seaonc presentation

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AN INTRODUCTION TO BUILDING FOUNDATIONS AND SOIL IMPROVEMENT METHODS

SEAONC 2008 Spring SeminarSan Francisco, 16 April 2008

Hadi J. Yap, PhD, PE, GE

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General Foundation Types

• Shallow Foundations– Spread footings: isolated, continuous– grid or waffle– Post-Tensioned Slabs (PT Slabs)– Mats

• Deep Foundations

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Factors to be Considered in Selecting Foundation Type

• Subsurface conditions• Column loads and spacing, basements• Site constraints

– noise– vibrations– proximity to existing improvements, slope, channel

• Economics

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Shallow Foundations

• Suitable where underlying material is strong

• Can be used in engineered fill if building load is light to moderate

• Mats can be used to span localized weak areas

• Mats can be used on weaker soil for structure with basements where net load (weight of structure minus weight of soil removed) is low

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Sources of Settlement• Immediate Settlement (sand and clay)

Occurs as the load is applied• Consolidation (saturated clay)

A slow process of squeezing water out of the pores in soft clay when loaded

• Liquefaction (saturated sand) Temporary loss of shear strength in loose sand due to a rise in excess pore water pressure during cyclic loading such as seismic

• Seismic Densification (dry/moist sand)Densification of loose sand above the groundwater level due to ground shaking

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Total and Differential Settlement

• Building can tolerate large total settlement if the differential settlement is within tolerable limits

• Where the total settlement is large, flexible connections should be provided to underground utilities where they enter the building

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Allowable Differential Settlement for Buildings

Angular Distortion = Differential settlement/Distance

Angular Distortion Limits (Bjerrum, 1963):

• 1/500 – safe limit where cracking is not permissible• 1/300 – limit where first cracking in panel walls is to

be expected• 1/150 – limit where structural damage to general

buildings is to be feared

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Design Parameters for Spread Footings

• Minimum width• Minimum embedment depth• Allowable bearing pressure• Allowable passive pressure• Allowable base friction coefficient

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Spread Footing Excavations

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When mat is to be considered

• When total footprint area of spread footings is more than, say, 50% of building footprint

• To reduce total and differential settlement

• To bridge areas of weak subgrade

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Design Parameters for Mat Foundations

• Minimum embedment depth • Allowable bearing pressures• Allowable passive pressure• Allowable base friction coefficient• Subgrade modulus

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Mat Subgrade and Mud Slab

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Mat Rebars

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Deep Foundation Types

• Drilled Piers/Cast-In-situ-Drilled-Hole [CIDH] Piles

• Driven Piles (Concrete, Steel H)• Tubex Piles• Auger Cast Piles• Torque Down Piles• Micropiles

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Drilled Piers

• Can use one large diameter pier in lieu of several smaller, driven piles

• Lengths can be adjusted in the field – reduce waste/build-up

• Derive axial capacity mainly from skin friction• Need to use casing and/or drilling fluid if

groundwater and/or loose soil is present

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Drilled Pier Installation

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Driven Precast, Prestressed, Concrete Piles

• Economical in San Francisco Bay Area

• Can be used where soft soil, non-engineered fill, or high groundwater level, is present

• Fabricated at yard – good quality control

• Moderately high capacity – up to 344 kips for 14”square piles using 6,000 psi concrete

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Concrete and Steel Piles

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Driven Steel H-Piles• More expensive than driven concrete piles• Suitable where depth to bearing soil layer

varies; can conveniently be cut and spliced• Design must consider corrosion• Moderate to high capacity – up to 456 kips

for HP14X89 using 50 ksi steel• Lateral resistance varies with load direction

relative to pile axis

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Soil Improvement

If poor soil conditions are encountered:

• Bypass poor soil, use deep foundations• Remove poor soil, replace with engineered fill• Improve soil properties in place

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Factors to be Considered in Selecting Soil Improvement Method

• Soil type; fines content (silt- and clay-size)• Area and depth of treatment• Soil properties – strength, compressibility• Proposed structure and settlement criteria• Availability of skills, equipment, materials• Adjacent improvements• Economics

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Primary objectives of soil improvement

CLAY– Increase bearing capacity or slope stability– Reduce foundation settlement

SAND- Reduce liquefaction potential- Increase bearing capacity- Reduce foundation settlement

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Mechanisms of Soil Improvement for Clay

• Consolidation- Preloading

• Reinforcement- Soil-Cement Columns- Vibro-Replacement Stone Columns- Geopiers® and Vibro Piers™

• Mixing- Soil-cement columns

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Mechanisms of Soil Improvement for Sand• By vibration

– Impacts at surface: Dynamic compaction– Depth vibrator: Vibro-compaction

• By vibration and displacement of backfill- Vibro-replacement stone columns- Vibro Piers™

• By displacement of backfill material- Compaction grouting

• By binding particles- Permeation grouting (e.g. ultra-fine cement)

• By mixing– Soil-cement columns

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Soil Improvement Methods

XPermeation GroutingXCompaction GroutingXXGeopiers® and Vibro Piers™XXStone ColumnsXVibro-CompactionXXSoil-Cement ColumnsXDynamic Compaction

XPreloading

SandClayMethod

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Preloading

• Performed by placing fill over soft clay• Improve foundation soil for buildings,

embankments, runways, bridge abutments• Type of preloads: earth fill, water, vacuum • Use prefabricated vertical (wick) drains to

reduce preloading time • Wick drains: plastic core wrapped in

geotextile; generally 4” wide and 1/8” to 3/8”thick

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Wick Drain Installation

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Preloading (cont’d)• Typical wick drain spacing is 3 to 6 feet,

depending on soil permeability and time available

• Typical preloading period is 3 to 6 months, depending on soil permeability and degree of consolidation to be achieved

• Construction monitoring: settlement (settlement plates/probes), pore water pressure (piezometers), lateral movement (inclinometers)

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Source: ASCE, Geotechnical Special Publication No. 69, 1997

Preloading with Wick Drains and Instrumentation

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Dynamic Compaction• Involves repeated dropping of heavy weights

onto ground surface• Effective for sand, waste, and rubble fills• Pounders: concrete blocks, steel plates, or

thick steel shells filled with concrete/sand• Typical weight of pounders: 6 to 30 tons,

depending on the depth of soil to be improved

• Typical drop heights: 40 to 100 feet

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Dynamic CompactionSource: Hayward Baker

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Dynamic Compaction (cont’d)

• Most effective for soil with less than 25% fines (silt- and clay-size particles; material passing #200 sieve [0.075 mm opening])

• Typical improvement depth is 10 to 30 feetD ≈ 0.5 √(WH)

where:D = improvement depth in mW = pounder weight in metric tonH = drop height in m

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Dynamic Compaction (cont’d)

• Ground Vibrations< 0.5 inch/sec to prevent cracks in walls< 2.0 inch/sec to prevent structural damage

• Construction monitoring– Induced settlement– Ground vibration– Ground heave – Pore water pressure– Verification testing (SPT, CPT)

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Scaled Energy Factor versus Particle Velocity

Source: FHWA, Dynamic Compaction, 1995

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Soil-Cement Columns• Mixing in-situ soil with cementitious materials using

mixing shafts consisting of auger cutting heads, auger flights, or mixing paddles

• Produce soil-cement columns with higher strength, lower compressibility, and lower permeability than the native soil

• Used to improve bearing capacity and slope stability, and as shoring walls

• Typical compressive strength of cylinders ranges from 15 to 300 psi

• Typical permeability of mix ranges from 10-6 to 10-7

cm/sec

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Soil-Cement ColumnInstallation

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Soil-Cement Shoring Wall

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Soil-Cement Wall Installation

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Steel Beam Installation

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Vibro-Compaction

• Densifying granular soil by inserting a vibrating probe into the ground

• Probe spacing ranges from 6 to 14 feet• Suitable for sand with less than 15% fines (silt- and

clay-size particles) • Vibrator is a torpedo shaped horizontally vibrating

probe, 10 to 15 feet long, and weighs about 2 tons. The probe penetrates to the design depth under its own weight assisted by water jetting

