1. 1. Foundation pit drainage and dewatering methods

In the process of earthwork excavation, when excavating a foundation pit (or trench) whose bottom elevation is lower than the groundwater level, groundwater will continuously seep into the pit because the aquifer of soil is cut off. The existence of groundwater not only makes the earthwork excavation difficult, time-consuming and the slope easy to collapse, but also makes the foundation soaked by water, disturbing the foundation soil, causing uneven settlement of the building after the project is completed, and cracking or damage the building. Therefore, in the foundation pit excavation, effective measures should be taken to reduce the groundwater level according to the engineering geology and groundwater hydrology, so that the foundation pit excavation and construction can reach the waterless state and ensure the engineering quality and smooth progress.

In the process of foundation pit and trench excavation, there are many ways to reduce the groundwater level. There are generally two ways to reduce the groundwater level: setting various drainage ditches and using various well point systems. Among them, setting open (dark) ditches and collecting wells for drainage is the most widely used, simplest and most economical method in construction, and various well points are mainly used for dewatering of large-scale deep foundation pits.

1. 1. 1.

I. Drainage method

The characteristic of sump drainage is to set up sump and drainage ditch. According to the different characteristics of the project, there are the following methods:

1. Open ditch and collecting well drainage

2. Layered open ditch drainage

3. Deep open ditch drainage.

4. Underdrain drainage

5. Use engineering facilities to drain water

Second, the choice of drainage equipment

Power pumps are widely used in foundation pit drainage, such as motor pumps, electric pumps, vacuum pumps and siphon pumps. When selecting the pump type, the displacement of the intake pump is generally 65438+ 0.5-2 times of the water inflow of the foundation pit. When the water inflow q of foundation pit

1. 1.2. Well point dewatering method

When a large area of foundation pit is excavated in the soil-rich layer below the groundwater level, it is difficult to drain it by using the general open ditch drainage method. When the silty sand layer is encountered, there will be serious mud boiling, mud sloshing and sand flowing, which will not only make the foundation pit not deep, but also cause a lot of soil erosion, make the slope unstable or the nearby ground collapse, and even affect the safety of adjacent buildings in serious cases. In this case, it is generally necessary to artificially reduce the groundwater level for construction. Various well point drainage methods are usually used to artificially reduce the groundwater level. Before foundation pit excavation, a certain number of well-point filter pipes or tube wells deeper than the bottom of the pit are buried along the periphery or one side and two sides of the excavated foundation pit, and water is pumped from them by connecting the main pipeline or directly connecting the pumping equipment, so that the groundwater level drops below 0.5- 1.0m at the bottom of the pit, so that earthwork excavation and foundation construction can be carried out without water, which can not only avoid a lot of water gushing. At the same time, because the water in the soil is removed, the hydrodynamic pressure is reduced or eliminated, the stability of the slope is greatly improved, the slope can be steep, and the amount of earthwork excavation can be reduced; In addition, due to downward seepage, hydrodynamic pressure strengthens gravity, increases the pressure between soil particles, makes the soil layer at the bottom of the pit more dense, and improves soil properties. Moreover, well point dewatering can greatly improve the construction conditions, improve work efficiency and speed up the project progress. However, the one-time investment of well point dewatering equipment is high and the operation cost is large. Therefore, the construction period should be arranged reasonably and properly to reduce the operation time and drainage cost.

The types of well point dewatering methods are: single-layer light well point, multi-layer light well point, jet well point, electroosmosis well point, tube well point, deep well point, sand-free concrete tube well point and small open caisson well point. According to the type of soil, the position and thickness of permeable layer, the permeability coefficient of soil layer, water supply source, well point layout, required precipitation depth, adjacent buildings and pipelines, engineering characteristics, site and equipment conditions and construction technology level, etc. After comparison of technology, economy and energy saving, one or two of them can be selected, and well points and open drains can also be comprehensively utilized. Table 1 gives the soil permeability coefficient and precipitation depth suitable for each well point. You can learn from it.

Table 1 Scope of application of each well point

The permeability coefficient (m/d) of the soil layer in the well point category of the project reduces the water level depth (m).

1 single layer light well point 0.5-503-6

2 Multi-layer light well point 0.5-506- 12

3 Injection well point 0.1-28-20

4 Electroosmosis < point

5 Well Point of Tube Well 20-2003-5

6 exploration well point 5-25 > 15

Note: Sand-free concrete pipe well point and small open caisson well point are suitable for soil permeability coefficient 10-250m/d and precipitation depth of 5- 10m.

