The engineering measures of landslide control mainly include: surface drainage, underground drainage, dredging and load reduction, back pressure, retaining works (including anti-slide retaining wall, anti-slide pile, anchor cable, anchor rod, anti-slide key, anti-slide open hole, micro-pile, etc. ), soil improvement in sliding zone (grouting, jet grouting pile, sand pile), tamping to fill cracks, leveling the ground, environmental protection and greening, etc. Here, the following points are mainly discussed.
3. 1 Clear load shedding
Dredging and load reduction is a common engineering landslide control measure, which is easy to implement. It can be used in emergency engineering and permanent engineering, especially in the period of landslide formation, with obvious effect, which can not only effectively weaken the sliding force of landslide, but also reduce the supporting works and effectively ensure the construction safety.
3.2 Drainage
As the saying goes, "there is no water but there is no landslide", which shows that landslide is closely related to groundwater and surface water. Therefore, the treatment of landslide first focuses on the treatment of water, especially the water acting on the sliding surface.
Drainage includes surface drainage and underground drainage. Surface drainage usually includes the construction of intercepting ditch outside the landslide body and dendritic drainage ditch inside the landslide body to intercept the surface water flowing to the landslide and discharge the surface water inside the landslide body, so as to avoid surface water infiltration, increase the weight of the landslide, soften the sliding zone, reduce its strength and reduce the sliding resistance of the landslide itself. Surface drainage is widely used as a direct and effective measure. To remove groundwater, it is best to intercept it outside the sliding body when the recharge and discharge of groundwater are found out. The measures to drain groundwater are often used to drain the shallow stagnant water in the slope, and are generally suitable for landslide control of soil cutting and embankment. A landslide treatment project of K253+000 ~ K253+ 190 in expressway adopts drainage holes to discharge groundwater; Rapids and intercepting ditches discharge surface water. Ying Qin is good at supervision and management during the construction of drainage works to ensure smooth drainage and seepage prevention.
3.3 anchor rod
In recent years, bolt support technology has been more and more used in slope reinforcement and landslide control projects. A landslide treatment project of an expressway K253+000-K253+ 190 has the characteristics of high, difficult and dangerous construction with a slope height of 80m and a harsh construction environment. Choose a drilling rig with light weight and convenient for aerial work. After investigation, the down-the-hole drilling rig produced by Xuanhua was selected, and the gas source equipment was Fu Sheng SP-46500.
3.3. 1 hole-making bolt
The hole must be perpendicular to the slope, with a hole depth of 9.5m, an anchor length of 0.5m and an aperture of 130mm. After the hole is formed to the designed depth, attention must be paid to discharging the residual sediment in the hole to ensure the quality of hole cleaning and the good adhesion of anchor slurry to the hole wall.
3.3.2 Anchorage processing and installation
The anchor rod is welded side by side by 2 Ф 32 threaded steel bars, and a centering bracket must be set every 2m to center it in the anchor hole. Grouting pipe must be the same length as the anchor rod, and the length of the anchor rod is 9m, which shall be sent into the hole by manpower for installation. K253+000-K253+ 190 Slope * * There are 324 anchor rods with a total length of 29 16m. There must be a supervision station for anchor installation, which can not only ensure the processing quality and length of anchor, but also check the hole depth and hole cleaning quality of anchor again.
Bolt grouting design
The cement slurry strength is more than or equal to ≥25Mpa, and the cement used is 425# ordinary portland cement with a water cement ratio of 0.4. During grouting, the grouting pressure (reaching 1- 1.5Mpa) and the fullness of grouting (returning the slurry from the bottom of the hole) must be ensured, and the whole process should be supervised.
3.3.4 Orifice pressure beam construction
The orifice pressure beam construction adopts reinforced concrete frame beam or anchor rib beam, and the anchor head is sealed to prevent anchor corrosion.
3.4 prestress
The application of prestressed anchor cable in landslide control has the advantages of fast construction speed and obvious effect. There are 249 anchor cables in K253+000-K253+ 190, with a total length of 5 103m.
3.4. 1 construction process
3.4.2 Selection of equipment and materials
The main equipments are: 15.4m3/min, mobile screw air compressor with air pressure of 0.9Mpa; MG-50A drilling rig and KQJ- 100B DTH drilling rig; JZ350 mortar mixer; SB 10 hydraulic mortar pump; ZB4-500 electric oil pump; YCD 150 hydraulic jack, etc.
