The trenching process is the most important in the construction of underground continuous walls, which often accounts for more than half of the trench construction period. Therefore, doing a good job of trenching is the key to improving the construction efficiency of underground continuous walls and ensuring the quality of the project. With the continuous improvement of the requirements for construction efficiency, new equipment continues to emerge and new methods are also developing. At present, the advanced and efficient underground continuous wall trenching (hole) machinery widely used at home and abroad mainly includes grab trenching machines, hydraulic trenching machines, multi-head drills (also known as vertical multi-axis rotary trenching machines), and rotary pile hole drilling machines, among which the most widely used is the hydraulic grab trenching machine.
Commonly used trenching machinery and equipment are mainly divided into three categories according to their working mechanism: grab, impact, and rotary. Correspondingly, there are also three main basic trenching methods: ⑴ grab trenching method; ⑵ impact drilling trenching method; ⑶ rotary drilling trenching method.
Grab bucket slotting machines have become the main equipment for underground continuous wall slotting in China, and there are more than 100 units (most of which are imported equipment). The grab bucket slotting machine usesa crawler crane to hang the grab bucket, which is usually clam type. According to the mechanical structure characteristics of the grab bucket, it can be divided into wire rope grab bucket, hydraulic guide plate grab bucket, guide rod grab bucket, and mixed grab bucket. The grab bucket cuts the soil with its bucket teeth, and the cut soil is stored in the bucket body. After it is taken out from the slot section, the bucket is opened to unload the soil, and the soil is excavated and slotted repeatedly in this way.
This slotting method is widely used in the construction, subway, and other industries. The underground continuous walls of major cities such as Beijing, Shanghai, Tianjin, and Guangzhou mostly adopt this process. For example, the underground continuous walls of projects such as Beijing National Grand Theater, Shanghai Jinmao Tower, Tianjin Hongji Building, Nanjing Xinjiekou Subway Station, and Shanghai World Financial Center all adopt the grab bucket construction process.
When using grab bucket slotting, you can make a single slot or multiple slots, and the slot section length is generally 3.8 to 7.2m. Single grabbing means grabbing one slot width at a time; multiple grabbing means each slot width is formed by three or more grabbing. Usually, the length of a single-sequence grab is equal to the maximum opening of the grab bucket (about 2.4m), and the length of a double-sequence grab is less than the maximum opening of the grab bucket.
Applicable environment: The strata have wide adaptability, such as clay, sandy soil, and gravel soil with N＜40. Except for large pebbles and bedrock, general covering layers are all acceptable.
Advantages: Low noise and low vibration; strong grooving ability of grab bucket and efficient construction; except for the early clam grab bucket and rope guide grab bucket, most of them are equipped with inclination measurement and deviation correction devices (such as deviation correction hydraulic push plate) to adjust the verticality of the grooving at any time, and the grooving accuracy is relatively high (1/300 or less).
Disadvantages: The excavation depth and hard layer are limited, which reduces the grooving efficiency. It needs to be used together with other methods.
Equipment: Wire rope grabs – such as Italy’s SOILMEC and Casagrande, Germany’s BAUER, LEFFER and WIRTH, and Japan’s MASAGO all produce various types of wire rope grabs; hydraulic guide plate grabs – such as the DHG and GB types produced by Germany’s BAUER, the MHL and MEH (super-large, with a maximum closing force of up to 1725kN, which can excavate up to 150m deep and 3.0m thick underground walls in sand and gravel foundations) produced by Japan’s MASAGO, and the HSWG grab produced by Lieberher; guide rod grabs – such as France’s KELLY, Italy’s KRC and Japan’s CON series; mixed hydraulic grabs – such as Italy’s SOILMEC’s BH-7/12 and MAIT’s HR160 grab.
The earliest underground continuous wall in the world was built by impact drilling (such as the Italian ICOS method – impact drilling, positive circulation slag removal), and the same is true in my country. With the continuous improvement of construction technology, the impact drilling method is no longer dominant. However, if it is combined with modern construction technology and equipment, the impact drilling method still has advantages that cannot be ignored.