1.2. Slope stability

When excavating the foundation pit, slope excavation can be carried out if conditions permit. Compared with vertical excavation after supporting structure, slope excavation is more economical in many cases. When excavating the slope, the earth slope should be determined correctly. For the foundation pit with a depth of less than 5m, the numerical value of earthwork slope can be found in relevant specifications and documents, and for the earthwork slope of deep foundation pit, it is sometimes determined by slope stability checking calculation, otherwise accidents will occur if it is not handled properly. In the excavation of deep foundation pit slope in China, there have been some landslide accidents. Although there are no landslides, some of them have caused excessive deformation and affected the normal construction. For deep foundation pit with supporting structure, the knowledge of slope stability checking calculation should also be used when checking the overall stability.

Theoretically, there are two methods to study the stability of soil slope. One is to determine the stress state of soil by elastic, plastic or elastoplastic theory, and the other is to analyze the limit equilibrium by assuming that soil slides along a certain sliding surface.

For soil slopes with complex boundary conditions, the first method is difficult to get accurate solutions. Many people at home and abroad have done a lot of research work in this field and made some progress. In recent years, according to the elastic-plastic stress-strain relationship that is more in line with the actual situation, the deformation and stability of soil slopes can be analyzed by finite element method, which is generally called limit analysis method.

The second method is based on the limit equilibrium condition of soil along the imaginary sliding surface, which is generally called the limit equilibrium method. In the limit equilibrium method, slice method is the most commonly used method, because it can adapt to complex geometric shapes, various soil and pore water pressures. There are more than a dozen slicing methods. The difference is that the assumptions used to make the problem static are different, and the methods used to solve the safety factor equation are also different.

1.3. Earthwork excavation of foundation pit

After solving the problems of groundwater and slope stability, the earthwork excavation of foundation pit engineering in high-rise buildings should also solve a series of problems such as what method, what machinery and how to organize construction.

Before the earthwork excavation of foundation pit, detailed construction preparations should be made. The excavation method and the cooperation between manual excavation and mechanical excavation should be considered during excavation, and the foundation treatment of some special foundations should be considered after excavation.

1.3. 1. Construction preparation

Construction preparation for foundation pit excavation generally includes the following aspects:

1. Investigate the site to understand the actual situation of the project.

2. Level the site according to the design or construction requirements.

3. Do a good job in flood control and drainage.

4. Establish survey control network.

5. Set up temporary facilities for foundation pit construction.

1.3.2. Mechanical and manual excavation

In the process of excavation construction, the cooperation between manual excavation and mechanical excavation generally follows the following principles and methods:

1. For large foundation pit earthwork, mechanical excavation should be adopted. When the depth of foundation pit is less than 5m, it is advisable to use backhoe to excavate on the stop surface at one time. When the depth is more than 5m, it is advisable to excavate in layers or dig trenches with a shovel excavator, or set up a steel trestle. The lower earthwork is excavated on the trestle with grab excavator, and the soil is piled in the basement with small bulldozer. For large and deep equipment foundation pit or high-rise building basement foundation pit, multi-layer simultaneous excavation method can be adopted, and earthwork can be transported out by dump trucks.

2. In order to prevent overbreak and keep the slope slope correct, mechanical excavation shall be carried out to the designed pit bottom elevation or near the slope boundary, and 80~50cm thick soil layer shall be reserved for manual excavation and slope repair.

3. Manual excavation generally adopts layered and segmented balanced downward excavation. Check the sideline and slope of the deep pit (trough) every 1m, and correct the deviation at any time.

4. For the foundation pit with technical requirements and deep below the bedrock surface, the lateral controlled blasting method should be adopted, and loose blasting should be carried out before excavation, but the bedrock surface and slope should be controlled not to be damaged.

5. If the excavated foundation pit (groove) is deeper than the foundation of the adjacent building, a certain distance and slope should be kept during excavation to avoid affecting the stability of the foundation of the adjacent building during construction. If the requirements cannot be met, retaining walls or supports should be set at the foot of the slope for reinforcement.

6. When digging, pay attention to check whether there are graves, caves, culverts or cracks and faults (on the rock foundation) at the bottom of the foundation pit. If signs are found, report them in time and conduct exploration and treatment.

