Preliminary summary of geological work for acceptance of Sanmenxia foundation pit

Since 1957, Sanmenxia water control project has been transferred from the stage of survey and design to the stage of tight dam construction. In the process of foundation excavation of the main building, it is necessary to further find out whether the actual natural geological conditions are consistent with the geological data used in the design, and whether the foundation rocks of the design elevation meet the requirements of the dam foundation. And geologists should put forward specific opinions. Therefore, geologists have further undertaken one of the most important and complicated tasks-foundation pit sketch and foundation pit acceptance.

I. Geological observation during excavation

1) Before foundation pit excavation, geologists should first know the geological conditions of foundation pit in detail, especially the geological data of structural fault zone. In addition, all design drawings, excavation specifications and the division of concrete pouring blocks during construction should be obtained to understand the design and construction intentions.

2) During excavation, we should always observe whether there are new geological phenomena, such as water inrush, slope stability and new fault zone. If there is the possibility of landslide or landslide, the construction unit should be reminded as soon as possible.

3) In order to further find out the geological structure of main building sections, the weathering, cracks and fault zones of rocks at different elevations should be carefully observed during excavation. There are many structural fracture zones in Sanmenxia foundation pit, especially these fracture zones change greatly in horizontal and vertical directions. Therefore, it is necessary to understand them carefully, and at the same time make geological sketches, take photos, collect specimens and rock samples, and if necessary, take rock samples from special areas for experiments to find out the particle gradation, soluble salt, compressive strength and microscopic properties of the fracture zone, so that when the foundation pit reaches the design elevation, it can be compared with each other as the main reference materials for geological conclusions. During observation, some control points must be fixed at different elevations and placed on the structural geological map after measurement. These points should be numbered uniformly and marked with elevation. It is best to put the photographic, sampling and test data on the same picture for future analysis and research. If new faults and fracture zones are found, they must be put on the structural geological map, and at the same time, relevant information should be collected in detail, and this new situation should be told to the construction and design units so as to supplement or modify their design or excavation map in time.

4) In the process of foundation pit blasting, geologists should supervise the construction personnel to avoid the deterioration of artificial geological conditions (such as cracks caused by blasting). For example, in the process of blasting, it is forbidden to drill large-diameter holes and fire monitors with drilling rigs, and even if the geological conditions are not good, they can only be dug with picks, and so on. Although there are norms in the excavation, it may not be suitable for the geological conditions in this area. Therefore, geologists should assist constructors in selecting some representative sections (fracture zone, fracture-intensive zone, fracture-undeveloped section, etc.). ) Conduct blasting test at the initial stage of foundation pit excavation, and then modify the specification after gaining experience to make it suitable for the specific conditions of this section.

5) In the process of foundation pit excavation, geologists should help the constructors to solve the geological problems in the process of excavation, such as groundwater treatment, foundation pit slope size, etc. In addition, it is necessary to determine whether the bedrock at the design elevation meets the requirements of dam foundation. If the rock is still not good when the excavation reaches the design elevation, the excavation must be continued. At this time, geologists should conduct comprehensive analysis and research according to the collected geological data of foundation pit and previous investigation results (drilling holes, pit exploration, etc.). ), and then put forward suggestions on how much to dig, half a meter or 1 meter. In this way, the excavation work will not be wasted and the construction cost will not be affected. In another case, it is not excavated to the design elevation. However, the rock has met the requirements of the dam, and geologists should also suggest that excavation is not needed. In short, designers try their best to keep the building cost low and put it on a solid foundation, so geologists have the responsibility to help them solve these problems.

6) When the foundation pit is excavated to the final elevation, in order to further demonstrate the engineering geological characteristics of dam foundation rocks, representative rocks must be selected as test pieces and sent to the laboratory for physical and mechanical tests, such as compressive test, shear test, bulk density, specific gravity, porosity and water absorption. In addition to microscopic identification when necessary. In case of groundwater, the water inflow should be measured, and water samples should be taken for water quality analysis to determine whether it is corrosive to concrete.

7) After the first stage of foundation pit excavation, the observation data of fracture zone and fault should be compared with the previous observation data to obtain the detailed information of foundation pit water inflow. More importantly, whether the water permeability of structural fracture zone, fracture dense zone or fault has increased, and whether there are new geological phenomena such as particle loss (piping phenomenon) and concentrated leakage. In case of the above phenomenon, suggestions should be made to the construction personnel in time to avoid affecting the safety of buildings in the future.

Second, the field geological sketch work

1) Geological sketch of foundation pit is a work that reflects the engineering geological characteristics of dam foundation rock and all geological phenomena. Therefore, the geological schematic diagram of foundation pit is also a comprehensive geological map of foundation pit, which can not only reflect the development degree, distribution law, length, width, occurrence factors and traps contained in backfill soil, but also reflect the outcrop of groundwater and the location of previous exploration pits and holes. In short, all geological phenomena in the foundation pit should be reflected (as shown in figure 1).

