Comprehensive suitability evaluation of urban geological environment in Zhengzhou-Kaifeng area

CUI Zhen, YANG Po, ZHANG Zheng

1Henan Institute of Geological Survey, Zhengzhou 450000, China.

2Henan Urban Geological Engineering Research Center, Zhengzhou 450000, China.

Abstract: Suitability evaluation of urban geological environment is an important basic work in city planning and construction. With Zhengzhou-Kaifeng area as the research area, this paper discusses AHP and fuzzy comprehensive evaluation methods to classify impact factors at different levels, makes a mixing evaluation of the basic factors and comprehensive factors so as to correct the uncertainty of the weight of the basic factors in the initial evaluation and establish the corresponding evaluation index system, and carries out urban geological environment suitability evaluation in study area. Furthermore, this paper partitions the suitability of the research area according to evaluation results into five zones. It puts forward opinions and suggestions for the construction of research area, which have certain practical significances.

Keywords: Zhengzhou-Kaifeng; Geological environment; Suitability evaluation; Index system

Introduction

As China’s urbanization process is gradually accelerating, the engineering construction activities have affected the original physiographic conditions so that the urban geological environment confronts increasing pressure. Currently, the problems with the domestic urban geological environment,mainly include our limited knowledge about urban geological environment and our irrational planning and utilization of land before the urban geological environment, are taken into full consideration, so there are many potential hazards. For instance, one building in Xiamen was located on the ancient drift sand, but the liquefaction of sand foundation in the heavy rain after the excavation of foundation pit. It took huge amounts of money to improve the condition, but the project was finally suspended due to its threat to those surrounding buildings(CHEN Gui-zhong, 2014). This paper, with Zhengzhou-Kaifeng as the research area, examines all the geological environment-related factors that may affect the urban construction, and creates and analyzes the suitability evaluation index system of urban geological environment.

1 Overview of research area

The Central Plain Urban Agglomeration, which is located in the heart of Henan Province, is an urban agglomeration with Zhengzhou and several other cities at the central layer. With the tight social and economic connections inside, this urban agglomeration has become a central growth pole to boost the rising of the Central China and influence and drive the development of the Central and Western China. In this urban system, the first layer is Zhengzhou Metropolitan Area, which covers Zhongmu, a city under the administration of Zhengzhou, and Zhengzhou-Kaifeng Integrated Area. Zhengzhou-Kaifeng area is characteristic of the changeable landform and strata, the frequent occurrence of such geological disasters as collapse,landslide and earth sinking, and the widespread distribution of collapsed loess, liquefied sand and muddy soil. In this sense, the urban construction work has to deal with many problems in connection with soil and geological environment.Now there are lots of geological materials and research findings available, but we have found few researches on the suitability evaluation of geological environment in the urban construction process of Zhengzhou-Kaifeng research area, and there is no uniform and well-established evaluation index system and method yet.

2 Suitability evaluation of geological environment within research area

Since the suitability evaluation of urban geological environment is the fundamental work in the city planning and construction process, some standards and methods are integral for the overall suitability evaluation of geological environment quality within Zhengzhou-Kaifeng research area.The key to this evaluation work is to select the relevant evaluation index. After the urban geological conditions are subject to systematic analysis, the selection of right index system and the accurate extraction of performance parameters are the basic conditions for guaranteeing the authentic and objective evaluation results. A less rational evaluation system will result in wrong evaluation results, irrational development decisions and even serious safety accidents. The process of this suitability evaluation is shown in Fig. 1(CHENG Wen et al. 2012).

