Underpinning Foundations

Alexander Newman, P.E.

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Underpinning and Strengthening of Foundations

ASCE Continuing Education SeminarPresented by Alexander Newman, P.E.

Exponent Failure Analysis Associates, Natick, MA

(508) [email protected]

Copyright 2008 Alexander Newman


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Agenda Introduction: Why strengthen foundations? General approach and methods Adding supports Shoring and replacement Underpinning

- Pit underpinning

- Using drilled piers, micropiles, proprietary piers

Enlarging footings Other strengthening issues Modifying soil properties Conclusion, Q&A

Introduction

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Some Reference Sources FHWA-RD-75-130, Lateral Support Systems and Underpinning, V. III, April 1976

FEMA 172, NEHRP Handbook for Seismic Rehab. of Existing Buildings (1992)

FEMA 547, Techniques for the Seismic Rehab. of Existing Buildings (2006)

David B. Peraza, Getting to the Bottom of Underpinning, Structure, Dec. 2006

DoD UFC 3-301-05A (fmr US Army TM 809-05, Seismic Evaluation & Rehabilitation for Buildings, 11/1999), 3/2005

P. Beckmann, Structural Aspects of Building Conservation, McGraw-Hill Intl, London, 1995

Introduction


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Related Two-day ASCE Seminar Design and Strengthening of Shallow Foundations for Conventionaland Pre-engineered Buildings

Related ASCE Webinars Design of Building Foundations: Practical Basics Foundations for Metal Building Systems Design of Moment-Resisting Foundations for Pre-Engineering Buildings

Introduction

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Why Strengthen Foundations? Reasons for Renovating Existing

Foundations: Strengthening

- Original foundations were inadequate for vertical or lateral load (or later overloaded)

- Foundations were designed before loading was finalized (MBS)

- Additional loading is proposed

- Field errors

- Prior or current renovations (e.g., making big wall openings)


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Why Strengthen Foundations?

Reasons for Renovating Existing Foundations, Contd Deterioration from

- Aggressive chemicals (sulfates, acids, esp. in masonry mortar)

- Washout

- Loss of support caused by changes in water elevation

Lowering bottom of footing because of adjacent construction Adding weight for uplift prevention Remediation of heave or settlement

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General Approach and Methods

Before Strengthening, Try Analysis and Reason Undersized foundations may have completed settlement and will

perform OK unless changes occur in soil, ground water, or loading

Settlement in cohesionless and cohesive soils: The differences Using live load reduction Check for surplus soil bearing capacity

Can We Establish the Existing Pressure on Soil?


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General Approach and Methods Determination of In-Situ Foundation Pressure

FEMA 547

Per ASTM D1194 Pit > 3 x 3 Access tunnel > 18 wide Equipment:

Hydraulic ram w/press. gage

Load cell

1 thick plate 12x12

(4 min) dial gages to measure soil deformation

Best for sand/gravel, stiff clay

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General Approach and Methods Methods of Foundation Strengthening

Adding supports in lieu of foundation strengthening Shoring and replacement Underpinning

- Pit

- By drilled piers, micropiles, helical piers

Modifying soil properties Connecting to adjacent footings with deep tie beams

FEMA 547


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Adding Supports Adding Supports in Lieu of Foundation

Strengthening Often, the most cost-effective Consider first

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Adding Supports Adding Wall Foundations Alongside Existing

FEMA 547


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Shoring and Replacement

Using Needle Beams for Temporary Shoring For replacement or

underpinning

FHWA-RD-75-130, Lateral Support Systems and Underpinning, V. III, April 1976

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Shoring and Replacement Inclined Temporary Shoring

FHWA-RD-75-130, Lateral Support Systems and Underpinning, V. III, 1976


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Shoring and Replacement Foundation Placed in Wrong Location

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Shoring and Replacement Another Foundation Placed

in Wrong Location


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Underpinning

Underpinning A process that transfers load to a greater depth than original Used to lower or to enlarge footing Temporary support is expensive and may not be needed if soil is

good and foundation strong

Pit Underpinning vs. Using Micropiles, Etc.

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Underpinning Pit Underpinning of Walls

Some say: Under favorable conditions, can place pits 16 o.c.

Photo: David B. Peraza, P.E.


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Underpinning

Pit Underpinning Existing

Wall Foundations If OK w/o temporary support Approach pit; its size Pack soil behind sheeting 1st excavation pit, same depth Continue excavation Place concrete


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Underpinning

Pit Underpinning, Contd Transfer load using drypack or shims after > 24 hr for high-early

cement, 48 hrs for regular

Can settle < from:Concrete shrinkage

Soil deformation

Loss of ground

Deflection of existing structure FHWA-RD-75-130, Lateral Support Systems and Underpinning, V. III, April 1976


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Underpinning Example of Pit Underpinning of Wall

Photos: David B. Peraza, P.E.

