Shi 'an expressway is located at K27 1+345-K487+358km in expressway, Beijing-Zhuhai, which is an important passage from south-central China to Beijing, with a large cargo transportation volume. After opening to traffic at the end of 1997, the bottom of some bridges, culverts and access structures appeared void to varying degrees. Its diseases are mainly manifested as:

(1) The road surface at the approach slab sinks in different degrees, and the side at the bridge head basically does not sink, but the side adjacent to the road surface sinks heavily, resulting in cracks at the joint between the approach slab and the road surface, and the bridge head trips when vehicles pass.

(2) Because the middle part of the plate is empty, the plate vibrates violently when the vehicle passes by. The belt with serious gap has broken; On rainy and snowy days, the accumulated water under the slab continued to scour the subgrade, and mud gushed out along the abutment under the bridge. The existence of this disease seriously affects the stability of the structure and the safety and comfort of driving. In March 2003, we analyzed the disease and adopted the construction method of grouting under the plate to deal with it, and the effect was very good.

2. Causes and effects of cavity formation

2. 1 Abutment back packing is not dense.

Because the filling material at the back of abutment is not dense, especially the filling material of subgrade in high fill section is uneven and dense, the filling material settles under the action of vehicle load and vibration, resulting in the void at the bottom of slab.

2.2 rainwater inflow, filler loss

Due to untimely joint maintenance, rain, snow and water enter under the slab, and under the action of vehicle load, pumped slurry and mud flow out along the abutment, which leads to the continuous reduction of subgrade materials under the slab, and then the void is formed.

2.3 the destruction of vehicle overload

Under the action of vehicle load, especially overloaded vehicles, the slab bears huge vertical load, constantly replenishing and compacting the subgrade at the bottom of the slab, and finally forming void.

2.4 the impact of the gap on the strap

Because of the high compressive strength, low flexural strength and sensitivity to overload of cement concrete slab, once the slab bottom is empty, the subgrade will form uneven support, which changes the actual mechanism of the close adhesion between concrete slab and subgrade and makes the stress mode of elastic thin plate on elastic foundation become cantilever thin plate. Under the action of vehicle load, it breaks prematurely, and this kind of break is irregular, so it is difficult to maintain and deal with it. This accelerates the invasion of rainwater, increases the area and depth of the gap, and then increases the cracks. This repeated action will eventually lead to serious damage to the board and make it impossible to drive.

3. Grouting treatment

3. 1 Mechanism of grouting treatment method

Grouting technology for void at the bottom of cement concrete slab is a technical measure to restore the close adhesion of slab bottom and ensure the uniform support of slab bottom, which has the following functions and effects:

1) Grouting treatment can enrich the gap at the bottom of the slab, restore the close adhesion at the bottom of the slab, improve the lateral support of the panel, make the stress state of the concrete slab conform to the design principle, and avoid the premature failure of the concrete slab.

2) Because the grouting material has good stability and strength, the slurry has good fluidity and strong permeability under pressure, which can improve the compactness and water stability of the base and subgrade.

3.2 Grouting Technology Construction Technology

3.2. 1 Main construction machinery and equipment. TXV- 100 geological drilling rig, BW-250/50 cement slurry pump and JW- 180 mortar mixer.

3.2.2 Arrangement and drilling of grouting holes. The arrangement of grouting holes and the number and depth of holes arranged in each plate directly affect the grouting effect. According to experience, the hole locations are generally arranged as 1m row spacing and 2m hole spacing. Shi 'an expressway is a two-way four-lane city, and bridge end transition slab is generally 3m long and 10.75m wide. Because the construction in expressway is continuous and the emergency parking zone is basically empty, the grouting holes are arranged in two rows, with the first row spacing at the bridge head 1m, the second row spacing at the junction of slab and pavement 1m, and five holes are arranged in the first row. Generally, the depth after drilling through the plate is 5 ~ 10 cm. Grouting flower tube should be 37 ~ 42 mm in diameter and 1.5 ~ 2.0m in length. Boreholes should be accurately positioned to minimize the inclination of grouting holes.

3.2.3 Selection of grouting materials and pulping. Cavity grouting at the bottom of slab is driven by the pressure of grouting pump, and the mixed filler is squeezed into the bottom of slab or infiltrated into the gap of subgrade through grouting pipe and pressure head, so the filled mixture is required to have good cohesiveness, high fluidity and small shrinkage, and it is required not to separate and bleed water during grouting.

