6.6 Shallow Foundations, Slabs, Deep Systems & Drainage

6.6 Shallow Foundations, Slabs, Deep Systems & Drainage

Foundations serve as the structural interface transferring the dead, live, wind, and seismic loads of a building into the underlying earth. In commercial construction, geotechnical site conditions dictate whether a project utilizes shallow foundations (spread footings, mat slabs) or deep foundation systems (drilled shafts, driven piles, micropiles). General contractors must understand the structural mechanics of foundation design, concrete slab-on-grade (SOG) jointing and moisture protection, deep foundation installation methods, and subsurface waterproofing and drainage assemblies.

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Shallow Foundation Systems & Georgia Frost Line Requirements

Shallow foundations transmit structural loads directly to the upper soil strata where allowable soil bearing capacities are sufficient (q_a >= 2,000 to 4,000 psf) to support the building without excessive total or differential settlement.

                           SHALLOW FOUNDATION TYPES

    Continuous Strip Footing       Isolated Spread Footing         Mat / Raft Foundation
     ┌────────────────────┐          ┌──────────────┐           ┌──────────────────────┐
     │    Bearing Wall    │          │    Column    │           │ Multiple Heavy Cols  │
     └─────────┬──────────┘          └──────┬───────┘           └───┬───────┬───────┬──┘
               │                            │                       │       │       │
     ┌─────────┴──────────┐          ┌──────┴───────┐           ┌───┴───────┴───────┴──┐
     │  Concrete Footing  │          │  Pad Footing │           │ Thick Reinforced Mat │
     └────────────────────┘          └──────────────┘           └──────────────────────┘

Types of Shallow Foundations

1. Continuous Wall (Strip) Footings: Continuous reinforced concrete ribbons running beneath exterior and interior load-bearing walls. They distribute linear dead and live wall loads evenly across the subgrade.
2. Isolated Spread Column Footings (Pad Footings): Square or rectangular stepped concrete pads supporting individual structural steel or cast-in-place concrete columns under concentrated point loads.
3. Combined & Cantilever (Strap) Footings: Used when an exterior column sits directly along a property boundary line where a centered footing cannot extend onto adjacent land. A stiff structural concrete grade strap connects the exterior footing to an interior footing to counterbalance eccentric overturning moments.
4. Mat / Raft Foundations: A thick, heavily reinforced monolithic concrete slab (often 2 to 6 feet thick) supporting the entire superstructure. Used on low-bearing capacity or highly variable soils to bridge localized soft pockets and minimize differential settlement.
5. Grade Beams: Reinforced concrete beams at or near ground level spanning between isolated spread footings, drilled shafts, or pile caps to carry perimeter curtain walls or masonry veneer.

Frost protection and footing depth

+Place exterior footings at the depth required by the adopted code, approved design, local climatic and geotechnical criteria, and undisturbed bearing conditions. Georgia does not use the author's universal south/central/north 12-, 15-, and 18-inch table. Confirm the jurisdiction's design criteria and the sealed foundation documents.

• Footings must always bear on solid, undisturbed native soil or certified engineered structural fill; footings must never bear on frozen soil, uncompacted fill, or organic topsoil.

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Slab-on-Grade (SOG) Construction, Moisture Barriers & Joint Detailing

Commercial concrete slabs-on-grade must resist heavy wheel loads, pallet racking point loads, moisture vapor transmission, and drying shrinkage cracking.

                      SLAB-ON-GRADE SUB-SLAB ASSEMBLY

   ┌─────────────────────────────────────────────────────────────────┐ ◄── Top of Slab
   │        Concrete Slab (with T/4 Contraction Joints)               │ (4" - 8" Thick)
   ├─────────────────────────────────────────────────────────────────┤
   │═════════ Minimum 10-Mil ASTM E1745 Class A Vapor Retarder ══════│ (Taped Seams)
   ├─────────────────────────────────────────────────────────────────┤
   │░░░░░░░░░ 4" Washed Crushed Stone (ASTM C33 No. 57 Gravel) ░░░░░░│ (Capillary Break)
   ├─────────────────────────────────────────────────────────────────┤
   │▓▓▓▓▓▓▓▓▓ Proof-Rolled, Compacted Native Subgrade (95% MDD) ▓▓▓▓▓│
   └─────────────────────────────────────────────────────────────────┘

Moisture Protection & Capillary Break Assembly

1. Capillary Break: A minimum 4-inch layer of clean, washed crushed stone (ASTM C33 No. 57 stone) containing no fine particles. This porous granular layer breaks capillary water suction from the underlying water table.
2. Vapor Retarder: Placed directly over the crushed stone base. Georgia commercial standards mandate a minimum 10-mil (0.010 in) or 15-mil extruded virgin polyolefin membrane complying with ASTM E1745 Class A (water vapor permeance < 0.1 perms). All seams must overlap a minimum of 6 inches and be sealed with manufacturer-approved vapor tape, with all pipe penetrations sealed with elastomeric pipe boots.

Concrete Slab Jointing Standards

Uncontrolled concrete shrinkage creates random structural cracking. Slabs must incorporate three distinct joint types:

• Sawcutting Timing: Control joints must be cut as soon as the concrete surface can support saw equipment without raveling or dislodging aggregates—typically 4 to 12 hours after placement for conventional wet saws, or 1 to 4 hours for early-entry dry-cut saws.

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Deep Foundation Systems: Drilled Shafts (Caissons) & Driven Piles

When near-surface soils have insufficient allowable bearing capacity (q_a < 1,500 psf), high groundwater tables, or heavy concentrated column loads, deep foundations transfer building loads to deep competent bearing strata or solid bedrock via end-bearing or skin friction.

