Device for Measuring Landslide Critical Angle

摘 要： The mountain landslide has high destructive effects， discussion of its landslide critical angle has always been one of the major concerns， and we designed a system that can automatically measure the landslide critical angle. This equipment consists of the box， water flow device， rotation inclined plane， rotation driving mechanism， scale and sensor. We can effectively analyze the dynamic characteristics between the water flow and the soil mass， in this way to predict the possible gradient for landslide to occur.

关键词： Experimental apparatus，Landslide，Critical angle

Preface

Landslide is a common natural disaster in the mountainous area. China has special geologic structure， its mountainous areas account for 2/3 of the national territorial area， and the severity and wide distribution of its landslide disasters are very rare in the world[1].There are four main factors that could affect the landslide stability： the geological conditions， geographic and geomorphic conditions， human activities， climate and run-off conditions. Among them， gradient is the main parameter used to describe geographic and geomorphic conditions， which can affect the soil erosion degree and method， and it is very necessary to measure the critical gradient of different underlying surfaces. Therefore， we conduct research on the critical features of soil landslide and predict the possibility of slope landslide and calculate the time of landslide occurrence through effective analysis of the dynamic characteristics between the water flow and the soil mass， which has very important theoretical significance and actual application value.

The landslide test can be used to repeatedly stimulate the whole development process of soil landscape difficult to observe in the field in the laboratory within a short time. Since the early 20th century， some West European countries have started the structural model test and gradually developed similar theories. Within China， for the research content of landslide simulation test， most scholars focus on the two aspects rainfall and underground water level research[2～8]. Liu Bo[9] used the large landslide physical model test system developed by Key Laboratory of Geological Hazards on Three Gorges Reservoir Area （China Three Gorges）， Ministry of Education， which can be used to measure the effect of rain. In order to accurately study the impact of impoundment， water level fluctuation and ground load of the Three Gorges Reservoir Region on the overall stability of Zhaoshuling Landslide， possible deformation and failure mechanism， Hu Xiuwen[10] used the high volume-weight and low volume-weight model material to stimulate the landslide process. Wen Gaoyuan et al. [11] used large indoor simulation test chamber to simulate the deformation and damage characteristics of compacted fill slope under 7d continuous artificial rainfall and follow-up 2h heavy rainfall. Hu Jinchuan el al. [12]

Therefore， by combining the actual situation， considering the impact of test method， objective and site， an indoor soil landslide test device is newly developed. The simulation test can be specifically set in accordance with the research content， which can truthfully reflect the landslide occurrence and development process， and it has three major advantages of accurate and reliable test results， big amount of information and high credibility. It has provided a new method for research on the relation between mountain landslide and slope angle.

1. Measurement principle and equipment composition

1.1 Measurement principle of landslide critical angle measuring equipment

The principle of this test is as shown in Fig.1. If the soil mass and landslide surface are static to each other， the contact place between two surfaces will form a strong binding force―static friction force （f=μN）， and one surface can only move against the other surface once this binding force is damaged. μ is the maximum static friction force coefficient， which is decided by the property， roughness and （possible） lubricant of sliding surface. When the moisture in soil mass reaches its bottoms， the soil texture of contact surface will have argillization， and the change of moisture will affect the friction coefficient.

On the slope， the force F that causes the sliding of soil mass is the component force of its own gravity in the slope direction （mg*sinθ）， and the pressure of soil onto the slope is also a component force of its gravity （mg*cosθ）. In accordance with the force of balance， we can obtain f=F， i.e.， μ=F/N. Therefore， the friction coefficient between soil and slope is μ=tanθ. The slope angle θ can be calculated in accordance with the following formula： θ=arcsin（H/L）， in which， H refers to the height of rotation inclined plane， and L is the length of rotation inclined plane.

1.2 Composition of landslide critical angle measuring equipment

Starting from the moisture infiltration characteristics at the landslide area， this design aims to discuss the mutual relation between moisture infiltration into soil and the slope surface， especially the impact of moisture change on the landslide critical angle. Therefore， this design provides measuring equipment that can automatically measure the change of soil landslide critical angle under the change of moisture change. This equipment consists of Mariotte bottle， tab ruler， hydraulic lifter， fixed tension rod， tension sensor， inductive switch， soil box， sliding table， underlying surface， camera， computer and water outlet.

2. Design and measuring procedure for main parts

2.1 Water supply device

The water supply device consists of water container and rotary speed controller. One rubber catheter with certain extensibility is used to connect the water container and rotary speed controller and adjust the speed of rotary speed controller， so that the controller could push the catheter to bring out water from the water container. It water flow can be calculated in accordance with the rotary speed：

q=（r-0.1932）/1.723

In which， q is the water supply flow rate， mm3/h； r is the rotary speed， r/min.

