Earthwork, Grading & Compaction

Every building we construct sits on the ground, and how we handle that ground determines whether the structure performs for decades or develops settlement cracks within months. This module covers the science and field practice of earthwork, grading, and compaction at a depth suitable for crew leaders and site superintendents. You will learn to identify soil types, select the right compaction equipment, interpret Proctor test results, read grading plans, and work confidently within Ontario's regulatory framework.

1. Understanding Soil Types

1.1 The Unified Soil Classification System (USCS)

Every geotechnical report on a church construction project will classify soils using the Unified Soil Classification System (USCS), standardized under ASTM D2487 (adopted in Canadian geotechnical practice). The system uses a two-letter code: the first letter identifies the dominant particle size (G = gravel, S = sand, M = silt, C = clay, O = organic, Pt = peat), and the second letter describes the gradation or plasticity (W = well-graded, P = poorly-graded, L = low plasticity, H = high plasticity).

Here are the primary USCS soil groups you will encounter on Ontario church construction sites:

1.2 Ontario's Glacial Legacy

Understanding why certain soils exist where they do helps you anticipate what you will find before the first excavator bucket hits the ground. Southern Ontario was covered by the Laurentide Ice Sheet until roughly 10,000 years ago. As the ice retreated, it left behind a complex layering of deposits:

• Glacial till — unsorted mixtures of gravel, sand, silt, and clay deposited directly by ice. Highly variable, sometimes within metres. Till plains dominate the landscape between London and Kitchener.
• Glaciofluvial deposits — sorted sands and gravels deposited by meltwater streams. These form the gravel pits that supply our aggregate. Eskers, kames, and outwash plains.
• Glaciolacustrine deposits — fine-grained silts and clays laid down in glacial lakes. The Champlain Sea clays east of Kingston are notorious for their sensitivity (Leda clay can lose 90% of its strength when disturbed).
• Organic deposits — post-glacial accumulation in poorly-drained areas. Always removed.

1.3 Frost Depth and Frost-Susceptible Soils

The Ontario Building Code (OBC) requires footings to be placed below the frost line to prevent heave. Frost depths vary across the province:

• Southern Ontario (Windsor to Toronto corridor): 1.2 m (4 ft)
• Central Ontario (Barrie, Peterborough): 1.4–1.5 m
• Northern Ontario (Sudbury, North Bay): 1.8 m+ (6 ft+)

Frost heave occurs when three conditions are present simultaneously: frost-susceptible soil, freezing temperatures, and available water. Silts (ML) are the most frost-susceptible because their pore sizes are small enough to draw water upward by capillary action but large enough to allow flow. Well-graded gravels (GW) and clean sands (SP, SW) are not frost-susceptible because water drains away before ice lenses can form.

1.4 Bearing Capacity by Soil Type

The OBC Table 9.4.4.1 provides presumptive bearing capacities for use when a geotechnical investigation is not required (Part 9 buildings only). For our Part 3 commercial church projects, a geotechnical engineer always determines allowable bearing pressure, but these values provide a useful reference for field personnel:

2. How Cohesive vs Granular Soils Behave

2.1 Granular Soils (Sands and Gravels)

Granular soils compact through particle rearrangement. Individual grains are relatively large, and densification occurs when vibration causes them to shift into a tighter packing arrangement. Think of shaking a jar of marbles — vibration causes the smaller pieces to fall into the voids between larger ones.

• Best compaction method: Vibration (vibratory smooth-drum rollers, vibratory plate compactors)
• Moisture sensitivity: Moderate. Granular soils compact well over a wider moisture range. A small amount of water acts as a lubricant; too much creates pore-water pressure that resists densification.
• Drainage: Rapid. Free water drains quickly, making granular soils forgiving in wet weather.
• Rebound: Minimal. Once compacted, granular soils stay dense.