7. The spoil should be shipped out in time. If it is necessary to temporarily pile soil or reserve it as backfill soil, the distance from the slope angle of pile soil to the pit edge should be determined according to the excavation depth, slope and soil type, and the dry dense soil should not be less than 3m, and the soft soil should not be less than 5m.

8. After the foundation pit is dug, the bottom of the pit should be leveled and trimmed. When digging a pit, if there is a small part of overbreak, it can be backfilled and compacted with plain soil, lime soil or gravel to basically the same density as the foundation soil.

9. In order to prevent the disturbance at the bottom of the pit, the exposure time should be reduced as much as possible after the excavation of the foundation pit, and the construction of the next working procedure should be carried out in time. If the next working procedure cannot be carried out immediately, the covering soil layer with a thickness of 15-30cm shall be reserved for foundation construction.

1.3.3. Local treatment of foundation

For the special foundation problems encountered during or after foundation pit excavation, local foundation treatment should be carried out. Here are some local treatment methods for special foundations.

I pit (filling, silting and grave) treatment

1 If the loose soil pit is small in the foundation pit, dig the soft virtual soil in the pit to the bottom of the pit to see the natural soil, and then backfill it with shaking soil similar to the natural soil at the bottom of the pit. If the natural soil is sandy soil, it shall be backfilled with sand or graded sand; if the natural soil is relatively dense cohesive soil, it shall be backfilled with 3:7 lime soil by layers. Natural soil is moderately dense plastic cohesive soil or newly deposited cohesive soil.

2 If the scarifying area is larger than the edge of the foundation trench, and the pit (trench) wall cannot dig the natural soil layer due to various conditions, the foundation trench within this range can be widened appropriately. When backfilling sand or gravel, each side of the foundation trench shall be widened according to the gradient of L 1: H 1 = 1, and L: 0 shall be adopted. Each side of the foundation trench is widened according to the slope of l1:h1= 0.5:1. When backfilling with 3:7 lime soil, if the pit is 2m long, the foundation trench may not be widened, but the contact between lime soil and trench wall should be compacted.

3 When the loose soil pit is large and the length is more than 5m, the soft soil in the pit will be excavated. If the soil at the bottom of the pit is the same as that at the bottom of the general trough, the foundation can be lowered, and both ends are connected by steps of 1:2, each step is not higher than 50cm, and the length is not less than 100cm. When the depth is large, backfill the rammed pit (groove) with lime soil layer by layer until the bottom is flat.

4 If the loose pit is deep and is larger than the groove width or 1.5m, after the groove bottom is treated, whether it is necessary to strengthen the strength of the superstructure should also be considered. Common reinforcement methods are as follows: 3 ~ 4 φ 8 ~ 12 steel bars are respectively arranged at the L ~ 2 skin brick on lime-soil foundation (or concrete foundation), at the12 skin brick under moisture-proof layer and at the first floor roof, and they span 1m at both ends of the loose pit.

5 For the loose soil pit with high underground water level, the soft loose soil in the pit (groove) should be excavated, and then backfilled with sand or concrete.

Second, the treatment of wells or sewage wells.

For 1 well, the water level should be reduced to the possible limit near the foundation. When in use, coarse sand, stones, pebbles or broken bricks should be compacted to 50 cm. Above the groundwater level. If there is a brick well circle, the brick well circle should be removed to below the pit (trough) bottom 1m above, and then compacted and backfilled to the basement (or floor bottom) with plain soil or lime soil in layers.

2 Within 5m from the foundation edge, the orange well shall be compacted by layers with plain soil, and backfilled to 1.5m below the floor, and the brick circle around the shaft wall shall be removed or the soft part shall be excavated, and then compacted by layers with plain soil or lime soil.

3. Under the dry well foundation, the strip foundation 3B or column foundation 2B shall be compacted by layers with plain soil, backfilled to 2m below the foundation, and the soft part around the shaft wall shall be excavated. When there is a brick well circle, the brick should be removed according to the regulations, and compacted in layers with plain soil or lime soil after heating.

Well 4 is in the corner of the house, but the foundation pressure of the well is not enough. In addition to backfill treatment according to the above methods, the foundation should also be strengthened, such as setting reinforced concrete slabs across the upper part. When the influence is not great, the method of picking beams from the foundation can be adopted.

