Preliminary study on deformation law of tailings dam?

What is the specific content of the deformation law of tailings dam body? Zhong Da Consulting will answer your questions below.

Deformation observation of upstream tailings dam is an important means of dam stability monitoring. By observing the deformation of the dam in vertical and horizontal directions, we can find the abnormal behavior of the dam in time and take measures as soon as possible to ensure the safety of the upstream tailings dam. However, the deformation observation data of the upstream tailings dam are often messy, and it is difficult to make an accurate judgment only by a single monitoring data, especially when analyzing the stability of the upstream tailings dam in safety evaluation. Faced with a large number of observation data, it is either discontinuous or irregular. How to judge whether the dam tends to be stable needs to know the general law of settlement and deformation of upstream tailings dam. In the following, taking the rock gully tailings dam in Shizhuyuan Mine as an example, the deformation law of the upstream tailings dam body is discussed.

Brief introduction of tailings pond

Yanchonggou tailings pond is located in the low mountain area on the northern edge of the middle section of Nanling Mountain. The terrain is high in the south and low in the north. Most of the banks are steep rock walls, and some of them are slopes, on which vegetation is developed.

The reservoir dam is a clay inclined wall rockfill dam with an initial height of 24m. In the later stage, the dam was built by tailings upstream method, and the slope was 1:4. The designed final stacking elevation of tailings is 600m, and the corresponding maximum dam height is102 m. The total storage capacity is 7.5 million cubic meters. At present, the reservoir has been piled up to an altitude of 583 meters, the total dam height is 85 meters, and the tailings are about 4.3 million cubic meters.

(1) Formation conditions in the reservoir area

According to the field drilling, the strata revealed are Quaternary diluvium+diluvium (Qdl+pl) and upper Devonian Xikuangshan Formation (Dx2+3) medium limestone.

Groundwater in the reservoir area is divided into upper water and karst fissure water.

(2) Tailings accumulation body

The tailings in the reservoir can be divided into five layers: loose tailings silt, slightly dense tailings silt, slightly dense tailings sub-sand, light and sub-viscous tailings in soft plastic-flowing plastic state, and heavy and sub-viscous tailings in soft plastic-flowing plastic state.

Second, the deformation observation results

Since 1990, 12 dam deformation observation points have been set up on Yanchonggou tailings dam, which are divided into three rows and observed continuously for 22 periods. Later, in 2004, four rows 17 observation wells were re-set, specifically: Row A and Row B were set at the initial rock filling position, with an elevation of 539.00m, and five observation points were used in Row A, which was set at 1990, with an elevation of 555.00 m; Row D is set at an elevation of 569.00m m. Up to May 2007, * * * has carried out eight deformation observations with intervals ranging from half a month to half a year.

3. Deformation law of upstream tailings dam

The upstream tailings dam is an artificial slope, which is affected by topography, accumulation time, ore drawing mode, seepage pressure and gravity of tailings, and its deformation is complex. For convenience, it is generally divided into vertical settlement and horizontal displacement, and its deformation has the following laws:

(A) Spatial distribution characteristics

Horizontal displacement. The horizontal displacement deformation law is:

(1) In the direction perpendicular to the dam axis, the horizontal displacement and deformation rate gradually increase from bottom to top;

(2) In the direction parallel to the dam axis, the horizontal displacement and deformation rate of the observation point in the middle of the dam is greater than that of the observation points on both sides;

(3) The drainage point D is controlled by ore drawing at the dam top, and the local horizontal displacement is negative.

It shows that the tailings dam is affected by the slope effect, which produces horizontal force, which leads to the gradual deformation of the tailings dam in the downstream direction. At the same time, when the upper load changes, the tailings dam shows the characteristics of adapting to the stress change and producing local deformation, which also shows that the vertical settlement is dominant here during this period, and the horizontal displacement is only the additional effect of the vertical settlement.

(2) Time evolution characteristics

1. Settlement deformation.

The settlement of (1) is closely related to the tailings deposition time. The longer the deposition time, the smaller the settlement value and the slower the speed. On the contrary, the greater the settlement value, the faster the speed. For example, there is little difference in the thickness of tailings accumulation between row C and row D, but the settlement of row D is more than twice that of row C;

(2) At the same observation point, with the passage of time, its settlement rate gradually decreases.

With the passage of time and the gradual heightening of the dam, the settlement center of the dam gradually moves upstream with the dam axis, while the vertical settlement rate of the downstream dam tends to be stable due to its upper load.

