There are two construction methods for deep mixing pile foundation reinforcement: powder spraying and shotcreting. When the natural water content of foundation is more than 60%, designers often choose powder spraying to reduce the water content of foundation. However, when the water content of natural foundation is high, the powder spraying method is adopted. Due to the action of wind pressure and airflow during powder spraying construction, plastic mud is liquefied due to disturbance during mixing, and its strength is too late to form, resulting in pile sinking. Through the construction site test, it is proved that it is effective to solve the problem of pile sinking by spraying concrete to build mixing piles, and the project cost has not changed much, which is feasible.

Keywords: deep mixing pile sinking and spraying subgrade engineering

I. Preface:

After nearly 20 years of development, due to the maturity of construction technology and construction machinery, deep mixing piles have been widely used in soft soil foundation reinforcement, slope support, foundation pit and dam seepage control. Deep mixing pile can improve the bearing capacity of soft soil foundation, reduce settlement and improve the stability of slope, which is rapid, economical and effective. Therefore, it is applied to soft soil foundation at the bridge head of highway to speed up the progress of highway construction and eliminate or alleviate the problems such as bump at the bridge head. Its construction methods are divided into two methods: powder spraying and guniting. When the natural water content of foundation is more than 60%, designers often choose powder spraying method to reduce the water content of foundation. Due to the ever-changing geological conditions, if the water content of silt is too high, the problem of pile sinking may occur by powder spraying. This paper analyzes the problems existing in the pile sinking project of dry jet grouting pile, and puts forward the treatment methods to discuss with peers.

Second, the engineering example

1, project introduction

A high-grade highway at K9+753~K 10+836 bridgehead186m is treated by powder jet grouting pile. DJM piles are arranged in a regular triangle with a pile diameter of 50kgcm, a pile spacing of 1.5m, an average pile length of 10m, and a cement mixing ratio of 15%. When the construction equipment is calibrated and the pile test scheme is approved by the supervision unit and the owner unit, the construction unit adopts the powder spraying method to test the pile. * * * A total of 59 piles were tested, of which 2 1 pile was sunk, and the depth of pile sinking was generally between1.1m ~ 4.5m. ..

2. Cause analysis of pile sinking

The reinforcement mechanism of cement deep mixing pile is to form a cement-soil pile with integrity, water stability and certain strength through a series of chemical reactions such as hydrolysis and hydration of cement, ion exchange and granulation between cement hydrate and soil particles, hardening and carbonization. Therefore, the causes of pile sinking can be analyzed from two aspects: geology and construction technology.

Geologically, the effect of cement strengthening soil is different because of the different soil quality of each geological layer. Generally speaking, soft soil containing clay minerals such as kaolinite, kaolinite and montmorillonite has better reinforcement effect, while cohesive soil containing illite, chloride and diaspore and cohesive soil with high organic matter content and low PH value have worse reinforcement effect. The difference of water content in each geological layer is also the reason why a series of chemical reactions between cement and soil lead to different strength formation speeds.

In terms of construction technology, cement and soil are not evenly mixed, and even cement and soil cannot be mixed. This is related to various construction parameters of construction machinery, such as drilling speed, bit speed, lifting speed, powder injection pressure and cement dosage. Through pile testing, suitable construction parameters should be found according to different geological layers, different soil qualities and different earth pressures, and strictly controlled to make the pile body uniform and prevent necking and neck breaking.

1) Geological aspects

In a row of 7 piles in the first pile test, 4 piles sank on the right side of the route. On the seventh day after the pile test, two sections of the pile were tested for core pulling, and it was found that two sections of cement on the pile body were obviously not solidified. On the tenth day after the pile test, the geology of the pile sinking place should be investigated, and the specific strata are as follows from top to bottom:

① Sand filling: river sand with a layer thickness of 0.5m 。

② Silty clay: grayish yellow, grayish brown, plastic, slightly wet ~ wet, gradually becoming soft and plastic with depth increasing, with layer thickness of1.2m.