5 Well is at the corner of the building. If it is difficult or uneconomical to press the foundation on the well with cantilever beam, the foundation can be extended outward along the length of the wall, so that the extended part falls on the natural soil. The total area of the foundation falling on the natural soil is not less than that of the original foundation in the well circle. At the same time, it can be properly reinforced or reinforced with reinforced concrete beams in the wall.

Well 6 is silted, but not dense. The soft soil below can be squeezed by big stones and then backfilled by the above method. If the well can't be tamped and compacted, a reinforced concrete cover can be added to the brick ring of the well for sealing, and the upper part can be backfilled.

Third, local soft and hard (height difference) foundation treatment

1 If the foundation part encounters bedrock, old wall foundation, old lime soil, large stones or structures, dig them out as much as possible to prevent the building from cracking due to uneven settlement caused by local tribes on hard objects, or chisel off hard objects to a depth of 30 ~ 50 cm, and then fill them with mixed sand for tamping.

If the foundation part falls on bedrock or hard soil layer, and part falls on soft soil layer. In soft soil, concrete or block stones are used to protect the wall (or pier), or cast-in-place piles are used to reach the bedrock. The foundation slab should be equipped with appropriate steel bars, or the bedrock below the foundation should be chiseled to a depth of 30 ~ 50 cm, and medium-coarse sand or soil-sand mixture should be filled as cushion to adjust the relative deformation of the foundation at the rock-soil interface, avoid cracks caused by stress concentration, or strengthen the rigidity of the foundation and superstructure to overcome the uneven deformation of the foundation.

The foundation falls on the inclined rock with large elevation difference, part of the foundation falls on the bedrock, and part of the foundation is suspended. Then, a concrete or stone retaining wall (pier) is built on the lower bedrock, and plain soil is compacted in layers in the middle for backfilling, or the upper bedrock is chiseled to make the foundation slab fall on the same elevation, or the lower bedrock is filled with low-grade concrete or rubble concrete.

Fourth, rubber soil, ancient rivers and lakes.

1 rubber soil treatment: the local water content of the foundation is close to saturation, and the foundation soil becomes "rubber soil" with trembling feeling after tamping. The foundation treatment method avoids direct ramming, and the water content of soil can be reduced by drying tank or mixing lime powder. If rubber soil has appeared, it can be compacted with a layer of broken bricks or gravel, or the vibrating part of the soil can be dug out and compacted with sand or graded sand.

2 Treatment of natural ancient rivers and lakes According to their causes, there are ancient rivers and lakes with uniform and dense soil, about 20% water content and less impurities, which have been subjected to long-term atmospheric precipitation and natural precipitation. Ancient rivers and lakes with relatively loose soil structure, high water content and heavy debris and organic matter can be left untreated if their bearing capacity is not lower than that of natural soil. For modern ancient rivers and lakes, loose soil with high water content should be excavated, compacted with plain soil or lime soil in layers as appropriate, or foundation reinforcement measures should be taken.

Artificial ancient rivers and lakes are treated by old filling and paid filling. The old fill is formed by long-term accumulation, containing bricks and tiles, plant ash and other sundries. The soil is uniform, dense and stable. The formation time of new fill is short, the settlement is unstable, the soil contains more brick fragments, plant ash and slag, the structure is loose and uneven, and the water content is generally greater than 20%. If the bearing capacity of the old fill is not lower than the natural soil in the same area, it can be left untreated. The new fill should be excavated in layers, compacted and backfilled with plain soil or lime soil, or measures to strengthen the foundation should be taken.

