NL2030211B1 - Method for monitoring stress gradient of concrete dam - Google Patents

Abstract

The present invention discloses a method for monitoring stress gradient of a concrete dam. The method comprises: 1. calculating the dam body principal stress distribution condition of a key part through monitoring data of a multi-directional strain gauge set arranged at the key part of the concrete dam; 2. arranging a plurality of stress monitoring sensors in the drilling hole in a serial connection manner; 3. calculating the spatial position coordinate value of each stress monitoring sensor from an opening of the drilled hole to the hole bottom; and 4. monitoring in real time according to the dam working condition, selecting actually-measured data for comparison and calculation, and acquiring a principal stress gradient path and a corresponding principal stress gradient value.

Description

METHOD FOR MONITORING STRESS GRADIENT OF CONCRETE DAM

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates to the technical field of water-power engineering, and more particularly to a method for monitoring stress gradient of a concrete dam.

[0003] 2. Description of Related Art

[0004] In recent years, concrete dam construction is developed rapidly, and 96 high dams with the height of 200 m or above are built in the world. The technical difficulties of various types of high dams and extra-high dams have exceeded the current standards, and no previous experiences are available for reference, so it brings uncertainty to the dam construction, and challenges are put forward for more effectively improving the safety management and control capacity and level of the high dams in the operation period. The operation safety problems of the high dams and the extra-high dams need to focus on the following points:(Jthe dam foundation and dam body structure bearing capacity, the stress distribution and gradient of key parts of dam bodies, etc. under the condition that the reservoir water thrust borne by the dams reaches up to ten million tons; @ the dam body stress redistribution under the action of strong vibration loads, the dam foundation deep and shallow layer structure face deformation, and the dam body transverse joint opening degree, etc. under the condition that dam sites are located in geological structure active areas, the seismic intensity is high; 3 the dam body material strength and durability under the loading-unloading reciprocating action under the condition that the high dam reservoir inter-year water level lifting amplitude is large, and the geological weak structure working conditions of the dam foundations and dam shoulders, etc. after reinforcement; and @ the influence of long-time high-power flood discharge on dam body vibration under the condition that the extra-high dams are provided with multiple dam body orifices, the discharge fall is large and the flow speed is high, and dam body holes, flood discharge energy dissipation facilities, etc. The stress is a physical quantity most sensitive to reflecting the mechanical property of the dam construction structure, that the stress exceeds the standard is the sign of internal cracks or local damage of the concrete dams. The dam body cracking will be caused by the tensile stress exceeding the standard, the dam body slippage is caused by the shear stress exceeding standard, and the local damage of the concrete dams can always have evidence in stress monitoring data. Therefore, stress monitoring is indispensable for evaluation of the dam safety.

[0005] The traditional stress monitoring instruments mainly comprise compressive stress meter, stress-free meter, unidirectional strain gauge, multi-directional strain gauge set, etc. According to the design solution, the traditional stress monitoring instruments are distributed on the main dam monitoring section in the vertical direction and the main dam monitoring section in the horizontal direction in a discrete single-point mode, so the internal stress of large-volume concrete of the dam is monitored, but the continuous stress distribution gradient of key parts of the concrete dam cannot be directly and continuously monitored; or due to the fact that the monitoring device is complex in structure, the method is tedious in step, implementation cannot be conducted in the construction period or the operation period, then the stress distribution and gradient change conditions of the key parts of the concrete dam under various working conditions cannot be found in time, and therefore support is provided for diagnosis of potential internal cracks or local damage.

BRIEF SUMMARY OF THE INVENTION

[0006] Objective of present invention: the present invention aims to solve the problems that the internal stress gradient of the current concrete dam can only be speculated through interpolation calculation through values of measured point which are distributed discretely and have limited space in the dam, and on-line accurate monitoring cannot be performed directly to obtain the continuous change condition of the stress gradient distribution of key parts of the concrete dam. The present invention aims to provide a method for continuously monitoring stress gradient of a concrete dam. The method is simple and feasible, can be implemented in a construction period or an operation period, and further realizes continuous monitoring of the stress gradient of the concrete dam to discover the stress distribution and gradient change condition of the key parts of the concrete dam under various working conditions.