The domestic impact drilling slotting methods mainly include impact drilling (drill splitting method) and impact reverse circulation (drill suction method). The impact drilling method adopts the method of impact crushing and tube extraction (i.e., mud does not circulate), that is, the impact drill uses a wire rope to suspend the impact drill bit for reciprocating lifting and falling movements, relying on its weight to repeatedly impact and crush the rock, and then uses a slag collection barrel with a movable bottom to remove the crushed soil slag and stone chips to form a hole. Generally, the main hole is drilled first, and then the auxiliary hole is split, and the main and auxiliary holes are connected to form a slot hole. The impact reverse circulation type replaces the impact drill with an impact reverse circulation drill. A slag discharge pipe (or a reverse circulation sand and gravel pump) is set in the center of the hollow sleeve drill bit to suck the mud containing drilling slag, which is returned to the slot hole after purification, greatly improving the slag discharge efficiency. After the drilling slag in the mud is reduced, the efficiency of the drill bit impact crushing is also greatly improved. The slot hole construction can be done by flat drilling or by dividing the main and auxiliary holes. The drilling and suction method of this impact reverse circulation drill is much more efficient than the drilling and splitting method of the old impact drill.
Applicable environment: It can be used in various soils, sand layers, gravel, pebbles, boulders, soft rocks and hard rocks. It is especially suitable for the construction of complex strata such as thick boulders and boulders. In such strata, its construction cost is much lower than that of grab slotting machines and hydraulic slotting machines. It is a method still used by domestic water conservancy departments in the construction of anti-seepage walls.
Advantages: The construction machinery is simple, the operation is easy, the cost is low, and it is an economical and applicable process.
Disadvantages: Low slotting efficiency and poor slotting quality. Main models: The impact drills mainly include YKC, CZ-22, and CZ-30, and the impact reverse circulation drills include the CZF series, CJF series, CIS-58, etc.
In my country, impact drills have been used in underground continuous wall construction for more than 50 years. The maximum wall depth of impact reverse circulation drills is 101m (Sichuan Yele Hydropower Station), and they have also played an important role in rock-embedded underground continuous wall projects such as the Three Gorges of the Yangtze River and the Runyang Yangtze River Bridge.
Rotary slotting machines are divided into vertical rotary type and horizontal rotary type according to the direction of the rotary axis.
(1) Vertical rotary type Vertical types are divided into vertical single-axis rotary drilling rigs (also called single-head drills) and vertical multi-axis rotary drilling rigs (also called multi-head drills).
Single-head drills are mainly used to drill pilot holes, while multi-head drills are mostly used to dig grooves.
a. Single-head drilling Single-head drilling rigs mostly use reverse circulation drilling technology and positive circulation can also be used for slag removal in fine-grained formations. Since various formations from soft soil to bedrock may be encountered during drilling, they are generally equipped with a variety of drill bits to meet the needs of drilling.
Single-axis rotary drilling rigs mainly include: CIS-60, CIS-61 in France, BG in Germany, GJD, GPS, GQ in my country, etc. There is also a rotary drilling method without mud circulation. Its working principle is that the machine applies strong power (torque) to make the drill bit, vibrating sinking pipe, shaking pipe, full casing, etc. cut and crush the rock (soil) body during the rotation process, and then use rotary buckets, spiral drills, impact grabs, and other equipment to directly dig the soil out of the hole. The main models are: CIS-71 of Soledans Company in France, KCC and MR-2 in Italy, KPC-1200 in Japan, and GJD-1500 in my country, etc.
