INTD 306: Foundation Systems

Foundation Systems

• A foundation system supports the building's load, transfers it to solid ground, and prevents excessive settlement or differential settlement.
• It also repels water accumulation, anchors the building to the ground, and keeps the basement dry and healthy.

Soil Classification

• Soil is classified into four groups: Sands and gravels, clays, silts, and solid rock, which has the highest bearing capacity.
• Soil type and bearing capacity (Kg/cm2) are determined through subsurface exploration and testing.

Subsurface Exploration Methods

• Borings and test pits are common exploration methods.
• Core borings involve removing soil samples at regular intervals, recording the type of material, depth, moisture content, and density.
• A minimum of four borings are taken, one near each corner of the proposed building.

Soil Treatment

• Drainage: Sloping the ground away from the building and installing a footing drain to minimize water-related problems.
• Fill: Removing unsuitable soil and replacing it with compacted new material (soil, sand, gravel, or other material).
• Compaction: Compacting existing soil to provide a suitable base for construction.

Concrete Joints

• Settlement joints (2 cm thick, filled with elastic materials) are used when: • Parts of a building are not equal in terms of dead load (D.L) and live load (L.L). • Soil is not homogeneous. • Site level changes as a building will be constructed on different foundation levels.
• Expansion joints are used for large construction areas, weather conditions, concrete equipment issues, and limited manpower.
• Concrete casting joints are used for the same reasons as expansion joints but have the disadvantage of creating weak points in the structure.

Location and Excavation

• Before excavating, determine subsoil conditions through test borings.
• Footings should extend through to undisturbed soil.
• Use a batter board to locate and retain the building's outline.
• Check local codes for minimum setback (building regulations).
• Mark the location of the outer walls of a building and assure square corners.

Plinth

• A plinth is a basic building element that distributes the load of columns evenly over the foundation.
• The plinth height is between 300mm to 450mm from ground level.
• A damp proof membrane (polyethylene sheet) prevents moisture transmission and concrete from gaining moisture through capillary action.

Foundation Systems Classification

• Foundations can be categorized into two broad divisions:
  1. Shallow Foundation/Spread footings (< 1.0 m)
  2. Deep Foundation/ Pile or caisson (> 2.0 m)

Shallow Foundation/Spread Footings

• A. Wall footing/strip: placed under a continuous foundation wall supporting a bearing wall.
• B. Isolated Pad/Independent: supports one column.
• C. Combined footings: support two or more columns in situations where columns are spaced close together.
• D. Mat or raft: used when soil bearing is low or loads are heavy in relation to soil pressures.

Deep Foundation/Pile or Caisson

• Used when soil is unsuitable for spread footings.
• Piles transmit building loads through unsuitable soil to more secure soil.
• Drilled piers are formed by drilling out a hole and filling it with concrete.
• Piles are usually placed in groups or in a line under a bearing wall.
• Pile foundation is used where good soil is at a higher depth (10 or 15 m) or soil has low bearing capacity.

Excavation Steps

• Step 1: Excavation involves removing earth to prepare the site for footings, foundations, and pipes for site drainage.
• Step 2: Forms are constructed on undisturbed soil at the bottom of the excavation.
• Rebar (reinforcing steel rods) is embedded into concrete.