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Vibrator and Water Jets

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Vibro-Compaction (cont’d)

• The action of vibrator and water jetting reduce inter-granular forces between soil particles allowing them to become denser

• The vibrator starts at the bottom of the hole and raised to treat the next interval; the procedure is repeated as backfill sand is added

• If backfill is not added, craters with diameters of 10 to 15 feet can form around vibrator

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Vibro-Replacement Stone Columns

• Extends the range of soil types that can be improved to silt and clay

• The probe is penetrated to design depth and gravel/crushed rock is placed in the hole as the probe is withdrawn in vertical increments of 2 to 5 feet

• A stone column is formed with the stone laterally compacted against the surrounding soil

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Vibro-Replacement Stone Columns (Cont’d)

• Three primary methods– Wet, top feed method: hole is saturated with

jetting water – Dry, top feed method: hole formed by probe

remains open without water– Dry, bottom feed: stone backfill is fed through a

hopper and tube to the bottom of the hole• Construction Monitoring

– Settlement and ground heave– Amount of stone backfill used– Verification testing (SPT, CPT)

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Wet Top Feed MethodSource: Bauer

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Dry Bottom Feed Method

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Geopiers® (Rammed Aggregate Piers)

• Locally installed by Farrell Design-Build Company• Typically 24 to 36 inches in diameter and 6 to 30 feet

deep, constructed by drilling and ramming crushed rock in 12-inch lifts

• The ramming equipment consists of excavators equipped with 2,000 to 4,000 lbs hydraulic hammers with beveled tampers

• The ultimate bearing capacity of a pier ranges from 100 to 300 kips in compression and 100-150 kips in uplift (with steel anchor). Allowable bearing capacity range from 5 to 8 ksf

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Completed Geopiers®

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Geopiers®

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Vibro Piers™• Installed using displacement and vibratory energy to

depths of 10 to 30 feet by Hayward Baker• Installation methods:

– Dry top feed method: stone is placed in pre-augered hole, densified in 6 to 12-inch lifts with a vibrator

– Dry bottom feed method: for high groundwater level. The vibrator with tremie pipe attachment are penetrated to the design depth to install and densify the stone in place. Little or no waste results from this method.

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Dry Top Feed Method

Source: Hayward Baker Inc.Dry Bottom Feed

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Compaction Grouting

• To compact loose soil or to produce control displacement to lift structure

• Involves injection of low-slump (less than 2 inches) grout (soil-cement mixture) which does not enter soil pores but remain in a homogeneous mass

• Grout material may consist of fine sand mixed with 12% cement and water to produce stiff, mortar-like mixture

• Grout pipe is installed to maximum treatment depth and grout is injected at high pump pressure as the pipe is withdrawn incrementally, forming a column of interconnected grout bulb

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Compaction Grouting

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Compaction Grouting (cont’d)

• Can be performed “stage down” or, more commonly, in a “stage up” process, as follows:– Advancing the grout pipe to the bottom of

treatment depth– Injecting the grout until refusal criteria is

achieved, based on injected grout volume, injection pressure, or ground heave

– Extracting the grout pipe to the next depth interval and injecting the grout

– Repeat the process until reaching the upper limit of treatment zone

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Compaction Grouting

Source: Hayward Baker

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Compaction Grouting (cont’d)

• Construction monitoring:– Injected grout volume– Pressure loss/ground surface heave– Verification testing: pre- and post-grouting

SPT/CPT

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Ultra-Fine Cement Grouting

• Uses micro-fine cement (particle size ranges from 1 to 10 microns); can penetrate fine sand

• Used to increase bearing capacity of sand under existing footings and/or reduce potential settlement

• Can be used to retain shallow excavation in loose sand

• Unconfined compressive strength can exceed 100 psi

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Ultra-Cement Grouting

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Elevator Pit Excavation