7.2 Soils, Footings, Deep Foundations & Retaining Walls

Soils, Footings, Deep Foundations & Retaining Walls

Quick Answer: Foundation design under IBC Chapter 18 requires matching foundation types (shallow spread footings, mats, driven piles, drilled shafts) to site-specific geotechnical conditions. Geotechnical reports per IBC §1803 are mandatory for Seismic Design Categories C–F, expansive soils (PI $\ge 15$), and high groundwater. Where allowed, IBC Table 1806.2 provides presumptive bearing capacities from 1,500 psf (clays) to 12,000 psf (bedrock). Shallow footings must extend below the local frost line and at least 12 inches into undisturbed ground (IBC §1809). Retaining walls holding $>4\text{ ft}$ of unbalanced backfill require engineered designs with a minimum Factor of Safety of 1.5 against overturning and sliding (IBC §1807.2.3). Sub-grade spaces must be dampproofed (moisture resistance) or waterproofed (mandatory where hydrostatic water tables exist $\le 6\text{ in}$ below slabs per IBC §1805).

---

1. Geotechnical Investigations & Site Reports (IBC §1803)

A geotechnical investigation conducted by an approved geotechnical engineer provides the technical basis for bearing capacity, settlement limits, groundwater mitigation, seismic liquefaction risk, and lateral earth pressures.

+-----------------------------------------------------------------------------------------+
|                        GEOTECHNICAL INVESTIGATION TRIGGERS                              |
+-----------------------------------------------------------------------------------------+
                                                                                           
  [ MANDATORY TRIGGERS (IBC §1803.5) ]                                                     
  - Seismic Design Category C, D, E, or F (IBC §1803.5.11 & §1803.5.12)                   
  - Suspected Expansive Soils (Plasticity Index >= 15 per IBC Table 1803.5.3)              
  - Deep Foundation Systems (Piles, Drilled Shafts, Micropiles per IBC §1803.5.5)          
  - Sites with Slopes Steeper than 1:3 (33.3% slope per IBC §1803.5.10)                    
  - Questionable Soil / Uncontrolled Artificial Fill / Organic Strata (IBC §1803.5.2)      
  - High Groundwater Table / Below-grade Basements (IBC §1803.5.4)                         

Expansive Soil Classification (IBC Section 1803.5.3)

Soils meeting all four of the following criteria are legally classified as expansive soils and cannot use standard shallow footing tables:

1. Plasticity Index ($PI$) of 15 or greater (determined in accordance with ASTM D4318).
2. More than 10 percent of soil particles pass a No. 200 ($75\text{ }\mu\text{m}$) sieve.
3. More than 10 percent of soil particles are less than $5\text{ }\mu\text{m}$ in size.
4. Expansion Index ($EI$) greater than 20 (determined in accordance with ASTM D4829).

Mandatory Inclusions in Geotechnical Reports (IBC §1803.6):

• Soil boring logs, core logs, and test pit profiles with sample recovery depths.
• Elevation of the seasonal high water table.
• Allowable soil bearing capacity ($q_a$) and expected total and differential settlement.
• Lateral earth pressure coefficients (active $K_a$, at-rest $K_o$, passive $K_p$, and equivalent fluid densities).
• Frost depth and recommended minimum footing embedment depths.
• Seismic site classification (Site Class A through F) and liquefaction potential evaluation.
• Special inspection and field compaction testing protocols.

---

2. Presumptive Soil Load-Bearing Capacities (IBC Table 1806.2)

Where the building official does not mandate a site-specific geotechnical investigation (typically minor single-story or standard low-risk commercial buildings on known uniform soil), the presumptive load-bearing values in IBC Table 1806.2 may be used.

+-------------------------------------------------------------------------+
|                   PRESUMPTIVE BEARING VALUE RULES                       |
+-------------------------------------------------------------------------+
|  1. Allowable Increases: A 1/3 increase in vertical bearing pressure   |
|     is permitted for load combinations including wind or seismic ONLY   |
|     where specifically authorized by the geotechnical engineer/code.    |
|  2. Lateral Sliding Resistance: Total sliding resistance equals either  |
|     (Vertical Load * Friction Coeff) OR (Cohesion * Contact Area),      |
|     but NOT both combined unless validated by geotechnical testing.     |
|  3. Isolated Embedment Depth: Lateral bearing pressure (psf/ft) may be |
|     multiplied up to a maximum of 15 times the designated table value.  |
+-------------------------------------------------------------------------+

---

3. Shallow Footing Design & Detailing (IBC §1808, §1809)

Shallow foundations transmit structural loads directly to soil strata situated immediately beneath the structure via continuous strip footings, isolated column pads, or mat/raft slabs.

