Analysis on the quality control in the secondary lining construction of the tunnel

During the construction of the secondary lining of the tunnel, lining materials, concrete mix ratios and construction methods should be selected reasonably, and construction experience should be constantly summed up, and the quality awareness of relevant personnel should be improved, and the surface defects of concrete should be controlled to ensure the quality of the project so as to achieve good social benefits and Economic benefits.

The tunnel lining structure can be roughly divided into five processes: tunnel bottom (foundation) treatment, base surface treatment, waterproof material laying, concrete lining, and polyurethane mortar jointing. During the construction process, quality control is mainly done in the following aspects:

1. The bottom of the tunnel is the foundation of the lining structure. If it is loose and not strong enough, the lining structure is prone to settlement and deformation, and the foundation surface is flat. It is the key to ensure the accuracy of the elevation and the elevation error to meet the specification requirements and ensure the realization of the design longitudinal slope. Therefore, the construction process In accordance with the design requirements, do the initial support. When there is no initial support, the flatness and compaction quality of the foundation surface should be removed to remove the scum to ensure that the foundation surface is solid and flat.

2. Base surface treatment is a key process for waterproofing. Monitoring and measurement analysis believes that after the surrounding rock and initial support deformation become stable, cut sharp objects on the surface, remove hard objects, fill up unevenness, and ensure that the base surface is flat The degree meets the specification requirements.

3. The protection and laying of waterproof materials is the quality focus in the construction process. Impervious materials are transported by special vehicles to avoid damage during transportation. After entering the field, check the geometric size, thickness, and strength for holes, pinholes, and flaws. The construction site is welded by professional and technical personnel of the factory using special tools. After the welding is completed, the quality of the weld is checked, whether there is any de-welding or missing welding, and each weld is checked for air inflation. The welding is required to be dense, wheel marks obvious, straight, and free. Wrinkles and waterproof materials are forbidden to step on, slip, slide, and walk, so as to avoid puncture damage to the plastic film. The waterproof board should be protected during the construction of the steel bar project, and the small damage detected before laying and concrete lining should be welded in time with a hand welding gun or repaired as required.

4. Measure before moving the lining trolley to ensure that the center line of the steel mold lining trolley is consistent with the center line of the tunnel, and the arch wall template is fixed after positioning, and the measurement is reviewed. Clean up debris, stagnant water and floating ballast at the foot of the wall, install a steel stopper template at the front end of the lining trolley, and install and fix the waterstop according to the design requirements; remove the waterstop protection mold at the construction joints of the upper group of lining concrete and conduct self-inspection Waterproof system settings.

5. The smoothness and cleanliness of the console car surface during the lining construction ensures the overall appearance quality of the tunnel surface.

Main factors affecting the quality of secondary lining construction

1. Irregular construction process or on-site operation

(1) The tunnel excavation is poorly formed and the lining concrete thickness is seriously uneven; under-excavation or initial support invades the lining boundary, resulting in insufficient lining concrete thickness; there is void behind the concrete lining of individual tunnels.

(2) The monitoring and measurement work was not carried out, and the construction time of the secondary lining was determined based on experience only. The safety and reliability were poor, which caused the secondary lining to bear the surrounding rock pressure beyond the design load.

(3) The measurement error of raw materials during the production of concrete is large, especially the random addition of admixtures, and the construction water consumption is not adjusted in time according to the actual moisture content of sand and stone, resulting in an increase in the water-to-binder ratio of concrete. The phenomenon of adding water during the transportation and pumping of concrete also exists from time to time.

(4) When the integral steel formwork trolley is used for construction, the concrete is not vibrated during pouring or the homogeneity of the leaking concrete is poor.

(5) Pursuing the construction schedule blindly. The demoulding time is arbitrarily advanced, so that the low-strength concrete bears excessive load and destroys the concrete structure. After demoulding, no moisture curing of the concrete was carried out.

(6) During summer construction, sand and stone materials are stacked in the open air without effective cooling measures, and the concrete entering mold temperature is high. The cold-proof and heat-preservation measures taken during construction in winter are not effective.

2. Poor quality of raw materials and unreasonable mix ratio design

Improper selection of cement varieties, poor stability, and mixing of different batches of cement. The gradation of crushed stone and sand is poor, the mud content exceeds the standard, and the content of stone powder in the crushed stone is too large, needles and flakes, which affects the cement and aggregate coagulation. When designing the mix ratio, neglect the negative impact of the increase in cement content on the performance of concrete. The selection of admixtures and admixtures lacks the guidance of professional and technical personnel, and often fails to achieve the expected results.

Quality control in the construction of secondary lining concrete

1. Positioning of the lining trolley

In the concrete construction of the secondary lining of the tunnel, if the lining trolley is not firmly in place, the phenomenon of dislocation and form running is extremely suitable. In order to prevent the occurrence of such problems, the construction technicians should focus on checking:

A. Before lining construction, the assembling design of the overall lining trolley should try to ensure the smooth connection between the plates. After the lining trolley is assembled, the structure size must be checked and adjusted to ensure that the structure size meets the design requirements.

B. The flatness of the steel formwork has an important influence on the appearance of the lining concrete. If the steel mold is uneven or too rough, holes and air bubbles will be left on the surface of the concrete after removing the mold.

