Case Study: FWD-Based Overlay Design on a 25 km NH Corridor in Haryana — How IRC 115 Backcalculation Saved 18% of the Maintenance Budget

Project Context

The corridor was a 25 km stretch of an NH segment in central Haryana, originally constructed to IRC 37 specification approximately 9 years before the survey, designed for 30 MSA traffic. The state PWD had flagged the corridor for overlay rehabilitation based on visual surface-condition assessment showing alligator cracking in the wheel paths, edge raveling, and longitudinal rutting up to 18 mm in patches. The contractor's initial proposal recommended a uniform 60 mm BC overlay across the full 25 km, costed at roughly ₹3.4 crore plus mobilisation and 18% GST.

Before signing off the change-order, the Project Director ordered an FWD-based structural evaluation under IRC 115 — the right call. Surface distress patterns alone underestimate structural variability. A pavement that looks similar end-to-end can have layer moduli varying 2-3x between segments due to sub-base saturation, subgrade variability, and historic patching. NKMPV was engaged for the FWD survey with a 14-day timeline from contract sign-off to final report submission.

Pre-Test Inspection and Methodology

Per IRC 115 Cl. 4 and ASTM D4694 procedure, every FWD campaign begins with a corridor reconnaissance to identify structural-zone boundaries (where layer composition or condition changes), test-frequency planning, and traffic-management coordination with the state PWD. NKMPV's pre-test reconnaissance covered the full 25 km in a single day:

• 5 distinct structural zones identified by visual inspection: km 0-4 (heaviest distress), km 4-9 (moderate), km 9-15 (good condition), km 15-20 (severe at junction approaches), km 20-25 (moderate)
• Test point spacing finalised: 100 m within each zone, 50 m at zone-transition segments, plus 5 control points at known good/bad reference locations
• Total test points planned: 250 across 25 km
• Traffic management arranged with state PWD — single-lane closure with rolling barricades, advance warning signs per MoRTH Section 700
• Pavement surface temperature monitoring planned at every 5th test point — FWD readings need temperature correction per ASTM E950 / IRC 115

The FWD trailer (KUAB / Dynatest / Carl Bro class equipment, depending on availability — for this project we deployed a 4-truck Dynatest 8002 with 7-geophone deflection-basin measurement) was calibrated against the in-house calibration weight stack the morning of mobilisation. Drop-load sequences were configured for 40 kN, 56 kN, and 80 kN to capture the deflection-basin response curves at multiple load levels — important for backcalculation accuracy.

Field Execution

Field deployment ran across 4 working days. Day 1 (km 0-7) ran under near-ideal conditions: 32 deg C surface temperature at start, light traffic, no overcast. Day 2 (km 7-14) hit a brief afternoon squall that forced 90 minutes of downtime — pavement surface readings during/after rain are not valid per IRC 115 Cl. 6 because the moisture changes asphalt stiffness. Day 3 (km 14-21) and Day 4 (km 21-25 + return-pass verification at 30 control points) completed without incident.

Total productivity: 250 test points in 4 days = 62.5 points/day, well within the 200-300 points/day FWD productivity range for open-corridor work. Each test point captured: 7 geophone deflection readings (D0 at load centre + D1 through D6 at increasing radial offsets), pavement surface temperature, ambient temperature, time stamp, GPS coordinate, chainage. Total raw data: ~3,500 deflection-basin records (250 points × 3 load levels × 7 geophones, plus quality-control replicates).

IRC 115 Layer-Moduli Backcalculation

Per IRC 115 Annexure 5 / ASTM D5858, layer-moduli backcalculation works backwards from the measured deflection basin to the elastic moduli of each pavement layer. We used KGPBACK (the KGPBACK iterative linear-elastic backcalculation engine commonly used for Indian-context FWD interpretation) with the assumed pavement structure: 60 mm BC + 100 mm DBM + 250 mm GSB + 200 mm WMM + subgrade. Layer thicknesses were verified from as-built drawings supplied by the state PWD.

Backcalculation results revealed sharp structural variability across the 5 zones:

The contractor's original 60 mm uniform overlay across all 25 km would have been overdesigned for km 9-15 (where the pavement was structurally sound and only needed crack sealing + microsurfacing) and underdesigned for km 0-4 and km 15-20 (where the existing layers had degraded too far for an overlay alone to restore the residual life).

