Foundation and foundation treatment

(1) Foundation and foundation treatment methods

Foundation and foundation are two different concepts.

The load-bearing structure of the lower part of the building usually buried in the soil layer is called the foundation.

Foundation is the part of rock and soil layer that bears the foundation load below the bottom of foundation. The soil layer under the foundation is called the bearing layer; Within the scope of foundation, the soil layer below the bearing layer is called the substratum, and the substratum with lower strength than the bearing layer is called the weak substratum.

There are two methods of foundation treatment: shallow treatment and deep treatment. The shallow treatment methods of foundation include mechanical rolling, heavy hammer compaction, vibration compaction and soil replacement cushion. Deep foundation treatment methods include: compaction method, sand drain preloading method, high-pressure jet grouting method, deep mixing method and so on.

Foundation can be divided into shallow foundation and deep foundation according to buried depth. Generally, the buried depth is about 5m, and the foundation constructed by general methods belongs to shallow foundation. Such as single foundation, strip foundation and raft foundation. When the foundation needs to be buried in deep soil and constructed by special methods, it belongs to deep foundation. Such as pile foundation, open caisson and diaphragm wall.

(2) DJM pile and bored pile construction

1.DJM heap

DJM pile adopts powder jet mixing method (combining rotary jet mixing with deep mixing), and uses powder materials such as lime or cement to fully mix the sprayed powder with soft soil to form a high-strength cement-soil pile or lime pile, which plays a role of composite foundation together with the foundation soil around the pile, and the reinforcement effect is remarkable. Powder jet mixing method can improve foundation bearing capacity, reduce settlement and increase slope stability.

Characteristics of (1) DJM pile

1) powder solidified material can absorb more water in soft soil foundation, which is more suitable for strengthening soft soil, extremely soft soil and peat soil foundation with high water content, and the effect is more obvious.

2) The solidified material is completely sprayed into the gap generated by the rotation of the stirring blade, and at the same time, it is adhered to the inside of the gap by the moisture of the soil. Under the stirring of the stirring blades, the curing agent is evenly distributed in the soil, and there will be no uneven dispersion, which is beneficial to improve the reinforcement strength of the foundation soil.

3) Compared with high-pressure jet grouting and deep mixing, the solidified material input into the foundation soil is much less, and there will be no slurry discharge and ground arching. 80 ~ 100 kg cement is needed to reinforce 1m3 soft soil, and only 40kg quicklime is needed under suitable geotechnical conditions.

4) During the construction process, dust will not overflow and pollute the environment, only air will be exhausted, which is better than deep mixing by rotary spraying and slurry spraying, and there is almost no material loss.

5) Powder can be a material or a mixture of many materials, so it has wide sources, low cost and strong adaptability to foundation soil reinforcement, and can be used as foundation reinforcement of buildings, supporting piles to prevent soil from sliding, retaining walls and foundations of hydraulic structures.

(2) the scope of application of deep mixing

Deep mixing method is suitable for treating silt, silty soil, silty soil and cohesive soil with high water content (generally 60% ~ 80%, the highest is 100% ~ 200%) and the standard value of foundation bearing capacity is not more than > 1.2MPa. Deep mixing is relative to shallow mixing, and the depth of shallow mixing treatment is generally 3 ~. However, the treatment depth of deep mixing method is generally greater than 5.0m, and the maximum reinforcement depth abroad can reach 60 m.

Powder jet mixing machinery is generally composed of mixing host, powder curing material supplier, air compressor, mixing wing and power part. The main performance indexes of domestic GPP-5 powder jet mixer are shown in Table 4-7, and the supporting machinery of powder jet mixer is shown in Figure 4-5.

Table 4-7 Technical Performance Index of GPP-5 Powder Spraying Mixer

(3) The construction process of powder jet deep mixing.

Mechanical positioning → drilling and spraying to the end → gunite stirring and lifting → repeated gunite stirring to the end → repeated stirring and lifting until the orifice → pore-forming → pumping and flushing pipeline → mechanical displacement → end.

