According to current pile foundation codes, the characteristic value of single pile bearing capacity consists of pile side resistance and pile tip resistance, considering only the reaction of the soil along the pile side and at the pile tip, with no consideration whatsoever of the influence of hydrogeological conditions on single pile bearing capacity.

1. Soil is divided into relative aquicludes and permeable layers. Generally speaking, a long pile penetrates several soil layers and involves several groundwater layers. Does the pile installation process change the groundwater occurrence conditions and environment? What effect do the changed conditions and environment have on the soil? What effect do they have on single pile bearing capacity?

2. What if the pile penetrates confined water?

3. If the pile does not penetrate confined water but is not far from the roof of the confined aquifer, what happens if it penetrates the confined water under load? Will it cause instability and failure of the soil at the pile tip?

4. Does groundwater act as a lubricant between the pile side and the soil, thereby reducing single pile bearing capacity?

Views from the Field

1. This is rather complex. In principle, changing the hydrogeological conditions changes the groundwater pressure, and thus changes the effective stress of the soil.

2. Under normal circumstances, the influence on single pile bearing capacity is small or relatively negligible. Of course, special conditions require special treatment. I believe that the influence of groundwater below the water table has already been taken into account when providing the side resistance and tip resistance parameters. Therefore, as long as the stratigraphic hydrogeological conditions are not disturbed by pile foundation construction, the influence of hydrogeological conditions need not be considered when calculating single pile bearing capacity.

3. The so-called lubricating effect of water on the pile is relatively small and can be disregarded; however, if confined water is penetrated, that is indeed a problem. This should be avoided during both the investigation and pile testing stages; I have never encountered such a situation.

4. This is indeed the case, especially when the water level changes, the bearing capacity of the pile foundation varies greatly. This has been confirmed especially for pile foundations constructed near the Yangtze River, because the silty sand in this area has a certain hydraulic connection with the Yangtze River water. Due to the tidal effect of water level fluctuations, the pile bearing capacity measured for piles constructed at different times (day and night) after pile installation also differs, and the difference is quite large!

5. There should be an influence; the main question is how large it is and how to deal with it if it must be considered. Have the side resistance and tip resistance data provided in the code already taken hydrogeological conditions into account?

6. First, this influence does indeed exist. However, in engineering it is generally ignored. The reason: aquicludes generally have a certain thickness, and after the pile penetrates, the pile-soil interface is relatively tight and generally does not transmit water, unless the pile body itself is a loose structure; even if there are a few channels, the clay fraction (which should be present in the impermeable layer) can clog them. This is similar to discussing whether bearing capacity increases when the pile tip rests exactly on a small boulder—too microscopic, and generally not discussed.

7. It is better to make the pile foundation safer, so that the adverse factors caused by changes in hydrogeological conditions can be offset.

8. In permeable layers (silty sand), the pile side resistance changes only under dynamic water pressure. This situation occurs in construction near the river, mainly because water level fluctuations cause fluctuations in the recovery time of soil friction and negative friction during ebb tide. In the Changzhou area, there is generally a silty sand layer 7–15 m below natural ground level, and foundation pit dewatering also has an influence. I once conducted a pile foundation bearing capacity test on an engineering project. Because there were pumping wells nearby, when testing according to the rest period specified in the code, the bearing capacity of the earlier test pile was exactly short of the friction resistance of the sand layer compared with the bearing capacity estimated from the geotechnical investigation report. During testing of the engineering piles, because the pumping wells had stopped working, the bearing capacity matched the report estimate.

9. The influence of groundwater is mainly caused by fluctuations in its water level, and the amplitude is generally limited. The key to analyzing this problem lies in understanding the mechanism of side resistance.

10. The problem is rather complex. For rock-socketed piles, this issue can be disregarded because their bearing capacity is mainly provided by the stratum at the pile tip, and pile side resistance is not considered in design. For pile foundations where side resistance is considered, groundwater conditions must be taken into account, for example, whether parameters such as side resistance decrease in a saturated state and by how much. If the groundwater level at the pile cap is relatively shallow, the buoyancy effect of groundwater should be considered. These are all issues that must be considered in design. Site stratigraphic conditions are also important; for example, if the site stratum is underconsolidated soil, the influence of negative skin friction must be considered. Geotechnical design should not rigidly adhere to codes; rather, on the basis of a thorough mastery of the codes, it should comprehensively consider the actual site conditions so as to achieve safe, economical, and reasonable design.