What are the design requirements of pile foundation?

What is the design of pile foundation? What are the requirements? Please read the article edited by Zhong Da Consulting Company.

With the development of economy, all kinds of high-rise buildings have sprung up in cities. As the basic part of high-rise buildings, they often occupy a large proportion in the whole construction investment. Pile foundation is often used in high-rise foundations. How to choose a reasonable pile foundation form plays an important role in ensuring safety, saving investment and reducing costs. This requires our designers to carefully analyze the survey report of each building and choose an optimal foundation scheme. The author discusses the problems worthy of attention in pile foundation design from the following aspects.

1. Importance of static load test in pile foundation design

At present, the design process of pile foundation is often limited by time. Firstly, the design value of single pile bearing capacity is determined according to the parameters provided by geological report, and the pile foundation design and construction are directly carried out according to this estimated single pile bearing capacity. After the construction of engineering piles is completed, test piles are selected for static load test. This process is quite unscientific. Everyone will be very happy if the result meets the forecast requirements. Otherwise, it will be very difficult to fill the pile because the project has been completed, and sometimes it will bring considerable inconvenience to the construction because the geological report is inconsistent. There are two main problems here, which are illustrated by the following examples. First of all, according to the standard value of soil friction around the pile and the standard value of pile tip bearing capacity provided by the geological report, the standard value of bearing capacity of single pile in the site is calculated according to JGJ94-94 specification, which is empirical and should not be directly adopted. In recent years, the author found that the actual bearing capacity of most piles is greater than the calculated value, and some of them are quite different. Therefore, the actual bearing capacity obtained from the test pile will produce greater economic benefits than the bearing capacity estimated in the survey report.

For example, the author once designed Du Nan Linglongwan Garden Residence in Suzhou Industrial Park, which is a high-rise residence with one underground floor and eighteen floors above ground. According to the geological survey report, D500 prestressed pipe pile with a length of 20m is proposed. According to formula 5.2.8 of JGJ94-94, it is estimated that the design value of single pile bearing capacity is about 1.400 kN. According to the three destructive test piles I requested, the actual bearing capacity of single pile can reach 65,438. Secondly, in the case of uneven site or deviation of geological report value, it will bring great difficulties and unnecessary waste to carry out engineering pile construction directly according to geological report without pile test. For example, in a five-story commercial and residential building in Weiting, according to the geological report, precast square piles are used, with a pile length of 10m and a pile diameter of 400x400, and the ultimate standard value of single pile bearing capacity is about 1350kN. In actual construction, almost every pile is pressed to 2000kN, but it has reached the pile strength of precast piles, so split piles are adopted in the construction process.

All engineering piles that fail to reach the design elevation in static load test meet the design bearing capacity, that is to say, if the pile is tested first, the pile length can be shortened by at least 1.5m, and the bearing capacity of the pile will not be reduced without splitting the pile. As can be seen from the above, static load test is a very important link in the process of pile foundation design. Because the quality of the secondary project directly affects the determination of pile foundation form, pile specifications and pile burial depth, it also has a close influence on the construction difficulty. Obtaining accurate data through scientific experiments can make the design scheme more reasonable, feasible and economical, far exceeding the benefits obtained by shortening the construction period.

2. The importance of pile type and pile length design in pile foundation design.

Reasonable selection of pile type and pile length in pile foundation design will have great influence on foundation design, and reasonable selection of pile type and pile length will produce huge economic benefits. In the design of "Kunshan Huadi" residence, due to the consideration of time (there are ready-made D400 prestressed pipe piles), Party A requires the use of D400 prestressed pipe piles. According to the geological report, the pile length L= 16m, the ultimate standard value of single pile bearing capacity is 850kN, and the cost of the basic part of the budget is about 160 yuan /m2, which accounts for a considerable proportion of the total housing cost. In the subsequent design, the author changed the pile type to 250x250 precast reinforced concrete square pile, and the ultimate standard value of single pile bearing capacity was about 600kN. The local construction price of precast square piles is only about 50 yuan /m, while the unit price of prestressed pipe piles is about 100 yuan/m. After using square piles, the estimated cost is about 90 yuan /m2, which has obvious comprehensive economic value.

