01 - What Are the Key Points in the Design Principles for Soft Soil Foundations?

First, we need to understand that soft soil foundations are characterized by low bearing capacity, high compressibility, poor permeability, and long settlement stabilization time. Their design must follow the core principles of "safety and reliability, economic rationality, technical feasibility, and adaptation to local conditions," while also taking into account construction convenience and environmental coordination. Below are the key points on design principles that I have summarized for everyone, shared for reference only.

(1)In soft soil foundations, especially in coastal areas with deep soft soil, hard strata are generally buried very deep, but it is still possible and very important to select relatively good medium- to low-compressibility soil layers as the pile tip bearing stratum.

(2)There have been many accidents in soft soil areas where pile foundations were damaged by negative skin friction, for various reasons. For example, some areas are covered with recently deposited underconsolidated soil layers; mountain excavation or hydraulic fill reclamation is adopted; large-area ground surcharging occurs during use; large-area soil-displacing pile driving causes excess pore water pressure and soil heave; and groundwater is lowered in adjacent sites.

It should be noted that the occurrence and harm of negative skin friction can be prevented and reduced. The problem lies in the designers' and constructors' ability to predict it in advance and whether they can take timely countermeasures.

(3)Soil-displacing pile driving also causes many accidents in soft soil areas. First, precast pile joints may be pulled apart, and piles may shift laterally and heave; cast-in-place driven-tube piles may break or neck. Second, adjacent buildings, roads, and pipelines may be damaged. During design, pile types and techniques should be selected according to local conditions, and the problems of driven-tube cast-in-place piles should be avoided as much as possible. For the driving of precast piles and steel piles, measures should be taken to reduce pore pressure and mitigate soil-displacement effects, including installing plastic drainage boards, pre-boring for pile driving, setting stress-relief holes, and controlling the pile driving rate.

(4)The issue of the impact of foundation pit excavation on completed piles. In soft soil areas, considering the favorable working conditions for pile foundation construction, the construction sequence of first completing piles and then excavating the foundation pit is often adopted. Because the foundation pit excavation is uneven, forming a "pit within a pit," soil creep and sliding may push the foundation piles out of alignment or break them; some horizontal displacements have exceeded 1 m, leading to serious quality accidents. Such accidents have been common from south to north since the 1980s. Therefore, when excavating a foundation pit in a soft soil site where piles have already been completed, balanced excavation must be strictly implemented, the height difference should not exceed 1 m, and soil must not be stockpiled at the pit edge, so as to ensure that the completed foundation piles do not undergo horizontal displacement or breakage due to soil sliding.