2.2 Hydraulic lifter

The hydraulic lifter is the dynamic device that can make the sliding table rise， which can provide stable rising speed. The hydraulic lifter consists of the hydraulic electric jack， it has height of 130cm， its maximum supporting capacity is 5t， and its highest lifting height is 430cm. Therefore， its adjustable scope is 130mm-560mm.

2.3 Height measuring instrument

The height measuring instrument uses tape with a precision of 1mm. Its overall measuring length is 5 meters， which can completely satisfy the measuring requirement of test.

2.4 Soil box

For the convenience of observing the location change of moisture flow in the soil， the soil box is made of transparent organic glass. It is certain strength and rigidity， which does not tend to break or deform during the test， and its specification is 300mm*300mm*400mm （length*width*height）. There is no seal on the upper or bottom surface： the upper surface is for the convenience of filling the water， and the bottom surface is to make the soil directly contact the sliding table， which will be convenient for the simulation of the contact between soil and rock surface.

2.5 Sliding table

The sliding table is the main part used to measure the soil landslide critical angle， which consists of two parts： the organic glass table and the holding stand.

The organic glass table has a volume of 1000mm*500mm*80mm （length*width*height）， and it is transparent for the convenience of observing the moisture infiltrating into the bottom of soil. There is also an outlet at the edge of bottom to discharge extra water within the glass table.

The second one is a weighing platform， which is used to place the steel stand for organic glass table， and it is stainless steel stand made of aluminium alloy. Considering the soil box will only be put on the top， and the box full of soil can be as heavy as 50kg， it is very difficult for a single glass table to carry its weight， so it is placed onto the steel hold to prevent the glass table from breaking due to heavy weight. Its size is the same as the base area of glass table， which is 1000mm*500mm.

2.6 Camera device

The camera device is aigoDC-v800 digital camera， which is placed right ahead of the sliding table. It is connected to a convenient computer， which can automatically transfer photo to the computer. It can be used to conduct real-time monitoring of moisture change in the soil. When the soil moisture reaches the three positions of up， middle and down， it can be used to measure the relation of soil moisture change and landslide critical angle.

2.7 Computation system

The computation system consists of the two parts of hardware and software， and the hardware part uses Lenovo K49 portable computer， which has fast data processing speed. The software uses the Soil Infiltration Rate Automatic Measuring Software independently developed by Lei Tingwu. This automatic measuring system can automatically estimate the complete process curve of soil infiltration performance changing with time in accordance with the measured surface moist surface and computation model， especially the initial high infiltration performance of soil. During the image processing process， the comprehensive distortion error correction method is used to ensure the measuring result of moist soil area has high precision， which can ensure the soil infiltration process curve is accurate and reliable.

2.8 Measuring procedure of the landslide critical angle measuring equipment

Specifically speaking， the experiment method that uses the equipment in this design to measure the landslide critical angle consists of the following steps：

（1） Fill soil into the box mentioned above， and place it onto the rotation inclined plane；

（2） Adjust the tension sensor to a tense state， or align a light sensor with the upper edge of soil box bottom；

（3） The water flow device will fill the constant water flow into the soil mass；

（4） Start the rotation driving mechanism to rotate the rotation inclined plane upward；

（5） When the moisture in soil reaches the bottom， the camera will take photos of the soil bottom for an interval of Δt （e.g.： taking continuous photos， 20 seconds/time）， and then transfer it to the computer for analysis；

（6） When the soil box slides down， i.e.， at time tn， immediately stop the continuous rotation of the rotation inclined plane， record the height of rotation inclined plane on the ruler―H， and obtain the landslide critical angle θ；

（7） Calculate the moisture infiltration rate in in soil at time tn in accordance with the following formula， （unit： mm/h）：

In which， q refers to the water supply flow of the water flow device， mm3/h； ΔAn is the increased moist area on surface during tn-tn-1， mm2；

（8） Evaluate the influence of soil moisture infiltration rate on the landslide critical angle.

2.9 Error analysis

By using the equipment and method in this design， the system errors mainly include （take the light sensor and hydraulic jack for example）： the height error caused （H） by the time delay of light sensor sending signal （0.03s） and the speed of hydraulic jack （14mm/s）， its measured height will have slight error from the actual height：

H=t*v=0.03*14=0.42mm

Note： during the actual measurement， the general risen height H is between 400-600mm， while the error H caused by system is 0.42mm， which is controlled around 1/1000. Therefore， the error of the measured value of its angle is also around 1/1000， and the error caused to the system can be ignored.

