3.2 Geotechnical Investigation & Soil Classification Systems

Quick Answer: Geotechnical site analysis evaluates subsurface stratigraphy, allowable bearing capacity, and pedological characteristics to determine structural feasibility, drainage performance, and vegetative viability. The Standard Penetration Test (SPT) measures soil density and consistency via blow counts (N-values) using a 140-lb hammer dropped 30 inches; the Unified Soil Classification System (USCS) categorizes soils into coarse-grained (gravel/sand) and fine-grained (silt/clay) groups based on grain size and Atterberg limits; and Proctor compaction tests define the Optimum Moisture Content (OMC) required to reach specified field densities (typically 95% under hardscape and 80–85% in planting beds).

Subsurface Geotechnical Investigation Methods

Landscape architects rely on geotechnical engineering reports to determine whether a site can support proposed structures (retaining walls, pergolas, pedestrian bridges, pavements) and how subsurface hydrology influences planting and stormwater infiltration. Subsurface exploratory investigations utilize three primary diagnostic techniques:

1. Exploratory Soil Borings

Drilled using truck-mounted hollow-stem augers or rotary wash rigs, soil borings extract continuous or interval samples at varying depths across the site. The spacing and depth of borings depend on the proposed program:

• Building pads and structural retaining walls: Borings spaced 50 to 100 feet apart, extending to competent bedrock or 1.5 to 2.0 times the proposed footing width beneath foundation bearing depth.
• Paved roadways and parking lots: Borings spaced 100 to 200 feet apart, extending 5 to 10 feet below proposed finished subgrade.
• Stormwater infiltration basins: Minimum 2 to 3 borings or test pits per basin footprint, extending at least 5 feet below the anticipated basin invert to verify confining layers and water table separation.

2. The Standard Penetration Test (SPT) and N-Values

The Standard Penetration Test (ASTM D1586) is the most widely specified in-situ test in North American geotechnical practice. A standard 2-inch outer diameter split-barrel (split-spoon) sampler is driven into the bottom of the borehole using a 140-pound (63.5 kg) hammer dropping freely from a height of 30 inches (760 mm).

• The sampler is driven a total distance of 18 inches, divided into three consecutive 6-inch increments.
• The number of hammer blows required to drive the sampler through the first 6-inch increment is recorded as the seating drive and is discarded because the borehole bottom is subject to disturbance and slough.
• The sum of the blows required to drive the second and third 6-inch increments (the final 12 inches) constitutes the SPT N-value (measured in blows per foot).
• When high-density bedrock or dense glacial till resists penetration, recording stops if 50 blows are reached within a single 6-inch increment (recorded as refusal, e.g., "50/2"").

3. Test Pits & Groundwater Monitoring Wells

• Test Pits (Open Trenches): Excavated with a standard backhoe to depths of 8 to 15 feet. Test pits provide a continuous, direct visual inspection of soil stratigraphy, topsoil depth, fill-strata boundaries, presence of cobbles/boulders, unweathered bedrock depth, macro-pore root channels, and localized perched water tables that narrow boreholes often miss.
• Piezometers / Monitoring Wells: Slotted PVC casings installed within boreholes to record static groundwater elevations, seasonal high water table (SHWT) fluctuations, and hydraulic gradients over time. The SHWT is also identified pedologically by redoximorphic features (mottling) within the soil profile.

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Pedological Soil Composition & USDA Textural Triangle

An undisturbed, ideal loam soil exhibits a volumetric composition divided into solids and pore space:

• Mineral Particles (45%): Sand, silt, and clay derived from weathered parent bedrock or transported sediments.
• Organic Matter (5%): Decomposed plant and animal residue (humus), crucial for cation exchange, moisture retention, and soil aggregation.
• Pore Space (50% total): Split under ideal field capacity into 25% Air and 25% Water.

Ideal Soil Volumetric Balance:
┌────────────────────────┬────────────────────────┐
│   Mineral Matter (45%) │  Organic Matter (5%)   │
├────────────────────────┼────────────────────────┤
│      Air (25%)         │      Water (25%)       │
└────────────────────────┴────────────────────────┘
      Pore Space (50%)         Solids (50%)

USDA Particle Size Classification

Under the United States Department of Agriculture (USDA) pedological classification system, mineral soil particles are sorted strictly by effective spherical diameter:

• Gravel: Greater than 2.0 mm (coarse fragments excluded from textural class naming unless exceeding specific volume thresholds).
• Sand: 0.05 mm to 2.0 mm. Sand grains are visible to the naked eye, feel gritty, have minimal specific surface area, and provide high permeability with zero cohesion.
• Silt: 0.002 mm to 0.05 mm. Silt particles feel smooth and floury when dry and slippery/non-sticky when wet. Silt is highly frost-susceptible due to high capillary suction.
• Clay: Less than 0.002 mm (< 2 microns). Microscopic, plate-like crystalline minerals with high negative surface electrical charges, vast specific surface area, high water-holding capacity, and pronounced plasticity and cohesion when wet.

The USDA Soil Texture Triangle

The USDA soil texture triangle defines 12 major textural classes based on the relative percentages of sand, silt, and clay. Sieve analysis (ASTM D422 / C136) determines particle size distribution for particles larger than the No. 200 sieve (0.075 mm), while hydrometer sedimentation analysis measures the settling rate of finer silt and clay fractions in water under Stokes' Law.

An ideal horticultural loam consists of roughly 40% Sand, 40% Silt, and 20% Clay. This proportion provides an optimal balance between water drainage (sand pores), moisture retention (silt pores), and nutrient cation storage (clay colloids).