The Optimised Design Recommendation

Per IRC 115 Cl. 8, the overlay-design output translates the backcalculated layer moduli into a residual-life prediction and, against the design traffic, the required overlay thickness to restore design life. NKMPV's recommendation, segment by segment:

• km 0-4 (4 km): 50 mm mill + 80 mm DBM + 40 mm BC overlay. Cost: ~₹72 lakh. (Heavier intervention than original 60 mm BC plan, justified by structural inadequacy.)
• km 4-9 (5 km): 30 mm crack-sealing + 50 mm BC overlay. Cost: ~₹48 lakh.
• km 9-15 (6 km): 12 mm microsurfacing + crack sealing only — NO overlay. Cost: ~₹24 lakh. (Major saving — original plan was ₹82 lakh of unnecessary BC.)
• km 15-20 (5 km): 50 mm mill + 50 mm DBM + 40 mm BC overlay at junction approaches (km 15-16, 17-19 patches), 60 mm BC overlay elsewhere. Cost: ~₹76 lakh.
• km 20-25 (5 km): 60 mm BC overlay (matching original plan). Cost: ~₹60 lakh.

Total revised design cost: ~₹2.80 crore vs the contractor's original ₹3.40 crore. Net saving: ₹60 lakh = 17.6% of the original budget. The state PWD accepted the FWD-based design with no rework or further verification — IRC 115 backcalculation reports are routinely accepted by NHAI / state PWDs as the basis for overlay-design certification.

The Report Structure

The final NKMPV report was 64 pages, structured for direct PWD acceptance:

• Executive summary with overall verdict and segment-by-segment recommendation table
• Pre-test reconnaissance with photographs, structural-zone boundaries, traffic-management plan
• Field methodology referencing IRC 115 / ASTM D4694 procedure, equipment specifications, calibration certificate
• Raw deflection-basin data tabulation per chainage (D0 through D6, 3 load levels, surface temperature)
• Temperature correction application per ASTM E950 / IRC 115 Annex 4
• Backcalculation methodology and software (KGPBACK) configuration
• Layer moduli results per chainage with statistical zone-mean and standard deviation
• Residual-life prediction per IRC 115 Cl. 8
• Overlay-design recommendation per IRC 115 with cost-impact analysis
• NABL accreditation certificate TC-14144 (ISO/IEC 17025:2017) appendix
• Equipment calibration certificates as appendices

The report was submitted on Day 12 (within the contracted 14-day timeline) to the state PWD's regional Engineer-in-Chief. Acceptance came in 5 working days with no requests for clarification. The contractor revised his BoQ per the segment-specific design and the overlay work was tendered against the corrected design within 3 weeks.

Lessons for Procurement Teams

Three observations from this project that apply broadly to overlay procurement:

1. Visual surveys consistently misjudge structural variability

Surface distress patterns are visible. Layer-moduli variation is invisible. Two pavement segments that look identical can have backcalculated moduli varying by 2-3x. The km 9-15 segment of this corridor looked indistinguishable from the rest at surface level — but its layer moduli were 60% higher, meaning a uniform overlay would waste ₹82 lakh of bituminous concrete on a structurally sound pavement. FWD reveals the structural variability that visual surveys miss.

2. FWD report acceptance hinges on calibration paper trail

PWDs and NHAI scrutinise calibration certificates because backcalculation is mathematically sensitive to load magnitude — a 5% error in load-cell calibration produces a ~10% error in backcalculated layer modulus. NABL accreditation TC-14144 (ISO/IEC 17025:2017) means the load cell, geophones, and DAQ system are all on a documented calibration cycle traceable to national standards. Without this, the same backcalculation produces challengeable numbers. NABL is the difference between a report PWDs accept on submission and a report that triggers re-test demands.

3. Specify FWD before overlay tendering, not after construction starts

The Project Director's call to commission FWD before signing off the contractor's design is what saved ₹60 lakh on this project. Once construction starts, design changes become extras, change-orders, and disputed bills. FWD-based overlay design at the tender stage costs ~₹5-8 lakh for a 25 km corridor and routinely identifies 15-25% cost reductions plus correct localised heavier-intervention recommendations where they matter. The ROI is consistently 8-15x.

Engage NKMPV for Your Overlay Design

NKMPV is NABL-accredited (TC-14144 under ISO/IEC 17025:2017) for FWD testing per IRC 115 / ASTM D4694. Reports accepted by NHAI, state PWDs, World Bank-funded overlay programmes without additional verification. We mobilise FWD field crews from our Pinjore HQ within 24-72 hours of confirmed quote and deliver final reports within 10-15 working days of test completion. View our FWD service →, see indicative FWD pricing, or call +91-82953-60108.

Note on this case study: project specifics — corridor name, exact chainage range, contracting authority, contractor — are anonymised per the concession agreement. The methodology, instrumentation, deflection-basin numbers, layer moduli, and cost-saving figures reflect actual NKMPV field practice and IRC 115 backcalculation outcomes. For procurement enquiries with named project references, contact via our press kit.