1500mm spherical grab used together with casing oscillator for cased bored pile foundation work

 Using a spherical grab in tandem with a casing oscillator is the definitive standard for the "All-Casing Method" (or Benoto Method). This powerhouse combination is specifically deployed for drilling deep, large-diameter bored piles in highly unstable, collapsible, or demanding ground conditions (such as loose sands, gravels with heavy cobbles, and water-bearing strata).Here is a technical breakdown of how these two components work together, their operational sequence, and why they are paired.The Roles of Each Component1. Casing OscillatorThe casing oscillator sits at the surface, anchored either to the ground or to a host crane/drilling rig. Mechanism: It clamps onto the heavy steel casing strings and uses high-torque hydraulic cylinders to oscillate (rotate back and forth in an arc) while simultaneously applying a massive vertical downward force. Function: It drives the casing deep into the ground ahead of or simultaneously with excavation, creating a continuous structural shield that prevents the borehole walls from caving in. 2. Spherical Grab (Hammer Grab) Suspended from a duty-cycle crawler crane positioned directly over the casing oscillator, the spherical grab is the primary excavation tool.Mechanism: Unlike standard cylindrical or rectangular bored pile grabs, a spherical grab features heavy, rounded, cup-like jaws (spherical shells) often outfitted with high-strength replaceable teeth. It is typically operated via a double-rope system (one for lifting/lowering, one for opening/closing) or hydraulically.
Function: It drops inside the casing to dig, break up, and extract the soil.Why the Spherical Grab is Paired with an Oscillator Perfect Geometry Match: The spherical shape of the grab jaws matches the inner curvature of the circular steel casings perfectly. This allows the grab to clean right up to the casing walls without damaging the inner face or getting wedged.High Splitting and Closing Forces: When driving casings into dense gravelly soil, boulders, or cohesive clays, standard augers or light grabs fail. The massive dead weight (often 7 to 15+ tons) and the high closing torque of a spherical grab allow it to chip away at hard layers and securely trap loose cobbles or soupy materials. Multi-Clamping Capability: Because the double-rope or hydraulic spherical grab can open and close repeatedly at the bottom of the hole, the operator can repeatedly punch and grab to break up stubborn ground before hoisting it out. Step-by-Step Operational Sequence The operation is a continuous cycle of driving and digging:
[Drive Casing] [Drop Spherical Grab] [Excavate Soil]

[Repeat to Depth] Step 1: Driving the Starter Casing The casing oscillator clamps the first section of casing (the starter casing, fitted with a hardened cutting shoe at the bottom). The oscillator begins twisting and pushing it into the ground. Step 2: Excavation inside the Casing The spherical grab is lowered in the open position down the center of the casing. It plunges into the soil under its own weight. The operator closes the spherical jaws, biting a huge volume of earth or rock fragments, and hoists it out to discharge. Important Geotechnical Note: To prevent a "piping" failure or boiling at the base (where high hydrostatic pressure forces loose sand upward into the casing), a positive water head is strictly maintained inside the casing during excavation. The hydrodynamic design of the spherical grab allows it to fall rapidly through this water without excessive drag. Step 3: Advancing the Borehole
As the grab clears out the inside, the casing oscillator continuously presses the casing further down. Once a casing section is fully driven, a new section is bolted or welded on top, and the process repeats until the target design depth is achieved. Step 4: Concreting and Casing Extraction Once the design depth is reached and the base is cleaned, the reinforcement cage is lowered. Concrete is poured through a tremie pipe. As the concrete rises, the casing oscillator reverses its vertical function-using its massive lifting cylinders to oscillate and extract the casing sections one by one.