Underwater concrete is poured by conduit method. The conduit is a threaded connection conduit with a sealing ring with an inner diameter of 280mm, with a length of 0.5~2.5m and a minimum length of 5.0m m. Before use, the conduit should be assembled and numbered, and water tightness, pressure bearing and joint tensile tests should be carried out to ensure that the conduit is in good condition. During catheter lowering, keep the catheter in the center, with straight axis and smooth sinking, to prevent the reinforcing cage from being stuck and colliding with the hole wall, and make records when installing the catheter. Before pouring underwater concrete, check the thickness of sediment at the bottom of the hole again. When it exceeds 200mm, the hole should be cleaned for the second time to make the thickness of sediment at the bottom of the hole conform to the regulations. After the self-inspection is qualified, it shall be reported to the supervision engineer for inspection, and the concrete pouring construction shall be carried out with the consent of the supervision engineer. After the concrete is transported to the pouring site, check its workability and slump. Immediately after meeting the requirements, concrete pouring shall be started, and it shall be carried out rapidly and continuously without interruption, and the pouring speed shall not be less than 50m3. The first batch of concrete is poured in a 6m3 storage hopper, the distance from the lower opening of the conduit to the bottom of the hole is controlled at 25 ~ 40 cm, and the initial buried depth of the conduit is greater than1.0m. During the whole pouring process, the outlet should be buried in the previously poured concrete for 2 ~ 6m to prevent mud and water from rushing into the tube. After the first batch of back cover concrete was poured into the crane batching hopper, the 6m3 storage hopper was replaced by 1.5m3 storage hopper, and the concrete was directly poured by the concrete tanker, which accelerated the pouring speed of underwater concrete. In the process of pouring, the height of the concrete surface in the hole is often detected with a hammer, the buried depth of the conduit is adjusted in time, and the conduit is lifted slowly to avoid touching the reinforcing cage. In order to ensure the quality of the pile head, the concrete pouring elevation is 0.5 ~ 1.0m higher than the design value of the pile head, which should be chiseled before the construction of the pile cap. During the construction process, designate a special person to fill in the underwater concrete pouring record. When pouring concrete, the spilled mud should be introduced to a suitable place for treatment to prevent environmental pollution.
Second, the common problems of catheter water intake
1, the main reason
2. The first batch of concrete reserves are insufficient, or the concrete reserves are sufficient, but the distance between the bottom of the conduit and the bottom of the hole is too large, and the bottom of the conduit cannot be buried under the concrete at the back, so that mud can enter from the bottom.
2. 1 The joint of the conduit is lax, the rubber pad between the joints is squeezed out by the high-pressure airbag of the conduit, or the weld is broken, and water flows in from the joint or weld; Poor sealing of conduit joint and improper placement of gasket; The gasket is squeezed or damaged; Loose flange bolts.
2.2 The conduit is lifted too hard, or the sounding is wrong. The bottom of the conduit is beyond the original concrete surface, and mud is poured into the bottom.
3, treatment measures
In order to avoid water inflow into the catheter, corresponding measures should be taken in advance to prevent it. Once an accident occurs, the cause of the accident should be found out immediately and the following treatment methods should be adopted:
3. 1 If it is caused by the above-mentioned first reason, the conduit shall be hoisted out immediately, and the concrete mixture scattered at the bottom of the hole shall be sucked out by mud pump with the drill rod of the reverse circulation drill, or removed by air mud suction machine, hydraulic mud suction machine and grab bucket. If necessary, the reinforcing cage shall be hoisted out and removed by re-drilling. Then lower the skeleton and conduit again, and put in the first batch of well-stocked concrete for re-perfusion.
3.2 If it is caused by the second or third reason, the original pipe should be replaced, and a new pipe should be replaced according to the specific situation; Or insert the original conduit to continue irrigation, but before irrigation, the water and precipitated soil entering the conduit should be sucked out by suction and pumping. If a new pipeline is installed, the water in the pipeline must be drained by submersible pump before pouring concrete. In order to prevent the muddy water outside the conduit from penetrating into the original poured concrete and becoming the bottom of the conduit after pumping, the conduit should be inserted into the concrete to a sufficient depth, generally more than 200cm. Because it is impossible for the submersible pump to discharge all the water in the conduit, the cement content of the continuously poured concrete should be increased, and then poured into the conduit after the consistency is improved. Before diving, slightly shake the conduit or suspend the vibrator to vibrate for a moment to make up for the loss of fluidity of the original concrete. After pouring concrete, the normal mixture ratio can be restored. If the concrete surface is not deep below the water surface and has not solidified initially, a waterproof plug (specially made of concrete) can be set at the bottom of the conduit, and the conduit can be reinserted into the concrete (force is applied to the side of the conduit to overcome the buoyancy of water). After pouring concrete into the conduit, gently lift the conduit, use the self-weight of new concrete to remove the bottom plug, and then continue pouring.
