The choice between shallow and deep foundations depends on several factors, primarily related to soil conditions and structural loads. Critical quality control aspects differ for each type.

Factors Influencing Choice:

• Soil Bearing Capacity: If strong, stable soil with adequate bearing capacity is available near the surface, shallow foundations are preferred. If weak or compressible soil extends deep, deep foundations are necessary to reach stronger layers.
• Groundwater Table: A high groundwater table can complicate shallow foundation construction and may necessitate deep foundations to avoid water issues.
• Structural Loads: Heavy structures or those with concentrated loads often require deep foundations to distribute weight effectively.
• Settlement Tolerance: Structures sensitive to settlement may require deep foundations to minimize differential settlement.
• Site Conditions: Presence of expansive soils, collapsible soils, or bedrock at shallow depths influences the decision.
• Cost & Time: Generally, shallow foundations are more economical and quicker to construct if soil conditions allow.

Quality Control Aspects:

Shallow Foundations (e.g., Spread Footings, Rafts):

• Excavation: Ensure excavation depth and dimensions match design, and the base is level and free from loose soil or water.
• Subgrade Preparation: Verify proper compaction of the subgrade soil to achieve specified density. Field density tests are crucial.
• Reinforcement: Check correct placement, spacing, and cover of steel reinforcement as per drawings.
• Concrete Quality: Perform slump tests, cast cubes/cylinders for strength testing, and ensure proper placement, compaction (vibration), and curing.
• Drainage: Implement proper site drainage to prevent water accumulation around the foundation.

Deep Foundations (e.g., Piles, Piers):

• Boring/Driving: Monitor drilling/driving logs for consistency, depth, and refusal criteria.
• Pile Integrity Testing (PIT): Non-destructive test to check for flaws (cracks, necking) in the pile shaft after casting.
• Pile Load Test: A critical test where a pile is subjected to a load to verify its actual bearing capacity and settlement behavior.
• Concrete Placement (for cast-in-situ piles): Ensure continuous concrete pour (tremie method for wet concrete) to avoid segregation and voids. Take concrete samples for strength testing.
• Reinforcement Cage: Verify correct assembly, length, and placement of the reinforcement cage before concreting.
• Verticality: Check that piles are installed vertically within allowed tolerances.

Expansive soils (like certain clays) swell when wet and shrink when dry, causing significant ground movement that can severely damage foundations. Managing quality control for such sites requires special attention.

Key Quality Control Strategies:

1. Thorough Site Investigation:
   • Soil Testing: Conduct detailed lab tests (e.g., Atterberg limits, free swell index, swell pressure) to confirm expansiveness and determine the degree of potential swelling.
   • Moisture Profile: Understand the natural moisture content variations with depth.
2. Foundation Design Adaptations:
   • Deep Foundations: Often preferred to extend below the active zone (depth where moisture changes occur) into stable, non-expansive soil or bedrock. Quality control focuses on proper drilling, casing, and concrete placement to ensure the pile/pier is isolated from swelling pressures.
   • Stiffened Raft/Slab-on-Grade: If shallow foundations are used, they must be heavily reinforced and stiffened to resist differential movements. QC involves strict control of reinforcement placement and concrete strength.
   • Void Forms/Cushion Layers: Use compressible materials (void forms) beneath beams or slabs, or create a cushion layer of non-expansive granular material. QC ensures proper installation and thickness of these layers.
3. Moisture Control Measures:
   • Drainage: Implement excellent site drainage (sloping grades away from the foundation, French drains) to prevent water from reaching the expansive soil. QC checks slopes and drain installation.
   • Moisture Barriers: Install impervious barriers (e.g., plastic sheets) around the foundation to control moisture migration. QC verifies proper overlap and sealing.
   • Vegetation Control: Manage trees and shrubs near the foundation, as their roots can extract moisture, leading to shrinkage.
4. Construction Practices:
   • Excavation: Minimize the time excavations are open to prevent moisture changes.
   • Backfilling: Use non-expansive, well-compacted granular backfill around foundations. QC involves regular compaction tests.
   • Concrete Curing: Ensure prolonged and effective curing of concrete to achieve maximum strength and reduce permeability.
5. Monitoring:
   • Post-Construction Monitoring: In some critical projects, install settlement markers or moisture sensors to monitor ground movement and moisture changes over time.

Liquefaction is a phenomenon where saturated, loose granular soils (like sand or silt) temporarily lose their strength and behave like a liquid during an earthquake or rapid loading. This happens because the shaking causes the soil particles to lose contact with each other, and the water pressure between them increases, making the soil unable to support loads.

Implications for Foundation Design:

• Loss of Bearing Capacity: Foundations built on liquefiable soil can lose their support, leading to severe settlement, tilting, or even floating of structures (for buried tanks, etc.).
• Lateral Spreading: The liquefied soil can flow horizontally, causing large ground deformations and pulling foundations apart.
• Damage to Piles: Piles passing through liquefiable layers can be subjected to large bending forces, leading to structural failure.

