Typhoon-Resistant Foundations in the Philippines

1. Relevant Codes

1.1 National Structural Code of the Philippines (NSCP) 2015

Key points:

• Provides comprehensive guidelines for structural design, including foundations
• Specifies design loads (dead, live, wind, seismic) for different regions in the Philippines
• Outlines minimum requirements for materials, including concrete and reinforcing steel
• Provides specific provisions for typhoon-resistant design, including:
  • Wind speed maps for different return periods
  • Procedures for calculating wind pressures on structures
  • Requirements for connections and anchorage to resist uplift forces

1.2 National Building Code of the Philippines (NBCP)

Key points:

• Establishes minimum requirements for building design, construction, use, and occupancy
• Provides guidelines for site preparation and excavation
• Specifies minimum depths for footings to protect against frost and erosion
• Outlines requirements for waterproofing and damp-proofing of foundations
• Establishes standards for soil testing and allowable soil pressures

1.3 Association of Structural Engineers of the Philippines (ASEP) Guidelines

Key points:

• Provides supplementary guidelines and best practices for structural engineering in the Philippines
• Offers interpretations and practical applications of the NSCP provisions
• Includes recommendations for foundation design in various soil conditions
• Provides guidance on geotechnical investigations and soil improvement techniques
• Offers design examples and case studies relevant to Philippine conditions

2. Basic Principles

2.1 Minimum Footing Depth

1. Minimum footing depth: 1 meter below ground level

   • Purpose: To reach stable soil below the frost line and organic topsoil
   • Considerations:
     • Increase depth in areas with deep frost penetration or expansive soils
     • In coastal areas, consider scour depth due to wave action

2. Reinforced concrete strength: Minimum 20.7 MPa (3000 psi)

   • Purpose: To ensure durability and load-bearing capacity
   • Considerations:
     • Increase to 30 MPa (4350 psi) or higher for marine environments
     • Use sulfate-resistant cement in areas with high sulfate content in soil or water
   • Testing: Conduct regular cylinder tests to verify strength

3. Steel reinforcement: Grade 40 (280 MPa) or Grade 60 (420 MPa)

   • Purpose: To provide tensile strength and control cracking
   • Considerations:
     • Use Grade 60 for higher strength and potentially reduced steel quantity
     • In coastal areas, use epoxy-coated or stainless steel rebar to prevent corrosion
   • Placement: Ensure proper concrete cover (minimum 75mm for foundations)

4. Soil bearing capacity:

   • Conduct thorough geotechnical investigation to determine allowable bearing pressure
   • Design foundation size based on calculated loads and allowable soil pressure
   • Factor of safety: Use minimum 3.0 for bearing capacity calculations

5. Waterproofing:

   • Apply waterproofing membranes to all below-grade concrete surfaces
   • Install drainage systems around foundations to divert water

6. Seismic considerations:

   • Design foundations to resist lateral loads from earthquakes
   • Incorporate tie beams to connect individual footings for better load distribution

2. Foundation Types

2.1 Shallow Foundations

1. Strip footings:

   • Use: For load-bearing walls
   • Design: Continuous reinforced concrete footing under walls
   • Advantages: Simple to construct, economical for light loads
   • Considerations:
     • Width based on wall load and soil bearing capacity
     • Typically 2-3 times wider than the wall above

2. Isolated footings:

   • Use: For individual columns
   • Design: Square or rectangular pad of reinforced concrete
   • Advantages: Efficient use of materials, easy to construct
   • Considerations:
     • Size based on column load and soil bearing capacity
     • May require tie beams in seismic zones

3. Raft foundations:

   • Use: For poor soil conditions or heavy loads
   • Design: Reinforced concrete slab under entire building footprint
   • Advantages: Distributes load evenly, reduces differential settlement
   • Considerations:
     • Requires more concrete than other shallow foundations
     • May incorporate ground beams for added stiffness

