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Calculate compaction requirements and material quantities
Accurate subgrade preparation calculations for roads, pavements, and slabs. Get instant compaction levels, material volumes, and testing requirements for 2026 projects.
Professional soil preparation and compaction calculations
Calculate exact compaction specifications for subgrade preparation based on Australian Standards AS 3798 and AS 1289. Our subgrade preparation calculator determines required compaction levels, layer thickness, and roller passes for roads, driveways, and slab foundations.
Determine volumes for select fill, imported material, and stabilization requirements. Calculate quantities for aggregate base courses, crushed rock, and chemical stabilizers needed to achieve specified CBR values and bearing capacity in 2026.
Get comprehensive testing requirements including frequency, locations, and compliance criteria. Understand field density testing, plate load testing, and CBR testing schedules for project certification and authority approval.
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A subgrade preparation calculator is a specialized construction tool that determines the requirements for preparing the soil foundation (subgrade) beneath roads, pavements, slabs, and other structures. This calculator computes compaction specifications, material volumes for treatment or replacement, testing frequencies, and costs associated with achieving the required bearing capacity and California Bearing Ratio (CBR) values.
The subgrade preparation calculator helps engineers, contractors, and builders ensure their subgrade meets Australian Standards AS 3798 (Guidelines on Earthworks for Commercial and Residential Developments) and AS 1289 (Methods of Testing Soils for Engineering Purposes). Proper subgrade preparation prevents settlement issues, pavement failures, and structural problems in 2026 construction projects.
Typical pavement structure showing subgrade foundation layer
Compaction is the process of densifying soil to increase its load-bearing capacity, reduce permeability, and prevent future settlement. Different project types require different compaction levels specified as a percentage of Maximum Dry Density (MDD) determined through Standard or Modified Proctor testing per AS 1289.
| Project Type | Compaction Required | Reference Standard | Typical Application |
|---|---|---|---|
| Residential Driveways | 95% Standard MDD | AS 3798 Class 1 | Light vehicle traffic only |
| Slab Foundations | 95-98% Standard MDD | AS 2870/AS 3798 | House slabs, sheds, patios |
| Local Roads | 98% Standard MDD | AS 3798 Class 2 | Residential streets, access roads |
| Arterial Roads | 100% Modified MDD | AS 3798 Class 3 | Main roads, highways |
| Industrial Pavements | 100-103% Modified MDD | AS 3798 Class 4 | Container terminals, heavy industry |
| Airport Runways | 103% Modified MDD | CASA/AS 3798 | Aircraft movement areas |
Calculating subgrade preparation requirements involves determining the area to be treated, the depth of treatment, the volume of materials needed, and the number of compaction tests required for compliance verification. These calculations ensure your project meets engineering specifications and regulatory requirements.
AS 3798 requires minimum testing frequencies: residential projects need 1 test per 250m² or 1 per lot (whichever is greater), commercial roads require 1 test per 150m², and heavy duty pavements need 1 test per 100m². Each test location requires minimum 3 readings averaged for compliance.
Vibratory rollers compact 100-200mm layers effectively. Smooth drum rollers suit cohesive soils (clays), while padfoot rollers work better for granular materials. Plate compactors handle small areas and confined spaces. Multi-drum rollers achieve higher compaction in fewer passes for large projects requiring 98%+ density.
Optimal moisture content (OMC) is critical for achieving specified compaction. Too dry: soil doesn't compact well. Too wet: soil becomes unstable and weak. OMC typically ranges 10-15% for clays, 6-10% for sandy soils. Test moisture before compaction and adjust by watering or aerating as needed.
When existing soil doesn't meet required CBR or bearing capacity specifications, various treatment methods can improve subgrade performance. The choice depends on soil type, required improvement level, project budget, and construction timeframe.
Mechanical compaction is the most common and cost-effective method for improving subgrade. It works by reducing air voids between soil particles, increasing density and load-bearing capacity. Suitable for most soil types when existing soil quality is fair to good (CBR 4-8%). Equipment includes vibratory smooth drum rollers (8-12 tonnes), padfoot rollers for breaking up clods, and plate compactors for confined areas.
Compaction is performed in 150-200mm layers (loose thickness). Each layer is compacted with 4-8 roller passes depending on soil type and roller weight. Field density tests verify compliance every 250m² minimum. Cost: $8-15 per m² for residential projects, $12-22 per m² for commercial projects including equipment, labor, and testing.
When existing soil is very poor (CBR less than 2%), highly expansive clay, or contaminated, the most reliable solution is complete removal and replacement with suitable material. Excavate to required depth (typically 300-600mm), remove unsuitable material, and replace with select fill material meeting specified CBR values.
Select fill materials include crushed rock (CBR 80-100%), gravel (CBR 30-60%), or select sand (CBR 15-30%). Cost: $45-85 per m³ including excavation, disposal, import, and placement. This method adds $135-255 per m² for 300mm depth treatment. Higher initial cost but provides guaranteed performance and eliminates future settlement issues. Essential for building sites on reactive clay soils.
Chemical stabilization improves soil properties by mixing stabilizing agents like lime, cement, or proprietary polymers into existing soil. Lime stabilization works well for clay soils, reducing plasticity and increasing strength. Cement stabilization suits sandy soils and silts. Dosage typically 2-6% by dry weight of soil depending on soil type and required improvement.
Process involves pulverizing soil to 50mm maximum particle size, spreading stabilizer uniformly, mixing to 200-300mm depth using rotary mixer or grader, watering to OMC, compacting within 2-4 hours of mixing, and curing for 7-28 days before pavement construction. Cost: $18-35 per m² depending on treatment depth and chemical dosage. Achieves CBR improvements of 200-500% compared to untreated soil. Learn more at cement industry resources.
Geotextiles and geogrids provide tensile reinforcement over weak subgrades, distributing loads and preventing subgrade intermixing with overlying granular layers. Geotextiles (woven or non-woven fabrics) provide separation and filtration. Geogrids (open mesh structures) provide mechanical interlock and load distribution. Suitable when subgrade CBR is 2-5% and replacement is too expensive.
Installation involves leveling subgrade (±25mm tolerance), rolling out geosynthetic with 300-500mm overlaps, anchoring edges with pins or staples, and immediately covering with 200mm minimum granular material to prevent UV damage and tearing. Cost: $8-25 per m² installed depending on product type and strength. Enables construction on weak subgrades that would otherwise require deep excavation and replacement.
Quality assurance testing verifies that prepared subgrade meets design specifications. Australian Standards mandate specific testing frequencies, methods, and acceptance criteria. Non-compliant results require remedial works including additional compaction, material removal, or design modifications.
Subgrade preparation costs vary significantly based on existing soil conditions, treatment method required, project size, site access, and compaction specifications. Understanding cost components helps with accurate budgeting and value engineering during project planning.
| Activity | Unit | Rate (2026) | Notes |
|---|---|---|---|
| Site Stripping (topsoil removal) | per m² | $4 - $8 | 150-200mm depth, stockpiled on site |
| Subgrade Excavation | per m³ | $8 - $15 | Bulk earthworks, level access |
| Unsuitable Material Removal | per m³ | $18 - $35 | Includes excavation and disposal |
| Select Fill (imported) | per m³ | $35 - $65 | Delivered and placed, CBR 15-30% |
| Crushed Rock (roadbase) | per m³ | $45 - $75 | 20mm crushed rock, CBR 80%+ |
| Compaction (light) | per m² | $8 - $15 | 95% compaction, residential |
| Compaction (heavy) | per m² | $15 - $28 | 98-100% compaction, commercial |
| Lime Stabilization | per m² | $18 - $32 | 3-5% lime, 300mm depth |
| Cement Stabilization | per m² | $22 - $38 | 4-6% cement, 250mm depth |
| Geotextile Installation | per m² | $8 - $18 | Separation fabric, standard grade |
| Geogrid Installation | per m² | $15 - $35 | Biaxial geogrid, heavy duty |
| Field Density Testing | per test | $180 - $250 | 3 readings per location |
Understanding common subgrade issues helps prevent pavement failures, cracking, and settlement problems. Early identification during construction allows cost-effective remediation before pavement installation.