2.2 Cohesive Soils (Clays and Silts)

Cohesive soils behave completely differently. Clay particles are flat, plate-like structures with electrochemical bonds between them. They do not respond well to vibration because the particles are too small and too tightly bonded. Instead, cohesive soils require kneading (shearing action) or impact to remold and densify.

• Best compaction method: Kneading (padfoot/sheepsfoot rollers) or impact (jumping jack rammers)
• Moisture sensitivity: Extreme. The relationship between moisture content and achievable density is governed by a sharp bell curve. Being just 2–3% above or below optimum moisture content can make proper compaction physically impossible, no matter how many passes you make.
• Drainage: Very slow. Clay can take weeks or months to dry out. Once it gets wet in fall, it may not be workable again until late spring.
• Rebound: Significant on the wet side. Over-wet clay will appear to compact under the roller but spring back after the load passes.

2.3 The Moisture-Density Relationship

Every soil has an optimum moisture content (OMC) at which it achieves maximum dry density for a given compactive effort. This is determined by Proctor testing (covered in Section 5), but the concept is critical to understand in the field:

• Dry side of optimum: Soil is stiff and cloddy. The roller bounces. Clods resist breaking down. Air voids remain. Density cannot be achieved because there is insufficient moisture to lubricate particle movement.
• At optimum: Soil compacts smoothly. The roller tracks are clean-edged. Material knits together. Maximum density is achieved.
• Wet side of optimum: Soil is soft and sticky. The roller sinks or pumps (you see a wave traveling ahead of the drum). Pore-water pressure builds and prevents densification. The soil may appear dense immediately after rolling but rebounds once the load passes.

For clay soils typical of southern Ontario (CL), optimum moisture content is usually in the range of 12–18%, with maximum dry density around 1,750–1,950 kg/m³. A 2–3% deviation from optimum can reduce achievable density by 50–100 kg/m³ — easily enough to fail a field density test.

3. Modern Grading Techniques

3.1 GPS Machine Control

Most grading operations now use GPS (more accurately, GNSS — Global Navigation Satellite System) machine control. Here is how the system works:

1. Base station: A GNSS receiver set up on a known survey control point broadcasts real-time correction signals to the machine. This provides RTK (Real-Time Kinematic) accuracy.
2. Dual GNSS antennas on the machine: Two antennas mounted on the blade or bucket mast. The dual-antenna setup determines both position (where the blade is) and heading (which direction the machine faces). The system calculates the blade/bucket cutting edge position in 3D space, updated 10–20 times per second.
3. 3D design surface: The surveyor or engineer creates a digital terrain model (DTM) representing the finished grade. This is loaded into the machine's onboard computer.
4. Operator display: The cab-mounted screen shows the operator a real-time cross-section with the current blade position relative to the design surface. Typically, a color-coded system: red = too high (still cutting), blue = too low (gone past grade), green = on grade. The operator also sees a plan view showing the machine's position on the site.

Accuracy levels:

• GNSS (GPS/GLONASS) based: ±25–50 mm vertical, ±15–25 mm horizontal. Adequate for rough grading, subgrade preparation, bulk earthwork.
• Robotic total station (RTS) based: ±3–5 mm vertical and horizontal. Required for fine grading of floor slabs, curb and gutter, precision pad grading.
• Hybrid systems: Use GPS for coarse positioning and switch to total station for fine grading. Increasingly common on larger projects.

GPS machine control largely eliminates the need for traditional grade stakes. The surveyor sets up the base station, loads the design, and the operator grades to the digital model. A survey check is still required to verify conformance, but the iterative stake-check-regrade cycle is dramatically shortened. On a typical church site, GPS grading can reduce earthwork time by 25–40%.

