Advanced-small-conduit-free excavation method for tunnel in old loess stratum

Abstract

The invention belongs to the technical field of tunnels and underground engineering, and particularly relates to an advanced-small-conduit-free excavation method for an old loess stratum tunnel. For an old loess tunnel with the span smaller than 12 m, advanced small pipe supporting and radial anchor rod reinforcing measures are omitted, a three-step excavation mode that an upper step is provided with a temporary cross brace is adopted, the temporary cross brace is installed on the upper step along with excavation during three-step excavation, the temporary cross brace is dismantled along with excavation of a middle step, and single-side staggered excavation of the middle step and a lower step is achieved; the three-step excavation technology without advanced small guide pipes and 2.4 m short steps is researched and developed, the disturbance influence of pre-supporting is reduced, the construction period is shortened, the novel three-step excavation mode that the upper step is provided with the temporary transverse support is adopted, the working efficiency is improved by 12% compared with an original three-step temporary inverted arch method, the maximum accumulated settlement deformation of the vault is 35 mm, the maximum accumulated settlement deformation is reduced by 50% compared with similar tunnel construction, and the construction period is shortened. The size is obviously reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of tunnels and underground engineering, and in particular relates to a method for excavating an old loess stratum tunnel without an advanced small guide tube. Background Art

[0002] Old loess is a type of loess that belongs to the Early and Middle Pleistocene. It is a general term for the Wucheng loess (Q1) of the Early Pleistocene and the Lishi loess (Q2) of the Middle Pleistocene. Its macroporous structure is often compacted and generally has no collapsibility or only has slight collapsibility in the upper part of the Q2 loess.

[0003] For loess tunnels, the main factors affecting the engineering properties of the surrounding rock are: water content, anisotropy and primary vertical joints. The hydraulic properties of loess include: permeability, shrinkage and expansion, and disintegration. Due to the anisotropy of loess, the vertical permeability is generally greater than the horizontal permeability; in addition, the more clay content, the worse its permeability. The permeability coefficient κ of general loess is about i×10 -2 ~10 -3 m / s, the permeability coefficient of old loess is about i×10 -3 ~10 -5 m / s. Loess expands when it meets water and shrinks when it dries. Repeated cracks and peeling are likely to form. Due to the accumulation of loess, its horizontal shrinkage is greater than the vertical shrinkage, generally about 50% to 100%. The disintegration of various types of loess varies greatly. New loess will completely disintegrate after being soaked in water, while old loess will not completely disintegrate until some time has passed.

[0004] At present, during the excavation of loess tunnels, a small advance pipe is driven into the face according to a certain pattern to squeeze, bond and reinforce the loess at a certain depth within the tunnel contour to form a bearing ring. The construction of the small advance pipe takes a long time. In order to speed up the progress of tunnel construction, a method for rapid tunnel excavation for old loess layers has been developed based on the characteristics of old loess and on the premise of ensuring construction safety. Summary of the invention

[0005] The purpose of the invention is to increase the excavation speed of tunnels in old loess strata.

[0006] The present invention provides the following technical solutions: a method for excavating old loess tunnels without an advanced small-ducting tube. For old loess tunnels with a span of less than 12 m, the advanced small-ducting tube support and radial anchor reinforcement measures are cancelled, and a three-step excavation method with an upper step having a temporary cross brace is adopted. When excavating the three steps, a temporary cross brace is installed on the upper step as the excavation is carried out, and the temporary cross brace is removed on the middle step as the excavation is carried out. The middle step and the lower step are excavated with a single side offset.

[0007] Furthermore, when the three steps are excavated, the length of the upper step is 2.4 meters, the left and right sides of the middle step are staggered by 3.6 meters, and the left and right sides of the lower step are staggered by 4.2 meters.

[0008] Furthermore, when excavating the three steps, in the first step, the left side of the middle step is excavated 3.6 meters first, and the right side of the lower step is excavated 4.2 meters first; in the second step, the right side of the middle step is excavated until it is level with the left side, and the left side of the lower step is excavated until it is level with the right side; in the third step, the right side of the middle step is excavated 3.6 meters first, and the left side of the lower step is excavated 4.2 meters first; in the fourth step, the left side of the middle step is excavated until it is level with the right side, and the right side of the lower step is excavated until it is level with the left side; steps one to four are repeated.

[0009] Furthermore, temporary cross braces on the upper steps are arranged at intervals of 60 cm, and the temporary cross braces are connected to the primary supporting steel frame with bolts.

[0010] Furthermore, temporary cross bracing uses Ⅰ20 I-beam.

