Introduction to Rainwater Monitoring and Construction of Small Reservoirs and Dam Safety Monitoring

1、 Background introduction

2、 Site layout

Layout diagram:

Red: Dam axis

Blue: Monitor cross-sections, generally no less than 3, with a spacing of 20-50m within 300m and 50-10m outside 300m. Yellow: Monitor longitudinal cross-sections, generally no less than 4. One should be installed above the normal water level of the upstream dam slope.

Sectional schematic diagram:

Monitoring of water and rain conditions: Real time water level and rainfall data provide timely and accurate data support for water storage and regulation, flood prevention and drought resistance.

Rainfall: real-time rainfall, cumulative rainfall. Rainfall directly affects the level of water.

Water level: flood control level, normal storage level, design flood level, etc.

Image and video surveillance: Real time monitoring of water color, floating objects, abnormal situations, etc., making management more timely and efficient.

Voice intercom, shouting: alarm function, etc

Site construction:

1、 Conduct on-site investigation based on reservoir data, design plans, and drawings: mark and record installation locations (latitude and longitude, etc.), required number of water gauges, pole height, cross arm length, required cable length, benchmark elevation verification, confirm and record on-site power supply, on-site network signals, on-site traffic conditions, etc.

2、 Confirmation of survey data, formulation of construction plans, procurement of equipment and auxiliary materials, customization of poles, personnel organization, etc.

3、 Foundation excavation and pouring, cable trench excavation, etc.

4、 Equipment, pole assembly and fixation, cable laying and backfilling, etc.

5、 Equipment debugging and platform integration debugging.

GNSS monitoring: Using Beidou high-precision positioning technology, a statistical analysis model is constructed by observing the overall small deformation of the dam to predict the long-term trend of deformation. The observation accuracy can reach millimeter level.

Manual observation: Regularly observe the deformation status of the dam body using precision level instruments and other equipment, and summarize and compile the observation data

Site construction:

1、 Conduct on-site investigation based on reservoir data, design plans, and drawings: mark and record installation locations (latitude and longitude, etc.), required cable lengths, monitoring points, benchmark elevations, verify and record on-site power supply, on-site network signals, on-site traffic conditions, etc.

2、 Confirmation of survey data, formulation of construction plans, procurement of equipment and auxiliary materials, customization of poles (observation piers), personnel organization, etc.

3、 Foundation excavation and pouring, cable trench excavation, etc.

4、 Equipment, pole assembly and fixation, cable laying and backfilling, etc.

5、 Equipment debugging and platform integration debugging.

GNSS station:

Manual observation:

Seepage monitoring: Seepage not only causes the loss of water storage capacity in the reservoir, but also easily leads to seepage deformation such as piping and soil flow.

The water surface line of seepage in the dam body is called the infiltration line. Real time understanding of infiltration line elevation, combined with safe infiltration line elevation, improves the timeliness, timeliness, and scientificity of dam safety monitoring.

Diagram of infiltration line:

A key factor in the stability of earth rock dams is the position of the saturation line (the fully saturated zone of the soil inside the dam). In a safe dam, this saturation zone is well confined below the surface.

Site construction:

1、 Conduct on-site investigation based on reservoir data, design plans, and drawings: mark and record installation locations (latitude and longitude, etc.), drilling depth of pressure measuring holes, required length of pressure measuring pipes and cables, confirm and record on-site power supply, on-site network signals, on-site traffic conditions, etc.

2、 Confirmation of survey data, formulation of construction plans, procurement of equipment and auxiliary materials, customization of uprights and pressure measuring tubes, personnel organization, etc.

3、 Drilling of pressure measuring tubes, excavation and pouring of foundations, excavation of cable trenches, etc.

4、 Installation of pressure measuring tubes, installation of pressure gauge equipment, assembly and fixation of uprights, recording of installation verification forms, backfilling of cable laying, production of hole protection devices, etc.

5、 Equipment debugging and platform integration debugging.

Seepage monitoring: Collect seepage into the collection ditch for visual monitoring, grasp the seepage situation of hydraulic structures and their foundations, analyze and judge whether it is normal and the degree and reasons of possible adverse effects, and provide a basis for engineering maintenance, repair and safe use.

