Section 3106F — California Building Code (Title 24, Part 2)

GEOTECHNICAL HAZARDS AND FOUNDATIONS

3106F.1 General.¶

3106F.1.1 Purpose. This section provides minimum stan- dards for analyses and evaluation of geotechnical hazards and foundations under static and seismic conditions.

3106F.1.2 Applicability. The requirements provided herein apply to all new and existing MOTs.

3106F.1.3 Loading. The loading for geotechnical hazard assessment and foundation analyses under static and seis- mic conditions is provided in Sections 3103F and 3104F.

3106F.2 Site characterization.¶

Site characterization shall be based on site-specific geotechnical information. If existing information is used, the geotechnical engineer of record shall provide adequate justification.

3106F.2.1 Site classes. Each MOT shall be assigned at least one site class. Site Classes A, B, C, D and E are defined in Table 31F-6-1, and Site Class F is defined by any of the fol- lowing:

1. Soils vulnerable to significant potential loss of stiff- ness, strength and/or volume under seismic loading due to liquefiable soils, quick and highly sensitive clays and/or collapsible weakly cemented soils.

2. Adequate coverage of subsurface data, both hori- zontally and vertically, shall be obtained to develop geotechnical parameters.

3. Exploration shall be deep enough to characterize subsurface materials that are affected by embank- ment behavior and shall extend to depth of at least 20 feet below the deepest foundation depth.

4. During field exploration, particular attention shall be given to the presence of continuous low-strength layers or thin soil layers that could liquefy or weaken during the design earthquake shaking.

5. CPTs provide continuous subsurface profile and shall be used to complement exploratory borings. When CPTs are performed, at least one boring shall be performed next to one of the CPT soundings to check that the CPT-soil behavior type interpreta- tions are reasonable for the site. Any difference between CPT interpretation and subsurface condi- tion obtained from borings shall be reconciled.

6. Quantitative site soil stratigraphy is required to a depth of 100 feet for assigning a site class (see Table 31F-6-1).

7. Laboratory tests may be necessary to supplement the borings and insitu field tests.

2. Peats and/or highly organic clays, where the thick- ness of peat or highly organic clay exceeds 10 feet.

3. Very high plasticity clays with a plasticity index (PI) greater than 75, where the thickness of clay exceeds 25 feet.

4. Very thick soft/medium stiff clays with undrained shear strength less than 1,000 psf, where the thick- ness of clay exceeds 120 feet.

3106F.2.2 Site-specific information.

1. Site-specific investigations shall include adequate borings and/or cone penetration tests (CPTs) and other appropriate field methods, to enable the deter- mination of geotechnical parameters.

3106F.3 Seismic loads for geotechnical evaluations.¶

Section 3103F.4 defines the earthquake loads to be used for struc- tural and geotechnical evaluations in terms of design Peak Ground Accelerations (PGA), spectral accelerations and design earthquake magnitude. Values used for analyses are based on Probabilistic Seismic Hazard Analyses (PSHA) using two levels of seismic performance criteria (Section 3104F.2.1 and Table 31F-4-1).

3106F.4 Liquefaction potential.¶

The liquefaction potential of the soils in the immediate vicinity of or beneath each MOT, and associated slopes, embankments or rock dikes shall be evaluated for the PGAs associated with seismic performance Levels 1 and 2. Liquefaction potential evaluation should fol-

TABLE 31F-6-1 SITE CLASSES

SITE CLASS	SOIL PROFILE	AVERAGE VALUES FOR TOP 100 FEET OF SOIL PROFILE3
*SITE CLASS*	*SOIL PROFILE*	*Shear Wave Velocity,* *VS [ft/sec]*	*Standard Penetration Test,* *SPT [blows/ft]*	*Undrained Shear Strength,* *SU [psf]*
A	Hard Rock	> 5,000
B	Rock	2,500 to 5,000
C	Very Stiff/Very Dense Soil and Soft Rock	1,200 to 2,500	> 50	> 2,000
D	Soft/Dense Soil Profile	600 to 1,200	15 to 50	1,000 to 2,000
E1, 2	Soft/Loose Soil Profile	< 600	< 15	< 1,000
F	Defined in Section 3106F.2.1	Defined in Section 3106F.2.1	Defined in Section 3106F.2.1	Defined in Section 3106F.2.1

1. Site Class E also includes any soil profile with more than 10 feet of soft clay (defined as a soil with a plasticity index, PI > 20, water content > 40 percent and Su < 500 psf). 2. The plasticity index, P1, and the moisture content shall be determined in accordance with ASTM D4318 [6.1] and ASTM D2216 [6.2], respectively. 3. Conversion of CPT data to estimate equivalent Vs, SPT blow count, or Su is allowed.