3.2 Reading a Grading Plan

Even with GPS machine control, every superintendent must be able to read a grading plan. Key elements:

• Contour lines: Lines of equal elevation. Major contours (heavier lines) are labeled with elevation. The interval between contours (typically 0.25 m or 0.5 m) is noted in the legend. Close-together contours indicate steep slopes; widely-spaced contours indicate flat areas.
• Spot elevations: Specific points with a marked elevation (e.g., "FFE 342.50" for finished floor elevation). These govern building pad grades, catch basin inverts, and control points.
• Slope arrows: Arrows showing the direction of drainage. The slope percentage or ratio is noted (e.g., "2.0%" or "1:50"). All graded surfaces must drain away from buildings — minimum 2% within 2 m of the building per OBC 9.14.6.1.
• Swales: Shallow V-shaped or U-shaped drainage channels shown as paired contour lines bowing toward higher ground. Minimum grade in a swale is typically 1.0% to prevent ponding.
• Retaining walls / grade breaks: Shown where contour lines terminate abruptly. Indicate a vertical or near-vertical grade change requiring a structure.

3.3 Cut/Fill Calculations and Earthwork Balance

Every grading plan is designed to balance the volume of soil cut (excavated from high areas) with the volume of fill (placed in low areas). In practice, a perfect balance is rare, and several factors complicate the arithmetic:

• Shrinkage: When you compact loose soil, it occupies less volume than it did in the ground. Typical shrinkage factors: clay 15–20%, sand/gravel 10–15%, rock fill 25–30% (swell, then compacts tight).
• Swell: When you dig soil out of the ground and load it into trucks, it expands ("bulks up"). Typical swell factors: clay 20–30%, sand 10–15%, rock 40–65%. This affects trucking calculations — a cubic metre in the ground becomes 1.2–1.3 m³ in the truck.
• Stripping: Topsoil must be stripped and stockpiled separately. Typical strip depth is 150–300 mm. This volume is "lost" from the cut/fill balance.
• Unsuitable material: Any peat, organic soil, or material designated unsuitable by the geotech must be removed and cannot count as fill.

3.4 Ontario Excess Soil Regulation (O. Reg. 406/19)

Key requirements under O. Reg. 406/19:

• A Qualified Person (QP) must oversee soil characterization and sampling
• Soil must be sampled and analyzed per the Excess Soil Standard — testing for metals, PHCs, PAHs, VOCs depending on the site's history
• An Assessment of Past Uses (APU) must be completed for the source site
• All soil movements must be tracked through a provincial soil registry
• Receiving sites must meet applicable quality standards for the intended reuse
• Hauling records with GPS tracking are recommended for due diligence

4. Compaction Equipment Selection

Choosing the right compaction equipment is one of the most consequential decisions on any earthwork operation. Using the wrong equipment wastes fuel, burns hours, and still produces failing density tests. The following guide covers the main equipment categories and when to use each.

4.1 Smooth Drum Vibratory Rollers

The workhorse of granular compaction. A steel drum with an eccentric-weight vibration mechanism inside. Key specifications:

• Frequency: 20–45 Hz (vibrations per second). Higher frequency = more blows per metre of travel = smoother finish.
• Amplitude: 0.3–1.0 mm (the vertical displacement of the drum per cycle). Higher amplitude = more energy per blow = deeper penetration.
• Operating weight: 7–20 tonnes for ride-on models; 0.5–1.5 tonnes for walk-behind.

High frequency / low amplitude (e.g., 40 Hz, 0.4 mm): Use for thin lifts, fine-grained granular material, surface finishing, working near structures or utilities. The many rapid light blows densify the top 150–200 mm without disturbing deeper layers.

Low frequency / high amplitude (e.g., 25 Hz, 0.9 mm): Use for thick lifts of coarse granular material (Granular B, pit run). The fewer but heavier blows penetrate deeper, compacting 300–400 mm per lift. More energy reaches the bottom of the lift where it is needed most.