+-------------------------------------------------------------------------+
|                 SHALLOW FOOTING GEOMETRIC CONSTRAINTS                   |
+-------------------------------------------------------------------------+
                                                                           
       FINISHED GRADE                                                      
       ===============\                                                    
                      \                                                    
                       \                                                   
                        |  Min Depth >= Local Frost Line (IBC §1809.5)     
                        |  Min Depth >= 12 inches into undisturbed soil    
                        v                                                  
       +---------------------------------+                                 
       |        FOUNDATION STEM WALL     |                                 
       +---------------------------------+                                 
       |                                 |                                 
       |   +-------------------------+   |                                 
       |   |                         |   |                                 
       +---+                         +---+                                 
       |        REINFORCED CONCRETE      |  Footing Thickness (T)          
       |         SPREAD FOOTING          |  Min 6" plain / 8"-12" reinforced
       +---------------------------------+                                 
       |<---- P ---->|<--- W --->|<-- P ->                                 
       Projection (P) <= Thickness (T) for plain concrete                 

Key Shallow Footing Requirements:

1. Frost Protection (IBC §1809.5): Foundation footings, stem walls, and grade beams must extend below the established local frost line established by the local jurisdiction, or be frost-protected in accordance with ASCE 32 (Frost-Protected Shallow Foundations in climates with seasonal ground freezing).
2. Minimum Embedment Depth (IBC §1809.4): Footings must extend at least 12 inches (305 mm) below the undisturbed ground surface, regardless of frost depth.
3. Minimum Footing Thickness (IBC §1809.7): Plain concrete footings must have a minimum thickness of not less than 6 inches (152 mm). The projection ($P$) of plain concrete footings beyond the face of the foundation wall or column shall not exceed the footing thickness ($T$), unless structural flexural and shear reinforcing is designed per ACI 318.
4. Stepped Footings on Sloping Ground (IBC §1809.3):
   • Where the slope of the ground surface exceeds 1 unit vertical in 10 units horizontal (10% slope), the bottom surface of the footing must be stepped.
   • Stepped transitions must be monolithic with the footing, and the vertical step height must not exceed half the horizontal step length (maximum 1:2 step slope).

+-------------------------------------------------------------------------+
|                   STEPPED FOOTING ON SLOPING GROUND                     |
+-------------------------------------------------------------------------+
                                                                           
  Finished Grade  \                                                        
  -----------------\------------------------------------------------------- 
                    \                                                      
                     +------------+                                        
                     | FOOTING #1 |                                        
                     +------------+------+                                 
                     |   Step H   |      |                                 
                     |<-- <=L/2 ->|      |                                 
                                  +------+------------+                    
                                  |    FOOTING #2     |                    
                                  +-------------------+------+             
                                  |<-- Step Length L ->|                   
  Max Base Slope = 1:10 (10%)     | Max Step Ratio = 1V:2H   |             

---

4. Deep Foundations: Piles, Drilled Shafts & Micropiles (IBC §1810)

Deep foundations transmit structural super-loads through weak, compressible upper soil layers down to competent bedrock or dense bearing strata via skin friction, end bearing, or a combination of both.

+-------------------------------------------------------------------------+
|                    DEEP FOUNDATION CLASSIFICATIONS                      |
+-------------------------------------------------------------------------+
                                                                           
  [ 1. DRIVEN PILES ]           [ 2. DRILLED SHAFTS ]    [ 3. MICROPILES / HELICAL ]
  - Steel H-Piles (ASTM A36/572)- Cast-in-place concrete - High-capacity steel casing
  - Steel Pipe Piles (concrete) - Large diameter caissons- Grout-bonded friction piles
  - Precast Prestressed Concrete- Auger-cast displacement - Helical steel screw plates
  - Treated Timber Piles        - Dry or slurried shafts - Ideal for tight access

Deep Foundation Design Principles (IBC Section 1810.3):

1. Structural Capacity vs. Geotechnical Capacity: The allowable design load is governed by the lesser of the structural capacity of the pile material (concrete, steel, timber) and the geotechnical capacity of the supporting soil/rock strata.
2. Group Action & Spacing (IBC §1810.2.5):
   • Piles driving into friction soils interact; when spaced closer than 3 pile diameters (3D) center-to-center, pile group reduction factors must be applied to account for overlapping stress bulbs.
   • Minimum center-to-center spacing for driven piles: not less than 2.5 pile diameters (or 2.5 times the width).
3. Load Testing Protocols (IBC §1810.3.3.1.2):
   • Static Axial Compressive Load Testing (ASTM D1143) is required to verify design capacities exceeding presumptive tabular limits.
   • High-Strain Dynamic Pile Testing (PDA per ASTM D4945) may be approved to verify pile driving integrity and ultimate capacity.
4. Seismic Detailing in SDC C, D, E, F (IBC §1810.3.11): Piles must be securely anchored to the pile cap with structural rebar embedment capable of developing full tension uplift and lateral shear forces; reinforcement cages in cast-in-place shafts must extend through liquefiable strata into competent soil.