V. Treatment of quicksand Phenomenon, causes and treatment methods of quicksand.

When the excavation depth of the foundation pit is below 0.5m, water will be pumped in the pit, and sometimes the soil at the bottom of the pit will be in a flowing state. Due to the skyrocketing groundwater, it is called quicksand that cannot be dug deep. When the water level outside the pit is higher than the water level after pumping in the pit, when the hydrodynamic pressure of water pressure movement in the pit is greater than the floating weight of soil particles, the soil particles will be suspended and unstable, and will rush into the pit with the water, surge from the bottom of the pit or flow in from both sides, and become a flowing state. If the construction is forced to excavate, the pumping will be more and more difficult, and the greater the hydrodynamic pressure, the more serious the quicksand will be. The condition of quicksand is that the greater the hydraulic gradient or the greater the porosity of sand, the easier it is to form quicksand; The smaller the permeability coefficient of sand, the worse the drainage performance, and the easier it is to form quicksand; Sand contains more flaky minerals, such as mica and chlorite, which are easy to form quicksand. The way to take measures is to "reduce or balance the dynamic hydraulic power" so that the soil particles at the bottom of the pit are stable and not disturbed by hydraulic power. Commonly used sugar cubes are: a. Arrange the construction in dry season, so that the maximum groundwater level is not higher than 0.5m at the pit bottom; B. Digging in the water, that is, no pumping or less pumping, makes the water pressure in the foundation pit basically balanced with the water pressure outside the pit, and narrows the head gap; C. For more important projects or projects with serious quicksand, the underground water level at the well point can be artificially lowered to lower the underground water level in the foundation pit and nearby areas below the pit bottom, so that the soil surface at the pit bottom can be kept in a waterless state; D, driving sheet piles along the periphery of the foundation pit, and driving the impervious layer to prevent the water pressure outside the pit from falling within the into the pit and reduce the hydrodynamic pressure inside the pit.

1.4. Selection of foundation pit supporting system

As a supporting system to ensure the stability of foundation pit excavation, it includes retaining wall and support, in which the main function of retaining wall is to retain soil, while the function of support is to ensure the stability of structural system. If the structural strength of the retaining wall is sufficient to meet the requirements of excavation stability, there may be no supporting members in the supporting system, otherwise supporting members (or structures) shall be added. Improper selection or damage of any part of the support system will lead to the failure of the whole support system. Therefore, retaining walls and supports should be paid enough attention.

1.4. 1. Selection of retaining wall

Retaining wall structures commonly used in engineering have the following types:

1 steel sheet pile

2 reinforced concrete sheet pile

3 Bored pile retaining wall

4 H steel column (or reinforced concrete pile column) and wooden baffle support wall

5 Diaphragm Wall

6 deep mixing cement-soil pile retaining wall

7 jet grouting pile curtain wall

In addition to the above, there are manual digging piles (widely used in southern China) and precast driven reinforced concrete piles as retaining walls for supporting structures.

The selection of retaining wall of supporting system involves technical and economic factors, which can only be determined after technical and economic comparison from the aspects of meeting construction requirements, reducing adverse impact on surrounding areas, convenient construction, short construction period and good economic benefits. In addition, the selection of retaining wall of supporting structure should be determined together with supporting selection, groundwater level reduction and excavation scheme.

1.4.2. Selection of supporting structure

When the depth of the foundation pit is deep and the cantilever retaining wall can not meet the requirements in strength and deformation, it is necessary to add a support system. Support systems are divided into two categories: internal support and external support. External anchor of foundation pit can be divided into top anchor and soil anchor. The former is used for not too deep foundation pits, mostly steel sheet piles. At the top of the foundation pit, steel sheet pile retaining wall and anchor pile are bound and anchored with steel bars or steel ropes at a certain distance. Soil anchors are mostly used in deep foundation pits. See the chapter "Soil Anchor" for details.

The following are several common forms of support:

1 anchorage support

2 inclined column support

3 short pile diaphragm support

4 steel structure support

5 underground continuous wall support

6 underground continuous wall bolt support

7 retaining and slope protection pile support

8. Combined support of retaining and slope protection piles and anchor rods

9 central transverse top support of sheet pile

10 sheet pile central inclined top bracket

1 1 layered sheet pile support

1.5. Calculation of retaining structure system

Because of the complexity of soil structure and the discreteness or uncertainty of soil parameters, the load distribution law of retaining structure system is complex, and it is difficult or even impossible to achieve the same calculation accuracy as that of superstructure.

In recent years, different countries have different calculation methods and codes, but the calculation methods are quite different. Using different calculation methods, the calculation results of supporting structures such as pile length, bending distance and pull rod load can reach 50%, because the calculation of supporting structures involves not only calculation theory and method, but also many factors such as soil quality, water level, excavation depth, ground load and adjacent buildings, and the design calculation is complicated. There is no design calculation standard in China. Therefore, a relatively safe, stable, economical and reasonable support design must require designers to study various objective conditions, master some empirical data and experimental research data, and comprehensively use calculation theories and methods to design, so as to get more reasonable results.