The most advanced rotary drilling method is the full-rotation full-casing drilling method, which is characterized by continuous casing cutting and guaranteed drilling speed under very hard geological conditions (even rocks with a compressive strength greater than 250MPa). The main models are the German RDM type and the Japanese RT type (Shanghai Foundation Company introduced the RT-200AⅢ full-rotation full-casing drilling rig in 2006, which was used for the clearance of underground continuous walls that passed through the thick riprap layer on the riverside in the comprehensive renovation project of the Shiliupu area of the Bund, with good results). Its application potential in the field of underground continuous walls needs to be further explored.
b. Multi-head drill Vertical multi-head rotary drill uses two or more submersible motors to drive multiple drill bits under the drilling rig to rotate through a transmission device, cut the soil layer symmetrically at the same drilling speed, and discharge the slag into the vibrating screen by pump suction reverse circulation. Larger sand and gravel and blocky mud are discharged by the vibrating screen, and finer particles flow into the sedimentation tank with the mud. They are separated and discharged through the cyclone multiple times, and the clean mud is recycled. The section dug by the multi-head drill once is called the excavation section, and several excavation sections constitute a unit slot section.
Applicable environment: N < 30 cohesive soil, sandy soil, and other not-too-hard fine-grained strata. The depth can reach about 40m.
Advantages: no vibration and noise during construction, continuous roughing, and slag discharge can be carried out, no repeated drilling is required, the construction efficiency is high, the construction quality is good, and the verticality can be controlled between 1/200 and 1/300. It was widely used in the early 1980s and is a more popular construction method.
Disadvantages: Poor adaptability to trenching in gravel and pebble layers and when encountering obstacles. Equipment: The main models include Japan’s BW series (currently the only one produced abroad, the BWN type has a maximum digging depth of 130m and a wall thickness of 1.5m), my country’s SF type (Shanghai Foundation Company successfully developed in the late 1970s, SF-60/SF-80) and ZLQ, etc.
Multi-head drills have been challenged in recent years and have gradually been replaced by grab buckets and horizontal multi-axis rotary drills (slot milling machines), but they still have a market for fine-grained strata such as soil and sand.
c. Horizontal rotary type – slot milling machine Horizontal multi-axis rotary drilling rig has only two axes (wheels), also known as a double-wheel slot milling machine. According to the different power sources, it can be divided into two types: electric and hydraulic. The slot milling machine is currently the most advanced underground continuous wall slotting machine at home and abroad, with a maximum slotting depth of up to 150m and a slotting thickness of 800 mm~2800 mm at a time.
Advantages:
(a) It has strong adaptability to the stratum and can excavate silt, sand, gravel, pebbles, medium-hard rock, etc. It can also drill hard rocks with a compressive strength of about 200MPa when equipped with a special roller milling cutter;
(b) It has high construction efficiency and fast excavation speed. Generally, the sedimentary layer can reach 20~40m3/h (2~3 times higher than the grab bucket method), and the medium-hard rock can also reach 1~2m3/h.
(c) High grooving accuracy. The electronic inclinometer, guide adjustment system, and adjustable angle drum milling machine can achieve a verticality of 1‰ to 2‰.
(d) Large grooving depth, generally up to 60m, and up to 150m for special models;
(e) Concrete can be cut directly. No special connectors or special sealing measures are required for the connection of the first and second sequence grooves to form a good wall joint;
(f) The equipment has a high degree of automation, flexible operation, and easy operation. The electronic indicator monitors the entire construction process, automatically records and saves the inclinometer data, and can be printed out as engineering data after the construction is completed;
(g) Low noise and low vibration, can be constructed close to the building.
Limitations:
(a) The equipment is expensive and has high maintenance costs;
(b) It is not suitable for formations with isolated rocks or large pebbles and must be used in conjunction with impact drilling or blasting.
(c) It is sensitive to the falling of iron objects in the formation or the existence of steel bars in the original formation. The slot milling machine has excellent performance and has been widely used in developed countries. However, it has not been fully promoted in my country due to the construction cost and the number of equipment (since the first machine was introduced in the Three Gorges Project of the Yangtze River in 1997, the number of machines in the society has been less than 10 so far). Japan has completed a large number of ultra-deep foundation projects using slot milling machines, the deepest of which has reached 150 m, the thickness of which has reached 2.8 to 3.2 m, and the test excavation depth has reached 170 m.
In China, the slot milling machine has been successfully used to construct many projects such as the Three Gorges Project, Shenzhen Metro Station (embedded with a slightly weathered rock ground wall), Nanjing Zifeng Tower, and Shanghai 500kV Expo Substation.
Equipment: Hydraulic types include BC type from Germany’s BAUER (which has a large market share in my country), HF type from France, K3 and HM type from Italy’s Casagrande, TBW type from Japan, etc.; electric types include EM and EMX type from Japan’s Tone Company, etc.
(1) Working principle of slot milling machine The equipment mainly consists of three parts: lifting equipment (crawler crane), slot milling machine (milling cutter frame, 12m high), mud preparation and screening system, etc.
Its working principle is: to use power to drive the two drum wheels (also called milling wheels) installed on the frame to rotate in opposite directions to cut the rock (soil) and break it into small pieces. The pump suction reverse circulation system configured on the frame itself is used to suck out the soil and rock residue and mud mixture excavated through the sand suction port in the middle of the milling wheel and discharge it to the ground for centralized treatment by special sand removal equipment, and the soil and rock fragments are separated from the mud. The purified mud is pumped back into the slot for recycling, and this cycle is repeated until the final hole is slotted. Figure 11-17 shows the working principle diagram of the hydraulic double-wheel milling machine.
Milling wheel cutters can be selected according to different strata. There are three main types: standard tungsten carbide teeth (flat teeth), conical teeth with alloy inlaid tungsten steel heads (conical teeth), and wheel-shaped cutting teeth (rolled teeth) with rolling drill bits, which are suitable for rock excavation with maximum compressive strength of 60MPa, 140MPa and 250MPa respectively.
(2) Unit slot division principle and cutter design According to the slotting characteristics of the slot milling machine, taking the BC type as an example, when the two milling wheels are opened to the maximum (called 1 full cutter), the length of the milling head is 2.8m, and the range of the closed cutter is between 0.8~1.6m.
When the excavated soil (rock) body is in the following two situations, it is a suitable working environment for the slot milling machine:
a. Both sides of the pre-excavated soil body have not been excavated. At this time, the size of the pre-excavated soil (rock) body B=2.8m (1 full cutter);
b. Both sides of the pre-excavated soil (rock) body have been excavated. At this time, the size of the pre-excavated soil (rock) body of the milling machine must be between 800~1600 (one closed cutter), and the center of the milling wheel should be as close to the center of the soil (rock) body as possible during the slotting construction to avoid horizontal offset of the milling wheel during the slotting construction due to eccentricity, thereby ensuring the milling effect.
Due to the special requirements of the slot milling machine for pre-excavated soil, certain requirements are put forward for the size division of the unit slot segment. It can be seen from the standard knife method drawing below that considering the appropriate overlap length of the pre-excavation area, the unit slot segment division length is more appropriate at 2.8~5.6m. Usually, the first sequence slot (slot segment is constructed first) is cut with three cuts, and the slot segment division length is longer, and the second sequence slot (slot segment is constructed later) is cut with one cut, and the slot segment division length is shorter. Sometimes the first sequence slot can also be cut with one cut.
(3) Milling joints There is a more distinctive method for connecting the groove segments formed by the milling machine, called the “milling method”, as shown in Figure 11-21. That is when excavating the first sequence trench section, the trench section joint center line is extended by 10 cm~25 cm. When excavating the second sequence trench section, the milling machine is placed between the two first sequence trench sections to mill off the first sequence trench section. The excess concrete is overlapped in a zigzag shape to form a fresh concrete contact surface, and then the second-order groove concrete is poured. Due to the roughening effect of the milling cutter teeth, the second-order groove concrete can be well combined with the first-order groove concrete, with good water-tightness, forming a more ideal continuous wall joint form, called “milling joint” (or sleeve milling joint).
The first-stage concrete surface formed by the slot milling machine is shown in Figure 11-22. The milling joint construction process is simple, the method is mature, and the probability of accidents is very low. It has been widely used in large underground continuous wall projects at home and abroad. Another notable feature of the milling method is that it eliminates the steps of hanging and pulling out the joint pipe (box), thus avoiding the risk of breaking the joint pipe or burying the pipe. This can be said to be a favorable factor for the construction of ultra-deep underground continuous walls.