4.2 Padfoot (Sheepsfoot) Rollers

These rollers have protruding pads (feet) welded to the drum surface. The pads concentrate the roller's weight onto a small area, creating high contact pressure that kneads and shears cohesive soil. Key characteristics:

• Pad contact pressure: 1,400–7,000 kPa (versus 200–400 kPa for a smooth drum). This high pressure is essential for breaking down clay clods and forcing air out.
• Compaction from bottom up: The pads penetrate the lift and compact the material at the base first. With successive passes, the compacted zone works upward.
• "Walking out": When the roller begins to ride on top of the surface instead of sinking in, the lift is fully compacted. This visual indicator is one of the most reliable field checks for clay compaction.
• Vibration option: Many modern padfoot rollers include vibratory capability. Use vibration only for mixed soils (e.g., silty clay with sand). For pure clay, static padfoot is preferred — vibration can over-work wet clay and destroy structure.

4.3 Plate Compactors

Vibratory plate compactors are essential for areas inaccessible to rollers: around foundations, utility trenches, behind retaining walls, inside buildings.

• Small single-plate (60–100 kg): Effective compaction depth 150–200 mm. Suitable for bedding sand, granular backfill in shallow lifts.
• Medium reversible plate (200–500 kg): Effective depth 200–350 mm. The go-to for trench backfill with granular material. Reversible plates are far more productive because the operator does not need to turn the machine.
• Large plate (500–1,000 kg): Effective depth 300–400 mm. Used for pad compaction, behind walls, larger open areas where a roller cannot access.
• Limitation: Plate compactors are ineffective on cohesive soils. They vibrate, and clay does not respond to vibration. If you must compact clay in a confined area, use a jumping jack rammer instead.

4.4 Jumping Jack (Tamping) Rammers

Rammers deliver high-impact blows at a relatively low frequency (8–12 Hz). The shoe lifts off the ground 50–80 mm with each cycle, then slams back down. This impact energy is effective on both granular and cohesive soils, making rammers the most versatile compaction tool for confined spaces.

• Best for: Trench backfill (especially clay backfill), work around pipes and utilities, confined spaces behind walls, patching.
• Effective depth: 150–300 mm per lift depending on soil type and rammer weight (60–85 kg typical).
• Limitation: Slow and physically demanding for the operator. Not practical for large areas. Operators must be rotated regularly to prevent whole-body vibration injuries.

4.5 Pneumatic Tire Rollers

Multi-wheel rubber-tired rollers (7–11 tires on a single axle, operating at variable tire pressures). The rubber tires create a kneading action as they flex over the surface. Primarily used for:

• Sealing the surface of asphalt or granular lifts
• Proof rolling subgrade (the flexing tire action reveals soft spots)
• Compacting mixed soils where both kneading and pressure are needed
• Final passes on cohesive fill before a smooth drum finishes the surface

4.6 Equipment Selection Guide

5. Proctor Testing Explained

5.1 What Proctor Testing Does

A Proctor test determines the maximum dry density a soil can achieve at a given compactive effort, and the optimum moisture content at which that maximum density occurs. It produces the characteristic bell-shaped moisture-density curve that governs all compaction specifications.

5.2 Standard Proctor vs Modified Proctor

5.3 The Moisture-Density Curve

The Proctor test is run at five or more moisture contents, producing data points that plot as a curve. Understanding the features of this curve is essential:

• Dry side (left of peak): Density increases as moisture increases. Water lubricates particle movement and reduces friction. Soil on the dry side is stiff, hard to work, and cloddy.
• Peak (optimum moisture content): Maximum dry density occurs here. This is the target. In the field, soil at optimum feels moist but not sticky, holds together when squeezed but crumbles along clean edges.
• Wet side (right of peak): Density decreases as moisture increases. Water fills voids and creates pore-water pressure that pushes particles apart. No amount of additional compactive effort will improve density on the wet side — the soil must be dried first.
• Zero air voids (ZAV) line: The theoretical maximum density if every air void were filled with water (saturation). The actual compaction curve always falls to the left of the ZAV line because it is physically impossible to compact all air out of soil. If your field density test results plot close to the ZAV line, the soil is too wet.