2) When the foundation pit is excavated to the rock required by the design, the geological sketch can only be carried out after the rock is washed clean. In the past, when we were sketching, due to the urgency of the project, we often required to complete the sketching task in a very short time, otherwise it would affect the acceptance work and delay the concrete pouring. Therefore, we often work in two or more groups, with two people in each group assisting each other, one sketching and one recording (including mass production factors, width, filler, quantity, etc. , see the table below). Within the scope of sketch, it can greatly improve the work efficiency to divide the tape measure into a square grid of 2m×2m, and then sketch with a square bamboo frame of 2m engraved with scales.

Fracture statistics table

3) We must pay attention to the following points when sketching in the field: (1) When sketching, first make the grid points with red paint and make the points correct, otherwise the same crack will not be connected or the straight line will be drawn into a bend; (2) Sketch should be divided into primary and secondary, and not everything should be covered. Don't draw all those tiny cracks and cracks caused by blasting, which will complicate the sketch. When sketching, special attention should be paid to the areas that are harmful to the foundation, such as the range of fault zone and fracture zone, and the product of fault zone and fracture zone should be clearly expressed; (3) The relationship among fractures, fracture zones and fault zones should be carefully observed and clearly expressed. For example, the relationship between mutual cutting, changes in horizontal and vertical directions, etc. This is of great help to the study of geological structure; (4) After the sketch is completed, it must be connected with the previous field sketch. If you can't connect, find out the reason and correct it as soon as possible; (5) All plane sketches and section sketches shall conform to the design and construction drawings.

4) The scale of the field schematic diagram should be consistent with that of the indoor schematic diagram, so as to avoid errors in the diagram and bring trouble to the work. When choosing the scale, it is mainly determined according to the geological conditions and the complexity of the sketch area. Regardless of the plane sketch and section, its scale can generally be divided into 1: 50, 1: 100, 1: 200, etc. Now the scale adopted in Sanmenxia is 1: 100.

5) Drawing of foundation pit is a complicated work. Drawing is necessary when the foundation pit is excavated to the design elevation, or drawing the final surface when pouring concrete. The observation records of the middle elevation before reaching the design elevation can only be used as the main data for geologists to analyze rock weathering, cracks, fracture zones and fault zones in the future, and what is important is the final surface sketch data. The sketch data of the final surface is extremely valuable literature in geology and is needed by design, construction and geological units.

Third, the map arrangement in the industry

1. Plan sketch (Figure 1)

Figure 1 plan schematic diagram

Fig. 3 cross-sectional view

Fig. 4 longitudinal sectional view

Myth; legend

Engineering geological survey history of Sanmenxia water control project on the Yellow River

The schematic diagram of the geological plane of foundation pit is the official diagram after the sketches are arranged on the spot. According to the field records, supplement the dip angle, dip angle, width and fillings of cracks, fracture dense zones, fracture zones and fault zones. Extra-large fractures, fracture-intensive zones and fault zones should be numbered uniformly, and these numbers must be consistent with the past. If there are no numbers in the past, new numbers will be given in order. If there is branching phenomenon, the words A, B and C will be added to the original number to distinguish them, and then the range number of concrete pouring block will be added. At the same time, the position, direction and proportion of longitudinal and transverse sections and the schematic range of pouring blocks will also be displayed on the drawing (as shown in Figure 2).

Fig. 2 Schematic diagram of concrete pouring block

2. Vertical and horizontal sections (Figures 3 and 4)

The vertical and horizontal sections are compiled according to the observation data of the construction completion drawing, the geological schematic diagram of the foundation pit during excavation and the geological map during investigation. Its purpose is to check whether the design section reaches or exceeds the design elevation. In either case, the reasons should be explained in this section. On the other hand, in order to understand the excavation depth, the thickness of overburden and weathered layer, the vertical variation of fracture dense zone, fault and fracture zone, etc. The excavation depth of the fractured zone (from the actual excavation surface downwards) shall be indicated in the cross section. On the profile, the original ground wire, design excavation line and actual excavation line must also be displayed.

The longitudinal section (Figure 4) is taken through the center line of a row of pouring blocks. The cross section (Figure 3) depends on the complexity of geological structure after excavation and the excavation situation. Can be appropriately increased or decreased according to the specific situation. The cross section of Sanmenxia is made every 5 meters.

Four. Engineering geological conclusion

All kinds of drawings, instructions and acceptance certificates must be prepared before foundation pit acceptance. The acceptance certificate consists of five parts: geological sketch, geological profile, engineering geological conclusion, as-built drawing, blasting instruction and acceptance instruction (which is the responsibility of the acceptance Committee), among which the geological sketch, geological profile and engineering geological conclusion of foundation pit are the most important parts of the acceptance certificate. All materials except the acceptance statement are written after acceptance, and the others should be submitted to the acceptance committee before acceptance.

