This article should not have been written. But surprisingly, it often comes up in the actual design of pile foundations. Causing heated debates between Design Consultants, Investors, Examiners, Appraisers, and Acceptance 😔.
Typical situation
Column foot load on the foundation is 450T. The calculated bearing capacity of 1 pile is 100T. The design arranges 4 piles. The examiner says it is insufficient, it must be 5 piles.
The examiner’s argument: 450T/4 = 112.5T > 100T. Therefore, the pile exceeds its bearing capacity and fails. It sounds so reasonable and pleasing to the ears.
Wait a minute… 😵
In a controversial situation, who has the authority to judge? The Standard must be used as an arbitrator. The current effective standard is TCVN 10304:2014 “Pile foundation – Design standards”.
📚Dictionary
✒️Pile bearing capacity:
The maximum load that a pile can withstand.
Standard 10304 has a more confusing, but less misleading definition, in section 3.11:
Bearing resistance of a single pile:The ultimate resistance of the ground to a single pile under the condition of limiting excessive development of shear deformation in the ground.
According to the old standard TCXDVN 205:1998, Pile bearing capacity includes: ultimate bearing capacity Qu (u-Ultimate), allowable bearing capacity Qa (a-Allowable):
✒️Ultimate bearing capacity:
is the maximum load-bearing capacity of the pile before failure, determined by calculation or experiment.
✒️Allowable load capacity:
is the load value that the pile is capable of carrying, determined by dividing the ultimate load capacity by the specified safety factor.
Thus, the concept of Pile Bearing Capacity, according to the new standard, is equivalent to the Allowable Bearing Capacity of a single pile according to the old standard.
Why argue?
Due to the habit from the old standard 205. In which, the pile group effect is mentioned in section 3.9.3, 3.9.5 but does not specify the number. Therefore, just take it all according to the allowable bearing capacity of a single pile.
✒️Pile group effect:
Due to the interaction between piles in the group, the settlement of the group as well as the Bearing Capacity of the pile in the group will be different from that of a single pile.
Explanation: The surrounding piles contribute to compacting the soil. Making the soil around the pile withstand a larger load than when the pile stands alone. The more piles there are, the bearing capacity tends to increase.
The pile group effect will gradually decrease as the distance between the piles increases. The standard does not specify the distance between the piles, so the Group Effect is ignored. According to some textbooks, the number is 6d. (d is the pile diameter). Of course, to save 💰, the design always arranges the smallest possible pile spacing to minimize the size of the foundation. Therefore, the Group Effect always comes into play.
Everything has two sides, the disadvantage of the Pile Group Effect is that it increases settlement.
According to the new standard 10304, the allowable bearing capacity of the pile is determined by the formula:
$$Q_a=\frac{\gamma_o}{\gamma_n}\frac{R_{c,k}}{\gamma_k}$$
From one value of $R_{c,u} (R_{c,k})$, several values of $Q_a$ will be obtained depending on the cases of $\gamma_o,\gamma_n,\gamma_k$ taken according to section 7.1.11.
In the table above, the pile group coefficient depends on $\gamma_o/\gamma_k$ of each case. Because γn is the same. Take the case of a foundation with only 1 pile as a benchmark (coefficient equal to 1). For foundations with more piles, this coefficient shows how many times the load-bearing capacity of the pile in the group increases.
The table above describes the most common case, the base of the foundation is on weak soil (section 7.1.11.b). Because the soil is weak, pile foundations must be used. In less common cases, the base of the foundation is on good soil, or the piles are resistant to compression (section 7.1.11.a). Then γk remains unchanged, only γo is different, leading to the pile group coefficient of 1.15 for groups with more than 1 pile.
In the case of determining the pile load-bearing capacity from static compression tests, the coefficients γ are slightly different. Use the values in parentheses () of section 7.1.11.
In summary, the new standard 10304 has quantified the pile group effect.
Another case where piles are allowed to withstand loads greater than the design load capacity is when the foundation is subjected to wind loads. The load on the pile is allowed to exceed 20% (according to note 2 of section 7.1.11 of the standard). Note that this does not apply to earthquake combinations. In this case, the pile load is reduced compared to the case of only bearing vertical loads (self-weight and service loads). See TCVN 9386:2012 volume 2 section 5.4.1.2 (6). The coefficient is usually 0.9 (reduced by 10%).
Impact on the wallet💰
Answer to the controversial example above: take the 10304 standard as the referee. Apply the pile group coefficient for a 4-pile foundation of 1.15. That means each pile in a 4-pile foundation can withstand a load of 115T compared to only 100T when in a 1-pile foundation. So these 4 piles can withstand 4×115= 460T > 450T, bearing the load on the foundation.
Money: D400 diameter centrifugal piles 30m deep. Market price 550,000 VND/m depth. Including deposit and pressing labor. Reducing 1 pile saves 30×550,000 VND = 16,500,000 VND. The house has 100 such foundations. The argument that the auditors find appealing is costly for the investor 1.65 billion 💰💰💰 Not to mention the cost for each larger foundation if making 5 piles. The price to pay for the feeling of love is sometimes not cheap 🥰
However, when it comes to the emotional category, the Design Consultant should not argue. Convincing a biased heart is sometimes impossible. That’s when soft skills come into play, but that’s not the topic of this article😉
Note for the Engineer
- Clearly state the load-bearing capacity of a single pile. Be more careful and list the pile load-bearing capacity table corresponding to a foundation of 1 pile, 2-5 piles, 6-10 piles… This is actually unnecessary, as the new standard has defined the Pile Load-bearing Capacity above. But anyway, to avoid controversy.
- The bearing capacity of the pile material (reinforced concrete) must be greater than the load after multiplying the pile group coefficients and the coefficient of 1.2 allows for an increase when subjected to wind loads. In the above example, a pile when subjected to wind can be up to 1.2×1.15x100T = 138 tons.