For a typical Ontario CL clay, a Standard Proctor test might produce: maximum dry density = 1,830 kg/m³ at optimum moisture content of 15.5%. The Modified Proctor on the same soil might produce: maximum dry density = 1,980 kg/m³ at optimum moisture content of 12.8%. Note that Modified Proctor yields a higher density at a lower moisture content — the higher compactive effort drives more air out at a drier state.

6. Field Density Testing

6.1 Nuclear Density Gauge

The nuclear density gauge (NDG) is the most common field testing device on Ontario construction sites. It measures both soil density and moisture content simultaneously, giving results in about 60 seconds. Here is how it works:

Density measurement (gamma radiation): A Cesium-137 (Cs-137) source emits gamma photons into the soil. As photons pass through soil particles, some are absorbed and some are scattered back to the detector. Denser soil absorbs more photons, so fewer reach the detector. The gauge calibrates the count rate to report wet density in kg/m³.

Moisture measurement (neutron radiation): An Americium-241/Beryllium (Am-241/Be) source emits fast neutrons. When neutrons collide with hydrogen atoms (found primarily in water), they slow down dramatically (thermalize). A detector counts slow neutrons — more slow neutrons means more hydrogen means more water. The gauge reports moisture content as a percentage.

Two operating modes:

• Direct transmission: A rod containing the Cs-137 source is lowered into a pre-drilled hole (typically 150–300 mm deep). Gamma photons travel upward through the soil to a detector in the base of the gauge. This mode is more accurate because the photons pass through the full depth of the lift. Always use direct transmission for compaction testing.
• Backscatter: The source remains at the surface, and photons scatter back from the soil. Only measures the top 75–100 mm effectively. Used primarily for thin asphalt lift testing, not for compaction verification of earthwork.

6.2 Sand Cone Method

The sand cone test (ASTM D1556, used in Canadian practice) is a non-nuclear alternative. A hole is excavated in the compacted fill, and the excavated soil is weighed and dried to determine density and moisture content. The volume of the hole is measured by filling it with calibrated sand from a cone apparatus. While highly accurate, the sand cone method takes 4–8 hours for results (oven drying) versus 60 seconds for the nuclear gauge. It is primarily used as a verification or calibration check when nuclear gauge results are disputed.

6.3 Testing Frequency

OPSS 501 (Ontario Provincial Standard Specification for Compacting) specifies minimum testing frequencies. Best practice is to meet or exceed these:

• Structural fill (under footings, slabs): Minimum 1 test per 250 m² per lift, plus 1 test at each change in material or condition. A good standard: 1 test per 200 m².
• General site fill: Minimum 1 test per 500 m² per lift.
• Trench backfill: Minimum 1 test per 50 linear metres per lift, plus at every crossing and connection point.
• Granular base/subbase: Minimum 1 test per 250 m² per lift.

6.4 When a Test Fails

A failing density test does not necessarily mean the entire lift is deficient — but it must be addressed systematically:

1. Verify the test: Ask the technician to retest 1 m away. If the retest passes, the failure may be a localized anomaly (a clod, a pocket of different material).
2. Check moisture: If the field moisture is significantly above or below optimum, additional compaction will not help. The material must be conditioned (dried or wetted) first.
3. Rework the area: If moisture is near optimum, scarify the lift, recompact with additional passes, and retest.
4. Expand the investigation: If multiple tests fail, the problem is systemic. Common causes: wrong material, improper lift thickness, wrong equipment, equipment malfunction (check vibratory mechanism). Stop production and resolve the root cause before proceeding.
5. Document everything: Record failing test results, corrective actions taken, and passing retest results. The geotech engineer and project superintendent must be notified of all failures.

7. Intelligent Compaction (IC)

Intelligent Compaction is a significant advancement in quality control. IC-equipped rollers integrate accelerometers, GPS, and onboard computing to provide real-time measurement of soil stiffness across 100% of the compacted area — not just at spot test locations.

7.1 How IC Works

The roller drum contains accelerometers that measure how the drum's vibration pattern changes as soil stiffness increases. Three common measurement values:

• CMV (Compaction Meter Value): Developed by Dynapac. Measures the ratio of the first harmonic to the fundamental frequency of drum vibration. As soil stiffens, the drum's response becomes more complex (higher harmonics increase). CMV is dimensionless, typically ranging from 0–150.
• MDP (Machine Drive Power): Developed by Caterpillar. Measures the power the machine requires to maintain constant speed. As soil compacts and becomes stiffer, less energy is absorbed, so less drive power is needed. Reported in kJ/s.
• Evib (Vibration Modulus): Developed by Bomag. Calculates an elastic modulus (stiffness) directly from the drum-soil interaction, reported in MN/m² (MPa). This correlates most directly to engineering properties.

7.2 Operator Display and Documentation

The IC system displays a color-coded map of the compacted surface on the operator's screen. Red/yellow zones indicate low stiffness (under-compacted); green/blue zones indicate adequate stiffness. The operator can see exactly where to make additional passes and when a zone is complete. GPS coordinates tie every measurement to a specific location, creating a permanent record of compaction quality across the entire fill area.

7.3 IC vs Spot Testing

Traditional nuclear gauge testing checks perhaps 1 spot per 200–500 m² — meaning 99.9% of the fill area is unverified. IC rollers measure every square metre. This catches weak zones that spot testing misses. However, IC has limitations:

• IC values are relative stiffness measures, not direct density measurements. They must be correlated to Proctor density through calibration testing (running IC alongside nuclear gauge tests).
• IC measurements are influenced by the underlying layers, not just the current lift. A stiff lift placed over a soft subgrade may show misleading results.
• Most Ontario specifications still require conventional density testing. IC is used as a supplement, not a replacement.
• IC equipment costs a significant premium over standard rollers (typically $80,000–$150,000 additional for the IC package).

8. Ontario Granular Specifications (OPSS.MUNI 1010)

OPSS.MUNI 1010 (Material Specification for Aggregates — Base, Subbase, Select Subgrade, and Backfill Material) governs the granular materials used on virtually every Ontario construction project. Every superintendent must know these materials, their gradations, and where each is used.

8.1 Granular A

The primary base course material under floor slabs, pavements, and footings. Granular A is a well-graded, crushed limestone or granite aggregate with a 19 mm nominal top size.

Key properties: The 2–8% passing the 0.075 mm sieve (the "fines content") is critical. Too few fines and the material will not bind together or hold a smooth surface. Too many fines and the material becomes frost-susceptible, pumps under wet conditions, and is difficult to compact. Granular A must be crushed — a minimum percentage of particles must have fractured faces to provide mechanical interlock.

8.2 Granular B

Subbase material placed below Granular A. Available in three types:

• Granular B Type I: 50 mm top size, well-graded. Used as the primary subbase layer in road and parking lot construction. Can be crushed or natural (pit-run).
• Granular B Type II: 150 mm top size. A coarser, more open-graded material used for thick subbase fills, access roads, and platforms where rapid drainage is needed. Often used as the first lift over soft subgrade because the large stones bridge weak spots.
• Granular B Type III: Similar to Type II but with tighter gradation control. Less commonly specified on building projects.

On a typical church project, the pavement structure might be: 150 mm Granular A over 300 mm Granular B Type I over prepared subgrade. Under the building slab: 150–200 mm Granular A over compacted structural fill.

8.3 Granular M (OPSS.MUNI 1010)

Granular M is specified for pipe bedding and embedment. It is a clean, well-graded sand/gravel with controlled gradation to provide uniform support around pipes without damaging coatings or creating point loads. Typical top size is 19 mm, with maximum 5% passing the 0.075 mm sieve. For PVC pipe, the bedding must be free of stones larger than 19 mm to prevent point loading and cracking.