---

5. Retaining Walls & Lateral Earth Pressures (IBC §1807.2)

Retaining walls are vertical or near-vertical structures designed to retain soil, rock, or other materials where a change in ground elevation occurs.

+-------------------------------------------------------------------------+
|                   CANTILEVER RETAINING WALL ANALYSIS                    |
+-------------------------------------------------------------------------+
                                                                           
                                            | Surcharge Load (q)           
                                            v                              
                                +-------+   ============================== 
                                |       |                                  
                                | STEM  |  Active Earth Pressure (P_A)     
                                |       |  Equivalent Fluid Density (pcf)  
                                |       |  --->                            
                                |       |  ------>                         
  Passive Earth                 |       |  --------->                      
  Pressure (P_P)                |       |  ------------>                   
  <-------------                +-------+  --------------->                
  ==============+---------------+-------+--------------------------------+ 
                |     TOE       |  HEEL |        FOOTING SLAB            | 
                +---------------+-------+--------------------------------+ 
                |<--------- B (Base Width = 0.5H to 0.7H) -------------->| 

Lateral Earth Pressure States:

• Active Earth Pressure ($K_a$): Occurs when the retaining wall tilts or deflects slightly away from the backfill (typically $\Delta / H \ge 0.001$). Generates the lowest lateral pressure (typically 30 to 45 psf/ft equivalent fluid density for level granular soil).
• At-Rest Earth Pressure ($K_o$): Occurs when the wall is rigid and unyielding (such as a basement wall restrained at top and bottom by floor diaphragms). Generates higher pressure (typically 45 to 60 psf/ft equivalent fluid density).
• Passive Earth Pressure ($K_p$): Occurs when the footing/wall pushes into the soil in front of the toe or keyway, resisting sliding.

Stability Safety Factors (IBC Section 1807.2.3):

Retaining walls retaining more than 4 feet ($1,219\text{ mm}$) of unbalanced backfill must be engineered to satisfy two statutory stability limits:

$FS_{\text{Overturning}} = \frac{\sum M_{\text{Resisting}}}{\sum M_{\text{Overturning}}} \ge \mathbf{1.50}$

$FS_{\text{Sliding}} = \frac{\sum F_{\text{Resisting}}}{\sum F_{\text{Driving}}} = \frac{\mu \cdot N + P_{\text{Passive}}}{P_{\text{Active}} + P_{\text{Surcharge}}} \ge \mathbf{1.50}$

Retaining Wall Drainage Systems (IBC §1805.4.2):

• Retaining walls must be detailed with drainage to prevent the buildup of hydrostatic water pressure behind the stem.
• Standard drainage details require: continuous 4-inch perforated drain tile wrapped in geotextile filter fabric, embedded in a gravel drainage envelope along the base of the heel, and discharging to an approved daylight outlet or storm sump; OR weep holes not less than 4 inches in diameter spaced no more than 10 feet on center.

---

6. Foundation Walls, Dampproofing & Waterproofing (IBC §1805, §1807.1)

Subgrade walls and floor slabs enclosing interior habitable or usable spaces must be protected against moisture infiltration.

+-------------------------------------------------------------------------+
|                 DAMPPROOFING VS. WATERPROOFING MATRIX                   |
+-------------------------------------------------------------------------+
|  CRITERIA              | DAMPPROOFING (IBC §1805.2) | WATERPROOFING (IBC §1805.3)|
+------------------------+----------------------------+---------------------------+
|  Hydrostatic Pressure  | NO Hydrostatic Pressure    | HYDROSTATIC PRESSURE      |
|  Groundwater Condition | Water table > 6" below slab| Water table <= 6" of slab |
|  Primary Purpose       | Retards moisture vapor     | Prevents liquid water     |
|  Typical Materials     | Bituminous coating, acrylic| Continuous elastomeric    |
|                        | coating, 3/8" parging      | membrane, bentonite sheet |
|  Floor Application     | Under-slab vapor retarder  | Membrane under slab + seal|
+-------------------------------------------------------------------------+

Under-Slab Vapor Retarders (IBC Section 1805.2.1):

• Concrete slab-on-ground floors in enclosed spaces must be underlain by a minimum 10-mil ($0.25\text{ mm}$) vapor retarder complying with ASTM E1745 Class A (or standard 6-mil polyethylene).
• Joints must be lapped not less than 6 inches (152 mm) and sealed around all plumbing, electrical, and structural penetrations.