Soft spots indicate areas of inadequate compaction, excessive moisture, or weak soil pockets. Identified during proof rolling when loaded trucks cause visible deflection or pumping. Causes include poor drainage, high water tables, buried organic material, or variable soil types. Remediation requires excavating soft areas to firm material (typically 300-600mm depth), installing drainage if water is present, backfilling with select material, and recompacting to specification.
Expansive clays (reactive soils) undergo significant volume changes with moisture variations, causing heaving when wet and shrinkage when dry. Common in many Australian regions including Sydney, Melbourne, and Brisbane areas. Identified through soil classification tests showing high plasticity index (PI > 20) and shrink-swell potential. Treatment options include removal and replacement with non-reactive material (most reliable), lime stabilization to reduce plasticity, or using deeper foundations/thicker pavements designed for movement.
For slab foundations on reactive sites, refer to AS 2870 requirements for site classification (M, H1, H2, E). Our slab calculator helps determine appropriate slab designs. Geotechnical investigation is essential before construction on suspected reactive soils - cost $1,500-3,500 but prevents failure costing $50,000-200,000+ to rectify.
Under-compacted subgrade settles under traffic loads causing rutting, cracking, and pavement failure. Causes include compacting layers too thick (over 200mm), working soil at wrong moisture content (too wet or dry), insufficient roller passes, or using inappropriate equipment. Prevention through proper layer thickness, moisture control, adequate roller passes (minimum 6-8), and comprehensive field testing at specified frequencies.
Following industry best practices ensures subgrade meets specifications, passes quality testing, and provides long-term performance supporting pavements and structures throughout their design life.
Avoid compaction during heavy rain or when soil is saturated above OMC+2%. Protect freshly compacted subgrade with plastic sheeting if rain is forecast. In wet conditions, add 50-100mm extra drainage layer or use geotextile separation. In hot/dry conditions, spray water during compaction to maintain OMC. Work in morning when moisture is stable.
Maintain complete records including compaction test results, material delivery dockets, equipment calibration certificates, and daily construction logs. Photograph subgrade before paving showing test locations. Compile test results in compliance matrix showing specification requirements vs. achieved results. Submit to certifier/engineer for approval before proceeding with pavement works.
Schedule subgrade works during dry season if possible. Allow 7-day curing period for stabilized subgrades before trafficking. Limit time between subgrade completion and paving to minimize weather exposure - ideally pave within 48 hours of final compaction. Don't leave exposed subgrade over weekends or rain periods without protection.
Residential driveways require minimum 95% compaction relative to Standard Maximum Dry Density per AS 3798 Class 1 earthworks. This applies to subgrade supporting light vehicle traffic only (cars, light trucks). For driveways accommodating heavy vehicles (trailers, machinery), increase to 98% compaction. Compaction is achieved through 6-8 passes with appropriate roller equipment, working in 150-200mm layers at optimal moisture content. Field density testing verifies compliance - minimum 1 test per 250m² or 1 per lot. Use our subgrade preparation calculator above to determine exact requirements for your driveway dimensions.
Subgrade preparation depth depends on project type and existing soil conditions. Residential slabs and light driveways require 150-200mm treatment depth. Standard roads need 300mm minimum. Heavy commercial pavements require 450-600mm. If existing soil is poor (CBR under 5%), increase depth by 150-300mm or remove and replace with select fill. Treatment depth includes scarifying existing surface, removing unsuitable material, and compacting in layers. For reactive clay sites, AS 2870 may require deeper treatment or complete removal. Geotechnical investigation determines appropriate depth for specific site conditions - don't guess on important projects.
California Bearing Ratio (CBR) measures soil strength by comparing penetration resistance to crushed rock. Expressed as percentage: higher numbers mean stronger soil. Typical values: soft clay 2-4%, sandy clay 6-10%, well-graded gravel 30-60%, crushed rock 80-100%. CBR determines pavement thickness design - stronger subgrade needs less pavement thickness, saving significant costs. AS 1289 testing determines CBR values. Minimum CBR requirements: residential driveways 5%, roads 10%, heavy pavements 15-20%. If existing soil CBR is too low, improve through stabilization, replacement, or increase pavement thickness. Our calculator estimates treatment requirements based on existing and required CBR values.