Three. Frequently asked questions: the reinforcing cage floats.
1, reason
The floating of the reinforcing cage is mainly due to the fact that the concrete surface is close to the bottom of the reinforcing cage, and the concrete pouring speed (m3/min) is too fast from 3m below the bottom of the reinforcing cage to above 1m, which causes the concrete to escape from the bottom of the conduit and recoil upwards, and its jacking force is greater than the gravity of the reinforcing cage.
In order to prevent the reinforcing cage from floating, when the bottom of the conduit is 3m below the bottom of the reinforcing cage, 1m above the bottom of the reinforcing cage, and the concrete surface is 1m above the bottom of the reinforcing cage, the concrete pouring speed should be slowed down. The allowable maximum grouting speed is related to the pile diameter. When the pile length is less than 50m, please refer to Table 1.
Among the piles that are not fully reinforced, the floating of reinforcing cage is the most common quality accident, and there are two reasons:
(1) The concrete with poor quality, easy segregation, short initial setting time and large slump loss will cause the concrete surface to float or the reinforcing cage to be difficult to insert when it reaches the bottom of the reinforcing cage, resulting in jacking and floating. Or sometimes the concrete surface rises to a certain height in the reinforced cage, and the surface concrete begins to set initially, which will also make it float.
(2) Operation reason:
(1) The hole of the reinforcing cage is not firmly fixed, and it will float up when it is slightly impacted; Or the reinforcing cage is not firmly fixed, and the anti-floating reinforcement is broken;
(2) When the concrete surface reaches the bottom of the reinforced cage, the conduit is buried deeply, and the pouring amount is large, so the upward impact of concrete on the cage body is too large;
③ The buried depth of the catheter is too large. When the buoyancy of concrete increases, the reinforcing cage will be easily lifted.
(4) the concrete pouring speed is too fast, and the buoyancy of concrete is greater than the self-weight of steel bars;
⑤ If the concrete pouring time is too long, the plasticity of the upper concrete decreases or has been initially set, forming a hard shell, and the concrete surface rises and the reinforcing cage floats;
6. When pouring concrete, if the steel bar moves to one side, or because the conduit is not in the center, the conduit flange hangs on the reinforcing cage when lifting the conduit, so that the reinforcing cage floats;
2. Treatment measures
The floating caused by concrete quality should be avoided by improving the fluidity, initial setting time and pouring technology of concrete.
For the floating caused by operation, when pouring concrete, when the concrete is about to touch the bottom of the reinforcing cage, the pouring speed should be slowed down slightly, or the conduit should be buried properly (at this time, the lower opening of the conduit is below the lower end of the cage) to reduce the stress on the concrete surface; When the concrete surface enters a certain height in the reinforcing cage, the conduit should be lifted to a position where the opening under the conduit is higher than the lower end of the reinforcing cage by a certain distance.
3. Preventive measures
Once the reinforcing cage tends to arch, it is difficult to restrain it. Therefore, measures should be taken in advance during construction to prevent the above situation from happening. According to the experience of underwater concrete construction of bored piles at home and abroad, the following measures can be taken to prevent the reinforcing cage from floating: after the reinforcing cage descends to the design elevation, the four steel hooks hanging the reinforcing cage are welded (spot welded) on the casing to fix the skeleton; Steel bars (or steel pipes) can also be used to vertically press the reinforcing cage and weld the steel bars (or steel pipes) on the orifice, but pay attention to the vertical position to prevent the skeleton from being deformed. The bottom of the reinforcing cage is bound into a cross shape, and concrete blocks with a thickness of about 10cm are arranged on both sides; The concrete injected by the conduit is piled up on the concrete block, and the self-weight of concrete is used to prevent the steel bar from arching. For friction piles with steel bars at the lower end, a small amount of main reinforcement can extend to the bottom of the hole. In this way, when pouring concrete in the upper reinforcing cage, the floating of the reinforcing cage can be prevented or reduced because there is a certain adhesive force between the lower main plate and the concrete. Specific measures are:
(1) appropriately reducing the number of stirrups at the lower end of reinforcing cage can reduce the upward jacking force of concrete;
(2) The upper end of the reinforcing cage is welded on the casing, which can bear part of the jacking force and prevent floating;
(3) Set 1 ~ 2 reinforcing annular bars with a diameter not less than the main bar at the bottom of the hole, and use an appropriate number of traction bars.