[0007] Technical solution: a method for monitoring stress gradient of a concrete dam comprises the following steps:

[0008] (1) calculate the dam body principal stress distribution condition of a key part through monitoring data of a multi-directional strain gauge set arranged at the key part of the concrete dam, and solve the principal stress and the direction;

[0009] (2) drilling in the concrete dam according to an inclination angle and an azimuth angle converted according to the principal stress direction of the key part, and arranging a plurality of stress monitoring sensors in the drilled hole in a serial connection manner;

[0010] (3) calculating the spatial position coordinate value of each stress monitoring sensor from an opening of the drilled hole to the hole bottom by taking the coordinates of the opening of the drilled hole as an original point and through the inclination angle and the azimuth angle in the drilling direction and the unit length of the stress monitoring sensors arranged in a serial connection manner, and enabling the spatial position coordinate value to correspond to the number of each stress monitoring sensor; and

[0011] (4) after the stress monitoring sensors in the drilling hole are buried and enter a stable working state, monitoring in real time according to the dam working condition, selecting actually-measured data for comparison and calculation, and acquiring a principal stress gradient path and the gradient value of the corresponding principal stress are obtained.

[0012] Further, in the step (1), firstly, free dependent variables in the monitoring data of the multi-directional strain gauge set are removed, balance check and distribution of the multi-directional strain gauge set are performed, and then measuring point strain is converted into uniaxial strain; and normal stress is calculated by using the uniaxial strain, and shear stress is solved through the normal stress; and finally, a stress matrix comprising the normal stress and the shear stress is solved, and principal stress and the direction of the principal stress are obtained.

[0013] Further, in the step (2), firstly, an azimuth angle and an inclination angle of the principal stress direction of the key part of the concrete dam in the step (1) are solved, the hole is drilled in the concrete dam by adopting a geological drill, then the plurality of stress monitoring sensors are arranged in the hole in a series connection manner, the arrangement of the stress monitoring sensors meets the principle that the monitoring range covers the whole hole depth of the drilled hole, and the stress monitoring sensors are not connected and fixed.

[0014] Further, the outer side of each stress monitoring sensor is provided with a protective sleeve, two ends of the protective sleeve are provided with distance measuring extension rod members, adjacent stress monitoring sensors are connected end to end through the distance measuring extension rod members at the two ends of the protective sleeve, and the series connection length of the plurality of stress monitoring sensors is adaptive to the full hole depth of the hole drilled in the concrete dam. Preferably, the distance measuring extension rod members are made of a metal material with the same strength, rigidity and elastic modulus as the protective sleeve.

[0015] Further, in the step (3), firstly, the space coordinates of an opening of the drill hole are measured by use of an attached lead method, and the space coordinates of the geometric center position of each stress monitoring sensor are calculated through the inclination angle and azimuth angle of the drilling direction and the unit length of the stress monitoring sensors arranged in the series connection manner and by taking the space coordinates of the drill hole as a base point.

[0016] Further, the step (4) specifically comprises the following content:

[0017] (4.1) selecting a group of actually-measured data, selecting a measuring point op: with the maximum stress measuring value in series connection sensors, taking the measuring point o9: as an original point, taking the instrument length d of each stress monitoring sensor as a single search distance, taking the original point as a starting point, and respectively carrying out measuring value comparison on adjacent stress monitoring sensors which are arranged on the two sides in a series connection manner until the last stress monitoring sensor at the two ends is reached; and

[0018] (4.2) according to the comparison result, connecting coordinate values of the sensors along the line with the stress sequence decreasing as a main part to obtain a principal stress gradient path, and taking the stress measuring value corresponding to the spatial coordinate value of each stress monitoring sensor as a gradient value in 5 the principal stress direction.

[0019] Further, in the step (4.1), the content of taking the original point as a starting point, and respectively carrying out measuring value comparison on adjacent stress monitoring sensors which are arranged on the two sides in a series connection manner is as follows:

[0020] The forward search distance along the original point reaches md, the results of the m times of search comparison processes are all oc >0 ., i"=0,1,2,......m, m represents the number of times of search in the direction from the original point to o; =;; the reverse search distance along the original point reaches nd, the results of the » times of search comparison processes are all of >o7:1, 7i=0,1,2,.....M, n represents the number of times of search in the direction from the original point to o7-; wherein op =o9 =oy if md>3nd, the stress decreasing sequence of the forward search distance of md is the principal stress gradient direction; and if nd>3md, the stress decreasing sequence of the reverse search distance of nd is the principal stress gradient direction.

[0021] Preferably, daily value measurement fluctuation and random work abnormity of the stress monitoring sensors are considered, the search distance in the direction from the original point to o; .reachesmd, the search distance in the direction from the original point to oj. jreachesnd, and md=>3nd; the results of the mtimes of search comparison processes contain / times of o; >; .; and j times of oi <6; +5, i=0,1,2,...., m=htj, if h215j, the stress measurement values corresponding to jtimes of search are not brought into calculation, and the stress decreasing sequence of the forward search distance of meis still the principal stress gradient direction; if ?=j, it is considered that no significant gradient rule of stress distribution is formed in the forward search distance of md.

[0022] When nd>3md, in the results of # times of search comparison processes,

there are p times of a; >; +7and g times of oi <oi +, 7 =0,1,2,..... .n=p+q; if p >1.5q, stress measurement values corresponding top times of search are not included in calculation, the stress decreasing sequence of the reverse search distance of nd is still the principal stress gradient direction; if p=g, it is considered that no significant gradient rule of stress distribution is formed in the reverse search distance of 1d.

[0023] Further, in the step (2), the hole is drilled in the key part of the concrete dam in the principal stress direction, and after stress monitoring sensors are installed in a series connection manner, the step of backfilling cement paste in the drilled hole is included.

[0024] Benefit effects:

[0025] Compared with the prior art, the present invention has the following remarkable advantages: 1. the method is simple and easy to implement, can be implemented in the construction period or the operation period, meets the requirement for continuous monitoring of the concrete dam stress gradient, finds the stress distribution and gradient change conditions of the key part of the concrete dam under various working conditions in time, and provides support for diagnosis of potential internal cracks or local damage; 2. high-precision stress monitoring sensors with two ends connected with distance measuring extension rod members in a bolted mode are arranged in a series connection manner, so stress gradient monitoring can be well carried out on stress distribution parts such as a dam heel, a dam toe and the upper portion of a dam body of the concrete dam, the continuous actually-measured data of the stress gradient distribution rule in the concrete dam are realized, and therefore a solution is provided for truly mastering the internal stress and the gradient distribution rule of the internal stress of a large-size concrete structure project represented by the concrete dam.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0026] FIG.1 is a flow chart of a dam body principal stress distribution calculation method based on monitoring data of a multi-directional strain gauge set;

[0027] FIG.2 1s a structural diagram of a five-directional strain gauge set monitoring device instrument of the embodiment of the present invention;

[0028] FIG.3 is a series connection structure of a plurality of stress monitoring sensors in a hole drilled in a concrete dam;

[0029] FIG.4 is a partial enlarged drawing of stress monitoring sensors in the 5 FIG3;

[0030] FIG.5 is a layout schematic diagram of a device for continuously monitoring principal stress gradient of a concrete dam; and

[0031] Fig.6 is a flow chart of a calculation method of a principal stress direction and corresponding gradient value.

DETAILED DESCRIPTION OF THE INVENTION

[0032] The technical solution of the present invention will be further described in detail below with reference to the embodiments and the drawings.

[0033] A method for monitoring stress gradient of a concrete dam comprises the following steps:

[0034] Step 1, calculating the dam body principal stress distribution condition of a key part of a concrete dam through monitoring data of a multi-directional strain gauge set installed at the key part, and solving the principal stress direction. The specific implementation mode is as follows:

[0035] a. as shown in FIG.5, continuous monitoring is carried out by using the multi-directional strain gauge set 8 installed and buried at the key part of the concrete dam, the actually-measured data of the dam concrete strain of the corresponding part are acquired, and then actually-measured data s'= s- sy of free strain monitored by a stress-free gauge in the strain gauge set are removed, wherein s' represents 23 each-directional positive strain obtained by deducting stress-free strain and is 10%;5 represents each-directional strain gauge measured value of the strain gauge set and is, 10°: and sy represents measured value of the stress-free gauge and is 10%.

[0036] The free strain is deformation such as thermal expansion and contraction, wet expansion and dry contraction and self-generated volume caused by temperature and humidity change and cement hydration, etc. under the condition that concrete is not subjected to external force.