From the perspective of process flow, the process is relatively simple. Let's explain the above process. When the mixing bit is aligned with the pile position, start the powder-spraying mixing rig, rotate while drilling, inject compressed air, and stop tripping when stirring to the designed depth. Start the duster, so that the stirring head is lifted while rotating in the opposite direction, and the powder solidified substance is continuously sprayed and mixed evenly with the soil. Lift the mixing head 30 ~ 50 cm above the ground, turn off the duster to prevent the powder from overflowing the ground, that is, complete the construction of the pile (column) and transfer it to another pile position. If necessary, repeat the above steps to re-spray and re-mix.

Figure 4-5 Schematic diagram of supporting machinery for powder injection mixer

In the case of soft soil and liquefied soil with high water content and low strength, powder solidified material can be sprayed while drilling down and stirring, so as to improve the consistency of the soil and prevent the powder from being liquefied due to the pulse of compressed air.

According to the Code for Acceptance of Construction Quality of Building Foundation Engineering (GB50202-2002), the treatment quality of projects constructed by shotcreting method and deep mixing method can be tested by the following methods.

1) After 7 days of piling, the soil sample reinforced by the pile body is drilled by portable cone penetration test to observe the uniformity of mixing. At the same time, according to the number of portable penetration tests, the strength of pile body is judged by comparison method, and the number of test piles is not less than 2% of the number of completed piles.

2) In the following cases, sampling, single pile load test or excavation test should also be carried out. (1) For the piles that have doubts about the strength of the pile body during the penetration test, a test block should be drilled to determine the strength of the pile body. For piles with complex site or construction problems, a single pile load test should be carried out to test its bearing capacity; (2) For projects with strict requirements for overlapping adjacent piles, when the piles are cured to a certain age, several piles should be selected for excavation, and the appearance quality of the pile top should be checked.

3) After foundation pile excavation, check the pile position, pile number and pile top quality. If the requirements are not met, effective remedial measures shall be taken.

4) Settlement observation: For the building foundation reinforced by deep mixing, settlement observation should be carried out during construction, and for important buildings or buildings with strict restrictions on settlement, settlement observation should be carried out during use to evaluate the foundation reinforcement effect and serve as the basis for use.

2. Construction of bored pile

In the early construction of cast-in-place bored pile, the hole was excavated manually and passed through soft soil to reach the first effective bearing layer. This excavation work is usually supported by wooden baffles or steel rings and supported as the excavation progresses. This method of digging holes and building piles is not only time-consuming, but also will encounter some major construction problems when the pile foundation crosses the water-bearing cohesionless soil.

In the late 1940s, with the development of truck-mounted drilling rig and excavation machinery, the construction period was shortened and the cost was reduced. These efficient tools almost completely replace manual operation, except cleaning the bottom and side walls before encountering hard rocks and pouring concrete. Now the drilling rig has been able to drill into various rock formations. The drilling rig can drill to a depth of less than 50m, a diameter of more than 3m, and a bottom hole diameter of more than 6m. Therefore, as one of the main forms of deep foundation, bored pile is widely used in foundation engineering at home and abroad.

In recent ten years, the application of bored piles with large bearing capacity has become more and more frequent, and it has been more and more trusted by people, especially for heavy loads in modern high-rise buildings and industrial facilities, when the settlement criterion stipulates that the structure must be supported on hard soil or bedrock.

The high trust in bored piles is mainly due to the intuitive inspection and testing of the bearing layer. The failure of bored pile foundation due to insufficient bearing capacity is rare. When problems occur, it is often because the hole wall collapses, which leads to the decline of pile concrete quality, or because of quality problems such as drainage (water leakage) and concrete segregation. Relevant departments and construction units attach great importance to these problems and have formulated corresponding specifications, construction procedures and inspection procedures.

