Do You Know How to Solve Various Problems in Pile Foundation Testing Projects?

There are many types of pile foundations, and the construction techniques vary greatly. In addition, the strata vary in complexity. During construction, the pile body may have defects such as shrinkage, expansion, mud inclusion, segregation, and broken piles. Of course, mechanical excavation after construction and collisions can also cause shallow pile body defects. The presence of pile body defects will change the normal working properties of the pile foundation, thus creating potential dangers to the foundation. Today we have summarized the common problems of pile foundations in engineering. Let's take a look.

Pile foundation detection

Under what circumstances should verification and expanded detection be carried out

1. During low-strain detection, for rock-embedded piles, the time domain reflection signal at the bottom of the pile is a single reflection wave and is in the same direction as the hammer signal; the measured signal is complex and irregular, and it is impossible to evaluate it accurately; the pile body cross-section is gradually changing or changing, and the concrete cast-in-place pile with a large change range is always verified by static load method or core drilling method.

2. During high-strain detection, there are defects in the pile body, and the vertical bearing capacity of the pile cannot be determined, or the defects in the pile body have an impact on the horizontal bearing capacity; the single-click penetration is large, the same-direction reflection at the bottom of the pile is strong and the reflection peak is wide, the side resistance wave and the end resistance wave reflection are weak, that is, the waveform shows that the vertical bearing characteristics are inconsistent with the geological conditions in the survey report, and the static load method can be used for further verification;

3. The same-direction reflection of the bottom of the rock-embedded pile is strong, and there is no obvious end resistance reflection after 2L/C. The core drilling method can be used for verification.

4. Shallow defects in the pile body can be verified by excavation.

5. Precast piles with cracks in the pile body or joints can be verified by the high-strainvisit method.

6. When the quality problem of the pile body concrete is found in the single-hole core drilling test, it is advisable to add drilling verification in the same foundation pile.

7. For piles or Class III piles whose integrity category cannot be determined in the low-strain method test, static load method, core drilling method, high-strain method, excavation, and other appropriate methods can be used for verification and detection according to actual conditions.

8. When the single pile bearing capacity or core drilling method sampling results do not meet the design requirements, the reasons should be analyzed and the sampling should be expanded after confirmation.

9. When the sum of Class III and Class IV piles found in the random inspection of pile body integrity using the low-strain method, high-strain method, and ultrasonic transmission method is greater than 20% of the number of sampled piles, the original inspection method should be used (the ultrasonic transmission method can be replaced by the core drilling method) to continue to expand the random inspection among the uninspected piles.

What should be included in the pile foundation inspection report?

The inspection report should include the following:

1. Name of the client, project name, location, construction, survey, design, supervision and construction unit, foundation, structural type, number of layers, design requirements, inspection purpose, inspection basis, inspection quantity, inspection date;

2. Description of geological conditions;

3. Pile number, pile position, and related construction records of the inspected piles;

4. Inspection method, inspection equipment, and description of the inspection process;

5. Inspection data of the inspected piles, actual measurement and calculation analysis curves, tables,,s and summary results;

6. Inspection conclusions corresponding to the inspection content. The evaluation of the engineering pile bearing capacity inspection results should give the bearing capacity inspection value of each inspected pile, and based on this, give a conclusion on whether the single pile bearing capacity characteristic value under the same conditions of the unit project meets the design requirements.

What regulations should be followed in determining the characteristic value of the horizontal bearing capacity of a single pile under the same conditions as a unit project?

1) When the horizontal bearing capacity is controlled by the pile body strength, the statistical value of the horizontal critical load is taken as the characteristic value of the water-bearing capacity of the single pile.

2) When the pile is subjected to long-term horizontal loads and cracking is not allowed, 0.8 times the statistical value of the horizontal critical load is taken as the characteristic value of the horizontal bearing capacity of the single pile.

3) When the horizontal bearing capacity is controlled by the horizontal allowable displacement required by the design, the design requirement can be taken.