Fig. 1 Flow chart of the suitability evaluation of urban geological environment

2.1 Evaluation principles

The urban geological environment is a highly comprehensive dynamic system. Given its variety and complexity as well as the effects of different factors on geological environment quality, the selection of evaluation index shall comply with the following principles. (1) The principle of combination between suitability and limitation is an essential evaluation principle. (2) The principle of comparable evaluation results; when the evaluation factors are selected, the top consideration is to select those major factors that can affect the geological environment quality within the whole area, and to adopt the uniform evaluation standard.(3) The principle of sustainable use; it is necessary to note the variation of land property as induced by the changes to land use, and the possibility of land deterioration and pollution. (4) The principle of ecological priority; the eco-landscape resources should be protected within the research area to prevent the effects and destruction of regional ecological environment. (5) The principle of combination between comprehensive analysis and dominant factors (LIU Chang-li et al. 2012).

2.2 Selection of evaluation methods

The suitability evaluation of geological environment is a comprehensive space-related decision-making process that involves multiple levels and factors. Currently, the most frequently used methods and theories include analytical hierarchy process (AHP), factor comparison method, expert clustering method, quality coefficient method, multivariate statistical analysis,fuzzy gray clustering, fuzzy comprehensive evaluation, vector modulus, compound fuzzy distance and GIS decision support system (ZHOU Ai-guo et al. 2008). The urban construction environment within Zhengzhou-Kaifeng research area is characterized by the complexity and diversity of geological environment and conditions in hydro-geological projects, the frequent occurrence of geological disasters, the shortage of geological resources and the strong human engineering activities. Many factors should be quantitative enough in the suitability evaluation process of urban geological environment,analytical hierarchy method, and the proper combination between AHP and fuzzy comprehensive evaluation can address the requirement for qualitative and semi-quantitative evaluation. This paper uses these two methods in the suitability evaluation of urban geological environment in Zhengzhou-Kaifeng research area.

Fig. 2 Hierarchical chart of suitability evaluation of urban geological environment in Zhengzhou-Kaifeng Area

2.3 Determination of evaluation factors

The evaluation factors in the suitability evaluation of urban geological environment mainly include five primary evaluation conditions like physiographic condition, hydrogeological condition, engineering geological condition, issues of geological disaster and environmental geology, and geological resources, and twenty secondary factors.Specifically, the physiographic condition-related suitability indices of urban construction mainly include landform type and topographic slope. The hydrogeological condition-related suitability indices of urban construction mainly include shallow groundwater depth, shallow groundwater quality and anti-pollution performance of groundwater system. The engineering geological condition-related suitability indices of urban construction mainly include the corrosion of steel bars and structures of groundwater and soil,suitability of natural foundation at 5 m and 10 m below the ground surface, and suitability of engineering construction. The maturity of issues of geological disaster and environmental geology includes susceptibility of geological disasters,collapsed loess, liquefied sand, soft soil and groundwater pollution. The geological resourcesrelated suitability indices mainly include land use,groundwater development and exploitation,shallow geothermal energy (heat exchange of groundwater and buried pipeline) and underground space development and exploitation (surface,shallow and middle-level) (CAI He-sheng et al.1998; LUO Yu-chi et al. 2007; SUN Xiao-ling et al.2015). This paper makes a mixing evaluation of the basic factors and comprehensive factors to correct the uncertainty of the weight of the basic factors in the initial evaluation and increase the accuracy of the comprehensive evaluation of urban geological environment. After the interaction of different impact factors is considered, the hierarchical chart of the suitability evaluation of urban geological environment is created as a result(Fig. 2).

Based on the partition and selection of above-stated factors, YAAHP 10.3 is used to create the judgment matrix and obtain the weight of each factor. The weight of factors and the calculation and evaluation methods can be used to complete the comprehensive evaluation of suitability, and the figure is created through MAPGIS space analysis.

Fig. 3 Judgment matrix A-B

2.4 Judgment matrix and weight of factors

Creation of judgment matrix

Based on the partition and selection of above-stated factors, YAAHP 10.3 is used to create the judgment matrix (Fig. 3).