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low the procedures outlined in NCEER report [6.3], SCEC

[6.4] and CGS Special Publication 117A [6.5].

If liquefaction is shown to be initiated in the above evalua- tions, the particular liquefiable strata and their thicknesses shall be clearly shown on site profiles. Resulting hazards associated with liquefaction shall be addressed including translational or rotational deformations of slopes or embank- ment systems and post liquefaction settlement of slopes or embankment systems and underlying foundation soils, as noted below. If such analyses indicate the potential for par- tial or gross (flow) failure of a slope or embankment, ade- quate evaluations shall be performed to confirm such a condition exists, together with analyses to evaluate potential slope displacements (lateral spreads). In these situations and for projects where more detailed numerical analyses are per- formed, a peer review (see Section 3101F.8.2) may be required.

3106F.5 Slope or embankment stability and seismically induced lateral spreading.¶

induced lateral spreading. Slope or embankment stability related to the MOT facility, shall be evaluated for static and seismic loading conditions.

3106F.5.1 Static slope stability. Static stability analysis using conventional limit equilibrium methods shall be per- formed for site related slope or embankment systems. Live load surcharge shall be considered in analyses based on project-specific information. The long-term static factor of safety of the slope or embankment shall not be less than 1.5.

3106F.5.2 Pseudo-static seismic slope stability. Pseudo- static seismic slope or embankment stability analyses shall be performed to estimate the horizontal yield acceleration for the slope for the Level 1 and Level 2 earthquakes. During the seismic event, appropriate live load surcharge shall be considered.

If liquefaction and/or strength loss of the site soils is likely, the following shall be used in the analyses, as appropriate:

1. Residual strength of liquefied soils

2. Strengths compatible with the pore-pressure gener- ation of potentially liquefiable soils

3. Potential strength reduction of clays

The residual strength of liquefied soils shall be esti- mated using guidelines outlined in SCEC [6.4] or other appropriate documents as noted in CGS Special Publica- tion 117A [6.5].

Pseudo-static analysis shall be performed without con- sidering the presence of the foundation system. Using a horizontal seismic coefficient of one-half of the PGA, if the estimated factor of safety is greater than or equal to 1.1, then no further evaluation of lateral spreading or kine- matic loading from lateral spreading is required. 3106F.5.3 Post-earthquake static slope stability. The static factor of safety immediately following a design earthquake event shall not be less than 1.1 when any of the following are used in static stability analysis:

1. Post-earthquake residual strength of liquefied soils

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2. Strengths compatible with the pore-pressure gener- ation of potentially liquefiable soils

3. Potential strength reduction of clays

3106F.5.4 Lateral spreading – Free field. The earth- quake–induced lateral deformations of the slope or embankment and associated foundations soils shall be determined for the Level 1 and Level 2 earthquakes using the associated PGA at the ground surface (not modified for liquefaction). If liquefaction and/or strength loss of the site soils is likely, the following shall be used in the analy- ses, as appropriate:

1. Residual strength of liquefied soils

2. Strengths compatible with the pore-pressure gener- ation of potentially liquefiable soils

3. Potential strength reduction of clays

The presence of the foundation system shall not be included in the “free field” evaluations.

Initial lateral spread estimates shall be made using the Newmark displacement approach documented in NCHRP Report 611 [6.6] or other appropriate but similar proce- dures.

3106F.6 Seismically¶

induced settlement. Seismically induced settlement shall be evaluated. Based on guidelines outlined in SCEC [6.4] or other appropriate documents such as CGS Special Publication 117A [6.5]. If seismically induced settlement is anticipated, the resulting design impacts shall be considered, including the potential develop- ment of downdrag loads on piles.

3106F.7 Earth pressures.¶

Both static and seismic earth pres- sures acting on MOT structures shall be evaluated.

3106F.7.1 Earth pressures under static loading. The effect of static active earth pressures on structures result- ing from static loading of backfill soils shall be considered where appropriate. Backfill sloping configuration, if applicable, and backland loading conditions shall be con- sidered in the evaluations. The loading considerations shall be based on project-specific information. The earth pressures under static loading should be based on guide- lines outlined in NAVFAC DM7-02 [6.7] or other appro- priate documents.

3106F.7.2 Earth pressures under seismic loading. The effect of earth pressures on structures resulting from seis- mic loading of backfill soils, including the effect of pore- water pressure build-up in the backfill, shall be consid- ered. The seismic coefficients used for this analysis shall be based on the Level 1 and Level 2 earthquake PGA val- ues.

Evaluation of earth pressures under seismic loading, should be based on NCHRP Report 611 [6.6] or other appropriate methods.