Bored piles are classified according to the types of rocks and soils they support and the main components of bearing capacity. Generally, bored piles are divided into three main types according to their functions: friction piles in homogeneous soil, end-bearing piles in hard soil and end-bearing piles in rocks, as shown in Figure 4-6.

Figure 4-6 Main types of bored piles

(3) Double jet grouting pile construction

Figure 4-7 Schematic Diagram of Double Method

Double jet grouting pile, also known as double-tube jet grouting pile, is a double-channel double grouting pipe. When the double grouting pipe is drilled into the soil layer to a predetermined depth, the coaxial double nozzles at the bottom of the pipe simultaneously spray high-pressure slurry and air jets to impact and destroy the soil, that is, a high-pressure generator such as a high-pressure mud pump sprays slurry with a pressure of about 20MPa, and the slurry is sprayed from the inner nozzle at a high speed, and compressed air with a pressure of about 0.7MPa is sprayed from the outer nozzle. Under the joint action of the high-pressure slurry flow and the surrounding airflow, the energy for destroying the soil is significantly increased, and the nozzles are rotated and lifted while spraying, and finally formed in the soil as shown in Figure 4-7.

(4) Foundation grouting

Grouting method refers to the use of hydraulic, pneumatic or electrochemical principles, through the grouting pipe to evenly inject slurry into the formation. Grouting occupies the position of moisture and air between soil particles or rock cracks through filling, infiltration and compaction. After a certain period of manual control, the slurry will bond the original loose soil particles or cracks into a whole, forming a "stone body" with new structure, high strength, high waterproof performance and good chemical stability.

Grouting can improve the impermeability and integrity of grouted strata or buildings, improve foundation conditions and ensure the safe operation of buildings.

1. Classification

1) According to grouting slurry materials, it can be divided into cement grouting, cement mortar grouting, clay grouting, cement clay grouting, sodium silicate or polymer solution chemical grouting.

2) According to the function of grouting, it can be divided into seepage-proof curtain grouting, rock consolidation grouting, backfill grouting for filling the gap between tunnel concrete lining and rock, joint grouting for concrete dam, contact grouting for filling the gap between steel plate lining and concrete, gap between concrete dam and bedrock, and reinforcement grouting for filling concrete buildings or earth dikes and cracks or cavities in earth dams.

3) According to the composition of grouting stratum, it can be divided into rock grouting, karst grouting (see karst treatment), gravel layer grouting and silt layer grouting.

4) According to the grouting pressure, it is divided into: conventional pressure grouting less than 4MPa; High pressure grouting greater than 4MPa.

5) According to the grouting mechanism, it can be divided into press-in grouting with general pressure and split grouting with higher pressure, so as to enlarge the original cracks or form new ones.

design

Before design, we should do a good job in engineering geology and hydrogeological exploration, and master lithology, rock structure, cracks, faults and their fracture zones, weak interlayers, karst distribution and its fillings, permeability of rocks, layered grading of sand or gravel layers, groundwater burial and recharge conditions, water quality and flow rate, etc. When designing dam reinforcement grouting, we should find out the size and distribution of cracks and overhead caves. Large-scale grouting projects need to carry out on-site grouting tests, determine the hole depth, hole spacing, row spacing and row number of grouting holes, and select grouting materials, pressure, sequence, irrigation methods, quality standards and inspection methods. Grouting pressure is an important parameter, which not only ensures the grouting quality, but also destroys or lifts the grouted strata and buildings. Generally, the initial value is calculated by formula first, and the grouting pressure is finally selected through grouting test.

3. Tools

The main machines used for drilling and grouting are:

1) drill. The drilling diameter is 32 ~ 65 mm, and the drilling depth in the rock is less than 15m.

2) Core drilling rig. The drilling diameter is 75 ~ 1 10 mm, and the drilling depth in the rock is greater than 15m m.

3) grouting pump. According to its structure and working principle, it can be divided into reciprocating pump, diaphragm pump and screw pump. , mainly according to the pressure and flow required for grouting.

