(4)
Zhang Baosen Wang Zili
Institute of Hydraulic Research, Yellow River Conservancy Commission,
Zhengzhou 450000, China
Cui Jianzhong Zhang Xiquan
Yellow River Henan Bureau, Yellow River Conservancy Commission,
Zhengzhou 450000, China
Abstract: It is
difficult to research the developing mechanism on forming levee leak in the
Yellow River due to shortage of data used for reference. Some viewpoints are
given by experimental data analysis of prototype observation on leakage blocking
practice in situ, combining with the theories of geo-technique, hydraulics and
structural mechanics etc. The primary experimental analysis and research on the
developing mechanism of leak formation have been carried out in order to make
out the scheme of dealing with an emergency and leakage blocking for reference.
Keywords: levee leaks, prototype observation, dynamic pressure, velocity
The deep-water leak of river levee is one of the most serious, dangerous and difficult to repair conditions. It is easy to take place dike breach without repairing in time. Because of the Yellow River’s levee with amounts of sandy soil, big suspended difference by levee outside and various hidden perils, It is common occurrence that the samples of Yellow River’s dike breach have taken place due to hidden trouble itself to form leak in history.
For this reason, Yellow River Conservancy Commission pays great attention to the leakage blocking technique research. A number of hydraulic staffs have created lots of feasible methods in leakage blocking of levees for a few years. In order to deeply study the leakage blocking technique in deep water and accumulate practical experience, the forming and developing mechanism of Yellow River levee’s leak has been primarily studied according to Yellow River Conservancy Commission’s guide lines for starting practically, paying attention to research, accumulating the practical experience, training teams through leakage blocking experiment, summing up to make out some practical plans suitable to leakage blocking of Yellow River’s levees, improving to deal with emergency technique, combining with 2000’s flood control practice.
This practical experimental site is chosen in Yangqiao damaged works of Zhongmu County at the south bank of Yellow River. The reservoir levee is basically built comparison with the Yellow River’s levees. The soil texture is silty clay, the width of levee top is 7m, the slope by levee outside is 1:2, the levee’s height by riverside is 5m, its water depth is 4.5m. Through burying Ф150mm, Ф100mm, Ф50mm galvanization steel pipes in advance, the leak is formed by tractor to pull them out when practicing.
While forming the leak, the flow inside leak is pressure one, the outflow is free one, so the velocity of inside leak can be calculated according to the formula of free outflow in atmosphere of simple pipes,
V=μ(2gH0)0.5 (1)
In which:μ- pipe discharge coefficient after corrected. μ is related to the soil texture, compactness, leak length, leak diameter of levee stage etc confirmed by further experiment; H0 – height between the center of leak outlet and water surface by river side.
It can be seen from the formula above that the velocity inside leak is mainly depended on the height of leak outlet, without direct relation to inlet location. For Yellow River’s levee with big difference by levee outside, the leak outlet is usually lower, the outflow velocity is higher, so we must follow the principle with trapping leakage blocking by river side and conducting seepage and reducing pressure by levee outside. Otherwise it is no use for filter pressure reduction in outlet.
When the leak becomes a biggest one into free flow, it is burst virtually. The formula of velocity with burst is:
V=2/3(gH0)0.5 (2)
Where: H0–total head difference; g– acceleration of gravity.
If the leak inlet is covered by something (such as soft curtain), its pressure is:
P=rHjω (3)
In which: Hj– height between leak inlet center and water surface; ω– area of leak inlet；r – specific gravity in water.
When Hj is over 2.5m, the pressure is very high, which can usually make low intensity soft curtain with low intensity deformed, even broken. So the soft curtain must be chosen material with high intensity.
It is known that the leak can be produced the suction force, which the size of its value is directly related to the leakage blocking difficulty, but there is no accurate calculating method yet at present. Here the analysis and calculation are carried out by Bonorry’s equation approximately. Fig.1 is a sketch-map of levee leak, taking the river bottom as datum level. The energy equilibrium equation of section1-1 and 2-2 is listed as follows:
(4)
Where: H–total water depth; α1 ,α2 – corrected coefficient of kinetic energy, which is related to uniformity of velocity distribution on the section of leak. The less uniform the velocity distribution is, the higher the value is, which will be confirmed by experimental study further; V1 of section 1-1 section–approach velocity, which can be ignoring; h–height between the computing point inside leak and datum level; P2–intensity of pressure at computing point inside leak;V2–velocity at computing point inside leak; ζ–the local water head loss coefficient in leak inlet.