Construction problems

Leakage: In hydraulics, leakage is called a pipe burst. Common phenomena are bubbles or turbid water on the surface of the water outside the casing. The above phenomena mean that leakage has occurred.

Treatment method: Add cement or vibrate the casing to sink. The casing sinks no more than 1m. If the casing sinks more than 1m, or the casing sinks suddenly, construction should be stopped.

Hole collapse

Phenomenon: abnormal increase in slag discharge, sudden increase in drilling rig load, or sudden generation of a large number of fine bubbles on the mud surface in the casing.

Treatment method: stop drilling immediately when a hole collapses, lift the drill bit, and prevent the drill bit from being buried. Because after the hole collapses, the arch effect of the destroyed soil suddenly disappears, and the hole wall enters an extremely unstable state. At this time, the most effective way is to let the backfill soil stand still.

The rebar cage floats up

Phenomenon: The rebar cage cannot be put into place smoothly when it is lowered, which is the most common problem in the construction of long spiral bored piles. Sometimes the rebar cage is too long and stuck in the hole halfway down. It can neither go down nor be lifted up, and is forced to be cut from the empty mouth.

Treatment method:

1) The concrete is not poured in place, and the drill is lifted too fast or too slow in the middle. Strictly control: the time of the concrete and its filling coefficient to ensure that the concrete filling coefficient is within the required range;

2) The slump is too large or too small. Check whether the concrete slump meets the requirements. If the slump is too large, it is easy to produce water seepage and segregation. In a short time, the coarse aggregate sinks, and the cement slurry floats on it. The downward resistance of the steel cage increases, failing the steel cage to be in place; if the slump is too small, it will still increase the difficulty of lowering the cage, so the slump must be strictly controlled within 18-22cm during the construction process;

3) The quality of the steel cage is too poor or the design is unreasonable. Strictly follow the specifications of the steel cage in the drawing, and there should be no problem with excessive dragon bars. When lowering the steel cage, the bottom of the steel cage should be made into a pointed shape to avoid the phenomenon of inserting the side;

4) The power of the selected vibrating hammer should be appropriate. When the steel cage is lowered to 1-2m from the hole mouth, it should be sent to the designed elevation by a vibrating hammer with a casing. Generally, for piles with a diameter of 400mm and a pile length not exceeding 10m, a 1.8kW vibrating hammer can meet the requirements; for piles with a diameter of 600mm and a pile length less than 15m, a 2.2kW vibrating hammer is suitable;

5) Check whether the drill bit and drill rod diameter meet the requirements. If the drill bit is found to be severely worn, it should be repaired in time, and alloy blocks should be inserted on the outer edge to ensure that the hole diameter and protective layer meet the requirements. There should be enough space between the steel cage and the hole wall to allow the steel cage to be smoothly in place.

Deviation of hole

Phenomenon: Deviation of holes often occurs in the process of bored pile construction. Deviation of the hole may occur when encountering large isolated rocks or probe stones (slope rock surface) in the punching hole, when punching into the inclined rock surface at the junction of soft and hard strata, or when punching into the sand and gravel layer with large particle size difference when the hammerhead swings to one side at the place where the hole is expanded when the base of the bored pile machine is not placed horizontally or uneven settlement and displacement occur.

Treatment methods:

1) When installing the punching pile driver, the chassis and base should be level, the lifting pulley rim, the clamping hole of the wire rope that fixes the hammerhead and the center of the casing should be on a vertical line, and they should be checked and corrected frequently;

2) When the hammer head is found to be unstable in impact (swinging left and right), the impact should be stopped to check whether a deviated hole has occurred. If a deviated hole has occurred, it should be handled in time;

3) When a deviated hole is found, first find out the cause of the deviated hole and the size of the deviated hole. If it is caused by the hammer head hitting a solitary rock or a probe stone (slope rock surface), fill the hole with stones in time, and impact when the backfill reaches 1m above the rock surface.

Plum blossom hole (or cross hole)

Phenomenon: When punching a bored pile, the hole is not round.

