Korea’s rural road network — the 농어촌도로 system administered by county and provincial governments — includes hundreds of thousands of kilometers of agricultural access roads, most of them unsurfaced or deteriorated. When they fail under seasonal frost and heavy-load cycles, project managers face a choice between conventional reconstruction and Full-Depth Reclamation (FDR). This guide explains both methods honestly — what they involve technically, where each is appropriate, and what they cost in Korean project conditions.

What Each Method Actually Involves

Conventional Road Reconstruction

Conventional reconstruction involves: excavation of the existing failed road material; haulage to a disposal site; import and placement of new crushed stone sub-base and base course; compaction; and application of a new surface. This sequence requires excavation equipment, multiple haulage trucks, aggregate supply from a quarry, and surfacing equipment. Project duration is typically several weeks per kilometer, with full road closure required throughout. It is the appropriate choice when the existing material is contaminated, organically rich, or when the structural deficiency is in the sub-grade (natural soil below the road formation) rather than the road base.

Full-Depth Reclamation (FDR) / In-Place Soil Stabilization

Full-Depth Reclamation does not remove the existing road material — it converts it. The THOR ST soil stabilizer mills existing road surface and base to a depth of up to 200 mm, simultaneously mixing the milled material with water (from a connected water truck) and a chemical binder (lime or cement). The chemical binder reacts with the milled material to produce a stabilized base with significantly higher bearing capacity than the original deteriorated road. The mix is then graded and compacted in place.

No excavation, no material removal, no aggregate import. The existing road becomes the raw material for the improved structure. The DCW 2.2 binder spreader mounts at the front of the same CVT tractor as the THOR ST, distributing lime or cement immediately ahead of the milling rotor — achieving complete binder distribution and soil milling in a single forward pass. This single-pass combined operation is the foundation of FDR’s time efficiency and cost advantage.

Cost Comparison — FDR vs Conventional Reconstruction

ⓘ Cost ratios are general estimates from road rehabilitation literature. Actual ratios depend on aggregate haul distance, binder costs, and project scale. FDR’s advantage increases significantly with haul distance — the dominant variable in Korean highland project conditions.

When FDR Works — and When It Doesn’t

FDR Is Well-Suited When:

The existing road material is granular. Decomposed granite, crushed stone base course, and sand-gravel mixes react effectively with lime and cement. Korean highland rural roads built on decomposed granite and gravel subgrade — the dominant road base type in Gangwon-do and the Gyeongsang highlands — are generally well-suited to FDR treatment.

The road failure is in the base layer, not the sub-grade. FDR treats the constructed road base above the natural soil. If the failure is in the road base (the most common failure mode on Korean agricultural roads), FDR addresses it directly. If the structural failure is in the natural soil below the road formation, FDR of the base layer alone may not produce a durable result.

Surface rock has been pre-processed. If the road has significant surface boulders above 5–8 cm, a pre-processing pass with the THOR 2.4 stone crusher reduces road surface boulders to below the THOR ST’s optimal milling range before the stabilization pass. This combined THOR 2.4 pre-crushing + THOR ST stabilization sequence is recommended for Korean highland farm road rehabilitation projects with mixed rock and soil surface conditions.

FDR Is NOT Suitable When:

The road material is organic-rich. High organic content significantly inhibits the chemical stabilization reaction. Roads through former paddy land, wetlands, or areas with organic soil deposition are poor FDR candidates without pre-treatment.

The sub-grade is highly expansive clay. Highly plastic clay sub-grades swell and contract with moisture — cracking stabilized base layers from below. Lime sub-grade pre-treatment may be required before base layer FDR in these conditions.

Structural load requirements exceed FDR capacity. For roads carrying sustained very heavy vehicle loading at intensities beyond typical Korean agricultural truck traffic, FDR alone at 200 mm depth may not achieve the required structural capacity. In these cases, FDR may be combined with a bound surface layer or conventional heavy-duty construction may be specified.

Lime vs Cement — Which Binder for Korean Road Soils?

The binder choice is a technical decision made by laboratory testing of the specific road material before project commencement — there is no universal answer. As general guidance for the Korean soil types most commonly treated:

Binder application rates are determined by laboratory Proctor compaction and unconfined compressive strength (UCS) testing of site-specific soil + binder combinations before project commencement. General ranges: 4–8% cement or 3–6% lime by dry weight of treated material. Korea Watanabe can assist in identifying appropriate geotechnical laboratories for pre-project stabilization design testing.

The Complete Watanabe FDR System

The combined DCW 2.2 (front) + THOR ST (rear) system achieves complete single-pass soil stabilization: binder distribution and soil milling in one forward movement. No separate binder spreading pass, no excavation machinery, no aggregate import trucks needed.

Planning an FDR Project — What Happens Before the THOR ST Arrives on Site

Successful FDR projects in Korean rural road conditions share a common characteristic: thorough pre-project assessment and design. The following steps are standard practice for professional soil stabilization projects and are recommended for any Korean 농어촌도로 rehabilitation project using the THOR ST system:

Step 1 — Site Investigation and Soil Sampling

Trial pits at 50–100 m intervals along the road length, to a depth of 300–400 mm, provide samples of the existing road material for laboratory analysis. Assess the visual stratigraphy (road surface, base course layers, sub-grade), note any signs of sub-grade failure (spring-like deformation under load, wet spots, organic material), and take samples from each visible layer for laboratory testing. The visual inspection also identifies any sections with large surface rock that may require pre-treatment with the THOR 2.4 stone crusher before the THOR ST stabilization pass.

