1) Basic Grouting Reinforcement
For reinforcement when the foundation is damaged due to uneven settlement, frost heave, or other causes. The slurry mainly uses cement slurry, with a water-cement ratio of 0.5~0.6, or can use epoxy resin, etc.
First, drill holes at the crack locations; each side of the single foundation should have no fewer than 2 holes.
b. Beams can be drilled at a longitudinal spacing of 1.5 to 2.0 meters from the foundation, and not less than 2 rows.
c. During grouting work, first drill holes at the original crack damage location. The diameter of the grouting pipe can be 25mm. The angle of inclination of the hole with the horizontal plane should not be less than 30°. The diameter of the hole should be 2-3mm larger than the diameter of the grouting pipe. The spacing between holes can be 0.5-1.0m.
d. The casting pressure can be set between 0.1-0.3 MPa. If the slurry does not sink, gradually increase the pressure to 0.6 MPa. If the slurry does not sink within 10-15 minutes, stop the casting. The effective diameter for casting is 0.6-1.2 meters.
2) Increase the base area
The method of increasing the base area is applicable for reinforcement when the bearing capacity of the foundation or the size of the base area of the existing building does not meet the design requirements. Increasing the base area can be achieved by using a concrete jacket or a reinforced concrete jacket.
a. When subjected to eccentric compression on the base, an asymmetric widening can be applied; when the center of the bearing is under compression, a symmetric widening can be utilized.
b. Prior to pouring concrete, the original foundation should be roughened and cleaned, then apply a layer of high-grade cement slurry or concrete bonding agent to enhance the bonding strength between the new and old concrete foundations.
c. For the widened section, a compacted bedding layer with the same thickness and material as the original foundation bedding should be laid on the subgrade.
d. When using concrete sleeve reinforcement, the width of the foundation expansion on each side shall comply with the current national standard "Code for Design of Foundation of Buildings" GBJ7, regarding the permissible width-to-height ratio of rigid foundation steps. Anchor bars should be set at intervals along the height of the foundation.
When using reinforced concrete sleeves for reinforcement, the main reinforcement in the widened section should be welded to the original foundation's main reinforcement.
f. When widening the base of a column, it should be divided into separate sections of 1.5-2.0 meters in length, and constructed in batches, sections, and intervals.
g. When it's not advisable to use concrete or reinforced concrete sleeves to increase the base area, the original independent foundation can be changed to a strip foundation. The original strip foundation can be transformed into a cruciform strip foundation or a raft foundation. The original raft foundation can be modified into a box foundation.
3) Anchored static pile
A pile driving technique that involves pushing precast piles into the ground using a static pile driver, which utilizes the weight of the pile driver and counterweights on the pile frame.
Typically applicable to highly compressible clay layers or soft clay layers with light sandy content. When piles need to penetrate sandy soil layers of a certain thickness, the suitability should be considered comprehensively, taking into account factors such as the pile driving force and final pressure of the pile machine, the shape, thickness, density of the soil layers, mechanical properties of the upper and lower soil layers, pile type, pile structure, strength, specifications of the pile cross-section size, and pile arrangement, as well as the groundwater level and the duration and frequency of steady pressure before the final pressure.
Sometimes applicable in karst areas with shallow soil cover. Drilling with bored piles is difficult in these regions; driving pile with impact piles is prone to jamming; and rammed piles are susceptible to breaking. Only static pressure piles can be slowly driven in, displaying the pile resistance, but they should be used with caution in karst areas with well-developed caves and grooves, as well as in soil layers with numerous solitary rocks and obstructions.
Advantages
* Quickly stops settlement and tilting; no additional settlement will occur during construction.
* Pile driving can be performed in confined spaces, suitable for projects where heavy machinery cannot access.
* Underpinning of settled or cracked structures can be performed without disrupting production in the workshop or relocation of residents.
The transmission process and load-bearing performance are clearly defined, ensuring the actual bearing capacity of each pile is obtained, with reliable construction quality.
a. The construction process is:
Measurement & Alignment → Piling Machine Positioning → Hoisting & Feeding Piles → Centering & Straightening Pile Body → Piling → Pile Connection → Re-Piling → (Pile Delivery) → Stop Piling → Cut Pile Head.
b. Measurement and Positioning: Drive a short steel bar at the center of the pile. If construction is on softer ground, the pile machine's movement may compress the pre-driven short steel bar. Therefore, re-measure the pile position after the pile machine is basically in place.
c. Set the pile driver in place: Position the pile foundation at the pile location, align the center of the pile driver's clamp jaws (can be equipped with a center line gyroscope) with the base of the sample pile on the ground. After leveling the pile driver, recheck for accuracy, and then lower the long crawler (long barge) to bear the load.
d. Piling Installation: The length of precast pile sections for static pressure is generally within 12 meters, which can be directly lifted and fed by the pile driver's lifting mechanism, or equipped with a dedicated lifting mechanism for installation. The first pile (foundation pile) should use a pile with a pile tip. Upon arrival near the pile driver, it is typically lifted using a single-point lifting method. Employing a double jack (hoisting rope) with a small crossbeam (small horizontal beam) allows the pile to be vertically inserted into the pile clamp. When using the sulfur cement bonding method for pile connection, check the depth of the grout anchor holes before lifting and clean out any debris and standing water inside the holes.