C. Check whether the lining trolley is deformed during use. If it is deformed, it should be adjusted in time.

D. Check whether the support rigidity of the lining trolley is enough. If the support rigidity is not enough, the template of the trolley will converge and deform inward under the action of the mixed pressure, resulting in the staggered joints of the lining board. To deal with such problems, the support of the trolley should be strengthened, the number of support screws should be increased, and all supports should be fastened in place to ensure the overall force of the trolley is balanced.

E. Before the formwork trolley is in place, the construction technicians should carefully check whether the formwork is polished cleanly. The improper brushing of the formwork is an important reason for the local pitted surface of the mixed soil.

F. Excessive or uneven coating of mold release agent (use of waste engine oil is strictly prohibited) on the template will cause oil stains and inconsistent color after the lining concrete is demolded, which will affect the appearance quality. After applying the oil, it is advisable to keep the oil on your hands.

2. Concrete construction technology

The improper construction process of the concrete is the main reason for the honeycomb pockmarked surface of the secondary lining concrete surface.

A. Improper mixing ratio of concrete or inaccurate water consumption of sand, gravel, and cement materials, resulting in less mortar, more stones, coarse aggregate sinking, and slurry floating, resulting in honeycomb pitting on the concrete surface near the arching line. Construction The mix ratio should be adjusted at any time with changes in raw materials, climate, machinery and other conditions.

B. When pumping concrete, use the upper, middle and lower three-layer windows of the trolley to pour the concrete in layers. The drop should be less than 1m, and the two sides shall be poured alternately or symmetrically continuous full-section pumping. Strict attention should be paid to this point to prevent the bias voltage from deforming the template.

C. Master the correct vibrating method. If the vibrating time is too short, it is easy to form shrinkage cracks in the upper layer, and the strength is uneven. Prolong the vibrating time appropriately to make the vibrating compact; at the same time, the concrete vibrating can also have the effect of smoothly dissipating air bubbles. The distance between the vibrator and the template is controlled at about 10cm. If the distance is too small, the vibration will easily strike the template and cause vibration, and the vibration of the template will easily cause a large number of bubbles to concentrate on the template. If the bubbles cannot escape effectively, the appearance quality of the concrete will be affected.

D. Demoulding and curing time. When demoulding the secondary lining concrete, ensure that the concrete compressive strength is greater than 25MPa and the tensile strength is greater than 0.5MPa. After removing the mold, the concrete should be sprinkled with water for curing, and the curing should be longer than 14 days.

3. Waterproof and drainage construction

A. Before the waterproof and drainage construction, the base surface should be treated first, all kinds of foreign objects should be thoroughly removed, and all kinds of sharp protruding objects should be removed.

B. For the construction of vertical and circular permeable blind pipes, the key point is to control the installation position. The longitudinal drainage pipe of the tunnel is placed on the low side wall at the corner of the wall. The upper side of the drainage pipe is made of sand-free concrete as the drainage body, and the lower side is wrapped by a waterproof board. The width of the low side wall should be as narrow as possible, based on the principle of not occupying or less occupying the thickness of the second lining, generally about 15cm wide. The height of the low side wall is consistent with the design elevation of the bottom of the longitudinal drainage pipe, the cement content of the drainage body does not exceed 80kg/m’, and the particle size
16mm-32mm, the surface slope is controlled at 1:1 during construction. The vertical and circular permeable blind pipes are equipped with PVC pipes at the second lining to ensure that the outlet is easy to drain.

C. The key point of the waterproof board construction is to control the waterproof board and the geotextile closely to the base surface to prevent leaving a cavity behind. After the waterproof board and the hot-melt gasket are heat-sealed, the bonding and peeling strength of the two shall not be less than the tensile strength of the waterproof board. The welding seam of the waterproof board should be welded firmly, and air tightness inspection should be carried out to prevent missing welding in individual places, and hot air guns should be used to repair the missing welds.

D. The positioning and installation of the buried waterstop should be accurate. The hollow ring in the middle shall coincide with the construction joint. The waterstop is fixed on the stopper template. One end is installed first. When the concrete is poured, the other end should be fixed with additional steel bars to prevent the waterstop from shifting and affect the waterstop effect. When the waterstop is positioned, it should be kept flat on the interface. If there is any kinking phenomenon, it should be adjusted in time.

4. Rebar construction structure

Rebar blanking should be accurate to minimize the number of rebar joints. Rebar joints should be set at places with less stress, and they should be distributed. When binding steel bars, control the spacing between the steel bars and control the thickness of the concrete protective layer of the steel bars to prevent the exposed bars or the thickness of the concrete protective layer from being too large. Be careful not to puncture the waterproof board during the process of tying the steel bars.

5. Installation of pre-lined channel

Accurately bury all kinds of curved and straight precision channels according to the design drawings. Use the method of opening bolt positioning holes on the trolley template to locate the installation channels, and each channel must have 2 to 3 fixed points. The construction error of the channel embedded in the concrete surface, the inclined construction error of the channel and the parallel construction error between the two channels (each group) require high precision, and special attention should be paid to strengthen the control during the construction. The already constructed channel shall be protected and shall not be blocked by cement slurry and other debris. If any, it shall be cleaned up in time.

With the rapid development of engineering construction, more and more tunnels are being built. How to control the quality of the secondary lining of the tunnel not only achieves the design function, but also meets the requirements of internal reality and external beauty is the goal of engineering construction.