Fig.
1 Sketch map of levee’s leak
Formula (4) can be transformed into:
(5)
The left of formula (5) is the pressure difference between outside and inside leaks; the right is the function of velocity inside leak, in which its value is positive. It is shown that the pressure inside leak is always less than that outside leak. Thus the suction force is produced. The bigger the right items, the bigger the pressure difference, the larger the suction force. In actually, when the leak inlet is covered by soft curtain instantaneously, the incoming flow is cut suddenly, the instantaneous negative pressure can be produced inside leak, which has stronger suction on the soft curtain. So this point should be fully considered for choosing the materials of soft curtain. The larger the suction inside leak, the stronger the vortex in leak inlet produced.
Because there is the pressure difference between outside and inside of leaks, the suction has been produced in the leak. This suction can lead to the vortex appeared in the inlet. It is known from formula (5) that the crucial factor of the suction is velocity inside leak. It is known from formula (1) that the height of leak outlet mainly decides the size of velocity, the relation to inlet location as secondary under the condition of atmosphere flowed out. Then it can be deduced that there is few difference of suction under the same condition of velocity, the vortex’s intensity brought should be basically accordant. Thus it can be explained that why it is easy to find vortex in shadow water than in deep one.
The measured principle and tracing method with dye are shown. The solid salt is very easy to dissolve in water, the Na+ ion and Cl- ion dissolved in NaCl solution are very active, their electrical conductivities are very good to be measured.
The automatic data collecting system with computer is used in this measurement, composing of notebook PC, pH-206 ion measuring instrument，ion choosing sensor，cable and salt tracer, pressure source, etc.
It is shown from the
observed result of 9# leak in 2# basin that during the initial period
of leak development, that is the leak diameter below 50mm, the velocity is below
1.6m/s when the water-depth is up to 3.5m, the speed of leak development is
slower. It is known from the observed result of 4# leak in #3 basin
that the average velocity in leakage is higher although the water-depth is only
3.0m, its measured velocity in the beginning is up to 2.72m/s if the leak
diameter is above 100mm. But 10# leak in 2# basin of diameter with
100mm seems certain special, its beginning velocity is 1.14m/s. It is analyzed
from the reason that the bottom drainage pipe near the east of 10#
leak has taken place the seepage, in which leakage-blocked by piling up soil,
which its range effects on 10# leak inlet during the process of
storing water in 2# basin. Therefore, the measured velocity is lower.
The velocity observation of 9# leak, 10# leak in 2# basin and 3# leak, 4# leak in 3# basin is carried out by combining with leakage blocking. Because the surrounding well for pressure reduction is made in the inlet of 3# leak in 3# basin beforehand, the velocity has not been measured after opening the leak. The average velocity of the rest 3 leaks measured is between 1.10~1.5m/s or so, the velocity inside leak is reduced step by step with the enforcement of leakage blocking scheme. It is shown that the further increase of velocity has been basically controlled and further development of leak has been prevented by varieties of leakage blocking schemes. It is shown from the measuring results of leak diameter that the leak diameter is not changed basically. It can be judged that the scheme of leakage blocking is successfully under the condition of levee’s soil texture.
An experiment of leak development up to burst was carried out in 3# leak in 1# basin on June 25. During the experiment, the observation along the whole distance for the velocity inside leak, the expansion of leak diameter and variation of water stage in basin etc. has been enforced.According to observation in site and analysis on the changeable process of velocity, water stage and leak diameter, the developing process after the leak is formed under the condition of soil texture can be approximately divided into 3 stages:
Stage 1: slow developing stage of the leak (10:54~13:15). Stage 2: rapid expansion stage of leak diameter (13:15~13:55). Stage 3: burst stage (13:55~14:01).
It can be seen from the leak developing process and law of velocity variation mentioned above that under the condition of certain water depth, the soil texture and degree of compaction are depended on extend speed of leak diameter; the higher the content of clay，the larger the degree of compaction, the slower the speed of leak diameter extension, otherwise, the faster (during the leakage blocking practice in 1999, because the soil texture was sandy soil, it took only ten minutes from the leak formed to burst); the leak diameter and difference of water head are depended on velocity inside leak. The maximum velocity of leak takes place at an instant from semi pressure flow to free surface flow. Because the radar velocity meter is set up on the bulldozer not far from the bursting under levee, in case of accident, the bulldozer is taken away from the site before bursting 1 minute, the maximum value (4.4m/s) is only measured.
