7.2 Site Preparation, Earthwork, Surveying & Grading

7.2 Site Preparation, Earthwork, Surveying & Grading

Site preparation and earthwork form the foundational phase of general building construction. Contractors must execute precise site surveying, understand geotechnical soil properties, implement mandatory environmental controls, adhere strictly to Cal/OSHA excavation safety orders, and achieve rigorous soil compaction specifications.

---

Geotechnical Soil Reports & Soil Mechanics

Prior to foundation design and site grading, a licensed Civil or Geotechnical Engineer conducts subsurface exploration via soil borings or test pits to issue a Geotechnical Soils Report. The report establishes soil stratification, depth to groundwater, expansion potential, and allowable bearing capacities.

Soil Classification (USCS System)

Under the Unified Soil Classification System (USCS), soils are classified by particle size distribution and plasticity:

• Coarse-Grained Soils (Gravels and Sands): GW (Well-graded gravel), GP (Poorly-graded gravel), SW (Well-graded sand), SP (Poorly-graded sand). These non-cohesive soils exhibit high shear strength and excellent drainage.
• Fine-Grained Soils (Silts and Clays): CL (Lean clay of low-to-medium plasticity), CH (Fat clay of high plasticity), ML (Inorganic silt). Cohesive soils retain moisture and are susceptible to volumetric expansion and settlement.

Expansive Soils & CBC Bearing Capacities

Expansive soils contain clay minerals (such as montmorillonite) that expand significantly when absorbing water and shrink when drying. The Expansion Index (EI) (tested under ASTM D4829) rates soil swell potential:

Expansion Index (EI) Ratings:
  0 - 20   : Very Low
 21 - 50   : Low
 51 - 90   : Medium   (Triggers CBC Chapter 18 expansive soil mitigation)
 91 - 130  : High
   > 130   : Very High

When building on expansive clay (EI > 50), CBC Chapter 18 (Section 1808.6) requires specialized foundations, such as post-tensioned slabs, waffle slabs, continuous footings extended below the moisture-change zone (minimum 18-24 inches depth), or complete soil removal and replacement with non-expansive structural fill.

---

Site Surveying, Boundary Layout & Batter Boards

Precision building layout prevents catastrophic property line encroachments and zoning setback violations.

Layout and the 3-4-5 Rule

Contractors establish building corners from official property boundary monuments using optical levels, transit instruments, or total stations. To ensure building corners form exact 90-degree right angles, contractors apply the 3-4-5 Pythagorean Theorem rule ($A^2 + B^2 = C^2$):

$\text{If side } A = 15\text{ ft} \text{ and side } B = 20\text{ ft}, \text{ the diagonal } C = \sqrt{15^2 + 20^2} = \sqrt{225 + 400} = \sqrt{625} = 25\text{ ft}$

To verify that an entire rectangular building footprint is square, measure both diagonal distances across opposite corners. If the footprint is perfectly square, both diagonal measurements must be identical.

Batter Boards

Once corners are established, contractors erect batter boards—temporary wooden framing setups placed 4 to 6 feet outside the actual excavation perimeter. Builder's line (masonry string) stretched between sawkerfs on opposing batter boards establishes building lines and foundation wall centerlines. Because batter boards sit outside the cut line, string lines can be removed during excavation and re-stretched to instantly recover exact building corners.

---

Environmental Stormwater Compliance (SWPPP)

Under the California State Water Resources Control Board NPDES General Permit for Stormwater Discharges Associated with Construction and Land Disturbance Activities (Order No. 2009-0009-DWQ), projects disturbing 1 acre or more of soil must obtain permit coverage and implement a Stormwater Pollution Prevention Plan (SWPPP).

Best Management Practices (BMPs)

• Erosion Controls (Soil Stabilization): Hydroseeding, hydraulic mulch, or geotextile soil blankets applied to exposed cut/fill slopes before rain events.
• Sediment Controls: Silt fences installed along site contours (embedded in a 6-inch deep trench backfilled with gravel), fiber rolls (straw wattles) staked every 10-20 feet on slopes, and storm drain inlet protection bags.
• Tracking Controls: Stabilized construction entrances constructed of 3-to-6-inch angular crushed stone (minimum 50 feet long by 20 feet wide) to prevent vehicle tires from tracking mud onto public roads.

---

Cal/OSHA Excavation & Underground Utility Safety

Excavation work is governed by Cal/OSHA Construction Safety Orders (8 CCR Subchapter 4, Article 6, Sections 1539-1547).