4) Mud mixer. It is divided into cyclone type, paddle type and jet type. The mixer should ensure that the mud in the machine does not precipitate and the construction is uninterrupted.

5) Grouting plug. Made of rubber, tightly sleeved on the grouting pipe, the outer diameter is slightly smaller than the diameter of the drilling hole. After pressurization, the outer diameter increases to tightly block the upper or lower part of the grouting section.

4. Architecture

According to the hardness and integrity of the rock, the drilling method of the core drilling machine can be cemented carbide, steel particles or diamond. Stone powder or iron chips ground during steel particle drilling often block the cracks in the hole wall, which affects the grouting quality. When drilling holes in gravel layer, high-quality mud is often used to fix the borehole wall. When drilling holes in bedrock, the hole inclination should be measured in sections to analyze the grouting quality. In order to ensure the quality of rock grouting, the borehole should be washed with pressurized water before grouting, so as to flush the argillaceous filler in cracks or holes out of the hole or out of the grouting treatment range. According to the number of holes washed at one time, it can be divided into single hole washing and group hole washing. According to the washing methods, it can be divided into continuous washing with pressure water, pulsating washing and pumping washing with compressed air. Before grouting after punching, a simple water pressure test should be done in each grouting section, that is, a water pressure test in one pressure stage. Its purpose is:

1) to understand the permeability of rock strata and compare it with geological data.

2) According to the seepage situation, the materials of grouting section are reserved to determine the slurry concentration at the time of opening irrigation.

3) Check the approximate relationship between the permeability of rock stratum and the quality of dry materials actually poured into the grouting section per meter, and check whether there is any abnormal phenomenon.

4) Check the rule that the permeability of grouting holes in each sequence decreases gradually with the increase of sequence.

5. All kinds of grouting construction must be carried out in the prescribed order.

Grouting construction is generally divided into primary holes, secondary holes and tertiary holes. With the increase of ordinal number, the grouting holes gradually become dense. There are two single-hole grouting methods:

1) Full-hole one-time grouting method. The orifice is closed with grouting plug, and pressurized slurry is poured into the whole hole of rock fracture, which is suitable for shallow hole grouting.

2) Full-hole segmental grouting method. The whole hole is divided into several sections from bottom to top, one of which is separated from the adjacent section by grouting plug, and pressurized slurry is only injected into the rock fracture of this section, which is suitable for deep hole grouting. According to grouting methods, it can be divided into: ① pure pressure grouting method. The injected mud is pressed into cracks in the rock to prevent it from returning to the ground. For example, chemical grouting is mostly quantitative grouting, and this method is often used. ② Circulating grouting method. The injected slurry is larger than the slurry absorbed by the fracture, and the excess slurry returns to the mixer through the slurry return pipe. Its main advantage is that the slurry is not easy to precipitate, which is beneficial to ensure the grouting quality. Curtain grouting or consolidation grouting usually adopts this method.

6. Possible accidents during grouting.

1) Grouting is interrupted.

2) ground lifting.

3) Slurry stringing, bleeding or returning around the plug.

After the accident, immediately find out the reason, take timely treatment measures, and stop working when necessary. After grouting, each grouting hole shall be sealed by mechanical grouting. The quality of hole sealing is very important, which directly affects the safety of buildings.

7. Grouting construction steps and key points

1) The diameter of grouting hole is generally 75 ~ 1 10 mm, and the vertical deviation should be less than 1%. When the grouting hole has a design angle, the drill pipe angle should be adjusted in advance, and the inclination deviation should not be greater than 20.

2) When the drilling hole reaches the design depth, the closed mud must be injected through the drill pipe until the mud overflows from the orifice. When lifting the rod to a medium depth, the closed mud should be injected again, and finally the drill pipe should be completely lifted out.

