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Foundations & Retaining Structures

Overview


Foundations are structural elements that connect a structure to the ground that supports it and are typically composed of concrete, steel, and wood. Foundations can generally be classified into two broad categories: shallow foundations and deep foundations.

Shallow foundations transfer the structural loads to near-surface soils and are typically used for small to medium-sized buildings when near-surface soil conditions are adequate. Shallow foundations are used occasionally for some large structures when the near-surface soil conditions are favorable or in the presence of shallow bedrock. Due to their cost and ease of construction, shallow foundations are far more prevalent than deep foundations. Examples of shallow foundations include spread footings and mat foundations.

Deep foundations transfer some or all of the load to deeper soils and are considerably more expensive and complex than shallow foundation systems. Because the structural quality of soil usually improves with depth, deep foundations generally possess higher structural load capacities relative to shallow foundations. Hence, deep foundations typically are used when adverse near-surface soil conditions (i.e., soft clays, collapsible soils, and/or expansive soils) are encountered and structural load requirements are sufficiently large to warrant deep foundations, or when high lateral load capacity is needed. Examples of deep foundations include piles, drilled shafts, and caissons.

Retaining structures are engineered to retain soil and/or rock from an area, building, or structure and are commonly used to accommodate changes in grade and provide a right of way. There are several types of retaining structures, including gravity, cantilever, sheet pile, and anchored earth and mechanically stabilized earth (reinforced earth) walls. Gravity retaining walls use their weight to resist pressures and are usually made from heavy materials such as concrete. Sheet pile walls typically consist of steel sheet piles that are driven into the ground to support pressures. Cantilever walls are often shaped like an inverted T and are typically made from a thin stem of steel-reinforced, cast-in-place concrete. The pressures from the retained soils or rock are carried through the stem to the structural footing (bottom of the T), are are transferred to the soils below and in front of the wall.

Anchored earth structures have facing units tied to rods or strips that have their ends anchored into the ground. The facing units come in many different sizes, shapes, and materials. The end anchors also use various construction methods. The resistance of the ties to movement is controlled by the end anchors. Soil nailing and tiebacks are some common types of anchored earth. Mechanically stabilized earth (MSE) walls are a relatively new method to design and construct retaining walls, and are also referred to as reinforced earth and reinforced soil. Reinforcing strips are embedded fully into the soil mass along their full length to resist movement, and the outer facing is connected to a facing unit or constructed by wrapping the reinforcement at the face. More recently, segmental retaining walls (SRWs) have dominated retaining-wall design because of the reduced cost and improved efficiency of construction. Designers of SRWs continue to push the design envelope in terms of wall height, length of the reinforced zone, and quality of backfill soils.

Exponent has extensive experience in evaluating the performance of shallow and deep foundation systems and numerous types of retaining structures. Because our soil engineers and geologists can draw on our in-house expertise in the areas of structural, materials, and mechanical engineering, Exponent can provide a multi-disciplinary approach that will solve any foundation or retaining-wall problem.

Our services include: 

  • Emergency response in the event of a failure of a foundation or retaining structure 
  • Analysis of the cause(s) of foundation or retaining-wall failure, settlement, lateral movement, or cracking 
  • Analysis of the nature and extent of damage to real property from failure of a foundation or retaining structure 
  • Evaluation of differing site condition claims during construction of deep foundations and retaining walls 
  • Cost analysis, delay impacts, and acceleration evaluation for construction projects involving deep foundations and/or retaining structures 
  • Analysis and field evaluation of foundation or retaining-wall performance during earthquakes 
  • Evaluation of field test data (e.g., vertical and lateral load tests of deep foundation elements, pile driving records, pile test data) 
  • Field investigation of foundations and retaining structures, including: 
    • Determining the as-built condition of the foundation or retaining structure    
    • Geologic mapping
    • Documentation of the condition of buildings and structures affected by foundation or retaining-structure failure
    • Collection of subsurface data by excavation and logging of test pits, small-diameter borings and rock cores, and downhole logging of large-diameter borings
    • In-situ subsurface testing
    • Installation and monitoring of instrumentation to measure ground movement and groundwater level
    • In-house geotechnical laboratory testing for measuring rock and soil engineering properties
  • Analysis of the interaction of landslides with retaining walls
  • Development of repair recommendations