Site construction:

1、 Conduct on-site investigation based on reservoir data, design plans, and drawings: mark and record installation locations (latitude and longitude, etc.), collection trenches and cable lengths, confirm and record on-site power supply, on-site network signals, on-site traffic conditions, etc.

2、 Confirmation of survey data, formulation of construction plans, procurement of equipment and auxiliary materials, customization of poles, personnel organization, etc.

3、 Construction of infiltration trenches, excavation and pouring of foundations, excavation of cable trenches, etc.

4、 Measurement weir equipment, assembly and fixation of vertical poles, cable laying and backfilling, production of protective devices, etc.

5、 Equipment debugging and platform integration debugging.

3、 Construction Introduction

Water level gauge:

Install an upright water level gauge in accordance with the relevant requirements of the "Water Level Observation Standard" (GB/T50138-2010). And conduct regular manual observations to verify with automatic observations.

1. The water gauge is made of enamel or stainless steel material, consisting of a 1m high and 10cm wide gauge plate with a resolution of 1cm. (Other sizes can be customized).

Fix the water gauge plate onto the water gauge pile during use. Set up a set of water gauge piles along the water level measurement section, install water gauge plates, and form an upright water gauge.

2. The water gauge pile is made of concrete material and firmly fixed on the bank slope or upstream dam surface. The water level markings between adjacent water gauge piles should have a certain overlap (such as 20cm) to ensure that any water level is read.

3. After installation, use precision leveling measurement method to determine the zero point elevation of each water gauge. After reading the water level scale on the water gauge board, adding the zero point elevation of the water gauge is the water level elevation.

4. When measuring the cross-section of a building with a suitable vertical surface, a water gauge plate can be installed directly along the vertical surface of the building.

Radar water level gauge:

1. The on-site installation of the radar water level gauge is very simple. Install the water level gauge on the prefabricated pole support or dam body. As shown in the figure

(1) During installation, avoid areas with many floating objects on the water surface.

(2) There should be no other objects within a certain range below the water level gauge to avoid reflection and measurement errors.

(3) When installing instruments, be careful not to let the highest liquid level enter the measurement blind zone.

(4) Grounding the instruments and adding lightning protection measures.

(5) Install protective tubes around signal cables for protection.

Bubble water level gauge:

When the upstream slope is gentle and the dead water level cannot be observed by using a radar water level gauge on the dam body, a bubble pressure water level gauge can be used. The installation diagram is shown in the figure.

The bubble type water level gauge blows air into a fixed measuring point underwater through a gas pipe, balancing the gas pressure inside the blowing pipe with the static water pressure at the measuring point. The water level is measured by measuring the pressure inside the blowing pipe, and the sensor is placed above the water surface.

① The laying of the trachea is the most important step in the installation of bubble pressure water level gauges. The laying of air pipes should follow the following principles:

a) The trachea should descend smoothly and be laid in a straight line as much as possible.

b) The length of the trachea should not exceed 150m; the curvature of the curved part of the trachea should not be too sharp, and a smooth channel should be provided for the trachea.

c) The end of the trachea should be fixed inside the protective tube to prevent the trachea from floating in water and affecting the accuracy of measurement.

d) Use high-density concrete blocks with sufficient and stable insertion into key areas of the riverbank to secure the air pipe, prevent flood threats or collapse, or fix the air pipe on existing stable buildings.

② Fixed installation of air chamber

Before installing the protective tube, first fix the gas container (fix it on a tough metal with screws or welding), and then proceed with the installation directly. During installation, fix the metal tube on the water surface.

Rain gauge:

Rainfall observation points should be selected in open and flat surroundings, unaffected by sudden changes in terrain, trees, buildings, and smoke. There should be no tall crops, trees, etc. within a 20 meter radius of rainfall monitoring facilities. Obstacles should be avoided around the rain gauge. If it cannot be avoided, the distance from the obstacle to the rain gauge should not be less than twice the height of the obstacle to ensure that the observed rainfall on the site can represent the depth of precipitation on the horizontal ground.

① Check the integrity and sensitivity of the instrument before installation.

② Install fixed rain gauges, level and install securely.

③ Adjust the level of the tipping bucket bracket: Check and adjust the tipping bucket to make it level.

④ Install output signal cable: Connect to telemetry terminal, lock outer cylinder, and tighten screws.