③ Silty soil: dark gray, gray-black, soft plastic ~ flowing plastic, saturated, strong in viscosity, containing oil, uniform in lithology, with humus at the bottom, with a layer thickness of 6.0m

④ Silty clay: gray, bluish gray, soft plastic, saturated, hard and uniform, with a layer thickness of1.8m.

⑤ Mud mixed with sand: gray, bluish gray, soft plastic, containing a large amount of medium and coarse sand, with a content of 30% ~ 50%, loose, with a layer thickness of1.5m.

⑥ Sand layer: light gray, slightly dense to medium dense, saturated, mainly coarse sand, containing clayey soil, well graded, layer thickness1.9m.

⑦ Silty clay with sand: grayish yellow, brownish yellow, soft plastic, wet, with a content of 20% ~ 50%, uneven lithology and a layer thickness of 2.6m.

⑧ Sand layer: mainly medium coarse sand, grayish white, grayish yellow, medium dense, saturated, containing cohesive soil, well graded, with a layer thickness of 0.3m.

The physical and mechanical indexes of soft soil are very poor. The average water content of silt is 90%, the natural void ratio is 2.5 1, and the direct quick shear C=6.79Kpa, φ=7.36.

From the above geological investigation and analysis, there are mainly the following reasons:

(1) Because the water content of the silt in this section is 90%, the water absorption rate of the cement in the pile after powder spraying is limited (50 ~ 60 kg/m). Referring to the test results of similar projects, it can be seen that the water content reduction of the cement-reinforced soil is lower than the cement mixing ratio in a short time, that is, the water content of the silt-reinforced soil in this section is still above 75%, and the soil and cement are still in a plastic ~ soft plastic state when mixing. After spraying 50 ~ 60 kg/m cement for 9m, the self-weight increased by 4500~5400KN. The compression modulus of cement-soil in the state of fluid plastic to soft plastic is small, and the pile compression caused by its own weight is large; The shear strength of cement-reinforced soil is low, which leads to large lateral extrusion of its own weight;

(2) After the disturbed soil around the pile body sinks, the soil body produces downward negative friction on the side of the pile. When soil and cement are still in the state of fluid plastic ~ soft plastic, the compression modulus is small and the shear strength is low, pile sinking occurs under the action of negative friction.

(3) The sensitivity of silty soil in this section is relatively high, and the sensitivity is the ratio of unconfined compressive strength of undisturbed samples to unconfined compressive strength of remolded samples with the same water content. From the pile test site, the silt extruded from the surface of sand cushion at the pile test site is very thin, which shows that the remolding strength is very low and the sensitivity is very high.

(4) Powder spraying absorbs water in the pile, which makes the pore pressure of soil around the pile dissipate and sink, and increases the negative friction on the pile for a short time, while the strength of cement-reinforced soil is very low and the growth is slow.

Generally speaking, in the early stage of powder spraying, the strength of cement-reinforced soil can not bear the self-weight and negative friction of cement-reinforced soil, and pile sinking occurs.

2) Construction technology

In terms of construction technology, aiming at the problem of pile sinking, combined with adverse geological conditions, the reasons for pile sinking are found from the construction technology. Various construction parameters (drilling speed, bit speed, lifting speed, powder injection pressure, cement dosage, etc. ) It is effectively controlled in the process of pile testing. In the second test of 52 piles, different drilling speed, different bit speed, different lifting speed, different powder injection pressure and different cement dosage were used for strict control. Although the amount of powder sprayed in the test pile has increased to 75kg/m, the problem of pile sinking has not been solved.

Look at the pile test site again. There is a lot of silt on the surface of sand cushion at the pile test site. According to the analysis, the soft plastic ~ fluid plastic sludge is liquefied under the action of wind pressure and airflow during the powder injection construction due to the disturbance in the mixing process, and the liquefied sludge rushes to the surface, resulting in the reduction of sludge in the pile body and pile sinking.