The discharge coefficient μis calculated by taking the velocity and water-depth of different stages into formula (1):
μ= 0.1~0.2 (when the leak diameter≤5cm);
μ= 0.2~0.3 (when the leak diameter is 5~10cm);
μ= 0.3~0.4 (when the leak diameter is 10~25cm);
μ= 0.4~0.6 (when the leak diameter is 25~50cm).
There are two groups of each leak (diameter:Φ50mm,Φ100mm) and 4 sections (9# leak, 10# leak in 2# basin and 3# leak, 4# leak in 3# basin) chosen.3 sensors of water pressure with 0.1Mpa and 1 sensor of soil pressure with 0.1Mpa are buried on each section. The arrangement of dynamic water pressure observed devices and sensors burying are shown in Fig.2. It is noted that the permeable stone should be taken away from sensors when hole pressure meter is buried by 3# basin. In fact the pressure meter has played a role on soil pressure meter.
After burying pressure sensors, they should be measured at once and filled initial measured value into the textual research stable. During the construction period of closing levee or storing water, measuring one time in one or two days. In normal experimental stage, after the steel tube is pulled out to form the leak, the 4 pressure sensors of this section should be observed continuously at the same time by XP98 vibrating wire frequency measurer, The observed interval is 10~20s primarily.
The results of pressure observation during the construction are shown in Fig.3 and Fig.4. The results of dynamic water pressure observation shown in Fig. 5.
Fig. 3 Curve of pressure observation in 9# leak of 2# basin during the construction
Fig.4 Curve of earth pressure observation in 3# leak of 3# basin during the construction
Fig. 5 Curve of dynamic pressure observation in 9# leak of 2# basin during the experiment of leakage blocking
(1) It can be seen from Fig.4 that the results of earth pressure observation during the construction accord with the law of earth pressure distribution of levee. It is shown that the precision of measurement range of sensors chosen and burying method can meet the demands of design.
(2) The observation of dynamic water pressure in leak is observed by combining with leakage blocking process, the changeable law in the process of leak expanded and developed further has not be observed with shorter observation time.
(3) The observed data won is difficult to meet the demands of analyzing leak development’s mechanism because of fewer series of observation with dynamic pressure.
(4) It can be known from A1 curve in Fig.5 that dynamic pressure in leak inlet is maximum and reduces along the direction of leak outlet gradually under the condition of natural development of leak (without blocking leak ago). The pressure reduces rapidly after air shutoff, therefore its effect of leakage blocking is known from the pressure variation. After the closing well of pressure reduction outside river is built up, the pressure inside leak is very high but the velocity is very low.
Though some results have been won, they can not meet the demands of analysis and research on mechanism because the observation related to parameters is carried out in situ by first time and limited by the operating condition of leakage blocking during the development process of levee’s leak in the Yellow River.
The following conclusions can be obtained by experimental analysis mentioned above:
(1) The sensors must be buried after the digging height is more than 30~50cm. The sensors are easily damaged or cables broken by heavy machinery operation and by man-made damage if they are buried directly according to construction to designed elevation.
(2) It is suggested that the vibrating wire frequency measurer with the proper range of measurement is used for the observation of dynamic pressure, it is fixed by angle iron when burying, which compression face is parallel to tunnel wall. There are 4 sensors buried in each leak, which their locations are far 2m and 7m from the inlet separately, 5m from levee axis and outlet. The observed interval is 10s ideally.
(3) It is effective to adopt tracing measuring method for velocity observation in this experiment. It is suggested that the measurement should be used partly when observing in the future. The measuring location should be same with that of dynamic pressure sensors in order to observe the velocity distribution in the whole leak.
(4) It is certain feasible to use the method of resistant multiplex sensor buried for leak diameter observation during this experiment and its primary result has been achieved. It is suggested that the one-way resistant multiplex sensor is transformed into net shape resistant sensor, which can be used to measure the changeable process of leak diameter.
(5) The changeable features of dynamic pressure. The dynamic pressure is mainly related to water-depth, location of leak inlet and outlet, and sediment concentration, i.e. the deeper the water-depth, the lower the location of leak inlet and outlet, the bigger the dynamic pressure. The larger the sediment concentration, the bigger the pressure. When the dynamic pressure in leak inlet is maximum, it is reduced gradually along the direction of leak outlet to zero near levee axis. The flow pattern in initial leak belongs to pressure flow, with the development of hydraulic drop, its flow of outlet section is changed into free flow step by step, and the section of free flow is longer and longer. The flow is all changed to free flow up to bursting.
