GSP 131 Contemporary Issues in Foundation Engineering
Step 2: Review All Available Project Information and Geotechnical Data
Such data are well known to all foundation engineers and represent the same scope as for micropiles. For guidance regarding the planning and execution of subsurface exploration programs (AASHTO (1988), FHWA/NHI (2001), and FHWA (2002) documents apply. Particular care should be paid to studying the project-specific factors, e.g., liquefiable zones, expansive/dispersive soil deposits, and hazardous materials. Minimum guidelines for boring spacings are provided – as a first step – in Table 2.
Table 2. Guidelines for Minimum Number of Investigation Points and Depth of Investigation (Modified after FHWA-IF-02-034, 2002).
Deep Foundations (Micropiles for Structural Support)
MINIMUM NUMBER OF INVESTIGATION POINTS AND LOCATION OF INVESTIGATION POINTS For substructure (e.g., bridge piers or abutments) widths less than or equal to 100 feet, a minimum of one investigation point per substructure. For substructure widths greater than 100 feet, a minimum of two investigation points per substructure. Additional investigation points should be provided if erratic subsurface conditions are encountered.
MINIMUM DEPTH OF INVESTIGATION
In soil, depth of investigation should extend below the anticipated micropile tip elevation a minimum of 20 feet, or a minimum of two times the maximum micropile group dimension, whichever is deeper. All borings should extend through unsuitable strata such as unconsolidated fill, peat, highly organic materials, soft fine-grained soils, and loose coarse-grained soils to reach hard or dense materials.
For micropiles bearing on rock, a minimum of 10 feet of rock core shall be obtained at each investigation point location to verify that the boring has not terminated on a boulder.
For micropiles supported on or extending into rock, a minimum of 10 feet of rock core, or a length of rock core equal to at least three times the micropile diameter for isolated micropiles or two times the maximum micropile group dimension, whichever is greater, shall be extended below the anticipated micropile tip elevation to determine the physical characteristics of rock within the zone of foundation influence.
Larger boring spacings (e.g., 30 m grid or closer where problem soils/geology are encountered) and shorter lengths of rock core may be acceptable for private building work.
Step 3: Develop Applicable Loading Combinations
These are to be provided by the Structural Engineer as part of a design criteria package. Micropiles are typically not considered for applications involving large
lateral loads and
so the critical loading cases
loading condition is that will commonly
usually for axial compressive be considered include uplift,
seismic loads, and load imposed by downdrag.