With this method, we can generate the judgment matrix C-B1, C-B2, C-B3, C-B4 and C-B5. Based on judgment matrix, we can obtain T’s maximum eigenvalue and the corresponding eigenvector. All the eigenvectors can indicate the order of importance for all the evaluation factors.Suppose that A is the regular vector of the same level, XA=λmaxA, A generated following the solution of this characteristics equation can be regularized into the weighted value of x1, x2, …,xm. After λmaxis obtained, it is also necessary to check consistency and randomness. The consistency index (CI) is defined as follows:

The wolf pulled the bobbin, and the door opened, and then he immediately fell upon the good woman and ate her up in a moment,6 for it been more than three days since he had eaten. He then shut the door and got into the grandmother s bed, expecting Little Red Riding Hood, who came some time afterwards and knocked at the door: tap, tap.

where, CI is consistency index; λmaxis the maximum eigenvalue of judgment matrix A, n is the number of order of judgment matrix A.

The random consistency index (RI) should be introduced to measure if different judgment matrixes have satisfactory consistency. RI values of 1-14 judgment matrix is indicated in Table 1.

Table 1 RI Values of 1-14 judgment matrix

When the number of order is more than two,calculate the ratio of consistency. Only when CR <0.1, the judgment matrix has the satisfactory consistency and the evaluation results can be considered as well consistent. In this case,the final value is rational. The eigenvector of λmaxis obtained and then regularized into weight distribution. Otherwise, another judgment matrix should be created (HONG Zhi-guo et al. 2002).

The comprehensive evaluation of the suitability of urban geological environment in Zhengzhou-Kaifeng research area: A; consistency ratio:0.053＜0.1; λmax: 5.2375(2).

Table 2 Weight of primary evaluation factors

Table 3 Weight-Value table of evaluation factors

With the same method, we can obtain the following results: For physiographic condition B1,the consistency ratio is 0＜0.1, the weight of“comprehensive evaluation of the suitability of urban geological environment in Zhengzhou-Kaifeng research area: A” is 0.0333, λmax: 2.0; for hydrogeological condition B2, the consistency ratio is 0.037＜0.1, the weight of “comprehensive evaluation of the suitability of urban geological environment in Zhengzhou-Kaifeng research area:A” is 0.0634, λmax: 3.0385; for engineering geological condition B3, the consistency ratio is 0.021＜ 0.1, the weight of “comprehensive evaluation of the suitability of urban geological environment in Zhengzhou-Kaifeng research area:A” is 0.5128, λmax: 5.094; for issues of geological disaster and environmental geology B4, the consistency ratio is 0.037＜0.1, the weight of“comprehensive evaluation of the suitability of urban geological environment in Zhengzhou-Kaifeng research area: A” is 0.2615, λmax: 3.0385;for geological resource B5, the consistency ratio is 0.0503＜ 0.1, the weight of “comprehensive evaluation of the suitability of urban geological environment in Zhengzhou-Kaifeng research area:A” is 0.129, λmax: 7.4101. The testing results show that the consistency test of each judgment matrix meets the relevant requirements, and the obtained weights are correct.

The weight of impact sub-factor Ciin the Target Evaluation A can be obtained through the following equation:

where: Zi-the weight of constraint sub-factor Ciin the Target Evaluation A;

Xi-the weight of constraint factor Bi in the Target Evaluation A;

Yi-the weight of constraint sub-factor Ciin constraint factor Bi.

The weight of each evaluation factor is calculated (See Table 3).

2.5 Evaluation index and partition

2.5.1 Evaluation index

Based on the suitability evaluation method and principle of urban geological environment, there are five suitability grades, including great suitability, good suitability, normal suitability, low suitability and bad suitability. Each evaluation factor should go through the quantification and value-assigning process. The value assignment of suitability level of evaluation index is indicated in Table 4. See Table 5 for the suitability evaluation index of urban geological environment (TONG Xin et al. 2015; LIU Jian et al. 2014).