Quality Control (QC) and Mitigation Strategies:

If a site is identified as having liquefaction potential (through SPT, CPT tests, and seismic studies), QC focuses on implementing mitigation measures:

1. Site Investigation & Assessment:
   • QC: Ensure thorough geotechnical investigations using tests like SPT (Standard Penetration Test) or CPT (Cone Penetration Test) to identify liquefiable soil layers and their properties. Verify seismic hazard analysis.
2. Ground Improvement Techniques:
   • Compaction: Densifying the soil to make it less prone to liquefaction.
     • Methods: Vibro-compaction, dynamic compaction, stone columns.
     • QC: Monitor compaction effort, measure post-treatment density (e.g., using SPT/CPT) to ensure the target density is achieved.
   • Drainage: Installing drains to allow rapid dissipation of excess pore water pressure during shaking.
     • Methods: Gravel drains, wick drains.
     • QC: Verify proper installation depth, spacing, and material quality of drains.
   • Grouting/Chemical Treatment: Injecting materials into the soil to bind particles together and increase strength.
     • QC: Monitor injection pressures, volumes, and conduct post-treatment tests to confirm strength gain.
3. Foundation Design Solutions:
   • Deep Foundations: Extending piles or piers through the liquefiable layer to a stable, non-liquefiable stratum below.
     • QC: Ensure piles are designed for potential lateral loads from liquefied soil, and verify proper installation depth and connection to stable ground.
   • Raft Foundations: Designing a stiff raft foundation that can bridge over localized areas of liquefaction.
     • QC: Verify reinforcement detailing and concrete strength for the raft.

Instrumentation and monitoring involves installing sensors and devices in and around a construction site to measure ground movement, water pressure, stresses, and deformations. For large foundation projects, especially those with deep excavations or difficult soil, it's a vital part of quality control and risk management.

Contribution to Quality Control:

1. Real-time Performance Assessment:
   • QC: Provides continuous data on how the soil and surrounding structures are behaving during construction. This allows engineers to compare actual behavior with predicted behavior from design.
   • Benefit: Detects unexpected movements or pressures early, allowing for immediate corrective actions before problems become severe.
2. Safety Assurance:
   • QC: Monitors the stability of excavation walls, slopes, and nearby structures.
   • Benefit: Ensures the safety of workers, equipment, and adjacent properties by providing warnings if critical thresholds are approached.
3. Optimized Design & Construction:
   • QC: Data from instruments can validate design assumptions. If actual conditions are better than assumed, construction methods might be optimized (e.g., less shoring needed), saving time and cost. If worse, additional measures can be implemented.
   • Benefit: Allows for adaptive construction, where plans can be adjusted based on real-time feedback.
4. Dispute Resolution:
   • QC: Provides objective, recorded data on ground movements and impacts.
   • Benefit: Can be crucial in resolving disputes related to damage to adjacent properties or construction delays.
5. Long-term Performance Monitoring:
   • QC: For critical structures, monitoring can continue after construction to ensure long-term stability and identify any post-construction settlement.
   • Benefit: Provides valuable data for future projects and research.

Common Instruments Used:

• Inclinometers: Measure lateral ground movement.
• Piezometers: Measure groundwater pressure.
• Settlement Markers: Measure vertical ground movement.
• Strain Gauges: Measure stress and strain in structural elements.
• Extensometers: Measure displacement at various depths.
• Total Stations/GPS: For precise surveying of surface movements.

Geotechnical reporting is the process of documenting all soil investigations, lab tests, field tests, and recommendations related to the ground conditions of a construction site. It's a fundamental part of foundation quality control because it provides the essential information needed to design and build safe and stable foundations.

Importance in Foundation Quality Control:

• Basis for Design: It provides engineers with the necessary data (soil properties, strength, bearing capacity) to design appropriate and safe foundations. Without it, foundation design would be guesswork.
• Risk Management: Identifies potential ground-related risks (e.g., expansive soils, liquefaction, high water table) and suggests mitigation strategies, allowing for proactive quality control.
• Compliance: Ensures that the foundation design and construction comply with local building codes and engineering standards.
• Construction Planning: Helps contractors plan excavation, dewatering, and construction methods efficiently and safely.
• Problem Solving: If issues arise during or after construction (e.g., unexpected settlement), the geotechnical report serves as a crucial reference for investigation and remediation.
• Legal & Financial: Provides documented evidence of ground conditions, which can be important for legal purposes, insurance, and financing.

Information a Geotechnical Report Should Contain:

1. Project Description: Location, proposed structure type, and size.
2. Site Geology & History: General geological conditions, past land use.
3. Field Investigation Details:
   • Number and location of boreholes/test pits.
   • Drilling methods and depths.
   • Sampling procedures (e.g., SPT N-values, undisturbed samples).
   • Groundwater observations (depth, variations).
4. Laboratory Test Results:
   • Soil classification (e.g., grain size analysis, Atterberg limits).
   • Strength tests (e.g., unconfined compression, direct shear, triaxial).
   • Consolidation tests (for settlement prediction).
   • Compaction test results (Proctor values).
   • Chemical tests (e.g., sulfates, chlorides, pH, for concrete durability).
5. Analysis & Recommendations:
   • Soil Profile: Detailed description of soil layers encountered.
   • Bearing Capacity: Recommended allowable bearing pressure for different foundation types.
   • Settlement Analysis: Estimated total and differential settlement.
   • Foundation Type: Recommendations for suitable foundation types (shallow, deep) and their design parameters.
   • Groundwater Control: Recommendations for dewatering if needed.
   • Earthwork Recommendations: Compaction requirements for fills, excavation stability.
   • Seismic Considerations: Liquefaction potential, seismic design parameters.
   • Construction Considerations: Any special requirements or challenges.
6. Limitations: A clear statement of the report's scope and limitations.