8.4 Recycled Concrete Aggregate (RCA)

OPSS.MUNI 1010 permits the use of recycled concrete aggregate (RCA) in Granular A and Granular B applications under specific conditions:

• Maximum 50% RCA by mass in Granular A (some municipalities restrict this further)
• Must meet the same gradation requirements as virgin aggregate
• Must be free of deleterious materials (wood, steel, asphalt, drywall)
• RCA tends to self-cement (residual calcium in the crusite hydrates over time), which can be an advantage for base stability but a disadvantage if future excavation is anticipated
• Not permitted in pipe bedding (Granular M) due to alkalinity concerns with certain pipe materials

9. Fill Placement Procedures

9.1 Lift Thickness

The single most common cause of failed compaction tests is lifts placed too thick. Maximum lift thickness depends on the material type and compaction equipment. These are compacted thicknesses (loose thickness will be 20–35% greater):

9.2 Moisture Conditioning

Achieving proper compaction almost always requires moisture conditioning:

• Adding water: Use a water truck to spray the lift surface uniformly. For granular material, apply water immediately before compaction — the water acts as a short-term lubricant. For clay, water must be incorporated by mixing (discing, blading) and allowed to equilibrate for 12–24 hours before compaction. Simply spraying the surface of a clay lift does not work — the water will not penetrate the clods.
• Drying soil: Spread material in thinner lifts and disc or blade to expose surface area. Wind and sun will dry granular soils in hours. Clay may take days. On time-critical projects, mixing in dry granular material or lime can accelerate drying (lime addition requires engineering design and environmental approval).

9.3 Confined Area Placement

Backfilling around foundations, behind retaining walls, and in utility trenches requires special attention:

• Maximum lift thickness must be reduced to match the compaction equipment available (typically 100–150 mm with plates/rammers)
• Backfill should be placed symmetrically on both sides of a wall simultaneously to prevent lateral pressure imbalance
• Compaction within 600 mm of a concrete wall or pipe should use lightweight equipment only — heavy vibratory equipment can damage fresh concrete or displace pipe
• Granular material is strongly preferred for confined backfill because it is easier to compact in thin lifts and less moisture-sensitive than clay

9.4 Winter Fill Placement

Winter earthwork is possible but challenging in Ontario. Practical considerations:

• Temperature limit: At approximately –7°C, most granular materials begin to freeze in the stockpile and during transport. Below this temperature, earthwork operations are generally impractical.
• Frost penetration: Previously placed and compacted fill can freeze overnight. The frozen layer must be removed or thawed before the next lift is placed. Never bury a frozen layer — it will thaw and settle.
• Insulating blankets: On critical operations, insulating blankets or hay/straw cover can protect the working surface from overnight freezing. This adds significant cost and labour.
• Material sourcing: Ensure the aggregate supplier can load from the interior of the stockpile where material has not frozen. Frozen material at the edges of a pile is common even at moderate temperatures.
• Nuclear gauge accuracy: Below-freezing temperatures can affect moisture readings. Advise the testing technician of conditions so they can apply appropriate corrections.

9.5 Engineered Fill per OBC

The Ontario Building Code (OBC Section 4.2) requires that fill supporting a building be "engineered fill" — placed under the direction of a professional engineer (geotechnical engineer) with documented compaction testing demonstrating conformance with the specification. This means:

• A fill placement specification prepared by the geotechnical engineer
• Material approved by the engineer before placement
• Lift-by-lift compaction testing by a certified testing firm
• A compaction certificate issued by the geotechnical engineer upon completion
• This certificate is required by the building official before footing inspection approval

10. Subgrade Preparation

10.1 Proof Rolling

Proof rolling is a simple, effective method for evaluating subgrade uniformity before placing granular base. The procedure:

1. Use a loaded tandem-axle dump truck (minimum 30-tonne gross vehicle weight) or a loaded vibratory roller operated in static mode.
2. Drive the truck slowly (walking speed, approximately 3–5 km/h) over the entire subgrade area in a systematic pattern (parallel passes with 50% overlap).
3. Observe the tire tracks and surface response. The acceptance criteria:

• Pass: Tire rut depth ≤ 25 mm, no visible pumping, no surface wave ahead of the tire.
• Marginal: Rut depth 25–50 mm. The area should be scarified, moisture-conditioned, recompacted, and retested.
• Fail: Rut depth > 50 mm, pumping (water squeezing up), or visible deflection wave. The material is not suitable as subgrade without remediation (may require over-excavation, geogrid, lime treatment, or other engineering solutions).

10.2 Subgrade Reaction Modulus (k-value)

For slab-on-grade design, the structural engineer needs to know the subgrade reaction modulus (k-value, also called the modulus of subgrade reaction). This value, expressed in kN/m³ (or pci — pounds per cubic inch), represents the pressure required to produce a unit deflection of the subgrade. Higher k-values mean stiffer support and allow thinner slabs.

The k-value is typically determined by plate load test in the field or estimated from soil classification and compaction data by the geotechnical engineer. On most church projects, the structural engineer designs floor slabs based on a k-value confirmed by the geotech — typically in the 55,000–80,000 kN/m³ (200–300 pci) range for a standard 150 mm Granular A over compacted subgrade section.

10.3 Structural Fill vs Common Fill

These terms appear in every set of earthwork specifications, and the distinction matters:

• Structural fill: Material placed under or within the zone of influence of a building, footing, or structural element. Must meet a specified compaction level (typically 95–98% Modified Proctor), be approved material (usually select granular or approved native soil), placed in controlled lifts, and tested by a geotech. This is engineered fill per OBC.
• Common fill: Material placed in non-structural areas (landscaping, general site grading, berms). Lower compaction requirements (typically 90–95% Standard Proctor), wider range of acceptable materials. Testing frequency is reduced. However, even common fill must not contain deleterious materials (organics, waste, frozen material).

10.4 The Geotechnical Engineer's Role

On every project, the geotechnical engineer provides:

• Geotechnical investigation report: Issued before design, documenting subsurface conditions from borehole logs, lab testing, and groundwater observations. The superintendent must read this report — it tells you what to expect when you dig.
• Fill placement specification: Defines acceptable materials, lift thickness, compaction targets, testing frequency, and winter restrictions.
• Field inspection: The geotech must inspect footing bearing surfaces, structural fill operations, and any unexpected conditions. Best practice: the geotech's field representative should be on-site during all structural fill placement.
• Compaction certificate: A letter confirming that fill was placed and tested in accordance with the specification. Required for building permit inspections.
• Ongoing consultation: When conditions differ from the report (they always do to some degree), the geotech provides direction on remediation, alternative materials, or design changes.

Summary: Key Takeaways

Earthwork is not glamorous, but it is foundational — literally and figuratively. Churches serve their congregations for 50–100 years, and every floor crack, every settling door frame, every drainage problem traces back to how the earthwork was executed. As you advance from crew leader to superintendent, your ability to manage soil, moisture, and compaction will directly determine the quality and longevity of every building you deliver.

• Know your soil — USCS classification tells you which equipment and methods to use
• Moisture controls everything — test it, condition it, and never compact soil that is too wet or too dry
• Use the right equipment for the soil type — vibration for granular, kneading for cohesive
• Lift thickness matters more than number of passes — thinner is always safer
• Understand Proctor testing — know which standard is specified, and what the numbers mean
• Proof roll everything before placing base — it is the cheapest insurance on the project
• Never accept frozen material, never bury frozen layers, and never work beyond the practical temperature limit
• The geotech is your partner, not your adversary — involve them early and often