                              DEEP FOUNDATION MECHANICS

             End-Bearing Drilled Shaft                  Skin Friction Pile
           ┌───────────────────────────┐           ┌───────────────────────────┐
           │     Column Load (P)       │           │     Column Load (P)       │
           └─────────────┬─────────────┘           └─────────────┬─────────────┘
                         │                                       │
                         ▼                                       ▼
                ┌─────────────────┐                     ┌─────────────────┐
                │                 │                     │ │ ◄── Friction  │
                │  Drilled Shaft  │                     │ │               │
                │    (Caisson)    │                     │ │ ◄── Shearing  │
                │                 │                     │ │     Resistance│
                │                 │                     │ │               │
                └────────┬────────┘                     └────────┬────────┘
                         │                                       │
                         ▼                                       ▼
                ┌─────────────────┐                     ┌─────────────────┐
                │ Solid Bedrock   │                     │ Deep Sand/Clay  │
                │  (End Bearing)  │                     │  (End Bearing)  │
                └─────────────────┘                     └─────────────────┘

1. Drilled Shafts / Piers (Caissons)

Drilled shafts are large-diameter (24 to 96+ inches) cast-in-place concrete cylinders constructed by augering deep cylindrical holes into the earth, inserting rebar cages, and filling the shaft with structural concrete.

• Dry Open-Hole Method: Applicable in stiff, cohesive clays where the borehole walls remain stable without caving and no groundwater is present.
• Casing Method: Used in caving sands or water-bearing strata. A temporary or permanent heavy steel casing is driven or vibrated through unstable upper soils into an impermeable clay or rock socket, sealing out water while drilling progresses.
• Slurry Displacement Method: The borehole is filled with a dense drilling fluid (bentonite clay or synthetic polymer slurry) during excavation. The hydrostatic pressure of the slurry prevents hole collapse. Concrete is subsequently placed via Tremie Pipe (a sealed pipe submerged beneath the rising concrete), which displaces the lighter slurry upward out of the hole without diluting the concrete.
• Belling (Under-Reaming): Utilizing a mechanical belling tool to flare the bottom of the drilled shaft into a cone shape (up to 2 to 3 times the shaft diameter), greatly multiplying the end-bearing surface area on competent rock.

2. Driven Piles

Driven piles are prefabricated slender structural members driven into the ground using heavy impact or vibratory pile hammers until reaching refusal or designated load capacity:

• Timber Piles: Pressure-treated southern yellow pine (20 to 60 ft lengths; capacity 15 to 30 tons); economical for light waterfront and bridge approach structures.
• Precast Prestressed Concrete Piles: Square, octagonal, or cylinder concrete piles with high structural and axial load capacity (50 to 200+ tons); highly durable in marine environments.
• Structural Steel H-Piles (HP Shapes): Heavy structural steel sections that penetrate dense gravel, cobbles, and boulder layers to reach hard bedrock refusal (100 to 300+ tons).
• Steel Pipe Piles: Steel pipes driven either open-ended (rock coring) or closed-ended (with a welded conical point), and subsequently filled with high-strength structural concrete.

Pile Driving Verification & Load Testing

• Refusal Criteria: Defined as the point where pile penetration slows to a specified minimum under heavy hammer impacts (e.g., 10 blows per inch or 120 blows per foot).
• Engineering News Record (ENR) Dynamic Formula: R = (2 * W * h) / (S + 0.1) (For steam/diesel hammers) Where R = safe bearing capacity (lbs), W = hammer ram weight (lbs), h = hammer stroke (ft), and S = average penetration per blow (inches).
• High-Strain Dynamic Testing (PDA): The Pile Driving Analyzer uses accelerometers and strain transducers attached to the pile head to measure wave mechanics, verifying real-time capacity and pile structural integrity.
• Static Axial Compressive Load Test (ASTM D1143): Applying hydraulic test loads up to 200% of design load against reaction anchors over 24 to 48 hours to measure physical settlement.

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Specialty Deep Foundations: ACIP Piles, Micropiles & Helical Systems

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Subsurface Waterproofing, Dampproofing & Foundation Drainage

Water intrusion through below-grade foundation walls causes mold, concrete deterioration, and interior flooding. Contractors must distinguish between basic dampproofing and true waterproofing.

Dampproofing vs. Waterproofing (IBC Chapter 18)

IBC Section 1805 distinguishes dampproofing and waterproofing by the groundwater and hydrostatic condition and the approved water-control design. Verify the design water table, drainage or groundwater-control system, wall and slab use, joints, penetrations, and manufacturer assembly. Do not substitute a universal six-inch slogan or generic coating for the sealed documents.

Foundation Drainage System Engineering

To relieve hydrostatic pressure against basement walls and footings, an engineered perimeter drainage system must be installed:

1. Perforated Drain Pipe: Minimum 4-inch diameter perforated rigid PVC (SDR 35) or heavy-duty corrugated HDPE pipe placed alongside the outer edge of the footing, with perforations facing downward or horizontally.
2. Gravel Envelope: The pipe must be embedded in a minimum of 6 to 12 inches of washed crushed stone (ASTM No. 57 stone).
3. Geotextile Filter Fabric Wrap: The entire gravel envelope must be wrapped in a non-woven needle-punched geotextile filter fabric to prevent soil silts from migrating into and clogging the gravel voids.
4. Drainage Dimple Board: A prefabricated high-density polyethylene (HDPE) dimpled drainage board with bonded geotextile face installed vertically against the waterproofing membrane to channel water rapidly down to the footing pipe.
5. Positive Gravity Discharge / Sump Basin: The drainage collector pipe must slope continuously (>= 1/8" per foot) to discharge by gravity to daylight away from the building, or drain into a sealed interior/exterior sump pit equipped with an automatic duplex sump pump system.