---

7. Realistic Plan Review Scenario: Commercial Column Footing & Retaining Wall Sizing

Project Submittal Overview

A commercial plans examiner reviews the foundation plan and calculations for a new retail structure. The geotechnical report establishes an allowable soil bearing capacity $q_a = 2,500\text{ psf}$ (Class 4 Silty Sand) and a local frost depth of 36 inches.

+-------------------------------------------------------------------------+
|                     FOUNDATION PLAN DATA BREAKDOWN                      |
+-------------------------------------------------------------------------+
|  1. Interior Column Pad Footing F-1:                                    |
|     - Dead Load (D) = 80 kips (80,000 lbs)                              |
|     - Live Load (L) = 45 kips (45,000 lbs)                              |
|     - Proposed Footing Dimensions: 7.0 ft x 7.0 ft square pad, 16" thick|
|  2. Site Perimeter Retaining Wall:                                      |
|     - Stem Height: H = 8.0 ft retaining level sandy backfill            |
|     - Active Equivalent Fluid Pressure: 35 psf/ft                       |
|     - Base Footing Width: B = 5.5 ft                                    |
|     - Total Overturning Moment: M_OT = 12.0 kip-ft per linear foot      |
|     - Total Resisting Moment: M_R = 21.6 kip-ft per linear foot         |
+-------------------------------------------------------------------------+

Step-by-Step Plans Examiner Calculations

Step 1: Verify Column Footing F-1 Soil Bearing Pressure

• Total Service Load under ASD ($D + L$): $P = 80\text{ kips} + 45\text{ kips} = 125\text{ kips} = 125,000\text{ lbs}$
• Self-Weight of Concrete Footing ($7\text{ ft} \times 7\text{ ft} \times 1.333\text{ ft} \times 150\text{ pcf}$): $W_{\text{footing}} = 49\text{ sq ft} \times 1.333\text{ ft} \times 150\text{ pcf} = 9,800\text{ lbs} = 9.8\text{ kips}$
• Total Applied Load on Soil = $125.0 + 9.8 = 134.8\text{ kips} = 134,800\text{ lbs}$.
• Contact Bearing Pressure ($q_{\text{actual}}$): $q_{\text{actual}} = \frac{134,800\text{ lbs}}{7\text{ ft} \times 7\text{ ft}} = \frac{134,800}{49} = \mathbf{2,751 \text{ psf}}$
• Check against Allowable Bearing Capacity ($q_a = 2,500\text{ psf}$): $q_{\text{actual}} (2,751\text{ psf}) > q_a (2,500\text{ psf}) \quad \mathbf{[NONCOMPLIANT]}$
• Correction Required: The footing is undersized. Sizing a $7.5\text{ ft} \times 7.5\text{ ft}$ pad ($56.25\text{ sq ft}$): $W_{\text{footing}} = 56.25 \times 1.333 \times 150 = 11,250\text{ lbs} \rightarrow P_{\text{total}} = 136,250\text{ lbs}$ $q_{\text{actual}} = \frac{136,250}{56.25} = \mathbf{2,422 \text{ psf}} \le 2,500\text{ psf} \quad \mathbf{[COMPLIANT]}$

Step 2: Verify Retaining Wall Overturning Safety Factor (IBC §1807.2.3)

• Overturning Moment $M_{\text{OT}} = 12.0\text{ kip-ft/ft}$.
• Resisting Moment $M_R = 21.6\text{ kip-ft/ft}$.
• Factor of Safety against Overturning: $FS_{\text{OT}} = \frac{M_R}{M_{\text{OT}}} = \frac{21.6}{12.0} = \mathbf{1.80} \ge 1.50 \quad \mathbf{[COMPLIANT]}$

---

8. Common Plan Review & Exam Traps

• Trap 1: Neglecting Footing Self-Weight in Soil Bearing Checks. Plans examiners must ensure that the weight of the concrete footing and soil overburden is added to the column/wall service loads when evaluating allowable soil bearing pressures.
• Trap 2: Using Presumptive Bearing on Expansive Clays. Table 1806.2 values cannot be used if the soil exhibits expansive properties ($PI \ge 15$). A geotechnical investigation is mandatory.
• Trap 3: Inadequate Frost Depth in Heated Buildings. Footings for exterior canopies, detached trash enclosures, and exterior retaining walls are unheated and must extend to full local frost depth regardless of the building's main footprint frost protection strategy.
• Trap 4: Specifying Dampproofing Where Water Table is High. If the seasonal high water table rises within 6 inches of the lowest basement floor level, bituminous dampproofing is illegal; full continuous hydrostatic waterproofing per IBC §1805.3 is mandatory.
• Trap 5: Missing Safety Factor of 1.5 on Retaining Walls. Retaining walls must be checked for BOTH sliding ($FS \ge 1.5$) and overturning ($FS \ge 1.5$). A wall that is safe against overturning may still fail in sliding if a keyway or passive resistance is omitted.