The bottom of the reinforcing cage is welded firmly, and the practice proves that the floating of the reinforcing cage is effectively overcome.
Fourth, hole collapse
If it is found that the water (mud) level in the casing suddenly rises and overflows the casing, and then suddenly falls and bubbles appear, it should be suspected as a sign of well collapse, which can be detected by sounding probe or sounding hammer. If the sounding hammer was originally hung on the concrete surface and has not been taken out, it can't be lifted up now, or the surface depth measured by the sounding probe can't reach the original depth, and there is a big difference, so the hole collapse can be confirmed.
The hole collapse may be caused by water leakage around the casing foot, and the water level in the hole decreases, or when the tidal river rises, the water level difference in the hole decreases, and the original hydrostatic pressure cannot be maintained, and the hole collapse may be caused by the accumulation of heavy objects around the casing or mechanical vibration.
After the hole collapse occurs, the reasons should be found out and corresponding measures should be taken, such as maintaining or increasing the water head, removing heavy objects and eliminating vibration. To prevent further hole collapse. Then, the mud collapsed into the hole is sucked out by a mud suction machine; If the hole collapse does not continue, normal perfusion can be resumed. If the hole collapse still does not stop, and the hole collapse part is deep, it is advisable to pull out the conduit, drill out the concrete, and grab out the steel bar at the same time, just to keep the hole position, and then backfill it with clay mixed with gravel. After the backfill soil is settled, drill holes to form piles.
When drilling in loose silt, silt or quicksand layer, control the footage and select high-quality mud with large specific gravity, high viscosity and high cement ratio; If the orifice collapses, backfill, re-drill or lower the steel casing to1m; Below the unfolded position; If the hole collapse is not serious, increase the proportion of mud to continue drilling; if the hole collapse is serious, backfill the mixture of sand and clay to drill again.
Five, fill the short pile head
1, reason
The elevation of cast-in-place pile top is not enough. First, the construction control is not strict, and the concrete will stop pouring before it reaches the design elevation. On the other hand, although the elevation reaches the design value, the elevation of the pile top is insufficient because of the thick laitance layer of the concrete at the pile top.
(1) At the end of grouting, the slurry residue is too thick, so it is difficult to judge the slurry residue or the concrete surface with the sounding hammer, or the sounding hammer is too light to sink to the concrete surface, resulting in false measurement, so that the catheter is pulled out to terminate grouting, resulting in short pile head accidents.
(2) When pouring concrete, the collapse of the hole wall is not found, and the probe of the sounding hammer or sounder can't reach the concrete surface, which is the most dangerous situation, sometimes the whole process is as short as several meters.
2, preventive measures and treatment measures
(1) During grouting, attention must be paid to whether there are signs of hole collapse. If there is any hole collapse, it should be treated as above before grouting.
(2) The hammering shall not be lower than the specific gravity and shape specified in the specification. For cast-in-place piles with high mud density, the specified value of hammering specific gravity must be taken. Even when the slump of concrete is still large, the heavy hammer may sink into the concrete for tens of centimeters, and the result of sounding error is only that the depth of the conduit buried in the concrete surface is ten centimeters more than that of the actual majority; However, the slump of the first batch of concrete will be smaller and smaller in the later stage of pouring, and the depth of the heavy hammer sinking into the concrete will be smaller and smaller, so the sounding can still be accurate.
(3) At the end of perfusion, add clear water to dilute the mud, and take out some precipitated soil.
(4) Using thermistor or induction probe sounder.
(5) Insert the iron box sampler into the suspicious horizon for sampling and identification.
The treatment method can refer to the above extension casing according to the specific situation; Or add a protective tube inside or outside the original protective tube, press it into the poured concrete, then pump water, remove slag and pour ordinary concrete; Or loosen the sludge and soft layer with high-pressure water, then clean the sludge on the concrete surface with a sludge suction machine, and then pour underwater concrete into the conduit.
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