3. Test result

3.1Simulation test of the impact of soil moisture change on landslide

Researches on the change of moisture location within the slope and the impact of slope moisture infiltration ability on the landslide critical angle are conducted at the same time. In other words， when the moisture reaches the four different locations of M1， M2， M3 and M4， the sample’s infiltration rate and the landslide critical angle caused by it will be measured successively， which will be recorded in the test data.

（1） Supply of constant water flow

The supply of constant water flow is realized by using the water container， catheter and flow control device （control the water flow through the control of rotation speed）. In accordance with the test requirement， the constant water flow is set at 10.8L/h to simulate the soil moisture condition under heavy rain （2mm/min）. A linear water distributor is used to evenly distribute the water flow into the soil sample.

（2） Measurement of soil infiltration rate

Take R1-1 for example， which is the sample with a volume weight of 1.20g/cm3， and the water flow reaches the upper part.

Measurement process of soil infiltration rate： place soil sample

start the constant water supply device measure the soil infiltration rate

record and process test data.

Place soil sample： because a large amount of soil sample is used， the soil amount is high （54kg maximum） and the soil box does not have a bottom， the filling process of soil sample is conducted on a smooth landslide table.

Start the constant water supply device： after placing soil onto the landslide table， start the constant water supply device， set a rotation speed of 56r/min， and the constant water flow is 10.8L/h. Record the time.

Measure the soil infiltration rate： in the soil box， when the soil sample moisture line reaches the upper part of soil box （at 13cm）， start to measure the soil infiltration rate. First of all， connect the camera and soil infiltration rate measuring system software， and use the calibration target board to calibrate the test environment， which can be used to correct the camera deformation and the proportion and deformation of measuring interval space， so that the soil infiltration measuring system can accurately calculate the actual moist area. Secondly， properly set various parameters in the measuring system software， for example. The camera can be set to shoot one picture every one minute， it takes 10 minutes in total， and the flow parameter （0.5L/h） and camera shutter time （0.1S） can also be set. Finally， open the water supply system， and set the flow as 0.5L/h in accordance with the test requirement. In the meantime， begin timing and shooting. After completing the test， save the data， and turn off the measuring system.

Process and record the test data： during the test process， the images of soil moisture area change shot by camera will be transferred to the computer in a real-time way， and after the measurement， it can automatically identify the moist area， and calculate the soil infiltration rate in accordance with the moist area increased every minute.

（3） Measurement of soil landslide critical value

Take R1-1 for example， which is the sample with a volume weight of 1.20g/cm3， and the water flow reaches the upper part.

Measurement process of soil i landslide critical value： place soil sample

start the constant water supply device measure the critical angle

record and process test data.

Place soil sample： because a large amount of soil sample is used， the soil amount is high （54kg maximum） and the soil box does not have a bottom， the filling process of soil sample is conducted on a smooth landslide table.

Start the constant water supply device： after placing soil onto the landslide table， start the constant water supply device， set a rotation speed of 56r/min， and the constant water flow is 10.8L/h. Record the time.

Measurement of critical angle： in the soil box， when the soil sample moisture line reaches the upper part of soil box （at 13cm）， turn off the constant water supply device. Start to use the hydraulic jack to slowly lift the height-adjusting test rack （used to change the slope angle） until the soil box just starts to slide， stop the hydraulic jack， and use the ruler to measure the height of sliding table from the ground.

Process and record the test data： after completing the measurement work. Record the data in time， i.e.， the height H1 and the time. Use the measured height H and the length L of sliding table to obtain the landslide critical angle：

θ=arctan（H/L）

In which， θ is the landslide critical angle， o； H refers to the adjusted height of test rack at critical landslide， mm； L is the bottom length of test rack， mm.

3.2Test results

In this experiment， in accordance with different positions reached by the moisture peaks （the four stages of upper part M1， middle part M2， bottom part M3 and forming water film M4）， measure the slope height at the critical point of soil landslide， and calculate different gradient values based on that.

In accordance with the above diagram， we can see that when the moisture peak reaches the upper part， the average value of its landslide critical angle is 24.1°； when it reaches the middle part， the average value of its landslide critical angle is 24.7°； when it reaches the bottom， the average value of its landslide critical angle is 33.4°； after forming the water film， the value of critical angle is 18.8°. When the location of moisture peak changes from the upper part to the middle part， the angle is increased by 2.5%； when the location changes from the middle part to the bottom， the angle is increased by 35.2%； from the bottom to forming water film， the angle declined by 43.7%. During the whole change of moisture peak position， we find that when it reaches the bottom， the value of landslide critical angle is the highest； after forming water film， the value of landslide critical angle is the lowest. This is mainly because it was sand surface before， when the moisture reaches the bottom， the soil is initially moisturized， which will increase the viscidity with the slope， it will change the coefficient of sliding friction， which will increase the coefficient of sliding friction between the soil and slope， and the critical value will increase. This proves that this set of test equipment has great feasibility.

