Cracks appeared in a large area of houses in Maoming City, Guangdong Province. Therefore, during the period of 1989, we conducted investigation and research with relevant personnel of Maoming Petroleum Industry Company. In addition to macroscopic investigation, 25 boreholes were arranged in Rifa's 13 villages, and soil samples with a depth of 5 ~ 100 m were analyzed. The research shows that expansive soil disaster is the main cause of building cracks.

Second, the characteristics of housing cracking

1) Cracks in houses appear in groups: not only in one village, but also in villages with cracks in groups. At present, cracks have been found in houses in 45 villages, which are distributed in the range of about 200km2.

2) Cracks generally change with the climate change: cracks open or increase in dry days, decrease or close in rainy seasons, and expand and contract repeatedly until the house is unusable;

3) Special crack forms: Oblique cracks are common in corners, gables, front and rear walls and partition walls, and are often inverted splayed, with vertical cracks mostly appearing in the middle of front and rear walls; Cross cracks generally appear in houses with severe deformation, which is the development of oblique cracks.

Third, climatic conditions.

Maoming has a subtropical maritime climate. According to the data of mine weather station 1956 ~ 1987, the annual average rainfall is 1560mm, and the annual average evaporation is 2085.5mm, which is greater than the rainfall. Calculated from the average values of 1978 ~ 1987 and 10, because this climate is seasonal alternation of wet and dry. It causes the change of water content of expansive soil, which leads to seasonal expansion and contraction. Cracks in houses open in the dry season and close in the rainy season, as do ground fissures.

Four. General situation of geology and geomorphology

Maoming basin is a Mesozoic-Cenozoic sedimentary basin, in which Cretaceous red sandstone and volcanic rocks are distributed, and the overlying tertiary (Paleogene) fluvial facies, lacustrine facies and coastal swamp facies are often exposed to the surface, forming Longgang hilly terrain. Quaternary slope deposits and alluvial deposits are distributed in low-lying areas, occupying a small area. Neogene (Neogene) Huangniuling Formation, Laohuling Formation and Gaopengling Formation have different lithology, but they are mainly gravel layer, sandy clay layer, clayey sandy soil layer and various clay transition layers and staggered layers, all of which belong to fluvial facies and lacustrine facies deposits. The cracked house is located on the cohesive soil layer, that is, the expansive soil layer in these three groups (Figure 1).

Maoming expansive soil is distributed in Longgang hilly area, and the terrain slope is generally 2 ~ 8. Most houses with serious cracks are located halfway up the mountain, while houses at the foot of the mountain and on the flat ground at the top of the mountain generally have no cracks or slight cracks. The reason is that the expansive soil layer in the middle of the mountain is unevenly distributed, and the water content changes greatly, resulting in a great difference in expansion and contraction. This expansive soil is also distributed in Guangxi, Yunnan and other neighboring provinces (regions), which shows that there is an environment conducive to the formation and evolution of this expansive soil in China.

Figure 1 stratum profile of Huangniuling in the upper part of open pit.

1-yellowish brown clayey sand; 2- iron ore deposit; 3- variegated clay (expansive soil); 4— yellow clayey sand; 5— Gray white clay (expansive soil); 6— yellow clayey sand; 7— Gray white clay (expansive soil)

Verb (abbreviation of verb) is the essence of geotechnical engineering

1) particle composition: See table 1 for the particle composition of Maoming expansive soil. As can be seen from the table, the content of particle size less than 0.002mm is generally more, which increases the activity of soil because of its high dispersibility.

Table 1 particle composition of Maoming expansive soil

2) Chemical composition: Table 2 shows the chemical analysis results of the samples. The content of alkali metal and alkaline earth metal oxides is low, only K20 content is slightly higher, mainly due to the high degree of rock and soil weathering in this area. In addition, because of the high iron content, Mossbauer spectrum is used to distinguish structural iron from cemented iron, in order to understand whether it has an effect on expansion and contraction and geotechnical strength. The analysis of LZ 1 1 and M T 13 shows that the content of structural iron accounts for 67.6% and 60.4% of the total iron, respectively, indicating that iron mainly exists in the form of structural iron and has little effect on expansion and contraction.

The cation exchange capacity of the sample is low, but it is still found that the larger the cation exchange capacity, the higher the expansion and contraction capacity.

Table 2 Chemical composition of Maoming expansive soil

3) Mineral composition: The results of X-ray powder diffraction (Figure 2), differential thermal analysis and infrared spectrum analysis show that the clay minerals are mainly kaolinite and illite, with a small amount of halloysite and chlorite. The semi-quantitative analysis results are shown in Table 3.

Figure 2 X-ray powder diffraction pattern

Table 3 Semi-quantitative results of clay minerals in Maoming expansive soil

4) Microstructure characteristics: Under the scanning electron microscope, the sample mainly presents four types of microstructure, namely, face-to-face layered structure, layered directional structure, honeycomb structure and overhead structure with overlapping edges.

Face-to-face layered structure is generally the structural feature of kaolinite-dominated soil, honeycomb structure is generally unique to illite-dominated soil, and layered directional structure and overhead structure are generally found in kaolinite-dominated and illite-dominated soil.

Compared with the expansive force Pe, it is found that the expansive potential of the soil with honeycomb structure is strong, followed by the layered directional structure, and the expansive potential of the soil with layered structure and overhead structure is weak.

Table 4 Physical properties of Maoming expansive soil

5) Physical and mechanical properties: Liquid limit in physical properties is an important index to distinguish expansive soil. As can be seen from Table 4, the average liquid limit is more than 49%, so it should generally be judged as expansive soil, and other physical indexes also meet the characteristics of general expansive soil.

6) Expansion and contraction characteristics: See Table 5 for expansion and contraction characteristics. These indicators are tested according to the requirements of China's "Technical Code for Building in Expansive Areas". The free expansion rate is the result of disturbed soil test, and it is a discriminant index. Its range value is 40% ~ 89%, and the expansion potential is weak to medium. The other three indexes are the results of undisturbed soil test. With these indicators, according to the calculation formula of the above specification, combined with the thickness of expansive soil layer and the range of water content in soil, the graded expansion and contraction can be calculated to determine the grade of expansive soil foundation.

Table 5 Expansion and Shrinkage Characteristics of Maoming Expansive Soil

Hydrogeological conditions of intransitive verbs

Expansive soil belongs to Huangniuling Formation, Laohuling Formation and Gaopengling Formation, with changeable lithology and complicated water-bearing conditions. It has the characteristics that clay aquifuge and gravel aquifer appear alternately, and confined water, semi-confined water and free water appear alternately.

The common feature of these three aquifers is that atmospheric precipitation is their main recharge source. Figure 3 shows that the aquifer water level varies greatly with seasons, generally 2 ~ 5m, which provides external conditions for the expansion and contraction of rock and soil.

Fig. 3 Water level curve of civil well

Seven. conclusion

Based on the investigation of the destruction characteristics of superstructure and cracks in a large number of houses, the geotechnical test of the lower foundation, and the analysis of the regional geology, landform, hydrogeology, climate and other environmental factors of the construction site, it can be concluded that the main cause of the cracks in Maoming houses is the expansive soil disaster. With this conclusion, we have a correct understanding of the renovation and repair of cracks in Maoming residential buildings.

(This article was originally published by China Journal of Geological Hazards and Prevention, Proceedings of the Second National Symposium on Geological Hazards and Prevention, Volume 5 Supplement, 1994 10, 20 1 ~ 206 pages; The authors include Li, Deng, Jiao Jingyao, Cai Shouni and Guo Fujiang)