The engineering geological conclusion is a short and capable document, and the words must be simple and clear, and what is written is primary, not secondary. Because the acceptance of the foundation pit is based on whether the rock below the design elevation can be used as the foundation, the excavated rock part is not described except the broken zone. According to the specific situation of Sanmenxia, we have compiled the following parts:

1. Rock

1) If there is only diorite porphyrite in the bedrock, it should be stated that the bedrock is gray-green, massive and hard Mesozoic diorite porphyrite in the form of bedrock, and then the dip angle, dip angle and thickness of the bedrock should be stated.

2) For diorite porphyrite, whether it belongs to phenocrysts, or which part is remarkable and which part is not, whether these phenocrysts are arranged and how to arrange them. Then the saturated ultimate compressive strength of diorite porphyrite is demonstrated according to the measured data.

3) This section needs to explain what kind of captured rocks are contained in diorite porphyrite, and their diameters are generally large, largest and smallest. In addition, it is necessary to explain whether the cementation between xenoliths and diorite porphyrite is close.

2. Geological structure

(1) structural fracture

1) First of all, it shows that structural fractures are divided into several groups, the occurrence factors of each group, the general length of fractures and their maximum and minimum length, and the degree of development.

2) In this paragraph, the approximate percentage of closed cracks and open cracks shall be explained, and the approximate width and length of open cracks and the attachments (iron oxide, calcite or clay film) on the two walls of cracks shall be explained.

3) Explain the general fracture rate of the whole batch (strips /m2). If the fracture rate of each batch is significantly different, it should be described separately.

4) Explain how each group of cracks cuts each other.

5) The distribution, length and width of calcite veinlets and their relationship with each group of fractures.

(2) Dense structural fracture zones

1) There are several tectonic fracture dense zones in the foundation pit, their distribution, quantity, occurrence factors, width and extension length.

2) Explain how far the rock in the concentrated tectonic fracture zone is cut by a fracture and what shape the rock takes after cutting.

(3) structural fracture zone and fault zone

1) There are several fracture zones in or in the foundation pit, and their number, occurrence factors, width, length and distribution.

2) Describe the distribution, thickness change, weathering degree, looseness degree and color of products (mylonite, structural clastic rock, structural breccia and structural massive rock) in the fracture zone respectively or comprehensively.

3) Whether the two walls of the broken zone are scratched, if so, the inclination and inclination angle should be recorded to judge which wall is pushed to which side.

4) Whether there are calcite veins in the fracture zone, whether the calcite veins are broken and whether the calcite veins themselves are scratched. If yes, the dip angle and dip angle should be recorded and compared with the scratches on the two walls of the fracture zone, and the degree of combination between calcite veins and rocks should also be explained.

5) Explain the situation of each broken zone in the foundation pit on the original ground and during excavation, and then compare it with the same broken zone in the foundation pit to further explain the vertical change of the broken zone.

6) Finally, it shows that these fractured zones meet the design requirements after several meters of excavation from the excavation face.

3. Groundwater

Explain its exposure position first, and then explain its exposure form (dripping exposure or long-flowing spring). The long-flowing spring should explain its water inflow (liter/second). If there is no groundwater in the foundation pit, it should also be indicated.

4. Conclusion

1) The first thing to explain is the natural foundation of one or part of the pouring blocks (piers, partition walls, combs, non-overflow dam sections on the left bank, power stations, apron, etc.) of the building. ) grayish green, with weak cracks or medium cracks. The hard Mesozoic diorite porphyrite is suitable as the foundation of these buildings.

2) If fissure water only drips from the weak points in the foundation pit, it should be said that the exposure of these groundwater has nothing to do with the foundation; If there is still a large amount of groundwater, the treatment method should be put forward here, but this aspect must be studied by geologists, construction teams, technical departments and chief engineers.

3) It shows that inclusions in bedrock are closely related to bedrock, and their existence has any influence on infrastructure.

4) It shows that the fault and fracture zone have reached the design requirements after treatment.

Five, foundation pit acceptance

When the excavation is completed and all kinds of acceptance documents have been completed, the construction unit shall first check on its own, and those that do not meet the standards must continue to be cleaned up. When no problems are found in the self-inspection, the acceptance committee is required to conduct acceptance.

After receiving the notice of acceptance from the construction unit. First, the on-site inspection team (composed of 1 geologists, 1 technicians and 1 quality inspectors) will conduct a comprehensive review. When rechecking, it is generally necessary to beat rocks with geological hammers or iron bars. Identify whether the excavation meets the requirements according to the sound made by the rock. Stupid rocks should be removed. In addition, in terms of excavation size and elevation, in addition to the construction completion drawing, the survey team of the technical department should also be invited to re-examine. If there is no problem, the acceptance Committee will agree on the acceptance time and notify all relevant units to carry out the acceptance.

The acceptance work of the acceptance committee is to accept the concealed works of dam foundation on behalf of the state. It is not a form but a very important work, and its work is directly related to the safety of the dam. Therefore, concrete can only be poured if it is considered completely qualified at the time of acceptance, otherwise it will continue to be cleaned, and the next working procedure unit can start work only after being notified after approval.

(originally published in Sanmenxia project 1958 1 1.5).