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Piles - Part 1. Introduction

posted Sep 6, 2021, 8:14 AM by jeffery jim
A request came in this afternoon from a friend and also a fellow consulting engineer to discuss about piles and geotechnic application. The inquiry is quite complicated and I will divide this question into parts so that there is continuity in this discussion which involves a very long article and revolves around various literature. Therefore, I will start with introduction to those who are quite new or not so familiar with piles and deep foundation.

2. Pile is generally a design option involving deep foundation and comes in various types, shapes and dimensions. There is no single pile system which is superior than the rest. Like most engineers, one have to consider design requirements as well as financial limits. Engineering is an empirical process which combines functionality, safety and cost efficiency. Without these criteria, any construction will not need engineer as overdesign will always lead to very high safety factor.

3. Foundation is essential as it is meant to transfer loads, bearings and/or stresses from the superstructure to the substructure and subsequently to the ground. For shallow foundation, these burdens are usually transfer directly to the ground surface and the ability or strength of the soil is called soil bearing capacity. This is sufficient when the overall load is low or small or the structure itself is not critical from other stresses. For larger loads (hereafter considered as a combination of building loads, stresses and bearings) may need to be considered for larger building, larger load or functions with higher performance during the construction period due to temporary staging or platform. When loads propagate from the top and require higher bearing capacity, shallow foundation may no longer be be economic or engineeringly sound. This requires a combination of skin friction from the soil and end bearing from hard soil or rock.

4. When deep foundation is the choice for a particular design, the specific type, size and dimension shall be determined. Deep foundation on pile is a slender structure with stilt-like shape penetrate deep into the ground. Due to its shape which is slender, this structural member is considered quite vulnerable if placed under extreme condition and/or under constant duress. Pile will have to react to vertical load most of the time and usually is considered and designed to be able to resist lateral load as well. Lateral load is vital as part of the design criteria as both the building and ground are dynamic in nature. Buildings sway when react to wind load as well as a reaction to its natural frequency. Ground on the other hand is constantly on the move when induced by seismic or earthquake, or even in a continuous progression when sitting on geologically unsteady materials such as colluvium. The other forces which which may not be considered in design would be the accidental load and hydrostatic pressure. This is something designers may consider or written off when considering on the safety factor and/or when deriving partial safety factor for each cases.

5. The pile must have adequate reinforcement bars and desired compressive strength for the concrete. Piles are usually categorized based on its capacity to carry load and classified in three groups based on SIRIM or Malaysia Standard. M Class which complies with MS standard and British/Euro Standard, J Class which complies with the older JKR standard, and the commonly available S class which is usually used for commercial purpose in most residential construction by developers. S class piles have reduced reinforcement percentage which does not mean it is inferior but maybe require additional considerations for certain engineering works. The main concern probably be when this class of pile is exposed to extreme lateral load and in certain soil type which may lead to possible failure when developing plastic hinge at certain length of the driven shaft.

6. As known to many, the vertical or axial load is the main concern when taking up load. The criteria of a pile depends on the type of pile mobilized and type of construction. Generically, pile are considered to have sufficient capacity when it is able to withstand double working (axial/vertical) load and certain degree of lateral force depending on geotechnical or structural application. For certain geotechnical applications; the slip, slide or rotation criterium may need to extend up to 3.0 for factor of safety. Hence, it is open to discussion or consideration depending on literature adopted by the engineer himself. Test shall be conducted to reaffirm the design considerations are appropriate and it is workable until twice the working load. The termination criteria for piles varies depending on the pile used. Driven pile are guided by its set criteria which is derived from Hiley Formula. Bored pile depth dictates by total length of the shaft and surface area which provides sufficient friction surface as load resistance, and the socket length depending on the end bearing and the capacity to clean the end of the shaft after completing boring works.

