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SDDC Pile Combined with Bored Pile for Landfill Foundation Treatment: Construction Technology

Abstract

Landfill sites are characterized by complex properties, significant thickness variations, low strength, high compressibility, strong corrosiveness, and pollution potential. These factors lead to substantial settlement during both operational and post-closure periods. This paper examines the application of Super Down-hole Dynamic Compaction (SDDC) piles combined with bored piles in landfill foundation treatment, using a case study from Xi’an. Field results demonstrate that this technique not only meets foundation bearing capacity requirements but also reduces construction costs, improves engineering quality, and delivers significant socio-economic benefits.

Keywords

Foundation; Pile; Landfill; Super Down-hole Dynamic Compaction (SDDC) pile; Construction technology

1. Introduction

Among China’s 668 cities (excluding county-level cities), two-thirds face waste containment challenges, with urban waste stockpiles occupying over 500 million square meters nationwide. Landfill foundations exhibit complex engineering behaviors due to heterogeneous composition, variable thickness, low strength, high compressibility, and environmental hazards. Settlement continues for 20-30 years after closure. This paper presents a Xi’an case study utilizing SDDC piles integrated with bored piles for landfill foundation stabilization.

2. Project Overview

The project site, located on the eastern bank of the Chan River in Xi’an’s eastern suburb,s covers 16,807 m² with partial basement construction (679.44 m² underground, 4,728 m² aboveground). The area contains 2-5m thick miscellaneous fill dominated by landfill waste with significant topographic variation.

Foundation Treatment Scheme:

  • Non-basement areas: ϕ1,600mm SDDC piles (7.5m length, triangular pattern at 2.8m spacing)

  • Basement excavation zone: ϕ1,600mm SDDC piles (3.5m length, same spacing)

  • Both zones supplemented with ϕ600mm bored piles (9.0m length)

3. Engineering Challenges

  1. Material heterogeneity: Variable deposition periods and formation conditions create complex engineering properties with uneven thickness distribution.

  2. Structural instability: Random waste composition and stacking methods result in high porosity (compression modulus <6MPa, bearing capacity 60- 120kPa) – significantly higher compressibility than natural soils at equivalent density.

  3. Under-consolidation: Loose structure with high void ratio leads to substantial long-term settlement potential.

4. Equipment and Methodology

4.1 SDDC Pile Equipment

Hammer Selection Criteria:

  • Material: Cast steel preferred (lower center of gravity, stable impact) over concrete

  • Shape: Circular hammers standard (square types cause alignment issues)

  • Base area: Proportional to hammer weight (increased energy requires larger static pressure)

  • Ventilation: 250-500mm diameter vent holes prevent pore pressure buildup

Implementation:

  • Used SANY SQH400 crawler rig (22m boom) with 10t cast steel hammer (ϕ1,150mm, 3.2m length)

  • Preliminary rotary drilling is required due to landfill material instability

4.2 Bored Pile Equipment

Construction Methods:

  • Displacement drilling: Suitable for ϕ<500mm piles (vibratory hammer with simultaneous concreting)

  • Soil removal: Implemented SR150C rotary drill (bucket diameter matches pile size) using cutting-edge excavation

5. Construction Process

5.1 SDDC Pile Workflow

  1. Site preparation: Clear obstacles, pre-drill unstable zones

  2. Marking: Lime lines for accurate pile positioning

  3. Drilling: Rotary pre-drilling + impact compaction (tolerance ≤50mm)

  4. Compaction parameters:

    • Upper half: 8m drop height, 4 blows/layer

    • Lower half: 5m drop height, 6 blows/layer

5.2 Bored Pile Workflow

  1. Verticality control: Stabilize the drill mast to prevent borehole enlargement

  2. Real-time monitoring: Adjust drilling speed based on subsurface conditions

  3. Emergency protocols: Address groundwater inflow/collapse immediately

  4. Concreting: Vibrate upper 5m zone continuously (<1.5m lift heights)

6. Quality Control Standards

  • Surveying: Pile center deviation ≤50mm

  • SDDC Piles:

    • Minimum ϕ1.8m finished diameter

    • Medium-dense to dense compaction (bearing capacity: 1,000kN characteristic/2,000kN ultimate)

  • Bored Piles:

    • Ultimate capacity >2,900kN

    • Concrete strength ≥design specification

7. Performance Evaluation

SDDC Piles:

  • Achieved 1,000kN characteristic capacity (180kPa composite bearing)

  • Medium-dense compaction throughout treatment depth

Bored Piles:

  • Exceeded 2,900kN ultimate capacity

  • Class I integrity rating

8. Conclusions

  1. SDDC methodology effectively reduces the void ratio through compaction, enhancing composite foundation performance.

  2. The technique demonstrates environmental benefits by incorporating construction waste (brick fragments, concrete debris) as backfill material.

  3. Field results validate this approach for landfill foundation treatment, providing practical reference for similar projects.

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