It can be seen that choosing a reasonable pile type has a great influence on the project cost. In the design of pile foundation, the selection of pile length is also very important. According to the investigation report, D500 prestressed pipe pile was adopted in the design of pile-raft foundation of a high-rise residential building. Optional pile length is 25m, single pile bearing capacity characteristic value Ra = 900kN, pile length 34m, single pile bearing capacity characteristic value Ra= 1300kN. 25m piles, about 290 piles are needed. For 34m piles, 200 engineering piles are needed. From the pile itself, the total number of engineering piles in the two schemes is equivalent, but let's analyze the corresponding raft design. When 25m piles are fully paved, the required raft thickness is about 1200mm, and when 34m piles are under the wall, the raft thickness can be reduced to 900mm, which has obvious economic benefits. Therefore, our designers must adopt multi-scheme comparison in pile foundation design and choose reasonable pile type and length, which has great influence on the rationality and economy of the whole foundation design. Of course, many factors such as construction feasibility should also be considered.

Three. Control and treatment of pile deviation

In the construction of pile foundation, the deviation of pile must be strictly controlled, especially for pile caps and strip piles, the deviation of pile position will produce great additional internal force, which will make the foundation design in an unsafe state. We mainly control two aspects of pile position deviation, one is vertical deviation. According to article 7.4. 12 of JGJ94-94, we control the allowable deviation of pile top elevation at -50~+ 100mm, but such a large deviation in actual construction will cause heavy construction tasks and losses. When the elevation of the pile top is higher than the design elevation, it is necessary to split the pile, especially for hollow piles such as prestressed pipe piles, it is difficult and uneconomical to split the pile with a cap at the top. When the elevation of the pile top is lower than the design elevation, it is necessary to repair the pile head, which not only affects the construction period, but also wastes money. This requires the construction unit to strictly control the elevation of the pile top in the construction process, so that the elevation of the engineering pile is consistent with the design as much as possible, especially in the construction process, the return of the pile after unloading must be considered, otherwise each pile will be higher than the design elevation without consideration. Our designers should also consider the construction error in the design process. The author suggests that according to the current construction quality, the deviation tolerance of about 2mm can be considered in the design, so that a large number of small deviation piles can be avoided, which is quite operable in practical projects and avoids a lot of unnecessary work. The second is the horizontal deviation of pile position.

According to Article 7.4. 1 1 of JGJ94-94, the deviation of each pile position is controlled. If the deviation of pile position is large during construction, it should be repaired in time. Here, for the pile foundation with 4~ 16 caps, the allowable deviation specified in Article 7.4. 1 1 of JGJ94-94 Code is 1/3 pile diameter or 1/3 side length, while according to Article 5 of GB50202-2002. In addition, for small diameter piles (D≤250), the author emphasizes that the deviation must be strictly controlled, not according to the above standards. The author suggests that the top of the pile can be controlled by 70mm. For strip caps, it is 50mm perpendicular to the direction of strip caps and 70 mm parallel to the direction of caps. Of course, these requirements must be made clear before construction. Of course, if the deviation of pile position meets the specification or design requirements, it only means that the pile foundation itself is qualified, and the overall eccentricity of the pile cap or the loss of foundation height caused by it must be dealt with separately. For the eccentricity of the pile, we can solve it by increasing the stiffness of the pile cap or tension beam and steel bar, which needs to be dealt with according to the specific situation in practical engineering.

Four. Treatment of special circumstances in construction

Because of the unknowability of stratum, many abnormal situations are often encountered in pile foundation construction, which requires us to analyze carefully and adopt appropriate methods to solve various problems according to specific conditions.

1) The pile foundation reaches the ultimate bearing capacity and cannot be pressed to the design elevation. There may be two situations here. One is that the geological report is wrong, and the actual bearing capacity of the pile is greater than the calculated value, so it is necessary to test the pile first to determine its reasonable pile length and bearing capacity. Secondly, it may be due to the soil itself, for example, the pore water pressure generated by saturated sand makes the pile foundation unable to be pressed in, which requires us to solve it from the construction measures. First of all, it is necessary to formulate a reasonable construction sequence, such as jumping piles, so that the water pressure generated by the piles constructed in advance will dissipate before the next pile construction; Secondly, for static pressure piles, we must choose construction machinery with sufficient pile pressing force to avoid the phenomenon of lifting the machine; In addition, measures such as introducing holes and setting drainage holes can be taken to reduce the interstitial water pressure as much as possible. Of course, when pressing piles, we must pay attention to controlling the pile pressing force within the ultimate strength range of the pile body, and pay attention to the influence of pressing piles and squeezing soil on the surrounding buildings.

2) During pile foundation construction, the pile pressing force is far lower than the design bearing capacity. D400 prestressed pipe pile is used in the small high-rise residence of Suzhou Xu Xu Apartment, with a pile length of 18m. According to the geological survey report, the design value of pile bearing capacity is 650kN, and the maximum pile pressing force of four consecutive piles is only 300kN, which is far less than the design bearing capacity. We have carefully analyzed the survey report, and think that the characteristics of each soil layer provided by the report are basically accurate, and the geological reports of other surrounding projects also prove the correctness of the survey report. Therefore, we analyze that the pile pressing speed of the pile pressing machine is relatively fast, and the cohesion of the soil layer is relatively small, so the pile directly cuts the soil when pressing the pile, resulting in low pile pressing force, and the soil can be consolidated after a period of time. The pile test after 15 days proves that our judgment is accurate and the test bearing capacity meets the design requirements. This also emphasizes the importance of static load test pile first from the side.

3) The static load test of pile foundation is unqualified. Due to the construction period, Party A requires that the pile test be carried out at the same time as the engineering pile. When the test pile meets appendix c.0.6 of JGJ94-94, carry out static load test. As a result, one of the three groups of test piles meets the design requirements, and the other two groups of test piles fail when they are lower than the design bearing capacity. This allows us to analyze these two groups of test piles from the aspects of design, construction and test, but after comparing the surrounding projects and analyzing the field construction test records, no special circumstances are found, that is, there are no mistakes in construction and test. The author compared the first group of qualified test piles, and finally found that the rest of the test piles in the latter two groups had enough rest time, but the construction of surrounding engineering piles could not be completed until two days before the test. It is completely reasonable that the soil around the test pile will be destroyed and not consolidated during the construction of the engineering pile, thus affecting the bearing capacity of the test pile. Therefore, when the stop time of engineering piles also meets the appendix c.0.6 of JGJ94-94, the static load tests of two test piles are carried out again, and the results are completely consistent with our judgment, and the test piles all meet the design requirements. This example tells us that there are many factors that affect the test pile results. We must carefully analyze all kinds of situations in engineering practice and find out the problems, instead of blindly handling them, causing unnecessary losses and waste.

4) Pipe pile crack treatment. Compared with precast pile, prestressed pipe pile has the advantages of high strength, short manufacturing cycle and material saving, and is widely used in engineering design, but it also has the disadvantage of poor shear resistance. In engineering practice, cracks often appear in the pipe wall due to verticality deviation or soil squeezing, which affects the quality. In a project in Kunshan, due to the low elevation of the natural ground, about 2m of soil was backfilled in the site before construction, and no proper measures were taken during construction, resulting in uneven lateral pressure of the pile driver on the pile. After construction, it is found that some piles have lateral deviation, and through small strain detection, it is found that these piles have cracks in different degrees, so how to deal with them is quite critical.

The above are collected and sorted by Zhong Da Consulting Company.

For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd