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Geotechnical Engineering

 

Checklist and Guidelines for Review of Geotechnical
Reports and Preliminary Plans and Specifications


Table 3 - Required Geotechnical Engineering Analysis

Soil Classification Embankment and Cut Slopes Structure Foundations
(Bridges and Retaining Structures)
Retaining Structures
(Conventional, Crib and MSE)
Unified AASHTO1 Soil Type Slope Stability2 Analysis Settlement Analysis Bearing Capacity Analysis Settlement Analysis Lateral Earth Pressure Stability Analysis
GW
A-1-a GRAVEL
Well-graded

Generally not required if cut or fill slope is 1.5H to 1V or flatter, and underdrains are used to draw down the water table in a cut slope.

Erosion of slopes may be a problem for SW or SM soils.

Generally not required except possibly for SC soils. Required for spread footings, pile or drilled shaft foundations.

Spread footings generally adequate except possibly for SC soils

Generally not needed except for SC soils or for large, heavy structures.

Empirical correlations with SPT values usually used to estimate settlement

GW, SP, SW & SP soils generally suitable for backfill behind or in retaining or reinforced soil walls.

GM, GC, SM & SC soils generally suitable if have less than 15% fines.

Lateral earth pressure analysis required using soil angle of internal friction.

All walls should be designed to provide minimum F.S. = 2 against overturning & F.S. = 1.5 against sliding along base.

External slope stability considerations same as previously given for cut slopes & embankments.

GP
A-1-a GRAVEL
Poorly-graded
GM
A-1-b GRAVEL
Silty
GC
A-2-6A-2-7 GRAVEL
Clayey
SW
A-1-b SAND
Well-graded
SP
A-3 SAND
Poorly-graded
SM
A-2-4A-2-5 SAND
Silty
SC
A-2-6A-2-7 SAND
Clayey
ML
A-4 SILT
Inorganic silt
Sandy
Required unless non-plastic.

Erosion of slopes may be a problem.

Required unless non-plastic. Required.

Spread footing generally adequate.

Required.

Can use SPT values if non-plastic.

These soils are not recommended for use directly behind or in retaining or reinforced soil walls.
CL
A-6 CLAY
Inorganic
Lean Clay
Required Required    
OL
A-4 SILT
Organic
Required Required    
MH
A-5 SILT
Inorganic
Required.

Erosion of slopes may be a problem.

Required. Required.

Deep foundation generally required unless soil has been preloaded.

Required.

Consolidation test data needed to estimate settlement amount and time.

These soils are not recommended for use directly behind or in retaining walls. All walls should be designed to provide minimum F.S. = 2 against overturning & F.S. = 1.5 against sliding along base.

External slope stability considerations same as previously given for cut slopes & embankments

CH
A-7 CLAY
Inorganic
Fat Clay
Required. Required.
OH
A-7 CLAY
Organic
Required. Required.
PT
---- PEAT
Muck
Required. Required.

Long term settlement can be significant

Deep foundation required unless peat excavated and replaced. Highly compressible and not suitable for foundation support
Rock
    Fills - not required for slopes 1.5H to 1V or flatter.

Cuts - required but depends on spacing, orientation and strength of discontinuities and durability of rock

Required for spread footings or drilled shafts.

Empirically related to RQD3

Required where rock is badly weathered or closely fractured (low RQD).

May require in situ test such as pressuremeter.

Required.

Use rock backfill angle of internal friction.

Remarks:

Soils - temporary ground water control may be needed for foundation excavations in GW through SM soils.

Backfill specifications for reinforced soil walls using metal reinforcements should meet the following requirements in insure use of non-corrosive backfill:

pH range = 5 to 10; Resistivity > 3000 ohm-cm; Chlorides < 100 ppm; Sulfates < 200 ppm; Organic content 1% maximum

Rock - Durability of shales (siltstone, claystone, mudstone, etc.) to be used in fills should be checked. Non-durable shales should be embanked as soils, i.e., placed in maximum 0.3 m (1 ft) loose lifts and compacted with heavy sheepsfoot or grid rollers.

1 This is an approximate correlation to Unified (Unified Soil Classification system is preferred for geotechnical engineering usage, AASHTO system was developed for rating pavement subgrades).

2 These are general guidelines, detailed slope stability analysis may not be required where past experience in area is similar or rock gives required slope angles.

3 RQD (Rock Quality Designation) = sum of pieces of rock core 4" or greater in length divided by the total length of core run.

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Updated: 04/07/2011

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