(6) Magnitude of velocity and its distribution. The magnitude of velocity inside leak is mainly related to water-depth, location of leak outlet position, leak diameter, soil texture, leak length etc. Under the conditions of other factors unchangeable, the deeper the water-depth, the lower the location of outlet, the higher the velocity. The smaller the leak, the lower the velocity; the larger the diameter, the higher the velocity. The velocity distribution inside the leak is non uniform, the velocity in leak inlet is lower and it increases along the leak direction. The velocity is biggest when the pressure flow is transformed into free flow.
(7) The variation process of leak diameter. The both sides of leak inlet and outlet are developed first, the leak inlet is extended upward with suction damage, and the outlet (Fig.7) is extended downward with scouring damage, the collapse crown is expanded with horizontal scouring damage.
(8) The changeable relationships between dynamic pressure and velocity. It can be seen from Fig.5 that A1>A2>A4, the dynamic pressure is maximum near leak inlet and minimum at leak outlet. It can be known that the dynamic pressure inside leak is reverse to velocity development, i.e. the dynamic pressure is maximum and velocity is minimum. The pressure energy is transformed into kinetic energy gradually along the direction of leak outlet, the velocity is maximum near levee axis. The speed of leak development is mainly related to the magnitude of velocity, soil texture, leak length and leak diameter. The soil texture is mainly including as clay content of soil, compaction quality of levee and water ratio.
(9) The process of leak development is a scouring one actually. At the outlet, the slope increases suddenly, the flow is very easy to form hydraulic drop and produce retrogressive erosion, and then gradually extend towards leak inside. The soil mass in both sides of leak is expanded by scouring, the top of soil mass is empty scoured loses stability to be collapsed by its own weight due to the bottom of soil mass is scoured empty. The collapsed soil mass is carried to downstream by flow, which the outlet’s shape is an oblate one, and is mainly developed downward. At the inlet, the angularity of initial leak inlet pattern is obvious, according to the principle of minimum energy loss, the part of angularity will be scoured first by the flow, and made it be suitable to flow streamline. Soil mass at inlet’s top is very easy to be collapsed and developed rapidly by its own weight, flow scouring and water pressure, its developed speed is faster than the downward one which is mainly limited by the outlet’s erosion datum and slowly. So the leak inlet develops upward mainly to form the flaring pattern with large outside and small inside gradually. At the beginning of leak development, its diameter is small, the resistance formed by uneven leak wall is relative large, adding the larger local resistance of initial leak inlet and longer length of leak to make water head loss largely, thus the velocity and suction inside leak are small. With the continuous expansion of leak diameter, the uneven leak wall can be scoured smoothly and make its relative resistance reduced by increase of leak diameter and its local resistance reduced by scouring of leak inlet while the velocity and section inside leak will be increased rapidly.
As mentioned above, after forming the leak, the vortex near inlet by river is caused to produce the larger section, taking the soil particle around leak’s inlet into the leak and away by flow. In the initial period of leak formation, the velocity inside leak is low relatively. When leak diameter is small, the uneven leak wall makes flow forming a series of vortex separated with energy dissipation to reduce the velocity inside leak. The leak body is scoured and destroyed along the contacted face with leak wall, which makes leak diameter expand unceasingly, the bigger the leak diameter, the higher the velocity and the faster the leak development. When the leak extends to a certain extent, the leak has lost an arching, the levee begins to be collapsed and destroyed by crack. Because the levee surface by riverside is immersed by water and easy to be scoured by flow, it is destroyed speedily. At the leak outlet near river outside, the soil particle at the leak bottom will be scoured by flow first and aggraded in concave-down place of slope foot along the slope surface’s movement downwards. The local hollow or rill is formed on slope surface step by step, and destroyed till slope surface downstream. The hydraulic drop is formed in leak outlet, the damage of outlet’s levee surface is accelerated, making the outlet move toward leak inside, the bottom erosion, both sides’ scouring and top collapse are main damaged modes. The changeable process of leak diameter is: to develop both inlet and outlet of leak first, inlet development upward with suction’s damage; outlet development downward with scouring damage, horizontal scouring damage and top collapse expansion.