The reasons why increasing the amount of powder injection can't solve the problem of pile sinking are:

(1) increasing the amount of powder injection means increasing the self-weight of the pile;

(2) Increasing the quantity of DJM piles leads to the increase of water absorption of DJM piles in the pile body, which makes the pore pressure around the pile body dissipate and sink rapidly, and the negative friction on the pile body increases in a short time. The strength of cement-reinforced soil has not been significantly improved because of the increase of powder injection. At the initial stage of powder spraying, the strength of cement-reinforced soil still cannot bear the self-weight and negative friction of cement-reinforced soil, and pile sinking occurs.

3. Processing method

Through the above analysis, pile sinking is caused by the disturbance of soil in the early stage of powder spraying, and the strength of cement-reinforced soil can not bear the self-weight and negative friction of cement-reinforced soil. Adding an appropriate amount of early strength agent into cement slurry can solve the problem that the strength of cement-soil can not bear the self-weight and negative friction of cement-soil at the initial stage of pile formation by powder jet grouting pile. Main reasons:

(1) early strength agent can rapidly improve the strength of cement-reinforced soil, and the early strength agent can be evenly mixed into cement slurry;

(2) When the cement slurry is injected into the soil, it becomes a cement-soil pile with integrity, water stability and certain strength through a series of chemical reactions such as hydrolysis and hydration of cement, ion exchange between cement hydrate and soil particles, granulation, hardening, carbonization, etc., the cement slurry itself has enough water, and it does not need to absorb water from the natural foundation, so that the pore pressure of the soil around the pile will not dissipate and negative friction will be generated on the pile.

Therefore, the shotcrete method is adopted, and an appropriate amount of early strength agent is added to the cement slurry to solve the problem that the strength of cement-reinforced soil can not bear the self-weight and negative friction of cement-reinforced soil at the initial stage of pile formation by shotcrete method, resulting in pile sinking.

Sprayed concrete construction parameters:

Pile diameter: 50㎝

Drilling speed: control in 2 ~ 3 gears (30 ~ 50 cm/min).

Reading of ammeter: I≥60A when entering the bearing stratum.

Continuous mixing time of shotcrete at pile bottom: ≥30s.

Lifting speed of sprayed concrete: ≤30cm/min

Spraying pressure: 0.6 ~ 1.0 MPa

Water cement ratio of cement slurry: 0.5

Content of early strength agent (cement content): 0.8%

Mixing time of cement slurry: ≥30 minutes (each mixing)

The construction technology of sprayed concrete mixing pile shall be implemented in accordance with Technical Specification for Design and Construction of Highway Embankment on Soft Soil Foundation JTJ0 17-96 issued by People's Republic of China (PRC) and Ministry of Communications. All pass through the silt and enter the bearing stratum 50㎝.

The above construction parameters are tested on site, and the core-pulling test of pile body is carried out seven days after the test. From the core-pulling results of the pile body, the continuity and integrity of the pile are good, and there is no pile sinking problem. The owner organized the design unit, supervision unit and construction unit to hold a special meeting on soft foundation treatment technology, and decided to change the original mixing pile construction method of K9+753~K 10+836 bridgehead 186m to shotcrete construction, and the original contract unit price remained unchanged. After the construction of this section was changed to shotcrete, there was no pile sinking problem, which proved that the mixing pile constructed by shotcrete was effective in solving the pile sinking problem. The project cost has not changed much, and it is economical and feasible.

Third, the conclusion

Deep mixing pile is used to treat soft foundation with high water content, and powder spraying is often used to reduce the water content of foundation. When the problem of pile sinking occurs by powder spraying, an appropriate amount of early strength agent is added to the cement slurry to solve the problem of pile sinking. Practice has proved that it is effective and economically feasible to solve the problem of pile sinking by spraying concrete to construct mixing piles.

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