1 Hydrogeological conditions of the site
1.1 General situation of the project
This project is the third station of the Olympic branch line of Beijing Metro. The foundation pit of the station is divided into two underground floors, the first underground floor is excavated by sloping, the second underground foundation pit is surrounded by bored piles, and the internal support is steel support system. Due to the high underground water level at the station site, the self-weight of the station structure and the covering soil on the roof can not meet the anti-floating requirements, so uplift piles are set under the floor of the structure, and squeezed branch cast-in-place piles are used as uplift piles. The pile top is located below the structural floor with an elevation of 27.8m m. In this experiment, three test piles (see Figure 1 for the location) will be driven by simulated engineering piles in the underground layer with the marked height of 35.5m in the foundation pit, and the vertical uplift performance of single test pile will be tested in situ.
1.2 geological overview
this project site is located at the northern edge of Yongding river alluvial fan. The distribution of strata from top to bottom is as follows:
Artificial accumulation layer: the total thickness of artificial fill layer is .7 m ~ 4. m, and the elevation of bottom layer is 4.34 m ~ 43.59 m..
quaternary alluvium: the total thickness of this large layer is 7.4 m ~ 13. m, and the bottom elevation is 3.51 m ~ 33.93 m.
Silty clay layer: the total thickness of this large layer is 3.8m～11.9m, and the elevation of the bottom is 22.56m～26.6m.
pebble bed.
1.3 Hydrological conditions
According to the data provided by Beijing Survey and Design Institute, there are three layers of underground water, namely phreatic water, interlayer water and confined water, which are mainly replenished by lateral runoff and overflow, and discharged by lateral runoff and artificial mining.
the ground water in the first floor is phreatic water, and the water level elevation is 36.28m～39.39m (the buried depth of water level is 5.m～8.2m).
the groundwater in the second layer is interlayer water, and the water level elevation is 25.46 m ~ 34.74 m (the buried depth of water level is 9.3 m ~ 18.7 m).
the groundwater in the third layer is confined water, with the water level elevation of 11.6m～19.69m (the buried depth of water level is 25.m～33.1m) and the head height of 2m ~ 5m.
2 pile foundation design and construction methods
2.1 Brief introduction of test pile design
This test designs 3 test piles and 4 anchor piles. The designed pile length is 24.5m, the pile diameter is 85mm, the concrete strength of the pile body is C3, and the design requires the characteristic value of the vertical uplift bearing capacity of a single pile to be 44kN. The pile diameter, concrete strength and main reinforcement specifications are the same as those of engineering piles. The test pile is driven in the underground layer of foundation pit excavation, and the elevation of the underground layer is 7.7m higher than the top elevation of the engineering pile, so the concrete only needs to be poured to the pile length of 24.5m during the construction of the test pile, and the upper part of 7.7m does not need to be poured.
2.2 Construction of squeezed branch cast-in-place pile (refer to Technical Specification for Squeezed Branch Cast-in-place Pile submitted for review by China Engineering Construction Standard)
1) Measurement, positioning and casing embedding;
2) Hole-forming equipment in place and drilling tool inspection;
3) inspection of hole formation, disk formation and disk support;
4) manufacturing, hoisting and hoisting the reinforcing cage, and hoisting the slurry guide pipe and hole cleaning;
5) Pouring underwater concrete and testing the quality of the pile.
in order to study the load characteristics of the squeezed branch uplift pile and the law of frictional resistance transmission between the squeezed branch uplift pile and the pile body, steel string steel bar stress gauges are embedded in the pile body above and below the branch plate and between the two branches, and the embedding position is shown in Figure 2.
3 test scheme
3.1 bearing capacity test
3.1.1 method, principle and equipment
According to the actual situation on site, the anchor pile method is adopted in this test. Anchor pile method test is a test method that uses anchor piles to provide reaction force to test piles. In this experiment, one reaction pile is driven on both sides of each test pile as anchor pile. The 7.7m upper part of the test pile is only equipped with main reinforcement without concrete pouring, and the main reinforcement is exposed to the ground for 1 m. The main reinforcement of the test pile is anchored on the loaded steel beam, and both ends of the steel beam are supported on the top of the reaction pile. Make sure that the center distance between the test pile and the reaction pile is not less than 3.4m[2]. The relative position between the test pile and the reaction pile is shown in Figure 3, and the middle two reaction piles are reused once. Each counterforce pile needs to provide a compressive bearing capacity of not less than 5kN. According to the investigation report, the calculation shows that the pile end of counterforce pile needs to enter the pebble layer 8mm, the pile length is about 25.5m, the pile top is located at an elevation of 35.5m, and the pile diameter and concrete strength of the pile body are the same as those of the test pile. Reinforcement shall be properly strengthened near the pile top to prevent excessive test pressure and broken pile head.
