According to GB50202-2018 "Standard for Construction Quality Acceptance of Building Foundation Engineering": foundation engineering must undergo trench inspection. After the foundation pit (trench) is excavated to the design elevation of the base and cleaned, the construction unit must jointly conduct trench inspection with the survey, design, construction (owner), and supervision units; only after passing can foundation engineering construction proceed.

Conditions for Trench Inspection

➤ Relevant technical personnel from all parties, including survey, design, supervision, construction, and the owner, shall jointly participate in the trench inspection.

➤ At the time of trench inspection, the site shall have available the geotechnical investigation report, light dynamic penetration test records (except where light dynamic penetration testing may be omitted), foundation design documents, and quality inspection reports for ground treatment or deep foundation construction, etc.

➤ Trench inspection shall be carried out after the foundation pit or trench has been excavated to the design elevation; where a protective soil layer is left, its thickness shall not exceed 100 mm; the trench bottom shall be undisturbed original soil.

Inspection Content

1. Inspection of Natural Foundation

➤ The inspection of a natural foundation shall verify the following:

1) Check the position, plan dimensions, and bottom elevation of the foundation pit against the investigation and design documents;

2) Check the geotechnical conditions and groundwater conditions at the pit bottom and pit sides against the investigation report;

3) Inspect for cavities, ancient tombs, ancient wells, hidden ditches, air-defense shelters, and underground buried objects, and ascertain their locations, depths, and characteristics;

4) Inspect the disturbance of the soil at the pit bottom and the extent and degree of such disturbance;

5) Inspect disturbance of the pit-bottom soil caused by freezing, desiccation cracking, water erosion, or soaking, and ascertain the affected range and depth.

➤ Before inspection of a natural foundation, light dynamic penetration tests shall be carried out extensively at the bottom of the foundation pit or trench, and the test data shall serve as the basis for trench inspection.

➤ Light dynamic penetration testing may be omitted under any of the following conditions:

1) When the confined water head may be higher than the pit-bottom elevation and penetration testing could cause water and sand boils;

2) When the foundation bearing stratum is a gravel or cobble layer and the thickness of the gravel or cobble layer below the base is greater than 1 m;

3) When the foundation bearing stratum is a uniform, dense sand layer and its thickness below the base is greater than 1.5 m.‍

2. Inspection of Ground Treatment Works

➤ For replacement fill foundations and dynamic compaction foundations, the inspection reports on uniformity and density of the treated ground and the bearing capacity test data shall be checked on site.

➤ For reinforced composite foundations, the pile positions, pile heads, soil between piles, and the construction quality inspection report of the composite foundation shall be checked on site.

➤ For foundations on special soils, the test data on treatment effects shall be checked on site, including collapsibility, seismic liquefaction, thermal insulation of frozen soil, water isolation for expansive soil, and improvement of saline soil.

3. Inspection of Pile Foundation Works

➤ When the design calculation considers the combined action of soil between piles, such as in pile-raft foundations and low pile caps, the soil between piles shall be inspected after excavation and cleaning to the design elevation.

➤ For hand-dug cast-in-place piles, the bearing stratum at the pile tip shall be inspected after the pile hole is cleaned. For large-diameter hand-dug piles, the geotechnical conditions at the bottom of each hole shall be inspected hole by hole.

Inspection Methods

The inspection method is usually primarily the observation method; for invisible soil layers below the base, it shall first be supplemented by the sounding (rod penetration) method to complete the inspection jointly.

1. Observation Method

➤ Observe the soil conditions on the trench walls and bottom, verify the excavation depth of the trench, preliminarily verify whether the soil at the trench bottom matches the investigation report, and observe whether the soil structure at the trench bottom has been artificially disturbed.

➤ Check whether the trench slope is stable and whether there are factors affecting slope stability, such as groundwater seepage, surcharge at the pit edge, or nearby disturbance (for soils difficult to identify, use a Luoyang shovel or other means to dig to a certain depth for careful identification).

➤ Check whether there are old building foundations, caves, ancient wells, buried pipelines, civil air-defense facilities, etc., in the trench. If such problems exist, trace along their alignment to determine their range, extension direction, length, depth, and width within the trench.

➤ During direct observation, a pocket penetrometer or other means may be used as an auxiliary aid for trench inspection.‍

2. Light Dynamic Penetration Test

➤ When using the light dynamic penetration test for trench inspection, the following shall be checked:

1) The strength and uniformity of the foundation bearing stratum; 2) Shallow buried weak sublayers or shallow buried protruding hard layers; 3) Shallow buried ancient wells, tombs, cavities, etc., that may affect the foundation bearing capacity or foundation stability.

➤ The light dynamic penetration test should preferably be implemented mechanically and automatically, and the inspection depth and spacing shall be in accordance with the table below. After the test is completed, the penetration hole positions shall be backfilled and compacted with sand.‍

[Table] Inspection Depth and Spacing of Light Dynamic Penetration Test (m)