7. There are several tests to be conducted but it is common to have test piles at area of concern or perhaps more when there is sudden or drastic change to geological or geotechnical consideration apart from statistically representation of (two percent) whole foundation capacity. Static Load Test (STL) and Dynamic Load Test (DLT) are test conducted on the conditioned pile. STL can be in many forms such as Maintained Load Test or Bi-Directional Static Load Test (BDSLT). DLT in the other hand is usually done via Pile Dynamics Analyzer (PDA) for high strain. These tests have their own benefits and DLT is usually a confirmatory test on the load capacity aside from checking pile's structural integrity. When talking about STL, it is preferable to conduct BDSLT where this test usually compliment PDA's integrity test with its earlier checking through sonic logging test. Ultrasonic pulses are introduced and integrity or defects can be determined through time of arrival from pulse generated in sections of piles. BDSLT may cost as much as MLT but it is time saving test and extremely low in risk.

8. MLT will ensure the pile have to ability to settled in certain depth when working loads are introduced. Residual settlement will confirm the pile's ability to be in elastic condition or perhaps requires further probe on elastic shortening shall the residual settlement indicate lack of movement after load is discharged. If test indicates any failure, test pile is usually abandoned and a new pile test location nearby is suggested. If test indicates failure of a pile on a working pile, remedy shall be made by enlargement or increasing in numbers of piles as part of a group pile. Failure in test requires engineer to return to the drawing board and redesign their piles and reconsider other options.

9. When the test pile failed, it can be due to many factors from design, ignorance, site investigation, exploration and test, construction method, other disturbances and/or combination of these factors. The most common problem is due to the inability to derive the soil strata and its capacity. This is the case when no additional or complementary boreholes nearby is allowed for in the construction cost. Soil profile changes every ten feet or three meter, while geological condition can change slightly within this span of distance. Worst, when engineers are not competent to predetermine the inter-bedding of a geological formation especially at vicinity which have chaotic soil dispersion mainly in alluvial region and close to water bodies.

10. The second most common problem is the independence in execution handed to third party laboratory when conducting exploration. This is usually the problem faced when the design engineer have no prior knowledge or failed to conduct desktop study for due diligence purposes when assigning borehole location(s). The chosen location may not be statistically representative and the failure of laboratory to explore deeper. Usually, the termination depth required is after five consecutive flights of N>50 soil or five consecutive successful rock coring. I personally will require at least six consecutive layers of soil N>50 or five consecutive rock coring if the Rock Quality Designation (RQD) may achieve 30% at certain stratum or six layers if none achieved RQD>30%. Recovery ratio is another indicator which I persistently get updated during the exploration process.

11. Pile failure during the driving process may happen when pile was driven into floater or perhaps it is due to QAQC issues where not fully cured concrete pile or broken pile (during pitching) is used. The wrong shoe can also cause issues when driving test pile.

12. One of the important questions is about selection the pile shape and its resistance when exposed to lateral load. Well, there are a lot of literature about this issue but then again, square pile have a multiplier of 1.2 and H piles, 1.5 when compare to a round pile of similar properties. Is this something we may need to consider when selecting a pile type? Yes, if you want to have a rigid design when applying for high ductility structure in seismically active region, square and/or H piles is/are best. Naturally, H piles is the preferred choice for many Japanese engineers when introducing raking piles for their building. In seismic area and/or when colluvium is a concern for smaller building, I recommend the use of round pile as I have discussed with a fellow friend and a doctorate holder. The reason behind the selection of shallow foundation with short round pile and smaller circumference would allow the raft foundation to swift based on the ground movement. It is almost like contiguous pile system where movement is permissible and desired without introducing much lateral stress which may end up with broken pile due to plastic hinge. Similar goes with bridge design, it is very much depending on semi-integral or fully integral bridge design.

13. I hope this short note will give you some insights about piles. In coming days or weeks, I will write more on piles with interesting coverage. You can comment below if you need a specific coverage on certain issue involving piles or deep foundation.

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