The compulsory engineering construction code general code for waterproofing of building and municipal engineering (GB 55030-2022) implemented on April 1 stipulates that the waterproofing design service life of underground engineering shall not be less than that of engineering structure, and the waterproofing grade shall be determined according to the waterproofing category and waterproofing service environment of underground engineering. What is the difference between this and the current technical code for waterproofing of underground works (GB 50108-2008)?

1. Technical code for waterproofing of underground works (GB 50108-2008) specifies waterproofing grade

Article 3.2.1 of the technical code for waterproofing of underground works (GB 50108-2008) specifies the waterproofing grade and waterproofing standard of underground works. Article 3.2.2 gives the scope of application of different waterproofing grades and specifies that the selection shall be made according to the importance of the project and the requirements for waterproofing in use.

Article 3.2.1 divides the waterproofing level into four levels, and takes the allowable water leakage, the size of the water leakage and the state of the water leakage as the waterproofing level standard. Article 3.2.2 specifies the scope of application of different waterproofing levels.

2. General code for waterproofing of buildings and municipal works gb55030 specifies the waterproofing grade of underground works

(1) Determination of waterproofing grade of underground works

According to the general specification, the waterproofing level is divided into three levels. The first level is the highest, the second level is the second, and the third level is the lowest. The basis for determining the waterproof grade is the waterproof category of the project and the use environment. In combination with the waterproof category of underground works in article 2.0.3 of the general specification and the provisions on the use environment in article 2.0.4 of the general specification, and according to the provisions on the waterproof grade in article 2.0.6, except for the underground works by the concealed excavation method, the waterproof grade of the underground works shall be determined according to the following table:

(2) Waterproofing category of underground works and applicable types of works

Because of the particularity and difficulty of waterproofing of underground works, the quality inspection qualification criteria of underground waterproofing works are judged according to whether there is water leakage and the degree of water leakage. Class A shall be free of water seepage, and the back surface of the structure shall be free of wet stains; Class B shall be free of dripping and line leakage, and there may be scattered wet stains on the back surface of the structure; Class C shall not flow or leak mud and sand. There may be a small amount of wet stains, sagging or dripping on the back surface of the structure. Based on the quality inspection standard of underground waterproofing works, the waterproofing category of underground works is determined according to the type of works and the importance of waterproofing function, that is, considering the impact of leakage on society, economy and environment. The influencing factors include the impact of leakage on the physical and mental health of users, the impact of leakage on the internal instruments, equipment, materials and other properties of the project, the impact of leakage on the normal use state, structural durability and structural safety of the project, and the maintenance cost and maintenance difficulty of the project after leakage. Therefore, when dividing the waterproofing category of underground works, the following situations shall be considered according to the impact degree caused by leakage:  the basement of civil buildings with personnel activities and the underground works sensitive to leakage shall be determined as class A;  Underground works that are not sensitive to leakage, equipment use or storage sites, and do not affect normal use, are determined as class C;  Underground works between the two are classified as class B. The determination of project waterproofing category and applicable project types can be determined by the project designer according to the actual situation of the project by referring to the table below.

(3) Waterproofing environment of underground works

a. Category of waterproof environment

Different environmental conditions have different effects on the waterproofing system. The factors affecting the waterproofing system of underground works mainly include groundwater, surface water infiltrating into the ground (such as rainwater, irrigation water, etc.) and capillary water absorption of soil, as well as acid-base salt and mold in close contact with the waterproofing system. Acid, alkali, salt and mold mainly affect the durability of the waterproof system. In addition to the durability, the capillary absorption of groundwater, surface water and soil is also an influencing factor of the failure probability of the waterproof system. In the waterproofing of underground works, groundwater is the most important influencing factor. Therefore, according to the relationship between the anti floating water level elevation of underground works and the bottom elevation of underground structural slab, article 2.0.4 of the general specification divides the waterproofing environment of underground works into class I and class II.

b. Anti floating water level

The anti floating fortification water level in the table refers to the maximum groundwater level that the construction project may suffer when it meets the anti floating fortification standard during the construction period and service period, or the maximum buoyancy that may be received on the bottom surface of the underground structure base plate when the construction project meets the most unfavorable combination of anti floating fortification standards during the construction period and service period according to the static conversion. The category of waterproof environment for underground works shall be determined according to the anti floating water level during the service period.

The anti floating water level during the service period can be determined according to article 5.3.3 of technical standard for anti floating of building engineering JGJ 476-2019:

This article specifies the basic principles for determining the anti floating water level, and compares and analyzes various possible situations. Since the determination of the anti floating water level is related to the safety of underground structures, the highest value of the groundwater level is taken as the anti floating water level.

c. Determination of waterproof environment category

The anti floating water level is used for checking the overall stability of the structure and calculating the buoyancy during the design of underground structural members. The waterproof water level is used for determining the waterproof requirements of underground works. The starting points of the two are different. For the same site, the water level is the same in some cases, but may be different in some cases.

The anti floating water level is generally the historical highest water level of the site or the highest water level in the service period provided in the water level prediction and consultation report. Therefore, it can also be understood that when there is groundwater in the depth range of the basement, it is a class I service environment, and the waterproofing is mainly based on groundwater; When there is no underground water, it is a class II environment, and the waterproof fortification is mainly based on the surface water penetrating into the soil layer around the underground project.

Article 2.0.5 of the general specification also stipulates that "for open cut underground works with class II waterproofing service environment, when the annual precipitation of the project location is greater than 400mm, it shall be selected according to class I waterproofing service environment. ”Therefore, in most parts of the country (except in the northwest and northeast), the waterproofing environment for open cut underground works is class I.

Author Zhang Wenhua

Professor, School of civil engineering, Zhejiang University of Technology

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