I. Introduction
In high fill strata, the fill material is mostly earth and rock from blasting and mountain excavation, often dumped fill, loose, with block stone content generally 50%~80%, some particle sizes over 1m. Therefore, during hole formation, problems such as hole collapse and necking are very likely to occur. If the hole wall collapses severely, some holes cannot be poured into piles and become waste holes.
II. Brief Introduction to Full Rotation Full Casing Construction Method
The full rotation full casing construction method, also known as the “BUROTO” method, uses steel casing for wall protection during hole formation. It has the characteristics of good pile quality, no mud pollution, environmental friendliness, and reduced concrete overfill coefficient. It can effectively solve the problems of hole collapse, necking, and high overfill coefficient that occur when using ordinary methods for cast-in-place pile construction in urban high fill and karst landforms. 2.1 Construction Machinery Configuration for Full Rotation Method Cast-in-Place Piles
Main equipment:
1. Full rotation drilling rig: hole formation;
2. Steel casing: wall protection;
3. Power station: provides power to the full rotation main machine;
4. Reaction fork: provides reaction force to prevent the main machine from shifting during full rotation;
5. Operation room: operation platform, personnel operating place.
Auxiliary equipment:
1. Rotary drilling rig or grab: soil removal, rock penetration, hole cleaning;
2. Pipe jacking machine: casing extraction, enabling continuous operation of full rotation;
3. Crawler crane: lifting and transporting main machine, power station, reaction fork, etc.; providing support for reaction fork; lifting and lowering reinforcement cage, concrete tremie pipe, performing grab soil removal, etc.;
4. Excavator: leveling site, clearing and transporting slag, etc.
2.2 Construction Process Flowchart of Full Rotation Method 2.2.1 Construction preparation, survey and setting out
Construction preparation mainly involves site leveling. Survey and setting out use a total station to mark the pile center points. After setting out the pile positions, certain protective measures must be taken. 2.2.2 Installing main machine, lifting and lowering casing 2.2.3 Casing drilling for soil removal, verticality measurement
While the casing is drilled into the stratum, a crawler crane is used to lower a grab bucket along the inner wall of the casing to the bottom of the hole for grab soil removal, or a rotary drilling rig is used for soil removal.
Verticality adjustment is required when the casing is drilled into the stratum. 2.2.4 Reinforcement cage installation 2.2.5 Concrete pouring, casing extraction
  III. Construction Case of Full Rotation Method in High Fill Strata
3.1 Project Overview
Taking the “Standard Factory Building of Phase I Construction Project of Guiyang Comprehensive Bonded Zone B2 Plot Standard Factory Building” using the full rotation method for pile formation as an engineering example. According to design requirements, the pile foundation adopts rock-socketed piles. The pile diameters constructed by the full rotation drilling rig are divided into two types: 1.8m and 2.0m, with an average pile length of 30m to 45m. The pile end bearing layer is moderately weathered argillaceous dolomite, with embedment depths of 6.0m and 8.0m respectively.
3.2 Engineering Geological Conditions IV. Analysis of Key and Difficult Points in Full Rotation Method Construction of This Project
Using traditional wall protection (plain concrete wall protection) and rotary drilling rig dry hole formation resulted in severe hole collapse and could not effectively cut or break block stones. As shown in Figures 1 and 2 below: V. Analysis of Pile Quality by Full Rotation Method
5.1 Analysis of Wall Protection Effect of Full Rotation Method in High Fill Strata 5.2 Analysis of Pile Position Deviation of Full Rotation Hole Formation 5.3 Quality Analysis of Core Sampling of Full Rotation Full Casing Cast-in-Place Piles 5.4 Statistical Analysis of Overfill Coefficient and Sediment Thickness of Full Rotation Full Casing Cast-in-Place Piles From Table 2, it can be seen that after pile formation using the full rotation method, the sediment at the bottom of the hole is about 1.5~3.2cm. According to the pile foundation code, the sediment thickness of end-bearing piles should not exceed 5cm, which fully meets the requirement. The concrete overfill coefficient is between 1.07 and 1.13, not exceeding 1.2, reducing waste of concrete material.
VI. Summary
Through this engineering example, we can draw the following conclusions:
(1) It solves the problems of hole collapse and necking that affect cast-in-place pile hole formation under high fill conditions with large block stones and silt layers.
(2) Pile position deviation is minimal, and pile verticality is controllable.
(3) From the core sampling results, the quality of concrete poured into the pile body is good, and the sediment at the bottom of the hole is small, thereby ensuring the bearing capacity of a single pile.
(4) It can construct large-diameter, long rock-socketed cast-in-place piles.
(5) It can effectively reduce the concrete overfill coefficient and save concrete material.
(6) The construction process does not use mud at all, and is environmentally friendly.
Hereby, some experience is provided for completing large-diameter cast-in-place pile construction under similar engineering geological conditions.