⑤ After installation, double check if the installation is correct and if the instrument is functioning properly,

And check if the measurement accuracy meets the requirements.

camera:

1. Video monitoring points should be set up at locations such as dams, spillways, and drainage culverts, with a focus on monitoring the entire dam, taking into account water gauges, spillway inlet (outlet), drainage culvert outlet, and leakage behind the dam.

2. The video monitoring point set on the dam crest can be considered to be jointly deployed with the rainfall monitoring station, with unified station construction and centralized power supply; Other video monitoring points can be set independently.

3. The camera should be installed in a location with a wide field of view and sufficient lighting, at a height of not less than 2 meters from the ground, and should have measures such as waterproof, dustproof, and anti-theft.

Gnss：

(1) Excavate a 60 * 60 * 80cm foundation pit at the selected location (the specific size depends on the site and design plan);

(2) Create a pouring template for the observation pier and make a 60 * 60 * 20cm square template (specific dimensions based on the site and design plan);

(3) The cement grade shall not be less than 325, and the concrete shall be mixed according to the C25 concrete strength standard;

(4) Place the ground cage and prepared formwork, pour concrete, and vibrate until the concrete is level with the formwork;

(5) Remove the mold after standing still for 12 hours.

(6) Equipment installation, fixing the upright pole, fixing the equipment on the forced centering device, and connecting cables.

(7) Equipment debugging, IP planning for each monitoring point, setting IP and port numbers for the host, debugging and recording for storage.

Observation pier requirements

1) Excavate to a relatively fixed location and pour reinforced concrete base and column body.

2) The base points are set on the slopes of both sides of the river, and the placement should be solid, reliable, and avoid natural and human influences. The base point adopts an integral reinforced concrete structure, and the height of the column is greater than 1.2m. For the working base point buried on the rock foundation, the base can be directly poured on the rock; For the working base points buried on the soil foundation or non fixed working base points added, the base must be buried below 0.6m in the ground or dam surface to the original soil layer.

3) The base part is 1m x 1m x 0.4m; the column body is a platform column connected to the base, with a top dimension of 0.3m x 0.3m and a bottom dimension of 0.6m x 0.6m. The working base point height is 1.5m, and the height of the observation pier and verification base point is 0.9m (specific dimensions depend on the design scheme).

4) The top is embedded with a forced centering chassis, and the center of error of the chassis is less than 0.2mm.

5) Pour a concrete structure on the base of the pier, and ensure that the structure is in close contact with the pier base, while burying leveling points.

6) If the sight line method is used, the center of the forced centering chassis of each measuring point shall be located on the sight line, with a deviation of no more than 10mm and an inclination of no more than 4 inches.

Base observation pier:

Horizontal displacement measurement point observation pier:

Vertical displacement measurement point observation pier:

Vertical displacement measurement observation pier:

bench mark:

The basic benchmark should be measured from the national third class and above benchmark using a benchmark not lower than the third class benchmark.

The benchmark is usually made of concrete and buried deep below the frozen soil line. The top of the marker stone is embedded with corrosion-resistant hemispherical metal markers.

Pressure measuring tube:

Construction sequence:

(1) Hole making

(2) Manufacturing of pressure measuring tubes

(3) Installation of pressure measuring tube

(4) Seal the hole

(5) Sensitivity test

(6) Pipe mouth protection

(7) Table record

(1) Hole making

Use a 150 type geological lead drilling machine for drilling, with a hole diameter of 100-110mm, dry drilling, and strictly prohibit using mud to fix the wall or circulating water for drilling. To prevent hole collapse, casing can be used to fix the wall. Record and describe the rock core. After the final hole is drilled, measure the inclination of the hole to accurately determine the position of the measuring point.

(2) Manufacturing of pressure measuring tubes

The pressure measuring tube consists of a permeable section and a conduit section. The permeable section can be made by processing conduit pipes, with an area opening rate of about 10% to 20%. The hole shape is not limited, but it must be arranged evenly and the inner wall must be free of burrs. The length of the permeable section is about 2 meters, and the external wrapping is sufficient to prevent soil particles from entering non-woven geotextile. The bottom of the pipe is sealed. The length of the conduit depends on the pipe material and the convenience of burial. The joints at both ends should be connected with external threads and clamps.

(3) Installation of pressure measuring tube

Before burial, a comprehensive inspection should be conducted on the drilling depth, bottom elevation, water level inside the hole, presence of collapse, quality of pressure measuring tube processing, length of each pipe section, joints, and pipe cap conditions, and records should be kept.

Before lowering the pipe, a filter material with a thickness of about 10m should be filled at the bottom of the hole. During the process of lowering the pipe, it is necessary to connect it tightly, hang it firmly, and keep the pipe body straight. After positioning, the bottom elevation and water level of the pipe should be measured immediately, and filter material should be backfilled outside the pipe, compacted layer by layer, until the design inlet height of this measuring point is reached. The length of the hole section from the bottom of the hole to the top of the filter material is the true inlet section of the pressure measuring tube (which can be larger than the permeable section of the pressure measuring tube body), and it is also the actual monitoring range of the pressure measuring tube. Therefore, it is necessary to strictly follow the design intent during burial, accurately measure and record the information.

The requirements for the filter material should not only prevent fine particles from entering the pressure measuring tube, but also have sufficient permeability. Generally, its permeability coefficient should be greater than 10-100 times that of the surrounding soil. Pure fine sand can be used for cohesive soil or sandy loam soil; A mixture of fine sand to coarse sand can be used for sand and gravel layers. Before backfilling, it is necessary to wash and air dry, and slowly enter the hole.

(4) Seal the hole

Sealing material, using bentonite balls (or highly disintegrable clay balls). Require that the permeability coefficient after deliquescence in the borehole is lower than the permeability coefficient of the surrounding soil. Soil balls are composed of different particle sizes with a diameter of 5-10mm and should be air dried. They should not be exposed to sunlight or baked. When sealing the hole, it is necessary to pour each particle into the hole, and if necessary, 10% to 20% homogeneous soil material can be added and compacted layer by layer. Do not dump in large quantities to prevent overhead. Compaction method should be used to backfill clay within 1-2m below the pipe mouth. After sealing to the design elevation, inject water into the pipe until the water surface exceeds the required surface of the mud ball section, causing the mud ball to disintegrate and expand.

(5) Sensitivity test

Sensitivity testing should be conducted after the installation and sealing of the pressure measuring tube. The inspection method adopts water injection test, which should generally be conducted during the stable period of the reservoir water level. Before the experiment, measure the water level in the pipe first, and then inject clean water into the pipe. If the surrounding area of the inlet section is loam soil, the injection volume is equivalent to 3-5 times the volume of the pressure measuring tube per meter; If it is sandy aggregate, it is 5-10 times. After injection, continuously observe the water level until it returns to or approaches the water level before injection. For clay loam soil, the sensitivity is considered qualified when the water level drops to the original level within five days and nights; For sandy loam soil, the sensitivity is considered qualified when it drops to the original water level within one day and night; For gravel gravel, it is qualified if the water level drops to the original level within 1-2 hours or the water level rises by less than 3-5 meters after injection.

(6) Pipe mouth protection

After passing the sensitivity test, install the pipe protection device as soon as possible. The pipe mouth protection device adopts a cast-in-place concrete base or brick masonry, pre embedded expansion screws, and then installed with a steel protective cover. The structure is simple, firm, and can prevent rainwater from entering and human and animal damage. It also has a locking function and is easy to open.

(7) Construction records

During the construction process, relevant forms such as drilling records, pressure measuring tube burial records, and pressure measuring tube verification forms should be filled out.

Osmometer:

(1) Install a pressure gauge directly inside the pressure measuring tube.

(2) Before burying, the pressure gauge must undergo indoor inspection and on-site verification.

(3) Before installation, the pressure gauge needs to be soaked in water for more than 2 hours to reach saturation and maintain saturation.

(4) When installing, first remove the water permeable stone at the end of the instrument and apply a layer of butter or Vaseline oil on the steel diaphragm to prevent rusting.

(5) When extending cables, it is necessary to connect the same colored core wires together, solder them firmly with tin, carefully carry out waterproof treatment, and ensure sealing performance.

(6) During the installation process of the pressure gauge, it is necessary to carefully fill out the pressure gauge installation record form and its verification form.

Collection/drainage ditch:

For reservoirs with seepage and open water at the downstream foot of the dam body, a drainage ditch should be set up at the location where seepage accumulates downstream of the dam foot. Adopting a rectangular cross-section, its length should be greater than 7 times the water head on the weir, and the total length should not be less than 2m (the length of the weir groove upstream and downstream of the weir plate should not be less than 1.5m or 0.5m respectively). Both sides of the weir should be parallel and vertical.

Measuring weir:

Install a measuring weir at the outlet of the drainage ditch to monitor its outflow (open flow). Install weir plates and measuring weirs at the seepage outlet to achieve automated observation of seepage volume.

Installation of measuring weir:

(1) The flow pattern at the weir mouth must be free flowing.

(2) The measuring weir should be located on the straight section of the drainage ditch, and the weir groove section should adopt a rectangular cross-section.

(3) The weir plate is flat, with local unevenness not exceeding ± 2mm, and local unevenness at the weir mouth not exceeding ± 1mm; the top of the weir plate is horizontal, with a height difference on both sides not exceeding 1/500 of the weir width, and the right angle error of the right angled triangular weir shall not exceed 30 "; The weir plate and side wall should be kept vertical, with an inclination of less than 1/200, and the unevenness of the side wall should be less than ± 5mm. The weir plate and side wall should be perpendicular to each other, with an error of less than 30 ". The local distance error between the two side walls should be less than ± 10mm. The weir plate is made of stainless steel plate, and the downstream edge of the overflow weir mouth is made at a 45 ° angle.

(4) The weir plate should be perpendicular to the walls on both sides of the weir groove and the direction of incoming water flow. The weir plate should be flat and level, with a height greater than 5 times the water head above the weir.

(5) The water gauge or measuring needle for reading the water head on the weir should be set at 3-5 times the water head upstream of the weir mouth. The ruler body should be vertical, and the difference between its zero point elevation and the weir mouth elevation should not exceed 1mm.

(6) Use an anti fouling tube with a barrier to make a static observation well at the installation location of the water gauge or measuring needle, and install a water measuring weir.

(7) After the installation of the measuring weir is completed, a detailed verification form should be filled out and archived for future reference.

Pole foundation:

1. Basic measurement → excavation of foundation soil → formwork erection → concrete pouring → pre embedded parts embedding → curing → formwork removal → earthwork backfilling.

2. The rainwater situation video standing pole foundation and rainfall standing pole adopt concrete independent foundation, with a foundation size of 800mm × 800mm × 1000mm and a height of 100mm above the ground. The concrete strength of the foundation is C25. (Specific dimensions depend on the site and design plan)

3. The foundation of independent video surveillance stations and other poles is made of concrete, with a size of 800mm × 800mm × 800mm, and the foundation is 100mm above the ground. The concrete strength of the foundation is C25. (Specific dimensions depend on the site and design plan)

Earthwork excavation:

(1) Main technical requirements for earthwork excavation:

1) Unqualified soil and debris on the surface of the foundation surface must be removed, and pits, trenches, ditches, etc. within the scope must be backfilled according to the filling requirements;

2) Excavation, excess soil, debris, waste residue, etc. should be transported to designated locations for stacking;

3) The actual excavation contour must comply with the requirements of the construction drawings or the opening line, horizontal dimensions, and elevation specified by the engineer on site, and the final excavation contour must not be under excavated.

(2) Excavation method

The excavation volume of a single foundation earthwork in this project is relatively small, mainly using manual excavation. After excavation, the earthwork is filled and balanced, and should be transported to the designated place for stacking in a timely manner to prevent erosion and soil erosion.

All foundation excavations should be carried out on non rainy days. During the rainy season, technical measures should be taken to ensure the quality of foundation engineering and safe construction, effectively preventing rainwater from washing away the dam and eroding the foundation soil.

Before construction, the construction site should be cleaned up first. The surveyors should set out the excavation boundary and mark the excavation height according to the design drawings.

(3) Earthwork backfilling

1) After the foundation excavation is completed, timely backfilling of soil and rock should be carried out to restore the original appearance. Earthwork backfilling should comply with the construction acceptance standards of foundation engineering and the requirements in the design drawings for construction.

2) Before filling the soil, remove any debris from the base. The backfill material should meet the requirements of the "specifications" and be compacted using a frog hammer. If conditions are not met, manual compaction should be used. The appropriate moisture content of each layer of fill should be controlled to improve compaction efficiency.

