1 crack type of expansive soil
According to the causes, it can be divided into primary cracks and secondary cracks; It can be divided into closed ground fissures and open ground fissures. According to the severity of the depth, it can be divided into deep and severe ground fissures and shallow ground fissures. The ground fissures we often see are all open deep serious ground fissures and shallow general ground fissures formed under the influence of external environmental factors on the basis of primary ground fissures, which belong to secondary ground fissures. The formation and development of cracks in expansive soil are determined by the interaction and interdependence between the engineering characteristics of expansive soil itself and external environmental factors.
Formation and development of ground fissures
2. 1 primary fracture
Expansive soil is rich in clay minerals such as montmorillonite, but its content is lower than that of montmorillonite.
2μ m clay particles, which are highly dispersed between coarse particles in the process of expansive soil deposition, then form a gel state due to the continuous evaporation and solidification of water, and the continuous evaporation and dehydration make its surface dry and shrink to produce cracks, and the continuous drying and shrinkage make the cracks deepen and expand. In the subsequent deposition, the cracks are closed by water supply, and the redeposition cycle is carried out. This is the whole process of primary cracks in expansive soil. Primary fracture is a vertical joint in expansive soil, and its fracture surface is smooth and waxy. Generally, it is closed without losing water.
2.2 Ground fissures
Expansive soil is mostly old clay, deposited in Q2 ~ Q3, and exposed in foothills, hills and secondary terraces due to crustal uplift and river cutting erosion. Under the complex environmental geological action, coupled with the influence of climate and man-made environmental factors, expansive soil has lost its original natural balance and needs to be readjusted. In the process of adjustment, some primary cracks have developed and expanded, forming what we call open deep cracks, serious ground cracks and general shallow cracks. This kind of ground fissure changes with the change of environmental factors, which is one of the most active and unstable factors in the destruction of buildings by expansive soil. We often say that expansive ground fissures refer to this kind of secondary ground fissures. The main environmental factors affecting the formation and development of secondary fractures are as follows.
(1) Climate, landform, physical geography and environmental factors
Climate, topography and natural geographical environment are important external conditions for the formation and development of secondary fractures. As we all know, with the change of precipitation, evaporation and temperature and humidity conditions throughout the year, the water content of expansive soil changes continuously within the range of atmospheric influence depth, causing vertical expansion and contraction deformation of expansive soil; However, its site on the slope and near the slope is different. In addition to vertical lifting (expansion and contraction) deformation, there is horizontal displacement along the slope direction. However, the reversibility of this horizontal displacement is very poor, and residual deformation often remains. With the passage of time, the ground fissures along the contour line in the slope position and slope position continue to expand and deepen, forming deep and serious secondary fissures with open development. If combined with the infiltration of atmospheric precipitation or other water, the ground fissure will further split and soften, and the soil cut by the ground fissure will creep to the foot of the slope under the action of its own weight, and even cause adverse geological phenomena such as shallow landslide.
(2) Special hydrogeological conditions are the main reason why ground fissures develop into bad geological phenomena.
Expansive soil is an old clay and an impermeable layer. Atmospheric precipitation, farmland irrigation, canal and pipeline leakage all leak through ground fissures. Shallow groundwater in expansive soil belongs to fissure stagnant water in ground fissures, and there is no stable water level. When the two boreholes are 30cm apart, there is water on the ground fissure and there is no water on the ground fissure. The water level on the same slope varies greatly. There is water in the borehole at the top of the slope and no water in the borehole at the waist of the slope. It can be seen that the shallow groundwater in expansive soil flows along the contour line in the ground fissure, and the special seepage law of groundwater in expansive soil causes the splitting, softening, erosion and scouring of the ground fissure. Results In addition to promoting the collapse and landslide of expansive soil slope, dissolution and erosion are the main reasons for the bad geological phenomena such as solution groove, soil hole, gully and karst in expansive soil area.
(3) Human factors are also the main conditions for the formation and development of secondary fractures.
Building roads, digging ditches, building houses, greening and irrigating farmland in expansive soil areas ... Human production activities have destroyed the original natural balance of expansive soil. In order to adapt to the new environment, the water in expansive soil is transferred according to the new environment, and the expansion and contraction deformation changes in the new environment, resulting in new differential deformation, and new ground fissures will occur in the process. For example, the indoor and outdoor temperature and humidity conditions have changed after building a house; Male and female sides; Surface flow state behind anterior chamber; Different evaporation conditions; This change of environmental conditions leads to uneven distribution of water content in expansive soil and different deformation ranges. The measured results also show that the deformation amplitude of the sunny side is large, and the relative deformation amplitude of the back to the sunny side is small. This differential deformation will produce new ground fissures, leading to cracking and damage of houses. For example, to build a house on a sloping land, it is necessary to dig and fill the site, which further destroys the original natural balance and increases its expansion, contraction and deformation. Coupled with horizontal displacement, it will lead to new ground fissures, which will also make the original general ground fissures develop into serious ground fissures and cause greater damage to houses. Judging from the damage forms of houses in expansive soil areas, apart from inverted splayed cracks, the walls often appear dislocation after cracking, and cracks appear on concrete floors and pavements. Slope collapse of cutting and channel. These phenomena are often caused by ground fissures, and the damage caused by ground fissures is often difficult to repair.