2.2 Deep Foundations

1. Pile foundations:

   • Use: For weak upper soil layers
   • Types: a. Driven piles: Precast concrete, steel, or timber driven into the ground b. Bored piles: Cast-in-place concrete piles c. Screw piles: Steel shafts with helical plates
   • Advantages: Can reach deep bearing layers, resist uplift forces
   • Considerations:
     • Choose type based on soil conditions, load requirements, and site constraints
     • Consider noise and vibration during installation for driven piles

2. Caisson foundations:

   • Use: For very heavy loads or deep bedrock
   • Types: a. Drilled shafts: Large diameter bored piles b. Pneumatic caissons: Pressurized chambers for underwater construction
   • Advantages: Can handle extremely high loads, suitable for difficult soil conditions
   • Considerations:
     • Require specialized equipment and expertise
     • Often used for bridges, high-rise buildings, and marine structures

3. Combined foundations:

   • Use: For varying soil conditions or complex structural requirements
   • Examples: a. Piled raft: Combines raft foundation with piles for added support b. Pile groups with caps: Multiple piles connected by a reinforced concrete cap
   • Advantages: Combines benefits of different foundation types
   • Considerations:
     • More complex design and construction
     • Allows for optimization of foundation performance and cost

When selecting a foundation type, consider:

• Soil conditions and bearing capacity
• Structural loads (vertical, lateral, and uplift)
• Site constraints and accessibility
• Local construction practices and available equipment
• Cost and schedule implications
• Long-term performance and maintenance requirements

3. Materials and Techniques

3.1 Formwork

• Use water-resistant plywood or steel forms
• Ensure proper bracing to prevent displacement during concrete pouring
• Apply form release agent for easy removal

3.2 Concrete Mix

• Use sulfate-resistant cement for coastal areas
• Water-cement ratio: 0.45-0.50
• Include waterproofing admixtures for improved durability

3.3 Reinforcement

• Minimum cover for reinforcement: 75mm for foundations exposed to earth
• Use epoxy-coated or galvanized rebar in highly corrosive environments

3.4 Insulation

• Extruded polystyrene (XPS) boards: Place on the exterior of foundation walls
• Thickness: 50-100mm depending on thermal requirements

3.5 Waterproofing

• Apply bituminous waterproofing membrane on exterior foundation walls
• Install perforated drainage pipes around the foundation perimeter

4. Foundations for Limestone Soil

4.1 Characteristics of Limestone Soil

• Highly permeable
• Potential for sinkholes and cavities
• Variable load-bearing capacity

4.2 Site Investigation

• Conduct thorough geotechnical surveys, including:
  • Ground Penetrating Radar (GPR)
  • Electrical Resistivity Imaging (ERI)
  • Core drilling and sampling

4.3 Foundation Design for Limestone

• Avoid shallow foundations if possible
• Prefer deep foundations that transfer loads to competent bedrock
• Consider ground improvement techniques:
  • Grouting to fill voids
  • Soil mixing to improve bearing capacity

4.4 Specific Foundation Types for Limestone

• Pile foundations:
  • Use driven piles with caution due to potential for sudden drops
  • Prefer bored piles or continuous flight auger (CFA) piles
• Mat foundations:
  • Use for moderate loads and where bedrock is within reasonable depth
  • Incorporate grade beams to distribute loads evenly

4.5 Additional Precautions

• Design for potential sinkhole formation:
  • Incorporate structural slabs that can span potential voids
  • Use geotextiles to prevent soil migration into cavities
• Implement robust drainage systems to prevent water accumulation
• Monitor foundation performance regularly for signs of settlement or sinkholes

5. Quality Control and Inspection

• Conduct regular soil tests during excavation
• Perform slump tests and concrete cylinder tests for each pour
• Inspect reinforcement placement before concrete pouring
• Conduct post-construction integrity tests (e.g., pile integrity testing)

Remember to adapt these guidelines to specific site conditions and local regulations. Always consult with a licensed structural engineer and geotechnical expert for final design and implementation.