for discounting. If this is true, the expected cost arising from the first occurrence can be easily calculated.

Let X=the cost per event at time t, then

where r is the discount rate.

The expected cost arising from the first (next) event is obtained by the sum over the cost of the event at time t which times the probability that the event occurs at the time.

The expected cost due to

For some kinds of events, the damage can occur repeatedly, for example, if rebuilding occurs after an earthquake. For these events, it would be of interest to calculate the expected cost due to all future events. First, one calculates the probability that one must wait for a time t₁ for the first event and for a time t₂ for the second event

Now the theory of conditional probabilities shows

From this, it is easy to show by induction that

From this, one can calculate the expected cost due to the first n events.

The expected cost due to the first n events＝

The expected cost for all future events is given by

LAND-USE CHARACTERISTICS AND COSTS IN TANGSHAN CITY

For convenient study, we classify, according to the requirements of Tangshan city planning, the land-use types in Tangshan as four classes and eight kinds, with their characteristics and costs shown in Tables 1 and 2.

Table  1.  Land-use characteristics

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Table  2.  Land-use costs

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But for all future time, n infinite number of events will occur with the probability equal to one.

THEMES OF GEOLOGICAL ENVIRONMENT COSTS CALCULATION

Costs Associated with Earthquake Ground shaking

According to earthquake analysis report completed by seismic bureau, the earthquake damage prediction work has been carried out, where the damage rate of buildings and property, the basic data from Table 1, and the costs associated with ground shaking for each land-use type are calculated.

The formulas for the expected loss caused by the ground shaking are shown as follows.

① Damage costs of building

where V_d is damage costs of buildings; V_b is value of buildings, calculated from Table 2; R_b is damage rate of buildings for each land-use type; P_e is probability of earthquake occurrence; D is discount rate.

② Damage costs of property

where V'_d is damage costs of property; V'_b is property value calculated from Table 2; R'_b is damage rate of property; P_e is probability of earthquake occurrence; D is discount rate.

Surface rupture

The following formula is employed in this paper to calculate the costs associated with surface rupture

where V_o is damage costs associated with surface rupture; V_b is value of buildings calculated from Table 2; F is percent of covered area of buildings; n is numbers of building per hectare.

Costs Associated with Karst Collapse

The following formula is used to calculate costs associated with karst collapse

where V_k is cost associated with karst collapse; P_k is cost rate of karst foundation treatment; V_b is value of buildings calculated from Table 2.

Costs associated with special soil foundation improvement

There are more than ten methods, whose economic indexes are available, for soft soil foundation improvement in Tangshan City. Therefore, the costs associated with soft soil foundation improvement can be calculated.

LAND-CAPABILITY MAPPING AND COMPUTER CARTOGRAPHY

A program LUMP is designed for land-capability mapping. The calculation of the land capability involves only one simple addition. The reasons for the calculation with the computer are shown as follows: the speed at which the calculations should be completed for a large number of cells, the ease with which minor changes can be made and calculation rerun, and the ease and speed with which maps are produced.

The series of land-capability maps include: (1) land-capability map for low-storied residential use; (2) land-capability map for multi-storied residential use; (3) land-capability map for high-storied residential use; (4) land-capability map for commercial use; (5) land-capability map for commercial use; (6) land-capability map for educational and public health use; (7) land-capability map for office use (the scales of all these maps are 1∶25 000)

CONCLUSION

The method for the assessment of land capability, designed to help apply earth-science information to land-use planning, illustrates the close relationship between geological environment and urban planning, and shows clearly its potential economic efficiency. In addition, this method, an important role on urban land planning, enhances the cooperation between geologist and urban planner.