3106F.8 Pile axial behavior.¶

3106F.8.1 Axial pile capacity. Axial geotechnical capac- ity of piles under static loading shall be evaluated using guidelines for estimating axial pile capacities provided in POLB WDC [6.8] or other appropriate documents. A min-

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imum factor of safety of 2.0 shall be achieved on the ulti- mate capacity of the pile using appropriate MOT loading.

If liquefaction or seismically-induced settlement is anticipated, the ultimate axial geotechnical capacity of piles under seismic conditions shall be evaluated for the effects of liquefaction and/or downdrag forces on the pile. The ultimate geotechnical capacity of the pile during liq- uefaction shall be determined on the basis of the residual strength of the soil for those layers where the factor of safety for liquefaction is determined to be less than 1.0.

When seismically-induced settlements are predicted to occur during design earthquakes, the downdrag loads shall be computed, and the combination of downdrag load and static load determined. Only the tip resistance of the pile and the side friction resistance below the lowest layer contributing to the downdrag shall be used in the capacity evaluation. The ultimate axial geotechnical capacity of the pile shall not be less than the combination of the seismi- cally induced downdrag force and the maximum static load.

3106F.8.2 Axial springs for piles. The geotechnical analyst (see Section 3102F.3.4.8) shall coordinate with the structural analyst (see Section 3102F.3.4.4) and develop axial springs (T-z) for piles. The T-z springs may be developed either at the top or at the tip of the pile (see Figure 31F-6-1). If the springs are developed at the pile tip, the tip shall include both the friction resistance along the pile (i.e., side springs [t-z]) and tip resistance at the pile tip (i.e. tip springs [q-w]), as illus- trated in Figure 31F-6-1. If T-z springs are developed at the pile top, the appropriate elastic shortening of the pile shall be included in the springs. Linear or nonlinear springs may be developed if requested by the structural analyst.

Due to the uncertainties associated with the develop- ment of axial springs, such as the axial soil capacities, load distributions along the piles and simplified spring stiffnesses, both upper-bound and lower-bound limits shall be estimated and utilized in the analyses.

FIGURE 31F-6-1 AXIAL SOIL SPRINGS [6.8]

3106F.9 Soil springs for lateral pile loading.¶

For design of piles under loading associated with the inertial response of the superstructure, level-ground inelastic lateral springs (p-y) shall be developed. The lateral springs within the shallow por- tion of the piles (generally within 10 pile diameters below the ground surface) tend to dominate the inertial behavior. Geo- technical parameters for developing lateral soil springs shall follow guidelines provided in API RP 2A-WSD [6.9] or other appropriate documents.

Due to uncertainties associated with the development of p-y curves for dike structures, upper-bound and lower- bound p-y springs shall be developed for use in superstruc- ture inertial response analyses.

3106F.10 Soil-pile interaction.¶

Two separate loading condi- tions for the piles shall be considered:

1 . Inertial loading under seismic conditions

2. Kinematic loading from lateral ground spreading Inertial loading is associated with earthquake-induced lat- eral loading on a structure, while kinematic loading refers to loading on foundation piles from earthquake induced lateral deformations of the slope/ embankment/dike system. Simulta- neous application of these loading conditions shall be evalu- ated with due consideration of the phasing and locations of these loads on foundation elements. The foundation shall be designed such that the structural performance is acceptable when subjected to both inertial and kinematic loadings.

3106F.10.1 Inertial loading under seismic conditions. The lateral soil springs shall be used in inertial loading response analyses. The evaluation of inertial loading can be performed by ignoring potential slope/embankment/ dike system deformations (i.e., one end of the lateral soil spring at a given depth is attached to the corresponding pile node and the other end is assumed fixed).

3106F.10.2 Kinematic loading from lateral spreading. Kinematic pile loading from permanent lateral spread ground deformation in deep seated levels of slope/ embankment/dike foundation soils shall be evaluated. The lateral deformations shall be restricted such that the struc- tural performance of foundation piles is not compromised.

The lateral deformation of the embankment or dike and associated piles and foundation soils shall be determined using analytical methods as follows:

1. Initial estimates of free field lateral spread deforma- tions (in the absence of piles) may be determined using the simplified Newmark sliding block method as described in Section 3106F.5.4. The geotechnical analyst shall provide the structural analyst with level-ground p-y curves for the weak soil layer con- trolling the lateral spread and soil layers above and below the weak layer. Appropriate overburden pres- sures shall be used in simplified pushover analyses, to estimate the pile displacement capacities and cor- responding pile shear within the weak soil zone.

2. For the pushover analysis, the estimated displace- ments may be uniformly distributed within the thick- ness of the weak soil layer (i.e., zero at and below the bottom of the layer to the maximum value at and

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above the top of the weak layer). The thickness of the weak soil layer used in the analysis (failure zone) shall not be more than five times the pile diameter or 10 feet, whichever is smaller.