In order to ensure that the grouting mixture can be pressed into the bottom of the slab smoothly, the cementing material requires the grouting mixture to have good pumpability and water retention, so the cement with good water retention and small bleeding is selected as the cementing material of the grouting mixture. Generally, 32.5 ordinary portland cement is selected.

Adding an appropriate amount of admixture to grouting mixture can not only reduce the cost, but also improve the fluidity of grouting mixture. Fly ash is the first choice for grouting admixture, because it contains a large number of hollow glass beads, which can solve the contradiction between reducing water-cement ratio to improve the strength of grouting hardening material and increasing water-cement ratio to improve the pumpability of mixture. Due to the secondary hydration between fly ash and cement hydrate, the hardened product is denser and stronger; Due to the water absorption of hollow glass beads, after the final setting of the mixture, a small amount of water can be continuously released to participate in health preservation and reduce shrinkage; Because the grouting mixture will shrink after hardening, an appropriate amount of expansion agent can be added to the grouting mixture.

When the consistency of grouting mixture is too large, the strength after hardening is high, but the pumpability is poor, which is easy to cause pipe plugging; Too thin and hard, the shrinkage is large, and it can't achieve the close-fitting effect. Therefore, the proportion of grouting mixture should be determined according to different seasons, and generally 1:0.33:0.06:0.04 (cement: fly ash: water: expansion agent, mass ratio) is adopted.

Pulping step: (1) feeding according to the procedure, accurately measuring and mastering the slurry performance; (2) The slurry should be fully stirred, and it should be continuously stirred before grouting to prevent it from settling again and affect the quality of the slurry.

Measurement of pressure. Excessive pressure during slab bottom void grouting will cause slurry to flow between slab and asphalt concrete surface layer, resulting in arching or fracture of asphalt concrete surface layer; If it's too small, it won't fit in. Grouting pressure should generally be controlled at 0.5 ~ 0.0 MPa.

Grouting. Grout is injected into the target layer through grouting equipment and slurry pipeline, and the bottom-up method should be adopted, and grouting should be carried out in sections from bottom to top. In the grouting sequence, the edge curtain hole should be constructed first, and then the reinforcement hole should be constructed. When the grouting pressure meets the requirements, the grouting shall be terminated. During grouting, grouting should be stopped when uplift or slurry leakage occurs, and the reasons should be analyzed. The grouting quantity should be reduced in the next grouting section, and the hole sealing device and grouting equipment should be checked.

4, the problems that should be paid attention to in the construction

1) Observe the depth of grouting holes, which should be adjusted appropriately according to the formation conditions of each hole.

2) Observe the grouting sequence and make appropriate adjustments according to the formation conditions, site conditions and grouting purposes.

3) Strictly control the mud mixture ratio, and pay attention to whether the flowmeter and pressure gauge are in good condition and accurate.

4) Grouting quantity control. According to the dosage of materials and water during slurry preparation, check and make records at any time.

5) Safety measures. Safety signs must be complete, and the construction in expressway is continuous, difficult and dangerous, so safety work is particularly important.

5. Experience and experience

5. 1 plugging. If there is a large overhang under the slab, the abutment taper slope and slope protection should be opened first, and stones should be plugged to reduce the grouting amount, and prevent expansion and slurry leakage.

5.2 Grouting method. Grouting is carried out by combining gravity flow with pressure grouting. Porous grouting is used, and the speed is from fast to slow, and the order is first low and then high. The pressure depends on the situation. The end standard of single hole grouting is self-overflow filling in adjacent holes, and then grouting in other holes to ensure the filling under the plate and make the plate have sufficient bearing capacity.

5.3 Selection and configuration of mud. Grouting materials should meet the requirements of injectability, rapid setting and high strength of consolidated body. , and the slurry configuration should be 1:0.33:0.06:0.04 (cement: fly ash: water: expansion agent, mass ratio) to be economical and practical.

5.4 The grouting project is an underground concealed project with poor intuition, so it is technically difficult to directly and quantitatively judge the grouting effect. If the grouting construction is carried out in strict accordance with the design requirements and construction specifications, and the grouting quantity and pressure meet the standard requirements, it can generally be confirmed that the grouting effect is good and the grouting engineering quality can be guaranteed.

In the process of construction, some improvements were made to the construction technology, which made the grouting technology more reasonable and economical in dealing with the void at the bottom of expressway passage. After grouting, when heavy vehicles pass by, the slab has no obvious vibration and mud does not gush out under the rain bridge, which prolongs the service life of the slab and ensures the stability of the structure and the safety and comfort of driving.

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