4. Conclusion

By starting from the moisture infiltration features at the landslide area， efforts are made to discuss the relation between soil moisture infiltration and slope， especially the impact of moisture change on the landslide critical angle. A set of measuring equipment that can automatically measure the change of soil landslide critical angle caused by moisture angle is designed， which can effectively analyze the dynamical features between the soil water flow and its soil mass， which can be used to predict the possibility of landslide and estimate its time.

During the actual measurement， the risen height H in indoor experiment is generally around 400-600mm， while the error caused to system―H is 0.42， so the error value is controlled around 1/1000， and the error of the measured value of its angle is also around 1/1000.

This experiment equipment has a low cost， the prototype test shows that this equipment has smooth operation and stable performance， but it requires a complicated procedure and a large amount of computation after recording the data， so certain error is inevitable. In addition， it has a small application scope. Because the field landslide factors are complicated， therefore， it requires further studying the estimation algorithm for landslide critical angle under external factors to improve the measurement precision. In the future， this equipment will develop toward the direction of intelligence and automation. In other words， a set of program will be designed to ensure that the computer can automatically record and analyze data and obtain scientific conclusion. A database for landslide critical angle measurement should also be built， so that the operator of this equipment can choose whether to upload the data to the data platform， so that the local data can form a time sequence， and the landslide can be more effectively predicted， which will make the academic researches more convenient.

References

[1] Liu Chuanzheng. Research on the Cause of Geological Hazard at the Three Gorges Reservoir Area of the Yangtze River and Its Evaluation[M]. Beijing： Geological Publishing House， 2007.03.

[2] Jiao Bintian， Lu Xiaobing， Wang Shuyun. Impact of Different Moisture Content and Density on the Landslide Startup[R]. The Chinese Society of Theoretical and Applied Mechanics. 2005 （CCTAM2005）.08.

[3] Fu Helin， Li Changyou， Guo Feng， Zhou Zhong. In-situ Test of Landslide Triggering Factors and its Impact[J]. Journal of Central South University （natural science edition）， 2009， 40（3）： 781-785.

[4] Zhan Liangtong， Wu Hongwei， Bao Chenggang， Gong Biwei. In-situ Monitoring of Unsaturated Expansive Soil Slope under Rainfall and Infiltration Conditions[J]. Rock and Soil Mechanics， 2003， 24（2）： 151-158.

[5] Hu Jinchuan， Xie Yongli， Wang Wensheng. Stair-step Loess Slope Stability Test under Rainfall Conditions[J]. Journal of Guangxi university （natural science edition）， 2010，35（1）： 83-89.

[6] Zhang Junfeng， Wu Xiangyao， Zhu Erqian. Experiment and Research of Layered Slope Landslide Caused by Water Level Change[J]. Chinese Journal of Rock Mechanics and Engineering， 2004，23（16）：2676-2680.

[7] Jia Guanwei， Zhan Liangtong， Chen Yunming. Model Test and Research of the Impact of Sharp Water Level Decline on the Slope Stability[J]. Chinese Journal of Rock Mechanics and Engineering， 2009，28（9）：1795-1803.

[8] Luo Xianqi， Liu Defu， Wu Jian， et al. Landslide Model Test and Research under the Rainwater and Reservoir Water[J] Chinese Journal of Rock Mechanics and Engineering， 2005，24（14）：2476-2483.

[9] Liu Bo， Luo Xianqi， Zhang Zhenghua. Model Test and Research of Qianjiangping Landslide at Three Gorges Reservoir Region[J]. Journal of China Three Gorges University （natural science edition）， 2005， 29（2）：124-128.

[10] Hu Xiuwen， Tang Huiming， Liu Yourong. Stability Physical Simulation Test and Research of Zhaoshuling Landslide at Three Gorges Reservoir Region[J]. Chinese Journal of Rock Mechanics and Engineering， 2005，24（12）：2091-2095.

[11] Wen Gaoyuan， Yao Pengyun， Zeng Xianming， et al. Failure Model Test and Research Compacted Fill Slope before and after the Rainfall[J]. Chinese Journal of Rock Mechanics and Engineering， 2005，24（5）：747-754.

[12] Hu Jinchuan， Xie Yongli， Wang Wensheng. Stair-step Loess Slope Stability Test under Rainfall Conditions[J]. Journal of Guangxi university （natural science edition）， 2010，35（1）： 83-89.

作者简介：李雪菱（1992-），女，研究生，主要从事水土保持方面研究

通讯作者：夏卫生，（1966-），男，教授，主要从事水土保持和土地资源管理方面研究。