Summary of Key Points for Foundation Pit Trench Inspection, These Six Points Are Essential!

Information required for tank inspection

(1) Geotechnical engineering survey report in the detailed survey stage;
(2) Structural drawings in the construction drawing stage with basic plan and general structural description;
(3) Other documents or records that must be provided.

Preparation work before tank inspection

(1) Check the structural description and geological survey report, and compare whether the foundation bearing capacity and bearing layer used in the structural design are the same as those provided in the report;
(2) Ask and check whether the building location is consistent with the survey scope;
(3) Check whether there are weak sub-berths in the venue;

(4) Whether the site is particularly uneven or whether the surveyor requires special treatment but the designer did not do so;

(5) Require the construction party to provide whether there are underground pipelines and corresponding underground facilities on the site.

Tank inspection process

(1) Based on the self-inspection of the construction unit passing the inspection. The construction unit submits an acceptance application after confirming that the self-inspection is qualified;
(2) The chief supervisory engineer or the project leader of the construction unit shall organize the project leaders and technical quality leaders of the construction, supervision, survey, design, and construction units to jointly carry out the work by the design requirements and relevant regulations.

Main contents of tank inspection

Different buildings have different requirements for foundations, and different foundation forms have different groove inspection contents

1. Natural foundation trench inspection

➤ The following contents should be inspected during the natural foundation trench inspection:

1) Check the location, plane dimensions, and pit bottom elevation of the foundation pit according to the survey and design documents;
2) Check the conditions of the rock soil and groundwater at the bottom of the foundation pit and around the pit based on the survey report;
3) Check the conditions of cavities, ancient tombs, ancient wells, underground ditches, air defense bunkers, and underground buried objects, and ascertain their location, depth, and shape;
4) Check the disturbance of the soil at the bottom of the foundation pit and the scope and degree of the disturbance;
5) Check whether the soil at the bottom of the foundation pit is disturbed by freezing, cracking, water erosion, or soaking, and the scope and depth of the impact should be identified.

➤ Before trench inspection of the natural foundation, light dynamic cone penetration inspection should be generally carried out at the bottom of the foundation pit or foundation trench, and the inspection data should be used as the basis for trench inspection.

➤ In case of one of the following circumstances, light power penetration detection is not required:

1) The pressure-bearing water head may be higher than the elevation of the bottom of the foundation pit, and contact exploration may cause water and sand to flow out;
2) When the basic bearing layer is a gravel layer or pebble layer, and the thickness of the gravel layer or pebble layer below the base is greater than 1m;
3) When the basic bearing layer is a uniform and dense sand layer, and the thickness below the base is greater than 1.5m.

2. Groove inspection for foundation treatment projects

➤ For replacement foundations and dynamically compacted foundations, the uniformity, compactness, and other test reports and bearing capacity test data of the foundation after treatment should be inspected on-site.

➤ For reinforced composite foundations, pile positions, pile heads, soil conditions between piles, and composite foundation construction quality inspection reports should be inspected on site.

➤ For special soil foundations, the treatment effect testing data on the collapsibility of the treated foundation, earthquake liquefaction, frozen soil insulation, expansive soil water isolation, saline soil improvement, etc. should be inspected on site.

3. Pile foundation engineering trench inspection

➤ When the joint interaction of soil between piles such as pile raft foundation and low pile cap is considered in the design calculation, the soil between piles should be inspected after excavation and cleaning to the design elevation.

➤ For manually dug piles, the pile end-bearing layer should be inspected after the pile holes are cleaned. For large-diameter bored piles, the geotechnical conditions at the bottom of the holes should be inspected hole by hole.

Tank inspection method

The trench inspection method usually mainly adopts the observation method, and for the invisible parts of the soil layer below the base, it must first be supplemented by the drill method.

Observation

➤ Observe the soil quality of the trench wall and bottom, verify the excavation depth of the foundation trench, preliminarily verify whether the soil quality at the bottom of the foundation trench is consistent with the survey report, and observe whether the soil structure at the bottom of the trench has been artificially destroyed.

➤ Whether the slope of the foundation trench is stable and whether there are factors that affect the stability of the slope, such as underground water seepage, pit side loading or close-range disturbance, etc. (For soil that is difficult to identify, Luoyang shovel and other methods should be used to dig to a certain depth to carefully identify it. ).

➤ Check whether there are old house foundations, caves, ancient wells, buried pipelines, and civil air defense facilities in the foundation trench. If the above problems exist, they should be traced along their direction to find their range, extension direction, length, depth, and width within the foundation trench.

➤ When conducting direct observation, a pocket penetrometer or other means can be used as an aid for groove inspection.

Light power penetration

➤ When using a light power cone probe to inspect the foundation trench, the following should be checked:

1) The strength and uniformity of the foundation-bearing layer;
2) Shallow burial in a soft underlying layer or shallow burial in a protruding hard layer;
3) Shallowly buried ancient wells, tombs, and cavities that will affect the bearing capacity or stability of the foundation.

➤ Light power cone testing should be implemented using mechanical automation, and the inspection depth and spacing should be implemented as shown in the table below. After the inspection is completed, the location of the touch probe hole should be filled with sand.

After inspecting the trench and analyzing the drilled data, if local abnormalities at the bottom of the trench are found, appropriate treatment methods should be adopted based on the soil condition of the foundation soil, engineering properties, and construction conditions to reduce or avoid uneven settlement of the foundation.

(1) Treatment of local hard soil: dig out the hard soil part to avoid uneven settlement. During the treatment, the backfill material should be determined according to the soil quality of the surrounding soil. If it is difficult to excavate all the way, a soft cushion layer should be used on the upper part to make the foundation settle evenly.

(2) Treatment of local soft soil: In the foundation soil, due to the influence of external factors (such as pipe seepage), differences in strata, or changes in water content, there is a large difference in soft and hard soil in the local foundation.

If the thickness of the soft soil is not large, the soil cushion method is usually used to remove the soft soil. Generally, a graded sand and gravel cushion is used, and the compaction coefficient is not less than 0.94; when the thickness is large, on-site drilling is generally used. Local foundation treatment from cast-in-place piles, concrete or masonry support walls (or piers) to bedrock.

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