[0011] Compared with the prior art, the advantages of the present invention are:

[0012] As a special surrounding rock, loess is traditionally considered to have low strength and easy to disintegrate and collapse; however, old loess has poor permeability, higher hardness than new loess, good uprightness and easy forming. Under the condition of short footage, there is a period of time for the rock mass to stabilize, ensuring that there is enough time for erection and anchor spraying operations without large deformation and block falling of the rock mass, thereby ensuring safety. The present invention has developed a three-step excavation technology of "no advance small guide tube + 2.4m short step", which reduces the disturbance effect of pre-support and saves construction time. The new three-step excavation method with temporary cross braces on the upper step is adopted, and the work efficiency is improved by 12% compared with the original three-step temporary inverted arch method. The maximum cumulative settlement deformation of the arch is 35mm, which is 50% less than that of similar tunnel construction, which is significantly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a cross-sectional diagram of the temporary cross brace installation location;

[0014] Figure 2 This is a longitudinal section diagram of the temporary cross brace installation location;

[0015] Figure 3 This is a schematic diagram of the single-sided staggered excavation of the middle step and the lower step.

[0016] In the figure: 1-upper step; 2-middle step; 3-lower step; 4-temporary cross brace; 5-unexcavated soil at the face. DETAILED DESCRIPTION

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] like Figure 1 , Figure 2 , Figure 3 As shown: a method for excavating tunnels in old loess strata without advanced small ducts. For old loess tunnels with a span of less than 12m, the advanced small duct support and radial anchor reinforcement measures are cancelled, and a three-step excavation method with an upper step and a temporary cross brace is adopted. When excavating the three steps, temporary cross braces are installed on the upper step as the excavation is carried out, and temporary cross braces are removed on the middle step as the excavation is carried out. The middle step and the lower step are excavated with a single side offset.

[0019] When three steps are excavated, the length of the upper step is 2.4 meters, the left and right sides of the middle step are staggered by 3.6 meters, and the left and right sides of the lower step are staggered by 4.2 meters.

[0020] When excavating three steps, in the first step, the left side of the middle step is excavated 3.6 meters first, and the right side of the lower step is excavated 4.2 meters first; in the second step, the right side of the middle step is excavated until it is level with the left side, and the left side of the lower step is excavated until it is level with the right side; in the third step, the right side of the middle step is excavated 3.6 meters first, and the left side of the lower step is excavated 4.2 meters first; in the fourth step, the left side of the middle step is excavated until it is level with the right side, and the right side of the lower step is excavated until it is level with the left side; steps one to four are repeated.

[0021] The temporary cross braces on the upper steps are set at intervals of 60 cm. The temporary cross braces are made of Ⅰ20 I-beams and are connected to the primary support steel frame with bolts.

[0022] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for excavating a tunnel in an old loess layer without an advanced small pipe, characterized in that: For old loess tunnels with a span of less than 12m, the advance small-duct support and radial anchor reinforcement measures are cancelled, and a three-step excavation method with an upper step and a temporary cross brace is adopted. When excavating the three steps, temporary cross braces are installed on the upper step as the excavation progresses, and temporary cross braces are removed on the middle step as the excavation progresses. The middle and lower steps are excavated with a single-sided staggered approach.

2. The method for excavating a tunnel in an old loess layer without an advanced small pipe according to claim 1, characterized in that: When three steps are excavated, the length of the upper step is 2.4 meters, the left and right sides of the middle step are staggered by 3.6 meters, and the left and right sides of the lower step are staggered by 4.2 meters.

3. The method for excavating a tunnel in an old loess layer without an advanced small pipe according to claim 2, characterized in that: When excavating three steps, in the first step, the left side of the middle step is excavated 3.6 meters first, and the right side of the lower step is excavated 4.2 meters first; in the second step, the right side of the middle step is excavated until it is level with the left side, and the left side of the lower step is excavated until it is level with the right side; in the third step, the right side of the middle step is excavated 3.6 meters first, and the left side of the lower step is excavated 4.2 meters first; in the fourth step, the left side of the middle step is excavated until it is level with the right side, and the right side of the lower step is excavated until it is level with the left side; steps one to four are repeated.

4. The method for excavating a tunnel in an old loess layer without an advanced small pipe according to claim 2, characterized in that: The temporary cross braces on the upper steps are set at intervals of 60 cm and are connected to the primary supporting steel frame with bolts.

5. The method for excavating a tunnel in an old loess layer without an advanced small pipe according to claim 3, characterized in that: Temporary cross bracing uses Ⅰ20 I-beam.

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