In tall underground cavern projects, when the geological conditions are poor, in order to more effectively maintain the stability of the surrounding rock, in addition to using general grouting anchors and shotcrete supports, the surrounding rock is usually also supported. Anchor cables with a length of 10 to 20m and a prestress value of 100t are installed to reinforce the surrounding rock.
Rationally setting up prestressed anchor cables within a certain range around the cave can adjust the stress of the surrounding rock of the cave, move the tensile stress area around the cave upward, and improve the stability of the surrounding rock of the cave. . Prestressed anchor cables are not only used to build large caverns in less stable strata, but can also be used to reinforce slopes and dams. The prestressed anchor cables currently used include:
Expanded shell type steel strand prestressed anchor cables; Expanded shell type high-strength steel wire bundle prestressed anchor cables; Secondary grouting type prestressed anchor cables.
The structure of the primary and secondary grouting prestressed anchor cables
The composition of the secondary grouting prestressed anchor cables is shown in Figure 12-15, which consists of an inner anchoring section, a tensioned section It consists of four parts: outer anchoring section and grout overflow section.
1. Inner anchoring section
The main function of the inner anchoring section is to provide sufficient anchoring force when the anchor cable is tensioned. The length of the inner anchoring section should be based on the required anchoring force. Determine size.
Figure 12-15 Secondary grouting prestressed anchor cable
The inner anchoring section includes steel strands, wire rings, positioning grout stop rings, grout overflow pipes, exhaust pipes, etc. composition.
The cement used for grouting the inner anchoring section should be fast-setting, early-strength, less corrosive to steel and high in strength. The initial setting time is 30min to 1h, and the final setting time is 1~1.5h.
According to tests, the length of the inner anchoring section is 1.5-2.5m when the tension of the anchor cable is required to be 50-60t in general rocks.
2. Tension section
The tension section is the main body of the prestressed anchor cable. It is made of steel strands extending from the inner anchor section that are tensioned and then filled with cement mortar. .
The length can be determined according to geological conditions and the span of the cavern. The grouting mortar can be ordinary Portland cement mortar.
During grouting, the air in this section is compressed to the overflow section, and then discharged through the exhaust pipe. Finally, the overflow section is also filled with cement mortar.
3. External anchoring section
The external anchoring section consists of an external anchor head and a pier. Its main function is to maintain the prestressed force of the anchor cable after tensioning, and to expand the outer end of the anchor cable. The pressure-bearing area of ??the surrounding rock to evenly transmit pressure.
4. Slurry overflow section
The slurry overflow section is set up to ensure that the tension breaking point is full, and is generally about 50cm long.
There are top caps, positioning rods and exhaust pipes in the overflow section.
The top cap is made of coiled and welded steel plates and is used to straighten the steel strand above the slurry stop ring.
The positioning rod is made of steel bars and is about 60cm long. It is used to maintain the length of the overflow section and fix the exhaust pipe.
2. Prestressed anchor cable construction technology
(1) Drilling: diameter 80~130mm, depth up to 60m (using down-the-hole drilling);
(2) Assembly and pushing of anchor cables;
(3) Grouting of inner anchoring section: early strength cement is used, and the mortar ratio is 1:1: (0.4~0.45) (c:s ∶w);
(4) Pouring concrete piers with openings: In order to tension and transmit pressure, concrete piers with openings must be poured. When pouring, first insert the positioning pipe into the drill hole, adjust the position and fix it, then set up the formwork and pour concrete mixed with early-strength cement;
(5) Tensioning of the anchor cable: use a jack Tension and fix;
(6) Grouting of the tensioning section: to protect the steel strands and permanently maintain prestress;
(7) Treatment of the external anchoring section: When the mortar poured into the tension section reaches a certain strength, cut off the excess part of the exposed steel strand of the anchor ring. Concrete is then sprayed on to prevent corrosion of the steel bars.