The slot segment joints constructed by slot milling machines in China also include rigid joints of I-beams (H-beams), such as the Shenzhen Metro Laojie Station and the Shanghai World Expo 500kV substation project. The ground wall joints in the Shanghai Tower project, the tallest building in Shanghai currently under construction, use two forms: steel joints and sleeve milling joints. Among them, milling joints are the first application in the region. In short, as an advanced underground continuous wall slotting equipment, the slot milling machine has the outstanding advantage of being much faster than traditional construction processes in hard layers and having high construction accuracy. It is believed that with the continuous market expansion and the deepening of localization expectations, it will surely become the backbone of underground engineering construction equipment.
As the development and utilization of urban underground space develop towards greater depth, underground continuous walls, as an important form of deep foundation and deep foundation pit retaining structure, have also tended to become deeper and thicker. The strata are becoming more and more complex. The trenching construction in complex strata has also developed from a single pure grabbing, pure punching, pure drilling, and pure milling method to a combination of multiple trenching methods. The latter can often achieve twice the result with half the effort compared to the former. —High efficiency, low cost, and excellent quality. The main working method combinations include grab buckets, which can also be used in conjunction with impact drills or drilling rigs to form the “grab and impact method” or “drill and grab method” (such as two drills and one grab, three drills and two grabs, or four drills and three grabs, etc.).
The “grab and punch method” uses an impact drill to drill the main hole and a grab bucket to grab the secondary hole. This method can give full play to the advantages of the two machines. The impact drill can drill into different soft and hard strata, while the grab bucket has high soil extraction efficiency. When the grab bucket encounters hard strata during the construction of the auxiliary hole, it can be replaced with an impact drill or heavy chisel (“grab chisel method”) at any time to overcome it. This method can significantly improve work efficiency by 1 to 3 times compared to using impact drilling alone, and it is also adaptable to a wide range of formations.
The “drill-grab method” is to use a drilling rig (such as a submersible electric drill) to first drill two guide holes on both sides of the grab bucket width, and then use the grab bucket to grab the soil between the two holes, which is more effective. Early clam-type grabs and rope-guided plate grabs had no deviation correction devices, so they were mostly made by drilling and grabbing, with the verticality of the guide hole directly controlling the verticality of the groove.
With the application of slot milling machines, new working method combinations such as “grasping and milling combination”, “drilling and milling combination”, and “milling, grasping and drilling combination” have emerged. For example, the underground continuous wall construction of the Shanghai 500kV Expo substation (wall depth 57.5m/wall thickness 1.2m) adopted the “grab and mill” construction method combination. This process is to use a grab bucket trencher to trench the upper soft soil layer. After entering the hard soil layer (or soft rock layer), the slot milling machine is used to mill the slot, which greatly improves the efficiency of slotting and excavation. During the slotting process of the slot milling machine, the upper completed slot wall is trimmed to ensure the entire slot wall verticality meets the requirements.
The 73.5m deep trench section of the anti-seepage wall of the upstream cofferdam of the Three Gorges Project Phase II was constructed using a combination of milling, grabbing, and drilling methods, i.e. the weathered sand in the upper part was milled with a hydraulic mill, the sand and gravel in the middle were grabbed with a grab bucket. The spherical blocks in the lower part were drilled with a hydraulic milling machine. In impact reverse circulation drilling for bedrock, the three methods play to their strengths and avoid their weaknesses, ensuring the quality and progress of trenching.
The developed combined methods in hard strata such as hard rock and boulder include “drilling” and “chiseling and milling”. “The drilling method is a process that uses a heavy chisel of 8 to 12 tons and cooperates with an impact reverse circulation drill rig. For example, in the north anchorage ground wall project of the Runyang Yangtze River Bridge (wall depth 56m/wall thickness 1.2m), This method has achieved high construction efficiency and low cost in hard rock and is worth promoting. The “chisel milling method” is a process that uses heavy chisels and hydraulic milling machines. The advantages are good through quality, low noise, and suitability for urban construction operations.
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