Table 4 Value-determined standard of suitability evaluation of geological environment index grade of urban construction

Fig. 4 Partition map of suitability of geological environment of urban construction in Zhengzhou-Kaifeng area

2.5.2 Evaluation partition

Based on the standard table of suitability evaluation of geological environment of urban construction, the single-factor partition map of 20 evaluation factors including landform type, suitability of engineering construction, and suitability of underground space is created. Meanwhile, based on the value-determined standard of suitability evaluation index grade of geological environment of urban construction, the partition results of 20 evaluation factors are subject to value determination. After the partition map of 20 single factors goes through the weighted calculation, the comprehensive score is used in line with the same percentage to generate five grades including great suitability, good suitability, normal suitability, low suitability and bad suitability. Finally, the partition map of suitability of geological environment of urban construction in Zhengzhou-Kaifeng area is drawn as in Fig. 4.

The comprehensive suitability evaluation of geological environment of urban construction in Zhengzhou-Kaifeng area has the partition result as follows:

(1) Great suitability area: The flat terrain, no distribution of special rock and soil, great suitability of engineering construction, high bearing capacity of foundation, and suitability for shallow geothermal development and underground space construction;

(2) Good suitability area: The flat terrain,considerable groundwater depth, no distribution of special rock and soil or small distribution of special soil, good suitability of engineering construction, and suitability for shallow geothermal development and underground space construction;

(3) Normal suitability area: The flat or relatively flat terrain, considerable groundwater depth, small distribution of special soil in some parts, and normal suitability of engineering construction;

(4) Low suitability area: The rolling terrain on the marginal part of loess tableland to the east of Mazhai Township, considerable groundwater depth,collapsed loess, and low suitability of engineering construction. The low water depth, the distribution of soft earth and liquefied sand within the Yellow River’s riverside depression along the belt of Langchenggang-Xinghuaying-Tubaigang in Kaifeng,and unsuitability for the shallow geothermal development and underground space construction;

(5) Bad suitability area: The considerably rolling terrain in Mangshan Mountain in the west,the low water depth within the shoal of Yellow River in the north and east, the distribution of soft earth and muddy soil, low eigenvalue of bearing capacity of foundation, and unsuitability for the engineering construction because the shoal of the Yellow River, as the flood flowing zone, is an important nation-level wetland protection area.

3 Conclusions

This paper considers the impacts of five evaluation factors including physiographic condition, hydrogeological condition, engineering geological condition, issues of geological disaster and environmental geology and geological resource, uses AHP and fuzzy comprehensive evaluation methods to conduct the suitability evaluation of urban geological environment in Zhengzhou-Kaifeng research area, and partitions the research findings into five zones, including great and good suitability zone, which covers about 1 669.7 square kilometers, 42.57% of the research area, the normal suitability zone, which covers about 1 112.5 square kilometers, 27.90% of the research area, and the low and bad suitability zone, which covers about 1 177.5 square kilometers, 29.53% of the research area.

This paper suggests that, in the construction and planning process, the infrastructure construction for the purpose of residence, public facilities,industrial warehousing, transportation and green space should be considered within the good suitability zone; the infrastructure construction for the purpose of small- and medium-sized industrial and civil buildings, transportation and green space should be considered within the normal suitability zone, but the industrial projects with considerable environmental impacts and possible pollution to water and earth should not considered; the shallow geothermal development and underground space construction should not be considered within the low suitability zone, and the large-scale industrial and civil buildings should also be avoided due to the possibility of small- and medium-sized collapse of loess and the landslide disaster.

The aforesaid evaluation results can be used as an essential reference in the planning and construction of Zhengzhou-Kaifeng area, and as the scientific basis for planning and layout,effective land utilization, underground space development, urban disaster relief and prevention,and scientific decision-making of Zhengzhou-Kaifeng area and even the whole Central Plains Urban Agglomeration.

Acknowledgements

This study was supported by China Geological Survey Project “Urban Geological Survey of Central Plains Urban Agglomeration” (1212011089048).