Treatment method:

1) Select mud with appropriate viscosity and relative density, and remove slag in time;

2) When using a low stroke, use a higher stroke for each impact, and alternately impact to adjust the hole shape;

3) After a plum blossom hole appears, fill the hole with clay mixed with flakes and pebbles and impact again.

Quality issues

1) Errors in measurement and layout, which may cause errors in the entire building or large deviations in the pile foundation.

2) The bearing capacity of a single pile does not meet the design standards for building pile foundations.

3) The pile body is tilted too much.

4) The joints of precast piles are disconnected.

5) Pile breakage accidents caused by the pouring process.

6) During the acceptance process, it is found that the pile position has too large deviations.

7) Due to the quality or operation of the concrete, the pile body has problems such as mud inclusion, segregation, and substandard strength.

8) Insufficient elevation.

Treatment measures

Supplementary sinking method

This method can be used when the precast pile is not deep enough into the soil, or when the pile is lifted up due to soil uplift.

Supplementary pile method

It is to supplement the pile according to the supplementary pile plan issued by the design unit in conjunction with the opinions of the design, supervisor, and owner, but this method has a large investment and a long construction period, and it is difficult to be recognized by all parties.

Supplementary delivery combined method

When the pile is driven into the foundation by means of a segmented connection, if the quality of the pile body does not meet the standard, then during the sinking process, it is possible that the connection node will be disconnected. At this time, the treatment of the pile foundation can be combined with supplementary delivery.

For doubtful piles, they should be re-driven to make the pile sink, which can tighten the disconnected pile connection again and make the joint have vertical bearing capacity; additional piles can be added by appropriately adding some complete piles so that the foundation can not only meet the bearing capacity requirements, but also increase the seismic load of the building foundation.

Correction method

If the pile body is found to be tilted during the piling process and the pile is not long and intact, or the pile body is tilted due to the excavation of the foundation pit but the pile body is still intact, the pile body can be partially excavated and then corrected using a jack.

Expansion of the pedestal

1) If there is a large deviation in the pile position: the original pedestal design size cannot meet the normative requirements of the structure, then the deviation of the pile position can be corrected by appropriately expanding the pedestal area.

2) Pile-soil interaction. If a single pile cannot meet the design bearing capacity requirements, it is necessary to expand the pedestal, and the upper load borne by the natural foundation and the pile must also be taken into account.

3) Pile foundation quality issues. Uneven pile foundation quality is prone to occur during construction. In order to prevent the pedestal layout cloud settlement caused by pile foundation quality issues in the later stage and to improve the seismic resistance of the building, an integral pile foundation pedestal can be used to improve the integrity of the foundation.

Composite foundation method This method

1) Use the principle of pile-soil interaction to properly treat the foundation, improve the bearing capacity of the foundation, and more effectively share the load of the pile foundation.

2) Make a soil replacement foundation under the pedestal. Before the construction of the pile foundation pedestal, dig out a certain depth of soil, replace it with sand and gravel filling layers, tamp it in layers, and then construct the pedestal on the artificial foundation and pile foundation.

3) Add cement soil piles between piles. When the pile-bearing capacity does not meet the design requirements, the method of dry spraying cement in the soil between piles to form cement soil piles can be used to form a composite foundation.

Modify pile type or pile sinking parameter method

1) Change the pile type. For example, change the prefabricated square pile to a prestressed pipe pile.

2) Change the depth of the pile into the soil. For example, when the prefabricated pile encounters a thick dense silt sand or silt soil layer during the penetration process, it is difficult for the pile to sink, and even a pile break accident occurs. At this time, the pile length can be shortened, the number of piles can be increased, and the dense silt sand layer (expansive soil layer) can be used as the bearing layer.

3) Change the pile position. If a hard, small underground obstacle is encountered during the pile sinking, causing the pile to tilt or even break, the pile position can be changed and the pile can be re-sunk.

4) Change the pile-sinking equipment. When the pile sinking depth does not meet the design requirements, a large-tonnage pile frame can be used to sink the pile using the heavy hammer low-hit method.

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