To sum up, the formation of ground fissures in expansive soil area is mainly due to the evaporation of water in expansive soil by the atmosphere, and the soil shrinks after water loss, resulting in ground fissures. However, the development, development and expansion of ground fissures are inseparable from the participation of boiling water for a moment. The above three main factors are also complementary and interactive, and jointly affect the formation and development of ground fissures. Ground fissures in expansive soil areas are the product of extremely complex environmental geological processes. However, the depth of the most serious deep ground fissure will not exceed the depth of atmospheric influence. Under the condition of flat terrain, the atmospheric influence depth of expansive soil area in China is about 5.0m. Generally speaking, shallow ground fissures are located in the depth range of strata which is greatly influenced by the atmosphere. See table 3 * 4 * 129- 1994 for the depth of the layer with strong atmospheric influence, and the maximum depth is 2.50m, which provides a basis for us to prevent the ground fissures from damaging buildings.
Investigation and evaluation of ground fissures
GJB2 129- 1994 "Technical Code for Barracks Building in Expansive Soil Area" clearly stipulates that adverse geological phenomena such as ground fissures, landslides, gullies, karst and soil caves should be investigated and the scope should be preliminarily delineated. Putting ground fissure investigation in the first place shows the importance attached to ground fissure investigation. Ground fissures are divided into two categories according to their size and severity: one is deep and serious ground fissures; The second is shallow ground fissures. These two kinds of ground fissures are open secondary ground fissures, and their forms are wedge-shaped vertically inserted into expansive soil layer. It is easier to identify deep and serious ground fissures in the field, because the opening of serious ground fissures is large, and some silt particles containing organic matter are brought into the ground fissures by wind and water infiltration, so that the surface of the scratched cracks is covered with a layer of gray powder particles. Generally, shallow ground fissures are less, because the upper opening is smaller. In the dry season, especially in the dry year (the year of severe drought), or when the local cracks are large or the walls, concrete floors and sidewalks are cracked and dislocated, ground cracks are easy to appear; In general, the ground fissures in the newly-built site are covered by loose topsoil and vegetation, so they are not easy to be found. Conventional boreholes are not easy to find because they cannot be arranged too closely. The author believes that the distribution and development of ground fissures in the code can not be clearly understood by drilling and exploring pits. If the distribution and development of ground fissures are not clear, it is impossible to evaluate the engineering geology of the site. This paper recommends a simple and easy "measured profile method" to find out the distribution and development of ground fissures. This method is along the slope direction of sloping field during exploration drilling; In the direction perpendicular to the slope, more than two rows of drilling holes should be arranged according to the size of the site. If the slope is multi-faceted, at least two rows of drilling holes are arranged on each slope along the slope, and each row of drilling holes is connected with the drilling holes by shallow exploration grooves. The depth and width of the groove are about 30 ~ 50 cm. According to the principle of digging out the topsoil and rich ginger layer from the original fresh expansive soil, the development and distribution of ground fissures are recorded while digging and accurately marked on the map. In order not to miss the ground fissures, white lime slurry is poured in stages after cleaning the trench, so that the slurry permeates downward along the ground fissures, and then the residue of white lime slurry in the trench is removed, so that the ground fissures infiltrated by white mortar can be easily found at the bottom and wall of the trench. Marked on the trench development map, classified and counted according to the width, and then divided the distribution range of ground fissures according to the topographic and geomorphological characteristics, and evaluated its engineering geology.
4 prevention and treatment of ground fissures
The prevention and control of ground fissures can be carried out according to the prevention and control measures stipulated in two technical specifications: GBJ 1 12-87 and GJB2 129- 1994. In particular, the prevention measures proposed in GJB2 129- 1994 are more detailed and concrete, and the experience of our army in fighting the harm of expansive soil for many years is summarized. Strict implementation of these two norms can reduce the economic losses in the construction of expansive soil areas. However, we must also realize that the research on expansive soil in China started late, and we should continue to explore and experiment to accumulate new experience for the implementation of the code. In the flat site, comprehensive improvement measures can be taken according to the methods introduced in the specification. On the slope, the slope can be treated first, and various effective measures such as retaining wall, slope protection anchor rod and soil nail can be adopted to prevent the creep and slip of the slope and prevent the formation and development of ground cracks. In the foundation design, pier foundation and pile foundation can be selected to increase the buried depth of foundation, and the environment can be improved widely and comprehensively to prevent the damage of expansive soil cracks. Pile foundation can be used for a class of important buildings in expansive soil area, and the pile length is 1.5 times the atmospheric impact depth of flat site; According to the gradient, the depth of atmospheric influence can reach 2 ~ 2.5 times. The depth of atmospheric impact can be determined with reference to Table C of GBJ 1 12-87. Overhead foundation can be used for indoor ground to prevent the influence of expansive soil cracks on the ground.
5 concluding remarks
Expansive soil is a kind of unsaturated soil, and its engineering geological characteristics of water absorption expansion and water loss contraction can only be presented in unsaturated state. If expansive soil is completely dry (completely contracted) or completely saturated (completely expanded), it will lose its unique expansion and contraction characteristics. 1In September 1995, the Paris International Conference on Expansive Soils decided to change its name to the International Conference on Unsaturated Soils, and the second conference will be held in Beijing on 1998. We should welcome the second international conference on unsaturated soils with fruitful research results.