The steel strand adopts low relaxation prestressed steel strand with ultimate tensile strength 1860Mpa, and the anchorage adopts OVM anchorage and clip. The cement is 525 ordinary portland cement (the test shows that R28 is greater than 25Mpa when the water-cement ratio is 0.52, which meets the design requirements).
3.4.3 Hole making and hole cleaning
Before hole formation, use geological compass for orientation to ensure that the included angle between prestressed anchor cable and horizontal plane meets the design requirements of 25o. The diameter of the anchor hole is 130mm, and the hole depth is1m. Grouting treatment measures (water cement ratio =0.35, early strength agent 2.5%) are adopted when the loose, broken and easily collapsed rock stratum is encountered during drilling.
3.4.4 Anchor cable processing and installation
The anchor cable is processed on the working platform. A single anchor cable consists of six ASTMA4 16-87a steel strands, and the cutting length of the steel strands is equal to the length of the anchor segment+the length of the free segment+the length of the tensioning end, and it is cut with a grinding wheel saw.
Steel strand shall be derusted and protected against corrosion, especially the free section. There are three ways:
(1) evenly coat anticorrosive materials on the steel strand;
(2) Install the plastic casing and handle the break and crack;
(3) The grouting of cement slurry is full and uniform, forming a rust-free environment.
The guide cone has two main functions:
(1) Ensure the smooth installation of the anchor cable;
② Avoid grouting pipe blocking. Therefore, the grouting pipe must extend into the guide cone.
Expansion rings should be installed evenly according to the designed spacing, and can be made of nylon or engineering plastics with small deformation modulus, which is not only beneficial to the prismatic structure of steel strand, but also can produce compressive stress on slurry when steel strand is stretched.
The protective net of the anchorage section should be carefully wrapped, which can not only protect the anchor cable, but also increase the bonding force between the anchor cable and the cement slurry.
Grouting should be done for the free section and anchorage section, otherwise the plastic sleeve of the free section will come out during grouting.
The installation of the anchor cable can be completed slowly by hand by sending the anchor cable into the hole.
Be diligent in checking the processing and manufacturing of anchor cable to ensure the processing quality, and supervise the installation at the same time, and check the quality and depth of anchor hole again with the length of anchor cable.
grout
Grouting should be carried out continuously with hole formation and anchor installation, so as to avoid sediment on the wall of anchor hole and affect grouting quality. When grouting, strictly control the water cement ratio and grouting pressure, and the grouting pressure can reach 1- 1.5 MPa. It is necessary to ensure pressure grouting, return slurry from the bottom of the hole to the orifice, and ensure sufficient grouting.
Tension and locking
When the poured cement slurry and tensioning platform reach the design strength, tension the anchor cable. In the landslide treatment of a expressway K253+000-K253+ 190 section, the designed bearing capacity of a single anchor cable is 900KN, which requires two times of three-stage tensioning. Each tensioning load holding time shall not be less than 15min, and the design locking prestress shall be more than 800KN, but not more than 900KN.
Anchoring seal
Anchor cables are connected by frame beams or ground beams. After the anchor cable is tensioned and locked, the redundant steel strand shall be removed 50mm away from the work clamp, and the anchor head shall be sealed with C25 concrete.
3.5 Anti-slide pile
Anti-slide pile is to set one or more rows of reinforced concrete piles in the front or middle of the landslide with thin sliding body and small thrust to stabilize the landslide. Because of its large anti-sliding force, small disturbance to landslide stability and convenient construction, it is widely used in landslide control at present and can be divided into:
① Embedded anti-slide pile;
② Cantilever anti-slide pile;
③ Anchor cable anti-slide pile. This paper mainly discusses the anti-slide pile with anchor cable.
Anchor cable anti-slide pile is to set anchor cable at the top or upper part of anti-slide pile to exert lateral tension and improve the stress condition of anti-slide pile. Compared with anti-slide piles, its cross-section can generally be reduced by 30%-50%. In a landslide treatment project in expressway K253+000 ~ K253+ 190 and K253+400 ~ K253+580,
In the construction process of anchor cable anti-slide pile, the following points should be paid attention to:
(1) landslide monitoring and observation;
(2) Observation of geological changes during excavation, and comparative observation of design sliding surface and actual sliding surface;
(3) properly handle underground water seepage;
(4) the section size and verticality of the pile;
(5) Construction safety;
(6) The requirements for anchor cables have been mentioned in prestressed anchor cables, and will not be described here.
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