Underground Service Alert (811 - USA North / DigAlert)

Under California Government Code Section 4216:

1. Excavators must notify Underground Service Alert (811) at least 2 working days (48 hours, excluding weekends and holidays) prior to initiating excavation.
2. The excavation site outline must be delineated in white paint or white flags prior to calling.
3. Utility operators mark underground infrastructure using standard APWA color codes (Red = Electrical; Yellow = Gas/Oil; Orange = Telecom; Blue = Water; Green = Sewer).
4. Markings remain valid for 28 calendar days.
5. Excavation within the 24-inch tolerance zone on either side of marked utility lines must be performed exclusively using hand tools (potholing) or vacuum excavation until exposed.

Cal/OSHA Soil Classification & Protective Systems

Any excavation 5 feet or deeper requires an approved protective system (sloping, benching, shoring, or trench shields) unless cut entirely into solid rock. Any excavation 20 feet or deeper mandates custom protective systems designed by a registered California Professional Engineer.

Cal/OSHA Trench Safety Rules:
 ➔ Egress (Ladders/Ramps): Required in trenches 4 ft or deeper within 25 ft of lateral travel.
 ➔ Spoil Pile Clearance: Minimum 2 ft setback from excavation edge for spoils and equipment.
 ➔ Atmospheric Testing: Required in trenches > 4 ft deep where oxygen deficiency (<19.5%) or hazardous gas may exist.

---

Soil Compaction & Cut/Fill Calculations

Compaction Standards & Field Testing

Engineered fill placement requires controlled moisture content and mechanical compaction to eliminate air voids. Laboratory maximum dry density is determined via ASTM D1557 (Modified Proctor test).

• CBC Compaction Requirement: CBC Chapter 18 mandates that structural fill placed beneath shallow foundations, footings, and slabs must be compacted to a minimum of 90% relative compaction (based on ASTM D1557). Fills placed under upper subgrade pavements or supporting heavy highway loads require 95% relative compaction.
• Lift Thickness: Structural fill must be placed in uniform horizontal layers (lifts) not exceeding 6 to 8 inches in loose thickness before compaction.
• Moisture Control: Soil moisture must be maintained within ±2% of Optimum Moisture Content (OMC) during compaction. Sheepsfoot rollers are utilized for cohesive clays; smooth-drum vibratory rollers are used for granular sands/gravels.
• Field Verification: Tested on site using a Nuclear Density Gauge (ASTM D6938) or Sand Cone Method (ASTM D1556).

Cut and Fill Volume Calculations

Earthwork calculations reconcile volumetric changes between natural ground, excavated soil, and compacted fill:

• Bank Measure ($V_B$): Volume of soil in its natural, undisturbed state.
• Loose Measure ($V_L$): Volume after excavation. Soil expands (swells) due to void creation ($V_L = V_B \times [1 + S]$, where $S$ is Swell Factor, typically 15-30%).
• Compacted Measure ($V_C$): Volume after field compaction ($V_C = V_B \times [1 - K]$, where $K$ is Shrinkage Factor, typically 10-15%).

Average End Area Method Formula

$V = \left( \frac{A_1 + A_2}{2} \right) \times \frac{L}{27}$

Where $V$ is volume in cubic yards ($ ext{yd}^3$), $A_1$ and $A_2$ are cross-sectional areas in square feet at two stations, and $L$ is distance between stations in feet.

Worked Earthwork Example: Average End Area Cut Volume

A grading contractor calculates cut volume between Station $0+00$ and Station $1+00$ ($L = 100\text{ feet}$).

• Cross-sectional cut area at Station $0+00$ ($A_1$) = $450\text{ sq ft}$.
• Cross-sectional cut area at Station $1+00$ ($A_2$) = $650\text{ sq ft}$.

1. Average Area: $\text{Average Area} = \frac{450 + 650}{2} = \frac{1100}{2} = 550\text{ sq ft}$

2. Bank Cut Volume ($V_B$): $V_B = \frac{550\text{ sq ft} \times 100\text{ ft}}{27\text{ cu ft/yd}} = \frac{55000}{27} = 2,037.04\text{ bank cubic yards (BCY)}$

3. Truck Haul Volume (Loose Measure $V_L$ with 20% Swell Factor): $V_L = 2,037.04 \times 1.20 = 2,444.45\text{ loose cubic yards (LCY)}$