Step 2 — Laboratory Stabilization Design

Soil samples undergo Atterberg limit testing (to determine plasticity index and binder type selection), particle size analysis (to confirm granular content suitability), and stabilization mix design testing with varying binder contents (typically 3, 5, and 7% cement or lime by dry weight). The target unconfined compressive strength (UCS) for the stabilized base is specified by the project engineer based on the design traffic load. The laboratory outputs the required binder type and application rate that achieves the target UCS — this is the number the DCW 2.2 is programmed to deliver on site.

Step 3 — Site Logistics Planning

Before mobilization, confirm: the water source and water truck fill point (ideally within 1–2 km of the working section to minimize truck fill-cycle interruptions); binder storage and loading point (bulk cement silo or lime delivery schedule); traffic management plan for road users during construction; and the designated compaction sequence and compaction equipment specification. Poor water truck logistics is the most common cause of production delays on Korean FDR projects — a poorly positioned water source creates fill-cycle interruptions that limit the THOR ST’s productive working time to well below its rated capacity.

Korean Project Examples — FDR in Practice

4.2 km Agricultural Road Rehabilitation, North Chungcheong (2025)

County road project: 4.2 km of heavily rutted 농어촌도로 in Jecheon-si. Existing road: compacted granular base with 60 mm asphalt overlay, base course failed across approximately 70% of the road length. Conventional reconstruction quote: 45% higher than FDR and required 8 weeks of full road closure. FDR approach with THOR ST + DCW 2.2 + cement stabilization (6% by weight, 160 mm treatment depth): project completed in 9 working days with one-lane traffic maintained throughout. Compaction completed daily; sections open to light traffic within 24 hours of treatment. County road office accepted the work on first inspection. Two additional sections subsequently awarded to the same contractor using the same method.

11 km Plantation Internal Road Network, South Jeolla (2024)

Private plantation road: 11 km of compacted earth and gravel access roads carrying year-round agricultural vehicle traffic. Annual grading cost and dust generation were recurring problems. THOR ST lime stabilization (4.5% lime by weight, 140 mm treatment depth) on primary access routes — South Jeolla heavy-clay soils confirmed lime-suitable by pre-project laboratory testing. Sections treated: maintenance-free for 14 months post-treatment (no regrading required). Dust generation eliminated on stabilized sections. ROI on the stabilization project versus annual grading cost calculated at less than 4 seasons.

Forestry Bureau Access Roads, South Gyeongsang (2025)

Forestry bureau contract: mountain timber transport roads in Hamyang-gun requiring structural upgrade for heavier logging truck loading. Road surface: mixed soil and surface rock. Pre-treatment with THOR 2.4 stone crusher to reduce surface rock above 8 cm before THOR ST stabilization pass. Cement stabilization at 180 mm treatment depth. Two full timber seasons carried on stabilized sections without surface maintenance required. Cost comparison: 52% lower than the conventional aggregate overlay alternative that had been used on adjacent sections in previous years.

Häufig gestellte Fragen

Pre-Project Checklist — Is Your Road Suitable for FDR?

Before commissioning a THOR ST soil stabilization project on a Korean rural road, the following site characteristics should be confirmed to ensure FDR is the appropriate method:

Korean Project Results — FDR Cost and Performance

Korea Watanabe’s THOR ST has been used on Korean rural road rehabilitation projects across Gangwon-do, North Chungcheong, South Chungcheong, South Gyeongsang, and South Jeolla. Across completed projects in varied Korean soil and road conditions, the consistently observed results are:

Cost reduction vs conventional reconstruction: Completed Korean projects have shown total project cost reductions of 38–58% compared to the conventional reconstruction alternative quoted for the same section. The range reflects the variation in aggregate haul distance — projects in areas with long haul distances to aggregate sources (highland Gangwon-do) show cost reductions at the higher end; projects near aggregate sources show reductions at the lower end.

Construction duration: On typical Korean 농어촌도로 sections of 500–2,000 m length, the THOR ST system completes the milling and stabilization pass in 1–4 working days depending on section length and water truck logistics. Conventional reconstruction of the same section typically requires 3–8 weeks including material procurement, excavation, base placement, and surface work.

Performance after treatment: Stabilized road sections that have been in service for 12–24 months show significantly reduced rut depth, reduced surface deformation under heavy agricultural truck loading, and effectively zero dust generation compared to pre-treatment conditions. Roads carrying heavy timber and agricultural transport loads have remained serviceable without maintenance intervention for 18–24 months post-treatment in reported Korean project cases.

CO₂ reduction: Eliminating aggregate haulage from the construction process removes the largest single CO₂ source in conventional road reconstruction — multiple truck deliveries of crushed aggregate over project duration. Korea’s national carbon accounting framework for public infrastructure projects increasingly recognizes in-place stabilization’s emissions advantage as a project evaluation criterion, supporting the case for FDR in publicly funded rural road programs.

Herausgeber: Cxm