The test equipment includes: reaction system, automatic load loading device and observation system for load, pile top uplift and reaction pile settlement. The reaction system is a reaction main beam with a length of 8m, and the force supply capacity of both ends is more than 1 kN. The observation system consists of four electronic displacement meters, two digital dial indicators, pressure sensors and static load tester. The layout diagram is shown in Figure 4.
3.1.2 test loading and unloading methods
The characteristic value of the vertical uplift bearing capacity of the test pile is 44kN, and the maximum loading value of the test is not less than 2 times of the characteristic value of the vertical uplift bearing capacity of the single pile, that is, not less than 88kN. The loading is carried out in 1 stages by using the slow dimensional loading method.
in this experiment, two jacks are respectively placed on the reaction pile. The pressure is measured by the force sensor placed on the jack, and the sum of the pressure readings of the two instruments is the top load of the test pile. Four electronic displacement meters are installed at the top of the test pile to measure the uplift of the pile top under various loads, and the settlement of the reaction pile is measured with a digital dial indicator.
3.1.3 observation of displacement
1) installation of reference beam
each test pile is equipped with two I-beams of I 32a as reference beams, which are supported on the reference pile with one end fixed and one end freely supported. The spacing between two benchmark piles shall be at least greater than 6.8m. Benchmark piles can be steel pipe piles or cast-in-place piles, and buried 2m underground. The displacement meter is fixed on the reference beam through the magnetic watch seat.
2) observation mode of pile top pullout [3]
a. After each level of load is applied, measure the pile top pullout at the 5th, 15th, 3th, 45th and 6min, and then measure it every 3min.
B. Stability standard for uplift of test piles: the uplift on the top of the pile within one hour shall not exceed .1mm, and it shall occur twice in a row (from the 3th minute after the graded load is applied, it shall be calculated according to the observation value of uplift for three times every 3min for 1.5h).
C. when the uplift rate of the pile top reaches the relatively stable standard, the next level of load is applied.
D. when unloading, each level of load shall be maintained for 1h, and the first level of load can be unloaded after the displacement on the top of pile is measured at 15th, 3th and 6th 1536min. After unloading to zero, the residual uplift on the pile top shall be measured and read for 3h, with the measuring time of 15 min and 3min, and then every 3min.
3.2 branch detection
3.2.1 formation exploration
a geological borehole with a depth of 34.m is arranged beside the test pile. SH3 drilling rig percussive drilling and casing wall protection. It is required to find out the stratum within the depth of 34.m below the ground and stratify it accurately.
3.2.2 Probe into the position, size and thickness of the supporting plate
Three core drilling holes are arranged on each pile, with an outer diameter of 1mm, and the holes are evenly and symmetrically distributed at a distance of 85mm from the center of the pile, and the depth of the holes shall be subject to drilling through the supporting plate. Record the depth and thickness of the support plate. All core samples of concrete with supporting plates are preserved.
4 problems that should be paid attention to in the test
4.1 Installation of steel bar gauge
There will be deviation between the actual tray position and the designed tray position of branch pile, and the steel bar gauge can be installed only after the tray position is determined according to the actual soil conditions of the site.
4.2 Installation method of steel bar gauge
Two methods can be adopted: binding bar welding and casing butt joint. When binding welding is used, attention should be paid to cooling the steel bar meter to avoid damage to the steel bar meter caused by too high welding temperature, resulting in inaccurate data; Casing butt joint connection, steel wire set should be reliably fixed to avoid loosening and damage.
4.3 protection of data line of steel bar gauge
when the data line of steel bar gauge is led out, the head of the data line should be placed on the annular surface of the pile reinforcement cage to prevent the concrete pouring equipment from being damaged. In addition, we should also do a good job in protecting the steel bar meter before construction.
4.4 data line identification
There are many measuring points in this test, and the pile is long and constructed in sections, so labels should be attached in advance. It is recommended to stick a waterproof label every .5m, and it is best for the conductor to protrude from the ground for 3m ~ 5m.
4.5 Pay attention to the protection of data line
The conductor of the steel bar gauge is fragile. In order to prevent the conductor from being damaged, the problem is solved by bushing the conductor. It is recommended to use fireproof metal sleeve, which has a certain strength, the wires can slide freely in the sleeve, and the tube also has the effect of preventing high temperature, which solves the problems of extrusion, pulling and high temperature well.
4.6 Pay attention to waterproofing in subsection construction
Because the pile body is long, the conductor must be disconnected during subsection construction, and waterproofing at the joint is very important. It is recommended to use a connecting sleeve. After the copper wire in the conductor is connected, it is sealed with a plastic sealing tube. Then it is sleeved with a sleeve, coated with waterproof glue, and both ends of the sleeve are screwed to death. At this time, the conductor is buckled in the sleeve to avoid being pulled off.
5 Conclusion
This test is the first in-situ static pull-out test of squeezed branch piles in China. The description of each link in the test and the analysis of matters needing attention during the test can provide reference for the test of similar foundation piles.
for more information about the engineering/service/procurement bidding, and to improve the winning rate, you can click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd