A.5 Surveys and Geotechnical Reports
Site surveys are generally prepared for GSA projects involving sitework. The survey may be contracted separately by GSA or may be included in the scope of the A/E for the project. The guidelines given here apply in either case. In cases where GSA contracts for the survey directly, the A/E may be requested to review the scope of work for the survey and recommend modifications to the technical requirements to suit the specific project site.
The criteria listed here are not absolute; they should be modified by the civil engineer to suit the particular conditions of the project. All surveys should be prepared and sealed by a surveyor licensed in the state where the project is located.
General Requirements. Surveys should generally contain the following information:
- Locations of all permanent features within limits of work, such as buildings, structures, fences, walls, concrete slabs and foundations, above-ground tanks, cooling towers, transformers, sidewalks, steps, power and light poles, traffic control devices, manholes, fire hydrants, valves, culverts, headwalls, catch basins or inlets, property corner markers, benchmarks, etc.
- Location of all adjacent and abounding roads or streets and street curbs within limits of work, including driveways and entrances. Type of surfacing and limits should be shown. For public streets, right-of-way widths and centerlines should also be shown.
- Location of all trees, shrubs, and other plants within limits of work. For trees, caliper size should be shown; dead trees should be indicated.
- Location of all overhead telephone and power lines within the limits of work and their related easements.
- Based on existing records, location of underground utilities, such as gas, water, steam, chilled water, electric power, sanitary, storm, combined sewers, telephone, etc. should be shown. Sizes of pipes (I.D.), invert elevations, inlet or manhole rim elevations should be indicated. Where appropriate, information should be verified in the field.
- Based on existing records, location of underground storage tanks or other subsurface structures.
- Topography field criteria should include such items as 300 millimeter or 600 millimeter (1 to 2 foot) contour intervals plotted on a grid system appropriate to the scale of the survey; elevations at top and bottom of ditches and at any abrupt changes in grade; periodic top-of-curb and gutter elevations, as well as street centerline elevations; elevations at all permanent features within the limits of work; ground floor elevations for all existing buildings.
- Bearings and distances for all property lines within the limits of work.
- Official datum upon which elevations are based and the benchmark on or adjacent to the site to be used as a starting point.
- Official datum upon which horizontal control points are based.
- If there are not already two benchmarks on the site, establish two permanent benchmarks.
- Elevations of key datum points of all building structures and improvements directly adjacent and across the street from the project site during both wet and dry season.
- Delineate location of any wetlands or floodplains, underground streams or water sources.
Geotechnical Investigation and Engineering Report
On most GSA projects geotechnical investigations will take place at three separate stages: during site selection, during building design, and during construction. The requirements for geotechnical work during site selection and during construction are described in other GSA documents. The requirements for geotechnical work for the building design are defined here. They apply whether GSA contracts for geotechnical work separately or includes the geotechnical investigation in the scope of the A/E services.
Purpose. The purpose of the geotechnical investigation during building design is to determine the character and physical properties of soil deposits and evaluate their potential as foundations for the structure or as material for earthwork construction. The type of structure to be built and anticipated geologic and field conditions have a significant bearing on the type of investigation to be conducted.
The investigation must therefore be planned with a knowledge of the intended project size and anticipated column loads, land utilization and a broad knowledge of the geological history of the area.
The guidelines given here are not to be considered as rigid. Planning of the exploration, sampling and testing programs and close supervision must be vested in a competent geotechnical engineer and/or engineering geologist with experience in this type of work and licensed to practice engineering in the jurisdiction where the project is located.
Analysis of Existing Conditions. The report should address the following:
- Description of terrain.
- Brief geological history.
- Brief seismic history.
- Surface drainage conditions.
- Groundwater conditions and associated design or construction problems.
- Description of exploration and sampling methods and outline of testing methods.
- Narrative of soil identification and classification, by stratum.
- Narrative of difficulties and/or obstructions encountered during previous explorations of existing construction on or adjacent to the site.
- Description of laboratory test borings and results.
- Plot plan, drawn to scale, showing test borings or pits.
- Radon tests in areas of building location.
- Soils resistivity test, identifying resistivity of soil for corrosion protection of underground metals and electrical grounding design.
- Boring logs, which identify:
Sample number and sampling method. Other pertinent data deemed necessary by the geotechnical engineer for design recommendations, such as:
– Unconfined compressive strength.
– Standard penetration test values.
– Subgrade modulus.
– Location of water table.
– Water tests for condition of groundwater.
– Location and classification of rock.
– Location of obstructions.
– Atterberg tests.
– Compaction tests.
– Consolidation tests.
– Triaxial compression test.
– Chemical test (pH) of the soil.
Engineering Recommendations. Engineering recommendations based on borings and laboratory testing should be provided for the following:
Recommendations for foundation design, with discussion of alternate solutions, if applicable, including:
- Allowable soil bearing values.
- Feasible deep foundation types and allowable capacities, where applicable, including allowable tension (pull-out) and lateral subgrade modulus.
- Feasibility of slab on grade versus structurally supported floor construction, including recommended bearing capacities and recommended subgrade modulus (k).
- Discussion of evidence of expansive soils and recommended solutions.
- Lateral earth design pressures on retaining walls or basement walls, including dynamic pressures.
- Design frost depth, if applicable.
- Removal or treatment of contaminated soil.
- Discussion of potential for consolidation and/or differential settlements of substrata, with design recommendations for total settlement and maximum angular distortion.
- Use and treatment of in-situ materials for use as engineered fill.
- Recommendations for future sampling and testing.
- Recommendations for pavement designs, including base and sub-base thickness and subdrains.
- Recommendations for foundation and subdrainage, including appropriate details.
- Discussion of soil resistivity values.
- Discussion of radon values and recommendation for mitigating measures, if required.
Geologic Hazard Report
A geologic hazard report shall be prepared for all new building construction in Regions of Low, Moderate and High seismicity, except for structures located in regions of Low seismicity designed to the Life Safety Performance Level. Geologic hazard reports are not required for minor or relatively unimportant facilities for which earthquake damage would not pose a significant risk to either life or property.
Required Investigation. When required by the project scope, a geologic hazard investigation which addresses the hazards indicated below should be performed.Whenever possible, a preliminary investigation should be performed in the planning stage of siting a facility, to provide reasonable assurance that geologic hazards do not preclude construction at a site. During a later stage of geotechnical investigations for a facility at a selected site, supplemental investigations may be conducted as needed to define the geologic hazards in more detail and/or develop mitigating measures. The scope and complexity of a geologic hazard investigation depends on the economics of the project and the level of acceptable risk. In general, major new building complexes, high-rise buildings, and other high value or critical facilities shall have thorough geologic hazard investigations. Small, isolated buildings need not have elaborate investigations.
Surface Fault Rupture. For purposes of new building construction, a fault is considered to be an active fault and a potential location of surface rupture if the fault exhibits any of the following characteristics:
- Has had documented historical macroseismic events or is associated with a well-defined pattern of microseismicity.
- Is associated with well-defined geomorphic features suggestive of recent faulting.
- Has experienced surface rupture (including fault creep) during approximately the past 10,000 years (Holocene time).
Fault investigations shall be directed at locating any existing faults traversing the site and determining the recency of their activity. If an active fault is found to exist at a site and the construction cannot reasonably be located elsewhere, investigations shall be conducted to evaluate the appropriate set-back distance from the fault and/or design values for displacements associated with surface fault rupture.
Soil Liquefaction. Recently deposited (geologically) and relatively unconsolidated soils and artificial fills without significant cohesion and located below the water table, are susceptible to liquefaction. Sands and silty sands are particularly susceptible. Potential consequences of liquefaction include foundation bearing capacity failure, differential settlement, lateral spreading and flow sliding, flotation of lightweight embedded structures, and increased lateral pressures on retaining walls. The investigation shall consider these consequences in determining the size of the area and the depth below the surface to be studied. An investigation for liquefaction may take many forms. One acceptable method is to use blow count data from the standard penetration test conducted in soil borings. This method is described in publications by H. B. Seed and I. M. Idriss, (1982), Ground Motions and Soil Liquefaction During Earthquakes: Earthquake Engineering Research Institute, Oakland, CA, Monograph Series, 134 p. and H.B. Seed et al, (1985) “The Influence of SPT Procedures in Soil Liquefaction Resistance Evaluations”: Journal of Geotechnical Engineering, ASCE 111(12): pp. 1425-1445.
Landsliding. New construction shall not be sited where it may be within a zone of seismically induced slope failure or located below a slope whose failure may send soil and debris into the structure. Factors which affect slope stability include slope angle, soil type, bedding, ground water conditions, and evidence of past instability. The geologic hazard investigation shall address the potential for seismically induced slope deformations large enough to adversely affect the structure.
Differential Settlement. Loosely compacted soils either above or below the water table can consolidate during earthquake shaking, producing surface settlement. The potential for total and differential settlements beneath a structure shall be assessed. If liquefaction is not expected to occur, then in most cases, differential settlement would not pose a significant problem to construction.
Flooding. Earthquake-inducing flooding can be caused by tsunamis, seiches, and dam and levee failures. The possibility of flooding shall be addressed for new construction located near bodies of water.
Duration of Strong Ground Shaking. Estimates of the duration of strong ground shaking at a site are defined by earthquake magnitude and shall be used to assess geologic hazards such as liquefaction and slope failure. Strong motion duration is strongly dependent on earthquake magnitude.
Estimates of the duration of strong ground shaking shall be based on the assumption of the occurrence of a maximum considered earthquake generally accepted by the engineering and geologic community as appropriate to the region and to the subsurface conditions at the site.
Mitigative Measures. A site found to have one or more geologic hazards may be used, provided the hazards are removed, abated, or otherwise mitigated in the design, or if the risk is judged to be acceptable. Examples of mitigative measures include: removal and recompaction of poorly compacted soils; use of special foundations; stabilizing slopes; and draining, compaction, or chemical treatment of liquefiable soils. The geological hazard report shall identify feasible mitigative measures.
Required Documentation. Investigations of geologic hazards shall be documented. As noted in the paragraph entitled “Required Investigation” above, a preliminary geologic hazard investigation shall be conducted and a report issued during the siting phase for a facility. However, unless the geologic hazard investigations have been documented in a stand-alone report, they shall be addressed in a section of the geotechnical engineering report prepared during the design phase of a project. The geologic hazard report, whether it is a separate report or a section of the geotechnical engineering report, shall as a minimum contain the following:
- List of hazards investigated, which must include the five described earlier in this section.
- Description of the methods used to evaluate the site for each hazard.
- Results of any investigations, borings, etc.
- Summary of findings.
- Recommendations for hazard mitigation, if required.
In some cases, estimates of site ground motions may be needed for assessment of geologic hazards such as liquefaction and slope failure.