SCA 08 Deep Foundations

Deep Foundations

The lecture explains how deep foundations transfer building loads through weak or unsuitable near-surface soil to deeper ground that can provide adequate support. It emphasizes that foundations are part of the building’s structural and architectural design—not a separate system added after the layout is settled.

1. What Foundations Do

• The superstructure and substructure form a continuous structural system. Loads from columns, walls, cores, floors, and building use ultimately reach the ground through the foundation.
• Foundations must do more than support gravity loads. They may also need to resist:
  • Sliding and overturning caused by lateral forces such as wind or earthquakes.
  • Uplift or buoyancy from groundwater.
  • Lateral earth pressure against underground walls.
• Footings spread loads over a larger area, reducing pressure on the soil. The basic relationship is pressure = force ÷ area.
• Soil is part of the structural system. Its bearing capacity, compressibility, layering, and groundwater conditions affect foundation performance. Even soil that can carry a load may compress enough to cause unacceptable settlement.

2. Loads and Soil Conditions to Consider

The lecture identifies several kinds of loading and ground response:

• Vertical loads: Building self-weight and occupancy loads travel from slabs through beams, walls, and columns to the foundation. Each significant load needs a credible path to the ground.
• Lateral loads: Foundations resist sideways movement through friction at the foundation–soil interface and passive pressure from surrounding soil. These resistances depend on ground conditions.
• Groundwater effects: Water can press against basement walls and push upward beneath basement slabs. Buoyancy may be particularly important during construction, before the completed building’s weight is present. Piles or ground anchors may be used to resist uplift.
• Settlement: Acceptable bearing capacity alone does not guarantee acceptable performance; soil deformation and settlement also matter.

3. Foundation Selection and Design Process

Before choosing a foundation, the design team needs information about the site and the building. The lecture outlines these considerations:

1. Investigate the ground
   • Identify soil types and layers.
   • Determine the depth and extent of stronger bearing layers.
   • Establish groundwater conditions and how they may vary seasonally.
2. Establish building loads
   • Consider the building’s weight, use, and load distribution, including concentrated loads from columns and structural cores.
3. Select a suitable foundation system
   • Compare the loads with soil capacity and settlement limits.
   • Consider groundwater, topography, neighboring properties, applicable codes, construction methods, and construction risks.
4. Develop the structural design
   • Specify dimensions, reinforcement, materials, and other structural requirements in coordination with engineering specialists.

Architects do not necessarily calculate every pile, but their plans and design decisions influence where loads concentrate and what foundation arrangements are feasible.

4. Shallow Versus Deep Foundations

• Shallow foundations are appropriate when competent soil lies relatively close to the surface. They transfer loads directly to the supporting soil, often through footings.
• Deep foundations are used when near-surface soil is weak, unstable, too compressible, or otherwise unsuitable. They transfer loads to deeper soil or rock.
• Deep foundations may be appropriate when:
  • A shallow footing would need to be impractically large.
  • Settlement would be excessive.
  • Additional resistance to lateral loads or uplift is needed.
  • Excavation space, access, or nearby buildings constrain construction.

A pile does not always need to reach bedrock; it needs a verified load-transfer mechanism that satisfies bearing and settlement requirements.

5. How Piles Transfer Loads

Piles generally develop resistance in two ways:

• End or base resistance: The pile transfers load through its tip into the soil or rock below.
• Shaft resistance: The pile transfers load through interaction and friction along its sides.

Piles are often grouped beneath a pile cap, which distributes loads from a column or grade beam to several piles. Closely spaced piles can interact through the surrounding soil, so group behavior and settlement must be considered—not just the capacity of each individual pile.

Pile systems may use different materials and configurations, including timber, steel sections, concrete-filled pipes, precast reinforced concrete, and other forms. The lecture also refers to friction piles, end-bearing piles, cased and uncased piles, pedestal piles, and micropiles.

6. Construction Methods and Site Constraints

• Driven piles: Prefabricated piles are driven into the ground. This avoids drilling spoil, but installation can create noise, vibration, and ground displacement that may affect neighboring buildings.
• Bored piles: Holes are drilled and then filled with concrete, often with reinforcement. They can be preferable where vibration needs to be limited, but require drilling equipment and spoil handling; casing or drilling fluid may be needed to keep holes stable.
• Pile size: “Mini” and “maxi” are not fixed universal engineering categories. Relevant factors include pile diameter, length, shape, material, capacity, access for equipment, and effects on surrounding buildings.

The choice between methods depends on site conditions and construction impacts, not simply on which pile is stronger.

7. Caissons and Drilled Shafts

The lecture distinguishes between different uses of the term caisson:

• In one context, a caisson or pondasi sumuran is a shaft excavated and built within, sometimes using stone and concrete. It may be economical where materials are available and heavy equipment is impractical, but can consume substantial concrete, be difficult to quality-control, and be challenging in muddy ground or under lateral loading.
• In another context, “caisson” refers to larger engineered elements such as drilled piers, which may be mechanically excavated, temporarily supported, reinforced, and concreted.

The key lesson is to understand the actual construction method and load-resisting mechanism rather than relying on the name alone.

8. Foundations, Building Layout, and Basements

• Column and core layouts shape how loads reach the foundation. A heavily loaded central core, for example, may require a different shaft, raft, or pile arrangement than a more evenly distributed column layout.
• Foundation decisions should therefore be coordinated with structural planning from the outset.
• A basement is not merely underground floor space. It functions as a structural box that must respond to building loads, soil pressure, and groundwater.
• Basement design and construction require coordination of:
  • Excavation sequence and support of surrounding soil.
  • Groundwater control and dewatering.
  • Retaining walls, foundations, columns, beams, and floor slabs.
  • Equipment access, basement footprint, floor levels, services, and neighboring buildings.
• A raft foundation provides a continuous base rather than separate isolated footings. It may act as a shallow foundation or be combined with piles as a piled raft. Its thickness and position must be coordinated with columns, walls, piles, basement levels, and services.
• Retaining-wall types described include:
  • Gravity walls, which resist soil pressure mainly through their own weight.
  • Cantilever walls, which use a reinforced-concrete stem and base slab.
  • Counterfort walls, which use triangular supports to reduce bending and improve stability.
• Deeper urban excavations may use other systems, such as diaphragm walls, secant pile walls, bracing, or anchors.

The lecturer cites the collapse of the basement at Siloam Hospital in Surabaya as an example of basement-construction complexity.

9. Class Exercise

Students are asked to choose one of two proposed sites for a 20-story building with two basement levels and prepare an annotated section or sketch showing a plausible deep-foundation strategy.

• Coastal site: Flat terrain, deep soft clay, high groundwater, possible seawater exposure, highly compressible soil, and nearby buildings.
• Sloping site: Steep terrain, thick loose soil, variable soil depth, competent rock at greater depth, and groundwater seepage along the slope.

The sketch should show:

• How building loads could be transferred into the ground.
• Which concentrated loads or structural locations need particular attention.
• How the site conditions affect foundation and basement construction.
• Architectural or construction decisions influenced by the site.

Speakers and Sources Featured

• Speaker: One lecturer or instructor, unnamed in the subtitles.
• Other identifiable speakers: None.
• Sources or examples mentioned: Diagrams and illustrations are referred to during the lecture, but no publication or external source is identified. The lecturer mentions the Siloam Hospital basement-collapse example.