AS 3798 specifies minimum testing frequencies: Class 1 (residential) requires 1 test per 250m² or 1 per lot, whichever is greater. Class 2 (commercial roads) requires 1 test per 150m². Class 3/4 (heavy duty) requires 1 test per 100m². Each test location requires minimum 3 individual readings averaged for compliance. Additional tests required if initial tests fail (within 3m of failure location after remediation). For a typical 500m² residential driveway, minimum 2 tests required. Commercial project 1,500m² needs minimum 10 tests. Testing costs $180-250 per location, so budget accordingly. Test results must achieve minimum 90% pass rate; overall average must meet specification (95%, 98%, etc.).
Existing soil is suitable if it meets minimum CBR requirements, has low plasticity (plasticity index under 20), is free from organic material, and can achieve specified compaction. Sandy clays, gravelly sands, and well-graded soils generally work well. Don't use topsoil (contains organics), highly plastic clays (expansive), very fine silts (poor drainage), or contaminated soils. If existing soil is borderline, consider stabilization with 2-4% lime or cement rather than complete replacement - costs 60-70% less than removal and replacement. Geotechnical testing ($800-1,500) determines if existing soil is suitable with treatment or if removal is necessary. Never assume soil is suitable without testing on commercial projects.
Common failures include: compacting at incorrect moisture content (too wet causes pumping and shear failure; too dry prevents adequate densification), layer thickness too great (over 200mm loose depth), insufficient roller passes (under 6 passes), working in unsuitable weather (rain, freezing), inadequate drainage causing water accumulation, mixing topsoil or organics into subgrade, over-compacting clay soils causing shear failure, and skipping proof rolling before paving. Prevention: follow AS 3798 requirements strictly, conduct proper testing, use experienced operators, maintain moisture at OMC±2%, and don't rush the process to save time. Failed subgrade requires excavation and reconstruction - costs 5-10 times more than proper initial preparation.
Subgrade preparation costs in 2026 range from $8-15 per m² for basic compaction of existing soil to $80-150 per m² for complete removal and replacement with select fill. Typical residential driveway (compaction only, 200mm depth): $10-18 per m². Standard road (scarify, import 100mm select fill, compact to 98%): $35-55 per m². Heavy pavement (remove 300mm unsuitable, replace with crushed rock, compact to 100%): $90-140 per m². Add $18-35 per m² for stabilization, $8-18 per m² for geotextile, plus testing costs ($180-250 per test location). Use our subgrade preparation calculator to estimate costs for your specific project based on area, treatment depth, and method required. Regional areas add 15-25% to metro rates.
Equipment selection depends on project size and soil type. Small areas (under 100m²): plate compactor ($100-150 per day hire) or walk-behind roller ($150-200 per day). Medium areas (100-500m²): ride-on roller 1-3 tonne ($350-550 per day). Large projects (over 500m²): vibratory smooth drum roller 8-12 tonne ($800-1,200 per day with operator). Cohesive soils (clays): smooth drum roller. Granular soils: padfoot or pneumatic roller. Also need water cart for moisture control ($250-400 per day), grader for leveling ($900-1,400 per day), and testing equipment (nuclear density gauge $8,000-15,000 purchase or $180-250 per test from lab). For projects over 2,000m², hiring contractor with all equipment typically more economical than DIY.
AS 3798 Guidelines on Earthworks for Commercial and Residential Developments, and AS 1289 Methods of Testing Soils for Engineering Purposes - essential references for subgrade specifications.
Access Standards →Information about soil testing requirements, laboratory procedures, field testing methods, and interpretation of results for subgrade assessment and quality control during construction.
Find Testing Labs →Technical resources covering compaction theory, equipment selection, moisture-density relationships, and field procedures for achieving specified compaction levels on various soil types.
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