Based on Terzaghi’s bearing capacity equation, there are three components in contribution to the bearing capacity:

(i) Surcharge pressure
Foundations are normally not placed directly on the ground level. Instead, they are installed at a depth below the existing ground level. The soil pressure arising from the depth of soils serves as a surcharge imposing a uniform pressure at foundation level.

(ii) Self-weight of soils
The self-weight of soils contribute to the bearing capacity and is represented by 0.5rBN_r (r=density of soils).

(iii) Shear strength
The shear strength of soils contributes to the bearing capacity and is
represented by cN_c.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

In final set table it is commonly to limit pile set between 25mm/10 blows and 50mm/10 blows. The reason of the provision for lower limit of set value of 25mm/10 blows is to avoid possible damage at pile toe owing to compressive stress reflected at the bottom of pile. The upper limit of 50mm/10 blows is established principally to avoid very heavy hammer impacting at pile head so that the pile head may be damaged owing to excessive compressive stress. These limitations are believed to be applied originally to precast concrete piles.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

The use of high grade and heavy steel pipes may not warrant the use of drop hammer. For pile driving of Grade 55C steel section, it requires heavier drop hammer with increased drop height so as to comply with penetration resistance at final set. As such, it is possible that such heavy hammer may damage the pile and endangers the worker who takes the final set record.

Though hydraulic hammer is commonly used as standard hammer for driving piles, some piling contractors prefer to employ drop hammers to take final sets. The obvious reason of such practice is that it is convenient for drop hammer to change energy input easily to cater for the range of penetration in final set. On the other hand, the use of hydraulic hammers for setting of piles requires the knowledge of transfer of energy to pile head and hence needs the input from wave equation analysis.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

Direct coring method is commonly employed to check the quality and workmanship of bored piles. It is further divided into two main types, namely interface coring (pile/rock interface) and full coring (entire concrete pile length). In the past, the implementation of full-depth coring usually takes place near the pile centre to prevent physical conflicts with reinforcement cage. When placing concrete by tremie method, concrete pouring by tremie pipe is carried out near pile centre and soft and weak materials are usually displaced to the sides of bored piles. As a result, the pile/rock interface at pile centre should indicate the best results. However, with the recent trend of using interface coring through reservation tube attached to the reinforcement cage, there is a high chance that interface coring gives poor results as the reinforcement cage may trap soft and weak materials and they are located far from the pile centre.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

Under the 45^o load spread rule, when the horizontal distance between the toe level of adjacent piles is smaller than the vertical difference between the piles, additional load is deemed to be added by the pile at lower founding level. Hence, when the pile at lower founding level is within the zone of 45^o spreading from the pile at higher elevation, further load checking is required for the lower pile.

This 45^o load spread rule is a common practice of foundation engineers and may not be incorporated in foundation codes. There are some situations where this rule may fail. For instance, when two piles at the same foundation level are situated close to each other, they shall have load effect on each other. However, based on 45^o load spread rule, it could not address this concern.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.