[0037] b.balance check and distribution are carried out on the multi-directional strain gauge set : a five-directional strain gauge set is taken as an example (shown in

FI1G.2), an unbalanced distribution amount Asi is distributed to each strain gauge to minimize the total error, namely As;=As;3=-d/4, As;=Ass=d’4, according to the actual condition that the sum of strains in three mutually orthogonal directions of one point in an elastic mechanical space is a constant sj+s3+s5=s2+s4+s5 and an unbalanced amount d=si+s3_S2+s exists in consideration of the influence of external factors.

[0038] c. the strain of a strain gauge set measuring point in a complex space stress state in the dam body of the concrete dam 1s converted into uniaxial strain:

Sp = Eels gv gle, va, tE VO Al - Ze)

[0039] wherein, &p represents the positive strain obtained by deducting stress-free strain from each direction of the strain gauge (set) and is 10%: eo represents the uniaxial strain corresponding to & and is 10%; and x represents a

Poisson ratio. 10040] d. by using concrete elastic modulus and creep test data, according to the uniaxial strain s’, the positive stress in each direction is approximately calculated by using a deformation method, the stress at the 7, moment IS:

ERTS Tae) Ace YOR EPG pee; , LEAS iE .

[0041] e. according to elastic mechanics, the positive stress calculation formula on any oblique section is as follows: ay =o di bo mt ton +2 Ime 20 mn 20 nl ;

[0042] The shear stress is solved as follows: i Ll, Lo

Ee Tie, + TL) TT. = C0, Tie, PO) OT, FO zie, +} 2 \ 2 , 2

[0043] f. a stress matrix is solved to obtain three characteristic roots oy, ©, 3, namely three principal stresses, wherein characteristic vectors corresponding to characteristic values represent the principal stress directions.

for Toy r, = le}: | 7, o, T,.

IT, en i

[0044] Step 2, determine the drilling depth according to an inclination angle and an azimuth angle converted from the principal stress direction of the key part and the involved given stress gradient monitoring range, drill a hole in the dam according to the structure and design requirements of the concrete dam, and arrange high-precision stress monitoring sensors with two ends bolted with distance measuring extension rod member in the drilled hole in a series connection manner according to the drilling depth so as to form a series connection monitoring instrument. The specific implementation mode is as follows:

[0045] a. the azimuth angle and the inclination angle of the principal stress direction of the key part of the concrete dam are solved according to the step 1, the hole is drilled in the concrete dam by adopting a geological drill, and the drilling part and the drilling route are subjected to design rechecking to avoid the original position for embedding the monitoring instrument and monitoring cables.

[0046] b. the hole must be cleaned and swept after being drilled to ensure that no slag and cement slurry exist in the hole.

[0047] c. the high-precision monitoring instrument is arranged in the holes and meets the principle that the monitoring range covers the whole hole depth of the drilled hole, and the sensors are arranged in a series connection manner and are not connected and fixed.

[0048] d. the high-precision stress monitoring instrument 5 is formed by connecting a plurality of high-precision stress monitoring sensors 1 in series, wherein the high-precision stress monitoring sensors 1 are not directly connected and fixed.

[0049] Specifically, as shown in FIGs,3-4, a protective sleeve 101 is arranged on the outer side of each high-precision stress monitoring sensor 1, and distance measuring extension rod members 2 are installed at the two ends of each protective sleeve 101; a plurality of high-precision stress monitoring sensors 1 are arranged end to end in the hole 4 drilled in the concrete dam, and the sensors arranged in the hole meet the principle of covering the whole hole depth of the drill hole.

[0050] In order to further expand the measuring range of the sensors and reduce the quantity of the sensors arranged in the hole to reduce cost, the distance measuring extension rod members 2 connected to the two ends of each sensor in a bolted mode should have the same strength and rigidity as the protective sleeve 101 on the outer side of each sensor, and therefore the distance measuring extension rod members 2 and the protective sleeve 101 on the outer side of each sensor are preferably made of metal materials with the same strength, rigidity and elasticity modulus. In the embodiment, the protective sleeves 101 are selected as steel sleeves, the distance measuring extension rod members 2 are selected as steel rod pieces consistent with the protective sleeves 101 in material, strength and rigidity, the steel rod pieces are firmly connected with the steel sleeves on the outer sides of the sensors through connecting bolts 102, and gasket rubbers 202 are further arranged on the distance measuring extension rod members 2.

