1. Overview
(1) Working process
Rammed-expanded pile is a kind of steel pipe (immersed pipe) cast-in-situ concrete pile with top driven, and the lower end of the steel pipe is equipped with detachable cast iron pipe shoes, which are filled with plastic concrete to make it reach the required elevation, and then the immersed pipe is pulled out to make the concrete flow out of the pipe to form a pile body. Similar to the vibration pile, the pile body concrete is vibrated by hammering on the pipe, the lower part of the steel pipe is filled with semi-dry hard concrete, and the rest is filled with sand and gravel to form the base to be expanded. After lifting the immersed pipe, a pile shoe is added on the top of the sand and gravel pile, and the pile shoe is driven down with a standard pipe until the new pile shoe hits the previously driven pile shoe, and the sand and gravel and the concrete below are squeezed out, thus forming an expanded pile.
The design idea of rammed expanded pile comes from Alpha patent pile of American Birken Piling Co., Ltd. and AG Terta pile of Switzerland. After the joint practice and unremitting exploration of pile foundation companies in Europe, Asia, America and other countries, a relatively stable rammed expanded pile type is finally formed.
(2) Characteristics of rammed expanded pile
1) After the rammed expanded process, the cross section of the pile end can be increased by 1.2 ~ 3.6 times, and the compactness of the bearing layer at the pile end can be improved. Due to the full play of its end bearing capacity, the rammed expanded pile can greatly reduce the amount of concrete and steel bars under the same load. The project cost is 3% ~ 4% lower than that of ordinary concrete pouring pile foundation.
2) Simple equipment, convenient operation, reliable technology and reasonable technology.
3) At the same time of pouring concrete and extubation, the concrete is pressurized to shape the pile, which eliminates the disadvantages of shrinkage, cracks, unconsolidated concrete, siltation, etc., which are common in ordinary cast-in-place piles, and ensures the engineering quality.
(3) Rammed expanded pile type
1) Rammed expanded pile with reinforced concrete pile shoes. Precast concrete pile boots mainly have the functions of sealing the bottom, preventing siltation and stopping water. Suitable for stratum with hard interlayer.
2) Rammed expanded pile without pile boots. Similar to Frank pile, it does not need precast concrete pile boots, but uses dry hard concrete to seal the bottom, which is suitable for strata without hard interlayer. This kind of pile has simple manufacturing method and low cost, and this kind of pile is widely used.
(4) Applicable conditions
Stratum: it is suitable for general cohesive soil, silt, muddy soil, loess and hard cohesive soil, and can also be used in the case of groundwater, and can be applied on the basis of high-rise buildings with 2 floors or less.
pile diameter and pile length: the pile diameter is from 325mm and 377mm to 45mm; The pile length ranges from 7 ~ 12m to 19 ~ 25m.
It is expected that it will replace bored piles with a pile length of less than 3m, a pile diameter of 6mm and an allowable bearing capacity of a single pile of less than 1kN. Rammed expanded pile has a very broad application prospect.
2. Equipment and tools used for rammed expanded pile
The pipe sinking machine adopts hammer pile driver or 1.8t guide rod diesel pile driver and static pile driver, and is equipped with two 2t slow hoists for pipe drawing.
figure 3-21 construction process of rammed expanded pile
The pile pipe is composed of an outer pipe (casing) and an inner pipe (rammed pipe) (figure 3-21), and the outer pipe is a seamless steel pipe with a diameter of 325mm (or 377mm); The diameter of the inner tube is 219mm, the wall thickness is 1mm, the inner tube is 1mm shorter than the outer tube, and the bottom of the inner rammer tube can adopt closed flat bottom or closed cone bottom.
3. Rammed expanded pile construction
1) The construction process of rammed expanded pile is shown in Figure 3-21: ① The frame is in place, and a layer of hard concrete with the same strength grade as the cast-in-place pile is padded at the pile position, and the pile pipe is put down and pressed tightly on it to prevent siltation. ② Drive the outer pile pipe and the inner rammer pipe sleeve into the design depth synchronously. (3) Pull out the inner rammer pipe and pour the first batch of concrete into the outer pile pipe, the height is h, and the concrete quantity is generally .1～.3m3. ④ Put the inner rammed pipe back into the outer pile pipe and press it on the concrete surface, and pull the outer pile pipe out at the end of h height (h
2). Ramming and expanding the cast-in-place pile can also be formed by the following two methods: ① The immersed pipe is composed of the pile pipe and an internal hammer, and after the immersed pipe reaches the design position under the action of vibration force and mechanical self-weight, it is poured with concrete, and the concrete in the pipe is rammed with the internal hammer to form an enlarged head; (2) Use a single pipe, press it with vibration to the depth required by the design, pour a certain height of under-reamed concrete into the pipe, and then lift the pipe upwards. At this time, the flap at the pile tip will open, and the concrete will enter the bottom of the hole. Then press and vibrate the pile pipe again, forcing the under-reamed concrete to squeeze to the lower part and around to form an enlarged head.
3) When there is groundwater or seepage, when the pipe is immersed, the back cover of the outer pipe can be made of hard concrete or anhydrous concrete, and the pipe plug is formed by tamping, and its height is generally 1 mm..
4) When the length of the pile is long or a reinforcing cage is needed, the concrete of the pile body should be poured in sections; When pulling out the pipe, the inner rammer pipe and the pile hammer should be pressed against the top surface of concrete in the outer pipe, while pulling out.
5) The depth of the enlarged head at the pile end entering the bearing stratum is not less than 3m; When using 2.5t hammer for construction, it is necessary to ensure that the number of rammed expansion hammers of each pile is not less than 5. When this number of hammers cannot be met, it must be fed again. The expansion head is made of hard concrete, and the slump should be about 1 ~ 3 cm.