Digression: The author previously worked in building construction, graduated from university in 1994, and was involved in construction until 2003, rarely touching structural design or geotechnical engineering. Until before the postgraduate entrance examination in 2003, I didn't know what geotechnical engineering was; I only knew soil mechanics and foundation engineering. After the exam, I gradually learned that geotechnical engineering is far beyond soil mechanics and foundation engineering as I knew them, and also learned that the 21st century is the era of geotechnical engineering. So I applied for geotechnical engineering at the Institute of Engineering Mechanics, China Earthquake Administration, with a research topic on foundation pit engineering. At that time, there were two directions: one was pile foundations, and the other was foundation pits. Through consulting teachers and reviewing materials, I learned that foundation pits would definitely be very popular in the future and would require more professional knowledge. Those working on piles might not understand foundation pits, but those working on foundation pits must understand piles. Ultimately, I chose the direction of deep foundation pits and became attached to it. It has been exactly ten years now, and I have designed nearly a hundred foundation pits, large and small. I want to say that geotechnical engineering is a very promising profession, and I hope everyone interested will join this industry!

Regarding how to do a good job in foundation pit design, I would like to write some superficial things based on my practical experience over the years. I welcome everyone's criticism and correction for any shortcomings or mistakes.

1. Foundation pit designers must be proficient in geotechnical engineering investigation

After graduating with a master's degree in June 2006, I went to Changzhou, Jiangsu, to work on foundation pit design. After becoming slightly familiar with a few projects, I could independently do foundation pit design, but I just couldn't understand investigation reports (it's normal for those studying geotechnical engineering not to understand investigation reports). I could select unfavorable boreholes, but I couldn't determine design parameters. Every time, I had to ask the chief engineer of the unit to decide. I had no impression of silt, clay, etc. Once, at an expert review meeting, an expert said there was a local silt interlayer, but I couldn't find it. From then on, I knew that not only for foundation pit design, but to do geotechnical engineering well, one cannot do without proficiency in geotechnical engineering investigation. At the end of the year, I happened to be dissatisfied with that unit, so I switched to a small unit. At that time, I mainly wanted to be able to do investigation.

So I went to Zhejiang and engaged in geotechnical engineering investigation and foundation pit design. Up to now, I have done investigation for more than a hundred projects, gained a certain concept of soil, and become proficient in foundation pit design. I also dare to make appropriate adjustments to some design parameters. You may have encountered situations where the parameters proposed in the investigation report for foundation pit design are unreasonable, sometimes conservative, sometimes unsafe. Now I can judge which are reasonable and which are not. Many times I adjust parameters and have received recognition from review experts. Actually, in geotechnical engineering investigation, the key is to have some perceptual understanding of soil, and this is crucial for us in geotechnical engineering.

2. Be sure to clarify the surrounding environment

Every time I design a foundation pit, regardless of size, I go to the site for a personal survey, mainly to see if there are houses, roads, pipelines, etc. around. I think the most terrifying are sewage pipes and rainwater pipes. These two types of pipelines have poor construction quality and leak all year round, leading to deteriorated soil. I have encountered several projects where leakage from sewage or rainwater pipes caused collapses. When considering the scheme, be cautious, leave room, and take corresponding measures. For gas pipelines and cables, even if the burial depth is known, be cautious about using soil nail wall schemes. You should know that the workers now are migrant workers; who knows where their steel pipes are driven. Last year, I participated in a project where the gas pipeline burial depth was roughly known to be 2.5 meters. A Hangzhou unit used a soil nail wall scheme, while I adopted a row pile scheme and didn't dare use a single soil nail. I told the client that during construction, workers couldn't possibly control it at 2.5 meters, and steel pipes could hit the gas pipeline. In the end, the construction unit still adopted my scheme. For areas close to residential houses, try to use highly reliable schemes, such as row piles or piles with internal bracing (except for residential houses with pile foundations).

3. Remember to grasp the major and let go of the minor

A foundation pit always has some sections with lower risk and some with higher risk. We must have a concept in mind: important places cannot have any problems and must be built quite solidly; otherwise, once an accident occurs, the consequences are severe. Other unimportant places can be saved; if they collapse, at worst we can use an excavator to dig the soil. Generally, construction units can accept this. If some construction units blindly want to save money and make various demands, it's better not to do such a foundation pit, because ultimately we designers will suffer.

4. Frequently go to the site to obtain first-hand information

For projects you design, be sure to track them from beginning to end. Every project is a good opportunity for us designers to observe the prototype. Going to the site frequently, first, increases perceptual understanding, and second, allows us to check construction quality. Especially for sections where accidents occur, be sure to summarize and learn lessons. Successful cases are our chips for increasing our strength. On site, there are often constructions not according to design, even quite risky, but ultimately nothing may happen. Thus we have a better idea. For example, if you designed a 1:1 slope, but actually it was excavated at 1:0.5, and nothing happened, then we have a better idea for future similar foundation pit designs.

5. Foundation pit designers must not make conceptual mistakes

I remember Professor Yu Diaomei of Tongji University said that geotechnical engineering is 90% experience and 10% theory, concrete is 50% experience and 50% theory, and steel structure is 10% experience and 90% theory. This shows how important experience is in geotechnical engineering. Successful experience is built on correct concepts. Sometimes it's not a big problem if soil nails are shorter or piles have fewer reinforcing bars, but what's terrible is using soil nail walls where they shouldn't be used, or using cantilever piles where deformation requirements are strict. Then no matter how long the soil nails are or how much reinforcement there is, it's useless.

Regarding the foundation pit accident at Hangzhou Xianghu Metro Station, I think there were conceptual mistakes in design. First, the bottom of the diaphragm wall did not pass through the muddy soil, and even more unimaginable, the reinforcement of the passive zone was canceled. Second, the first support used steel supports, and the ends of the supports did not form a fixed connection with the diaphragm wall. As a result, when the top of the diaphragm wall tilted backward, the steel supports detached. If the first support had been reinforced concrete, the situation might have been better. The establishment of correct concepts comes from extensive engineering practice and a certain theoretical foundation; both are indispensable.

6. Beginners in foundation pit design can try the "engineering analogy method"

For those new to the industry, they may not have much experience and lack confidence. I suggest using the "engineering analogy method": collect some local successful case schemes and study them carefully; or download some foundation pit examples with similar geological conditions from the Internet. Conference proceedings are also good; these examples have certain reference significance.

I hope to encourage everyone!