2. Horizontal displacement. The horizontal displacement of tailings dam increases gradually with time, and the deformation rate is 0.2 ~ 0.6mm/ month, which is affected by precipitation in rainy season. In the middle and late rainy season, due to the increase of groundwater seepage pressure, the horizontal displacement rate of each point tends to increase, but after the rainy season, the saturation line in the tailings dam body decreases, the seepage pressure decreases, and the horizontal displacement rate of each point falls back, indicating that the horizontal displacement of the dam body is mainly affected by the uneven stress of the slope, and the change of groundwater seepage pressure will cause the deformation rate.

(3) Deformation under special circumstances

Because the tailings pond is located in karst area, the dam body collapses many times at the elevation of 570.00m, and an annular deformation zone will be generated in the collapsed area, which will lead to large vertical settlement and horizontal displacement of the nearby observation points, and even large deformation of the horizontal direction shifting upstream.

The fluctuation of saturation line caused by normal and abnormal drainage facilities of dam body will also increase the settlement of dam body. For example, after the large mouth drainage well with an elevation of 540 around 1998 works normally, the saturation line here will decrease, and at this time, the settlement deformation rate and horizontal displacement rate of drainage point B suddenly increase, but then gradually return to the original speed.

Fourth, the deformation mechanism analysis

The tailings pond belongs to underconsolidated soil layer, and it still needs consolidation and compaction under the self-weight stress, so that the ground slowly sinks. Its main characteristics are compressibility and consolidation.

(a) compressibility

Tailings are three-phase bodies, and the compression under load includes the compression of tailings particles, the compression of water in pores and the shrinkage of pores. In general, the compression of tailings particles and water itself can be ignored, so the compression deformation of tailings accumulation body is mainly caused by the discharge of water and air in pores.

The difference of physical and mechanical properties between tailings sand and tailings mud is the most basic factor affecting the compression index; Another important factor is the density or void ratio of tailings sand and tailings mud in the sedimentary layer. The looser or weaker the initial state, the greater the compression under load. Due to the loose accumulation of tailings, high angularity and unique grading characteristics, its compressibility is greater than that of similar natural soil. Tail light sub-cohesive layer and tail heavy sub-cohesive layer are the most important strata for tailings dam settlement due to their high clay content, poor pore connectivity, low water permeability and strong compressibility.

(2) merger

Consolidation not only causes the settlement of dam and foundation, but also controls the stability of dam and foundation, which is one of the most important engineering properties in tailings pond engineering.

According to Karl Terzaghi's theory, the time rate of material consolidation can be divided into two stages: primary consolidation and secondary consolidation.

The main consolidation controls the dissipation rate of pore pressure under constant load, and the main consolidation of tailings sand occurs rapidly, and the consolidation coefficient of tailings sand mud is in the same order of magnitude as that of natural soil.

Under constant load, even after the dissipation of pore pressure caused by primary consolidation is basically completed, secondary compression often produces continuous deformation, which is caused by continuous particle rearrangement and inter-particle sliding under load.

(3) Others

Abnormal settlement and horizontal displacement of tailings dam may occur due to the sudden change of engineering geological and hydrogeological conditions or the influence of unfavorable engineering geological conditions. As far as Yanchonggou tailings pond is concerned, the worst engineering geological condition is the abnormal deformation caused by the collapse of the dam body caused by karst.

In addition, the increase of dam saturation line caused by natural disasters such as failure of dam drainage facilities, surface water scouring, man-made destruction, earthquake and flood may also lead to abnormal deformation.

The relationship between verb (verb's abbreviation) deformation and stability

1. The settlement and consolidation of tailings will gradually improve the physical and mechanical properties of tailings, which is very beneficial to enhance the stability of upstream tailings dam.

2. In space, the settlement of the downstream of the dam is smaller than that of the upstream, and the settlement of both sides of the dam is smaller than that of the middle of the dam. Moreover, controlled by the physical and mechanical properties of tailings accumulation layer, the absolute deformation value and deformation speed will be larger where the soft layer is thick, which is an important basis for us to study the stability of upstream tailings accumulation dam.

3. With the passage of time, the deformation of the upstream tailings dam will gradually slow down, and after the tailings dam stops stacking, its deformation will gradually stabilize. In this process, any abnormal deformation may be a sign of tailings dam instability or other accidents, which should be paid enough attention to.

Conclusion of intransitive verbs

The deformation of the upstream tailings dam follows a certain law, which is closely related to the tailings accumulation law, accumulation time and hydrogeological conditions where the tailings pond is located.

In order to find out the deformation of the upstream tailings dam and provide accurate information for the management and safety evaluation of the tailings pond, it is necessary to find out the hydrogeological and engineering geological conditions of the tailings pond area, and find out the deposition law and time of the tailings, which requires the establishment of a dynamic management database and a relatively fixed evaluation model in the daily management of the tailings pond to simplify and speed up the analysis.

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