[0051] Further, backfill cement paste 6 1s poured between the hole 4 drilled in the concrete dam and a device, and after the backfill cement paste 6 reaches the age, stress gradient actual measurement can be carried out on the concrete dam. Therefore, a flexible high-molecular polymer material pipe 1s wrapped outside the protective sleeve 101 of each high-precision stress monitoring sensor 1, and a hard high-molecular polymer material protective pipe 203 (such as a PVC protective pipe) is wrapped outside each distance measuring extension rod member 2, so the cement paste is isolated, and the influence on the measurement result is avoided.

[0052] As shown in FIG.5, a drill hole stress gradient monitoring instrument 501 arranged on the upper portion of the dam body in the arch direction, a drill hole stress gradient monitoring instrument 502 arranged in the middle of the dam body in the beam direction, a drill hole stress gradient monitoring instrument 503 arranged on the dam heel portion in the beam direction, a drill hole stress gradient monitoring instrument 504 arranged in the middle of the dam body in the beam direction and a drill hole stress gradient monitoring instrument 505 arranged on the dam toe portion in the beam direction are arranged in the dam body 7 (701 is a gallery in the dam) of the concrete dam.

[0053] e. monitoring signal quantities of all sensors are transmitted to an external automatic acquisition unit through a communication cable 3. According to the embodiment, a five-core hydraulic cable is adopted for monitoring signal transmission of each high-precision monitoring instrument and composed of a main communication cable and a plurality of branch communication cables 301, the branch communication cables 301 of all the sensors and the main communication cable are welded into the five-core cable to be led out of an opening of the drilled hole in a unified mode, and the situation that the diameter of the hole is too large and the backfill grouting quality is affected due to the fact that the number of cables in the hole is too large is avoided. And

[0054] f after the high-precision monitoring instrument is arranged in the drill hole, cement slurry needs to be poured for drill hole backfill. Before grouting, a sensor protection sleeve is wrapped by a flexible high-molecular polymer material pipe to be isolated from the backfill cement slurry; monitoring range measuring distance measuring extension rod members on the two sides of the sensors are wrapped by the hard high-molecular polymer material protective pipes to isolate cement slurry bonding, and it is guaranteed that backfill grouting does not affect the measurement result.

[0055] Step 3, calculate the spatial position coordinate value of each sensor from the opening of the drilled hole to the hole bottom by taking the coordinates of the opening of the drilled hole as an original point and through the inclination angle and the azimuth angle in the main drilling direction and the unit length of the stress monitoring sensors arranged in a serial connection manner, and enable the spatial position coordinate value to correspond to the number of each sensor. The specific implementation mode is as follows:

[0056] a. a transfer space is measured by adopting an attached wire method based on a spatial three-dimensional coordinate of the opening of the drilled hole through a working base point of a concrete dam hinge area specific equal-deformation monitoring control network. And

[0057] b. a spatial coordinate of a geometric center position of each stress monitoring sensor is calculated by taking the spatial coordinate of the opening of the drilled hole as a base point through the inclination angle and the azimuth angle in the drilling direction and the unit length of the stress monitoring sensors arranged in a series connection manner.

[0058] Step 4, after the stress monitoring sensors are buried and enter a stable working state; perform monitoring in real time according to the working condition of the dam. The actually-measured data of a certain time can be randomly adopted, the measuring point with the maximum stress measuring value in the series connection sensors is selected; under the conditions that the measuring point is taken as an original point, and the instrument length of the stress sensors is taken as the single search distance, the monitoring instruments are arranged in a series connection manner to compare the stress measuring values of the adjacent sensors in pairs until the last sensor at the two ends of the series connection monitoring instruments is reached. According to the comparison result, the coordinate values of the sensors along the line with the stress sequence decreasing as a main part are connected to obtain the principal stress gradient path, and the stress measuring value corresponding to the spatial coordinate value of each sensor is taken as the gradient value of the principal stress in the direction. The specific implementation mode is as follows:

[0059] a. monitoring can be conducted in real time according to the working condition of the dam after the drilling stress sensors are buried and enter a stable working state. According to the operation working condition of the dam, the representative actually-measured data of a certain time is adopted, the measuring point op with the maximum stress measuring value in series connection sensors is selected, and measuring value comparison is conducted on the adjacent stress monitoring sensors which are arranged on the two sides in a series connection manner with the measuring point op as the original point, the instrument lengthd of the stress monitoring sensors as the single search distance and the original point as the starting point until the last stress monitoring sensor at the two ends is reached.

[0060] b. the forward search distance along the original point reachessmd, the results of the m times of search comparison processes are all

G'>0i +1, i =0,1,2,......m, and m represents the number of times of search in the direction from the original point to oc; -direction;the reverse search distance along the original point reachesnd, the results of the times of search comparison processes are all o7>0;.,1=0,1,2,.....n m represents the number of times of search in the direction from the original point tog; ; direction, wherein, co; =o; =o;ifmd>3nd, the stress decreasing sequence of the forward search distance of md is the principal stress gradient direction; ifnd>3md, the stress decreasing sequence of the reverse search distance of nd is the principal stress gradient direction.

[0061] c. further, the actual situation is combined, daily value measurement fluctuation and random work abnormity of the stress monitoring sensors are considered, the search distance in the direction from the original point to &; -; direction reachesmd, the search distance in the direction from the original point to oi +; direction reachesnd, and md>3nd, the results of the m times of search comparison processes contain / times of o; >6; -; and j times of 6; <a; 7, i=0,1.2,......, m=h+}; if A >1.5j, the stress measurement values corresponding to j times of search are not brought into calculation, and the stress decreasing sequence of the forward search distance of md is still the principal stress gradient direction; if 4#=j, it is considered that no significant gradient rule of stress distribution is formed in the forward search distance of md.

[0062] When nd>=3md, it is referred to the above solution: the results of 7 times of search comparison processes contain p times of 6; >; ;, and g times of o; <7 +, i=0,1,2,......n=p+q; if p=1.5¢q, stress measurement values corresponding top times of search are not included in calculation, and the stress decreasing sequence of the reverse search distance ofnd is still the principal stress gradient direction; if p=q, it is considered that no significant gradient rule of stress distribution is formed in the reverse search distance of nd.

[0063] d. the coordinate values of the stress sensors along the line with the principal stress sequence decreasing as a main part are connected to obtain a principal stress gradient path, and the stress measuring value corresponding to the spatial coordinate value of each sensor is taken as a gradient value in the principal stress direction.

[0064] In summary, the stress gradient distribution condition of key parts such as a concrete dam heel, a dam toe and the upper part of the dam body can be actually measured through the plurality of concrete dam principal stress gradient continuous monitoring instruments, and the continuous monitoring data of the principal stress gradient of all parts in the concrete dam can be obtained under the conventional or emergency working condition, so the situation that the actual measurement capacity of the dam internal stress distribution rule is insufficient due to the fact that a solution of arranging the stress monitors in a discrete mode for a long time is changed, interpolation is carried out by means of discrete measuring point data to calculate the current situation of the dam concrete stress gradient distribution rule, an actual measurement sensing solution is provided for continuous change of the internal stress of the concrete dam under the emergency situation, and therefore, the monitoring capacity of the internal stress gradient of the concrete dam is further enhanced.

Claims (10)

Conclusions CLAIMS 1. A method for monitoring the stress gradient of a concrete dam, characterized in that it comprises the following steps: (a) calculating the main stress distribution of the main dam with the monitoring data of multi-directional strain gauge installed in the main part of the concrete dam, then calculation of the principal stress and direction; (b) drilling the concrete dam according to an inclination angle and an azimuth angle converted based on the direction of the main stress in the main part and then placing several serial stress monitoring sensors in the borehole; (c) calculating the value of the spatial location coordinates for each voltage monitoring sensor calculated from an opening of the downhole bore relative to the coordinate of the borehole, the calculation being based on the inclination angle and the azimuth angle in the direction of the hole, as well as in the unit length of the serial voltage monitoring sensor and corresponding to the number of voltage monitoring sensors; (d) After the stress monitoring sensor is buried in the borehole and operating stably, monitor and select actual data for contrast calculation in real time based on the dam conditions to obtain the path of the principal stress gradient and the gradient value of the corresponding principal stress. The method for monitoring the stress gradient of a concrete dam according to claim 1, characterized by comprising: first eliminating in step (a) the freely dependent variable from the monitoring data for each multi-directional stress gauge, implementing the verification and distribution of the balance for multi-directional tension meter, then convert the measuring point into uniaxial tension and use it to calculate the positive voltage and get the clipping voltage, the voltage matrix formed by the positive voltage and the clipping voltage, as well as the principal voltage and direction. 3. The method of monitoring the stress gradient of a concrete dam according to claim 1, characterized by comprising: first obtaining in step (b) the azimuth angle and the inclination angle in the direction of the principal stress in the major part of the concrete dam in step (a), drilling the borehole in the concrete dam with a geological drill and installing multiple serial voltage monitoring sensors in the borehole, the mounting of the voltage monitoring sensors being done so as to cover the entire depth of the hole 1s with the monitoring range, without connection or hooks between the voltage monitoring sensors. The method of monitoring the stress gradient of a concrete dam according to claim 3, characterized in that the outer side of each stress monitoring sensors is provided with a protective sleeve, two ends of the protective sleeve are provided with distance-determining extension rod portions, adjacent stress-monitoring sensors are connected through the distance-determining extension rod sections on both sides of the protection sleeve, and in addition the serial length of the voltage monitoring sensors is adapted to the whole depth of the hole in the concrete dam. 5. The method of monitoring the stress gradient of a concrete dam according to claim 4, characterized in that the distance-determining extension rod members are made of a metal material having the same strength, stiffness and elasticity as the protective sleeve. The method for monitoring the stress gradient of a concrete dam according to claim 1, characterized in that in step (c) first the spatial coordinate of an opening of the borehole is measured by a tethered plumb method and then the spatial coordinate on the geometric central position of each voltage monitoring sensor is calculated using the spatial coordinate of the borehole as a base point, depending on the tilt angle and the azimuth angle in the borehole direction and the unit of length of a serial voltage monitoring sensor. The method of monitoring the stress gradient of a concrete dam according to claim 1, characterized in that step (d) specifically comprises: (d.1) selecting a group of actual measured data and then selecting a measurement point. with the largest voltage reading in the sensors placed in series, considering the measuring point o, as an origin point and the instrument length J of each voltage monitoring sensor as a single search distance, taking the origin point as a starting point, and measuring values of adjacent voltage monitoring sensor arranged in series respectively compare on both sides of the starting point, until the last voltage monitoring sensor is reached on both sides; (d.2) depending on the result of the comparison, connecting the coordinates of the sensors, along a line according to which the reduction of the voltage mainly proceeds, to obtain the path of the principal voltage gradient, the voltage value corresponding to the spatial coordinates of each voltage monitoring sensor if the gradient value is taken in the direction of the principal voltage. The method for monitoring the stress gradient of a concrete dam according to claim 7, characterized in that in step (d.1), taking the origin point as a starting point, and respectively comparing measurement values of adjacent stress monitoring sensor arranged in series to either side of the start point is as follows: the forward search distance from the origin point is md and the results of the comparison after m search times are 6; >; -1i = 0,1,2,..., m, where m represents the number of searches from the origin point in the direction of catch; 7; the inverse search distance from the origin point is 72d, and the results of the comparison after searching n times are o;/>o0;-,i=0,1,2, Hm, where 7 is the number of searches from the origin point in the direction of o/:1, where at —oy —0y, if md>3nd md is the order of the incremental voltages for the advanced search distance md in the direction of the principal voltage gradient; if nd>3md, nd is the decreasing voltage order for the reverse search distance corresponding to the direction of the main voltage gradient. The method of monitoring the stress gradient of a concrete dam according to claim 7, characterized in that in view of the fluctuation in the daily measurement value of the stress monitoring sensors and the random working deviation, the search distance extends from the origin point in the direction of ©; +; to md, and the search distance extends from the origin point in the direction of o,.; to nd, and md>3nd, in the result of the equation after searching for m times there are h times catch; >o; -1 and j times of co; <o; .;,i=0.12,.., m=htj; as A2 1.5], the voltage readings measured in the search for j times are not included in the calculation, and the decreasing order of voltage for md forward search distance is still the direction of the principal voltage gradient; if h=j, it is assumed that no voltage is distributed in md forward search distance and no significant gradient rule; when md>3md, there are p-times of © />0;:; and g times of oi < 0 (+1, i-=0,1,2,...n=p+q, if p=1.5¢g, then the voltage readings measured when searching for p times are not included in the calculation and the decreasing order of voltage for reverse search distance nd is still the direction of the principal voltage gradient; if p=q, it is assumed that no voltage is distributed in the reverse search distance nd and there is no significant gradient rule is. The method of monitoring the stress gradient of a concrete dam according to claim 1, characterized in that in step (b) the borehole is drilled in the main part of the concrete dam in the direction of the principal stresses and then the serial stress monitoring sensors are installed and the drilled hole is also backfilled with cement.