3) Measures should be taken for construction on rainy days, and attention should be paid to timely drainage of accumulated water on the site. After rain, the moisture content of the soil material increases, and it should be dried early to control the moisture content for backfilling.

Concrete (reference):

1. Self mixed concrete is used for construction, and no admixtures are considered for the concrete. Some parts and winter concrete construction will be adjusted according to the instructions of the supervising engineer during the construction process.

2. The cement used is ordinary Portland cement of grade 32.5 or above.

3. Fine aggregate should be selected from natural sand that is hard, clean, and well graded. The particle shape should generally be square circular and should not contain active materials. Natural sand should be divided into two grades according to particle size.

4. Moisture content: The moisture content of the fine aggregate entering the mixer should be balanced and less than 6%, and sufficient storage and dewatering time should be considered.

Fineness modulus: The fineness modulus of fine aggregate should be controlled within the range of 2.4-2.8, and the average value of fineness modulus should not exceed 0 15.

5. Coarse aggregate shall be selected from hard, clean, well graded crushed stones with natural gradation and a maximum particle size not exceeding 40mm. The water for concrete mixing and curing is supplied by reservoir water, and its water quality meets the requirements for concrete mixing water.

6. All formwork concrete is poured horizontally continuously, and the concrete leveling is achieved by combining manual labor with vibrators. The areas near the formwork, dense steel bars, waterstops, and embedded parts are leveled manually with shovels, while other areas are leveled directly with vibrators.

7. Winter construction shall be carried out in accordance with the regulations: when the daily average temperature in mild areas is below 3 ℃, the construction period enters the low temperature season, and concrete construction in the low temperature season requires anti freezing and crack prevention.

8. Try to avoid pouring concrete in heavy rain and rainstorm. If it is really necessary to pour concrete in rainy days, adjust the mix proportion appropriately to ensure that the concrete strength is not affected.

Lightning protection grounding:

Direct lightning protection measures should be taken at each monitoring station, in accordance with the relevant provisions of GB50343-2012 "Code for Design of Lightning Protection of Building Electronic Information Systems" and GA/T670-2006 "Technical Requirements for Lightning Surge Protection of Security and Prevention Systems", with a grounding resistance of ≤ 10 Ω.

Generally, direct lightning protection is achieved through external lightning protection devices, including lightning rods, lightning strips, lightning nets, and lightning conductors, as well as down conductors and grounding module devices, which form a complete electrical circuit to discharge lightning current into the ground.

1. Install stainless steel lightning rods made of Ø 12 stainless steel. Three 40mm × 40mm × 5mm hot-dip galvanized angle steels are selected as vertical poles for the grounding network. Use L=1.5 meters, interconnect with 40 × 4mm hot-dip galvanized flat steel, and bury the electrodes at a depth of ≥ 0.8 meters. (Specific dimensions based on site and design drawings)

2. Layout of grounding body: First, excavate the lightning protection grounding body trench, then vertically drill holes to embed hot-dip galvanized angle steel. The length and spacing of the vertical grounding body should be processed according to the construction drawing requirements. Then, all vertical grounding bodies should be firmly welded with hot-dip galvanized flat steel according to the design requirements, and connected to the lightning rod with a down conductor, and then buried with soil. The down conductor is connected to the horizontal grounding body from the foundation side through a connecting body.

3. During on-site construction, a precision ground resistance meter is used for resistance measurement, and the grounding resistance is required to be ≤ 10 Ω.

Example construction:

Option 1: Use three 1.5m galvanized angle steels (50 * 50 * 5m) arranged in a triangular pattern and driven into the bottom. The angle steels should be buried at least 0.8m deep and spaced at least 1.5m apart

Option 2: Use three 1.5m galvanized angle steels (50 * 50 * 5m) arranged in a straight line and driven into the bottom. The angle steels should be buried at least 0.8m deep and spaced at least 1.5m apart

The lightning rod is made of 12mm galvanized round steel, with its top at least 600mm higher than the highest point of the pole. The lightning rod round steel is fixed to the pole with clamps.

The top of the three angle steels in both schemes are welded with galvanized flat steel (50 * 5mm), and one of the angles is welded with galvanized flat steel (50 * 5mm) protruding from the ground and welded with the bottom of the lightning protection round steel.