3. For a simplified analysis (see Figure 31F-6-2), the pile shall be fixed against rotation and translation relative to the soil displacement at some distance above and below the weak soil layer. Between these two points, lateral soil springs are provided, which allow deformation of the pile relative to the

FIGURE 31F-6-2 SLIDING LAYER MODEL [6.8]

3106F.11 Soil-structure interaction – Shallow foundations and underground structures.¶

and underground structures.

3106F.11.1 Shallow foundations. Shallow foundations shall be assumed to move with the ground. Springs and dashpots may be evaluated as per Gazetas [6.10].

3106F.11.2 Underground structures. Buried flexible structures or buried portions of flexible structures includ- ing piles and pipelines shall be assumed to deform with estimated ground movement at depth.

As the soil settles, it shall be assumed to apply shear forces to buried structures or buried portions of structures including deep foundations.

3106F.12 Underwater seafloor pipelines.¶

Geotechnical eval- uations of underwater pipelines shall include static stability of the seafloor ground supporting the pipeline and settlement and lateral deformation of the ground under earthquakes. If the pipeline is buried, the potential for uplift of the pipeline under earthquakes shall also be evaluated.

3106F.13 Symbols.¶

A = Site Class A as defined in Table 31F-6-1

B = Site Class B as defined in Table 31F-6-1

C = Site Class C as defined in Table 31F-6-1

CPT = Cone Penetration Test

D = Site Class D as defined in Table 31F-6-1

Dp = Pile diameter

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E = Site Class E as defined in Table 31F-6-1

F = Site Class F as defined in Table 31F-6-1

P = Applied load

PI = Plasticity index

p-y = Lateral soil spring

SU = Undrained shear strength

SPT = Standard Penetration Test

t-z = Axial soil spring along the side of pile

T-z = Composite axial soil spring at pile tip

q-w = Axial soil spring at pile tip

VS = Shear wave velocity

3106F.14 References.¶

[6.1] American Society for Testing and Materials

(ASTM), 2014, ASTM D4318-10 (ASTM D4318), “Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils,” West Consho- hocken, PA.

[6.2] American Society for Testing and Materials

(ASTM), 2014, ASTM D2216-10 (ASTM D2216), “Standard Test Methods for Laboratory Determina- tion of Water (Moisture) Content of Soil and Rock by Mass,” West Conshohocken, PA.

[6.3] Youd, T.L., Idriss, I.M., Andrus, R.D., Arango, I.,

Castro, G. Christian, J.T., Dobry, R., Finn, W.D.L., Harder, L.F. Jr., Hynes, M.E., Ishihara, K., Koester, J.P., Liao, S.S.C., Marcuson, W.F., III, Martin, G.R., Mitchell, J.K., Moriwaki, Y., Power, M.S., Robert- son, P.K., Seed, R.B., and Stokoe, K.H., II, 2001, “Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resis- tance of Soils,” Journal of Geotechnical and Geoen- vironmental Engineering, ASCE, Volume 127, No. 10, p. 817-833.

[6.4] Southern California Earthquake Center (SCEC),

March 1999, “Recommended Procedures for Imple- mentation of DMG Special Publication 117 Guide- lines for Analyzing and Mitigating Liquefaction in California,” University of Southern California, Los Angeles.

[6.5] California Department of Conservation, California

Geological Survey (CGS), 11 September 2008, “Guidelines for Evaluating and Mitigating Seismic Hazards in California,” Special Publication 117A, Revised Release.

[6.6] National Cooperative Highway Research Program

(NCHRP), 2008, “NCHRP Report 611: Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments,” Washing- ton, D.C.

[6.7] Naval Facilities Engineering Command (NAVFAC),

1986, NAVFAC DM7-02, “Foundation and Earth Structures,” Alexandria, VA.

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[6.8] Port of Long Beach (POLB), 2012 February 29,

“Wharf Design Criteria (WDC),” Version 3.0, Long Beach, CA.

[6.9] American Petroleum Institute (API), November

2014, API Recommended Practice 2A-WSD (API RP 2A-WSD), “Recommended Practice for Planning, Designing and Constructing Fixed Offshore Plat- forms – Working Stress Design,” 22nd ed., Wash- ington, D.C.

[6.10] Gazetas, G., September 1991, “Formulas and Charts

for Impedances of Surface and Embedded Founda- tions,” Journal of Geotechnical Engineering, ASCE, Vol. 117, No. 9. Authority: Sections 8750 through 8760, Public Resources Code.

Reference: Sections 8750, 8751, 8755 and 8757, Public Resources Code.

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