Pile Jacking Stop Conditions | Fundamentals for Structural Engineering
Jacking piles are a common type of foundation when constructing urban structures. The nature of the load-bearing structure of a driven pile is no different from a driven pile, only the construction method is different. Due to the nature of construction in densely populated areas, using the driving method to construct piles causes many disadvantages in terms of noise and vibration, causing damage to surrounding structures. Another advantage of driven piles, constructed by the downward compression force created by hydraulic jacks, is that the compression force can be measured throughout the construction process. This helps provide data for making construction and acceptance decisions, for design engineers to handle when needed.
Parameters
The construction design project needs to provide the following information: pile size, pile depth, load capacity of single pile P. For construction, it is necessary to add: Pmin, Pmax, Lmin.
Pmin: Minimum pressure, ensuring that the pile penetrates the soil layers to a depth sufficient to withstand the load P according to the design.
Pmax: Maximum pressure, not to be exceeded when driving the pile. The meaning is to ensure safety, not to break the pile due to pressure.
The design engineer specifies Pmin, Pmax based on experience. Usually, Pmin = (1.5-2)P; Pmax = (2-3)P and is smaller than the load capacity according to the $P_{VL}$ pile material. The larger the number, the stronger the pile must penetrate or get caught in the lens. For example, medium-dense sand, hard clay… In many cases, when designing a pile tip penetrating a sand layer over 10m thick, Pmax may need to be taken up to 90%$P_{VL}$ to overcome the resistance of the soil when lowering the pile.
Lmin: Minimum pile depth based on the calculation of the predicted pile bearing capacity according to the ground (the most common method is to calculate P according to SPT), taking into account the variation in depth of the boreholes according to the results of the geological survey.
When to stop pile jacking?
1️⃣If the depth has reached Lmin but the driving force has not reached Pmin, increase the driving force to Pmin then stop.
2️⃣If the driving force has reached Pmin but the depth has not reached Lmin, increase the driving force until it reaches Lmin then stop.
3️⃣If the driving force has increased to Pmax but the depth has not reached Lmin. Must stop and consult the design consultant: Is there any design adjustment needed.
How to stop pile jacking? Construction contractors and site supervisors need to pay attention to the provisions of TCVN 9394:2012 “Driving and pressing piles – Construction and acceptance standards”, article 7.8: The pressing force must be maintained long enough for the pile velocity to be ≤1cm/s in a distance ≤3 times the diameter (or edge) of the pile. Because in reality, there are many cases where the pressing force has just reached the above conditions and stopped pressing immediately even though the force has not been maintained, the pile can still go down, leading to unsatisfactory static compression test results.
Handling the situation
When falling into situation 3️⃣, what should the designer consider to make a decision on how to handle it. Is this smaller depth acceptable? As mentioned, the highest basis is the Static Compression test result, but how to decide before testing. Is it acceptable to compress the pile that has not reached Lmin? Or is it recommended to press another pile to reach Limin before testing?
At this time, it is necessary to perform the calculation of pile foundation settlement, with a pile depth smaller than Lmin. Settlement conditions according to TCVN 10304:2014 “Pile foundation – Design standards”:
– Absolute settlement is not less than 15cm
– Differential settlement between foundations does not exceed 0.003
(These limit numbers correspond to common cases of civil houses, other cases according to Appendix E of the standard).
The method of calculating the settlement of conventional pile foundations is Conventional Block Foundation, considering the entire pile cluster and pile cap as an equivalent single foundation causing settlement of soil layers at a depth from the pile tip down. The calculation results fall into:
🔲Case 1: ensuring settlement conditions, the design consultant can agree to reduce the depth to conduct a static compression test.
🔲Case 2: not ensuring settlement conditions, it is necessary to consider increasing the pressing force Pmax, which leads to an increase in pile size. Even consider the option of pilot drilling before driving or switching to bored piles. The reason is that at this time the soil is too hard, the pile cannot penetrate to the depth to work as desired by design.
After having the results of the static compression test, the design engineer re-specifies the values of Pmin, Pmax, Lmin in the Official Dispatch approving the construction of mass piles. This is the legal basis for the conditions for stopping pile driving for construction and acceptance. Static compression results have 2 cases🤭:
🔲Case 1: achieved. Can keep the values of P, Pmin, Pmax, Lmin and enter them in the document.
🔲Case 2: not achieved, pile is damaged. P can be reduced compared to the forecast at the time of design and calculated according to the static compression test results. Other numbers also need to be changed accordingly.
The above is a basic problem of pile construction, requiring the participation of judgment and handling of the designer. Some experiences as mentioned, hopefully can help you in many practical situations encountered.