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Engineering Classification Systems: USCS vs. AASHTO

Landscape architects work across two primary engineering classification systems when reviewing civil and geotechnical drawings:

1. Unified Soil Classification System (USCS - ASTM D2487)

The USCS is the universal standard for commercial site design, structural foundations, and retaining wall backfills. Soils are identified by a two-letter group symbol:

• First Letter (Primary Soil Type):
  • G = Gravel (>50% of coarse fraction retained on No. 4 sieve / 4.75 mm)
  • S = Sand (≥50% of coarse fraction passes No. 4 sieve)
  • M = Silt (Inorganic silt, fine fraction below A-line on plasticity chart)
  • C = Clay (Inorganic clay, fine fraction on or above A-line)
  • O = Organic silt or clay
  • Pt = Peat, muck, and highly organic swamp soils
• Second Letter (Gradation or Plasticity):
  • For Coarse-Grained Soils (<12% fines):
    • W = Well-graded (wide range of particle sizes, smooth gradation curve, high density when compacted; $C_u \ge 4$ for gravels, $C_u \ge 6$ for sands)
    • P = Poorly-graded (uniform particle size or gap-graded; low compacted density)
  • For Fine-Grained Soils (≥50% passes No. 200 sieve):
    • L = Low Plasticity (Liquid Limit < 50; lean clays, inorganic silts)
    • H = High Plasticity (Liquid Limit ≥ 50; fat clays, elastic silts)

2. AASHTO Soil Classification (M 145)

Used predominantly by state departments of transportation (DOT) and municipal street authorities to evaluate subgrade materials for highway pavements:

• Soils are grouped into A-1 through A-7.
• A-1, A-2, and A-3 are granular materials (≤35% passing No. 200 sieve), providing excellent subbase and subgrade support.
• A-4, A-5, A-6, and A-7 are fine-grained silts and clays (>35% passing No. 200 sieve), offering poor to fair pavement support and requiring substantial base aggregate thicknesses.

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Engineering Properties: Bearing Capacity, Shear Strength & Compaction

Allowable Bearing Capacity ($q_a$)

Bearing capacity represents the capacity of subgrade soil to support vertical foundation loads without undergoing shear failure or intolerable differential settlement, measured in pounds per square foot (psf) or kilopascals (kPa):

• Solid, unweathered bedrock (granite, basalt, sound limestone): 10,000 to 20,000+ psf
• Compacted well-graded gravels and coarse sands (GW, SW): 3,000 to 6,000 psf
• Stiff inorganic lean clays and compact silty sands (CL, SM): 2,000 to 3,000 psf
• Loose sands and medium-stiff clays (SP, ML, CL): 1,000 to 2,000 psf
• Soft clays, plastic silts, and loose uncompacted fills: < 1,000 psf
• Organic peats and mucks (Pt): Structurally unsuitable (< 500 psf); requires total excavation and replacement, or deep foundation systems (helical piers, driven piles).

Shear Strength and Mohr-Coulomb Criterion

Soil shear strength ($ au$) determines slope stability, retaining wall earth pressures, and footing resistance. Under the Mohr-Coulomb failure criterion: $\tau = c + \sigma' \tan(\phi)$ Where:

• $c$ = Cohesion (inter-particle electrostatic attractive forces, prominent in clays; sands have $c = 0$).
• $\sigma'$ = Effective normal stress (vertical intergranular contact pressure).
• $\phi$ = Angle of internal friction (inter-particle frictional resistance and interlocking; typically 28°–34° for loose to medium sands, 35°–45° for dense well-graded gravels, and 0° for saturated, undrained clays).

Soil Compaction, Optimum Moisture Content (OMC) & The Proctor Test

Compaction mechanically expels air from soil voids to increase dry density, shear strength, and bearing capacity while reducing permeability, liquefaction susceptibility, and future settlement.

Compaction Curve (Dry Density vs Moisture Content):
Dry Density (pcf)
       │           Maximum Dry Density (MDD)
       │                   ▲
       │                 ╱   ╲
       │               ╱       ╲
       │             ╱           ╲
       │           ╱               ╲
       │         ╱                   ╲
       └───────┼─────────────────────────►
             Dry of OMC    OMC    Wet of OMC
                        Moisture Content (%)

1. Standard Proctor Test (ASTM D698): Uses a 5.5-lb hammer falling 12 inches into a 4-inch mold, applying 12,400 ft-lbf/ft³ of compactive energy across 3 layers. Used for standard landscape grading, pedestrian plazas, and utility backfill.
2. Modified Proctor Test (ASTM D1557): Uses a 10.0-lb hammer falling 18 inches into a 4-inch or 6-inch mold, applying 56,000 ft-lbf/ft³ across 5 layers. Used for heavy vehicular pavements, commercial loading docks, and airfield subgrades.
3. Optimum Moisture Content (OMC): The precise moisture percentage at which water films lubricate soil particles, allowing them to slip past each other into their densest packing configuration. Compacting "dry of optimum" results in high shear strength but brittle behavior; compacting "wet of optimum" produces a rubbery, pumping subgrade that cannot achieve specified density.
4. Field Density Specifications:
   • Under structural slabs, retaining wall footings, and pavement subgrades: Typically 95% of Maximum Dry Density (MDD) per ASTM D698 or ASTM D1557.
   • Under lawn areas, landscape berms, and athletic turf: Restricted to 80% to 85% MDD to prevent root-limiting bulk densities and turf suffocation.

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