e. Centering and Straightening the Pile Body: After the pile is hoisted into the pile clamp, the operator is instructed by the foreman to slowly lower the pile until the pile tip is about 10cm from the ground. Then, tighten the pile body, slightly adjust the pile press machine to align the pile tip with the pile position, and press the pile into the soil by 0.5 to 1.0 meters. Pause the downward pressure, correct the verticality of the pile body from the two orthogonal sides of the pile, and only proceed with the formal pressing when the verticality deviation is less than 0.5%.
f. Piling: The pile is driven into the ground by the force of the main engine's pile cylinder stroke. The stroke length of the pile cylinder varies with different pile driving machines, typically ranging from 1.5 to 2.0 meters. Therefore, each downward stroke drives the pile approximately 1.5 to 2.0 meters into the ground. Then, release the clamp, lift, re-clamp, and press again, repeating this process to drive a single pile. When a pile reaches a height of 80 to 100 cm from the ground level at its top, it can be connected or placed in a pile pusher to drive it to the desired elevation.
g. Piling Connection: Common connection types include electric welding and sulfur concrete anchor joints. During electric welding, the interface mortar, rust, and oil污 must be cleaned before welding, ensuring the bevel surface has a metallic luster, along with a locating plate. If there are gaps at the joint, they should be fully filled and welded with wedge-shaped iron plates. The welding bevel groove should be welded in 3 to 4 layers, with each layer's slag thoroughly removed. Welding should be done manually symmetrically to prevent defects like bubbles and slag. The weld should be continuous and full, and it is only after the joint has cooled naturally for 15 minutes that pressure can be applied. Water cooling or applying pressure immediately after welding is prohibited.
4) Rootstock Method
Drilled using a small drilling rig to the designed diameter and depth, then a steel cage is placed along with grouting pipes, followed by the injection of cement slurry or cement mortar to form piles with crushed aggregate. The root piles can be either vertical or inclined as required, and can be single or bundled.
a. The application scope of the root pile method: Suitable for the repair and superstructure of existing buildings on foundations such as silt, clayey soil, sticky soil, silt loam, sandy soil, gravelly soil, and artificial fill, as well as for the restoration of ancient buildings, reinforcement of subway crossings, and other reinforcement projects.
b. Basic Construction Techniques
Hole Making
Generally, small drilling rigs are used for boring, with water or mud as the medium for circulating cooling and debris removal. During the drilling process, the water and soil are mixed together, forming a mud-like consistency. To enhance the bearing capacity of the root piles, the positive circulation method is often employed. When encountering harder soil layers, a hydraulic boring bit is switched to achieve the expansion of the borehole. While drilling through saturated soft soil layers, it's common to encounter sand layers. The drilling speed should be slow, relying on the core tube to rotate and grind against the surface of the sand layer, with the mud inside the borehole forming a mud skin on the borehole wall to protect it. In loose topsoil layers, a casing is used to protect the borehole, with the casing mouth typically 10 cm above ground level, drilling to the designed elevation.
* Drilling Cleaning: After bore formation, transform the drilling mud inside to achieve a required specific gravity of mud water or nearly overflow with clean water.
* Steel cage manufacturing
Rebar cages are determined by the design load for their steel content. The length of each rebar cage can vary depending on on-site conditions and the lifting capacity of the equipment. Generally, each section is 5~6 meters long. The joints of the rebar cages can be tied or welded, and the overlap length should meet the specifications. Since the diameters of the tree root piles are all relatively small, the concrete cover for the rebar is 1.5~2.9 cm. Special requirements are handled separately.
* Manufacturing of grouting pipes
The production of grouting pipes involves using an external reducer at the joint to ensure the outer tube wall is smooth and easy to pull out from the mortar. To prevent slurry from entering the pipe, seal the bottom with black duct tape or polyethylene tape. Above the bottom of the pipe, within a 1.0m range, form a flower-shaped pipe with 0.8cm diameter holes, spaced 10cm longitudinally, in four vertical rows. The grouting pipes are typically placed inside the reinforcing cage and then lowered into the borehole.
Grouting to form holes
After the steel cage and grouting pipe are submerged into the drilled hole, press in cement mortar. During the grouting process, the grouting pipe is generally embedded in the cement slurry for 2 to 3 meters to ensure the quality of the pile. After grouting, immediately add碎石 (5 to 25 cm) and stir with steel bars to evenly distribute the aggregates throughout the pile.
Gating
Inject well-mixed and sifted slurry, using cement mortar with a water-cement ratio of 0.4~0.5, and a mixing ratio of 0.5:1.0:0.3 (weight ratio: water:cement:sand). Use fine sand for the sand, with particle size uniformity and no grain larger than 0.5mm. Employ a two-shot grouting process; only after the initial setting of the first shot grouting can the second shot grouting be carried out. The working pressure for the first shot grouting is 1.5~2.0MPa, and for the second shot, it is 2.0~4.0MPa.