3) The grouting pressure is generally related to the covering pressure of the reinforcement depth, the load of the building, the slurry viscosity, the grouting speed and the grouting quantity. During grouting, the pressure changes, the initial pressure is small and the final pressure is high. In general, the pressure increases by 20 ~ 50 kPa per depth1m.

4) If secondary grouting is carried out, the viscosity of the chemical slurry should be small, so the self-sealing ring sealing device should not be adopted, and the expansion sealing grouting core pipe pressurized with water at both ends should be adopted.

5) After filling the chemical slurry, it is necessary to pull out the tube. If it is not pulled out in time, the slurry will solidify the tube and increase the difficulty of pulling out the tube. Tube puller should be used when extubating. When the plastic valve pipe is used for grouting, the height of the core pipe is 330mm at a time, and the height of the flower pipe should be 500mm at a time. After extubation, brush the grouting pipe in time to keep it unobstructed and clean. The hole left in the soil when pulling out the pipeline should be filled with cement mortar or soil material.

6) Grouting flow is generally 7 ~ 10L/min. When filling and grouting, the flow velocity can be appropriately accelerated, but it should not be greater than 20L/min.

7) Under the premise of meeting the strength requirements, ground fly ash or coarse fly ash can be used to partially replace cement, and the dosage should be determined through experiments, and the general dosage is about 20% ~ 50% of the cement quality.

8) In order to improve the performance of cement slurry, the following additives can be added when mixing cement slurry: ① The modulus of sodium silicate for accelerating the solidification of cement slurry should be 3.0 ~ 3.3. The content of sodium silicate should be determined by experiments, generally 0.5 ~ 3%. (2) Surfactants (or water reducing agents) that can improve the mud diffusion capacity and pumpability, such as triethanolamine. , its dosage is 0.3 ~ 0.5% of cement dosage. (3) In order to improve the uniformity and stability of mud and prevent the segregation and precipitation of solid particles, bentonite is added, and its dosage should not be greater than 5% of cement dosage. Before pressure injection, the slurry must be fully and evenly stirred, and it should be continuously and slowly stirred during grouting. Before pumping, the slurry should be filtered through a screen.

9) Pulping treatment. When the upper pressure of the soil layer is low and the lower pressure is high, the mud tends to rise upwards. The grouting depth is large, but the uplift is not obvious. However, if the grouting depth is shallow, the slurry will be lifted more and even overflow the ground. At this time, intermittent grouting method can be used, that is, after a certain amount of slurry is poured into the soil with large pores in the upper layer, the work is suspended and the slurry solidifies. After several repetitions, the released channel can be blocked. Or speed up the solidification time of the slurry, so that the slurry will solidify when it leaves grouting pipe. Practice has proved that there should be a soil layer not less than 1m thick above the soil layer to be reinforced, otherwise measures should be taken to prevent mud from rising. The concepts of grouting effect and grouting quality are not exactly the same. Grouting quality generally refers to whether the grouting construction is carried out in strict accordance with the design and construction specifications, such as varieties and specifications of grouting materials, performance of grout, drilling angle, grouting pressure, etc. , are required to meet the specification requirements, otherwise appropriate supplementary measures should be taken according to the specific situation, grouting effect refers to the degree to which the physical and mechanical properties of foundation soil can be improved after grouting. High grouting quality does not mean good grouting effect. Therefore, in the design and construction, besides some quality indexes, the grouting effect and inspection method should also be stipulated.

8. Quality assessment

A few days after foundation grouting, a certain number of inspection holes are usually drilled for hydraulic test. By comparing the changes of formation permeability coefficient and seepage before and after grouting, the construction data and water pressure test results are analyzed hole by hole, and then combined with other test observation data for comprehensive evaluation, and a practical quality evaluation is obtained. Check the grouting effect methods are:

1) engineering geophysical prospecting and detection.

2) Conduct core test inspection from the inspection hole.

3) Visual inspection of large diameter borehole.

4) In-hole photography or television inspection.