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Underpinning Underpinning Column Footings in Quadrants

Excavate and brace soil around footing Drive rebars through soil, place concrete Wait a few days, do other quadrants

Sequence: 1,3,2,4


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Underpinning Underpinning Column Footings in Quadrants, Contd

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Underpinning

Pit Underpinning: Potential Problems Rubble foundations: May not be feasible to underpin Perhaps place a retaining wall alongside for excavation support Use other methods (below)


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Underpinning

Pit Underpinning: Potential Problems, Contd High water table with silts and clays

Example: Hi-rise building next to church Underpinned perimeter, but interior foundations settled w/dewatering => cracks, settlement , church vacated

Sandy soilsMay settle if vibrated, both at perimeter and interior (from piles in

adj. bldg, soldier piles and lagging)Can collapse into pit excavation, building loses support

Source: David B. Peraza, Getting to the Bottom of Underpinning, Structure, December 2006

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Underpinning

Pit Underpinning: Reducing Problems Engage a monitoring firm (by owner), do a preconstruction survey Establish benchmarks on adjacent buildings Place vibration sensors there to monitor peak particle velocity

generated by construction

Place crack monitors over existing cracks Keep excavating contractor from excavating too much and too fast

ahead of underpinning work! Need close coordination (by GC?).

Source: David B. Peraza, Getting to the Bottom of Underpinning, Structure, December 2006


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Underpinning Underpinning by Drilled

Piers, Piles and Minipiles When soil cannot support pits or

bearing strata is too deep

Needle Beams and Drilled

Piers C-I-P piers in uncased holes or

piles for gravity load & uplift

Requires interior access by equipment

FEMA 172

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Underpinning

Underpinning by Drilled Piers

FEMA 547


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Underpinning

Wall or Column Footing Underpinned by Piles Placed Alongside


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Underpinning

Adding Piles/Piers to Existing Wall Footing

FEMA 547


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Underpinning

Underpinning Column Footings by Drilled Piers or Piles Through Footing If footing is large enough

US Army TI 809-05

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Underpinning

Two Piles/Cantilever Beam


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Underpinning

Underpinning by Jacked Piles When DL is large Use open-ended pipe or H section Place in pits made as in pit underpinning Fasten a steel plate at bottom of footing and to top of pile, place

jack in between and blocking

Splice pipe and continue


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Underpinning

Underpinning by Augered Pile Installed in Slot Specialized equipment

needed to cut the slot



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Underpinning Underpinning by

Augered Concrete Caisson With Bracket Pit needed for bracket For smaller loads:

C-I-P piers (say, 12dia) w/ column-typerebars & haunch


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Underpinning

Underpinning by Steel Pile with Bracket Pile can be augered or

driven



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Underpinning

Using Micropiles (Minipiles, Pin, Needle, Root Piles) Small diameter Design loads from 3 to 500+ tons Can be readily designed for tension/uplift loads Appropriate for a wide range of ground conditions Suitable for low headroom and restricted access Low noise and vibration Can penetrate obstacles

Source: Hayward Baker, Inc..

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Underpinning Micropiles: Typical Uses

Can be spaced closer, so existing structure span is less. Still, may have to stabilize existing masonry if piles are driven thru it.

FHWA-RD-75-130, Lateral Support Systems and Underpinning, V. III, 1976


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Underpinning

Typical Micropile Design Steps

Source: Hayward Baker, Inc.

1. Geotechnical study

2. Determine load to be supported

3. Design pile-to-structure connection

4. Design pile-to-soil or rock load transfer

5. Develop a pile testing program (typ. to 2x static load)

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Underpinning

Driven: Light loads thru soft soil Compaction grout: Shallow depth,

loose, sandy soils (by blast of compressed air)

Jet grout: High capacity, most soilswhere direct pressure grouting is not

possible

Types of Micropiles

Hayward Baker, Inc.


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Underpinning

Types of Micropiles, Contd

Hayward Baker, Inc.

Post grouted: All soils. Post-grouting enhances friction capacity of pile

Pressure grouted: High capacity; wide range of soils; enhanced friction capacity by densification & grout permeation of soil

Drilled, end bearing: Small diameter, can transfer high loads to till or rock

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Underpinning Typical Grouted Micropile Construction

Detail next

FEMA 547


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Underpinning Grouted Micropile Connection Details

Top plate for tension (placed deep enough into footing), bottom for compression

FEMA 547

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Underpinning

Example: Using Minipiles for Seismic Retrofit of Union Station Theater, St. Louis, Mo.

Hayward Baker, Inc.

Problem: Seismic upgrade of I-70/64 bridge column foundations was needed, but there was a theater was built around two of them Access could only be through a pair of 6 foot high doors

Solution: Minipile installation w/low overhead drill rig


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Underpinning Minipiles for Seismic Retrofit, Contd

Hayward Baker, Inc.

Minipile: pipe to rock, then drill into rock 18 Flush rock hole, place high-strength (150-ksi) bar & grout Bearing plate on top of bar + shear studs

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Underpinning Minipiles for Seismic Retrofit, Contd Bar placed in black corrugated plastic & space within grouted

Hayward Baker, Inc.

Existing column and excavated foundation

Completed minipile with cap and rock anchor

High capacity rock anchors with corrosion protection


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Underpinning

Example: Underpinning with Minipiles to Repair Settlement in a SE Florida Parking Garage.

Hayward Baker, Inc.

Problem: Three story precast parking garage; shallow foundations with 4 ksf Assumed soil: 0 - 25 firm sand / dense sand & limestone But: One interior isolated column began settling SPT at that location found isolated pocket of 0 13 sand fill / 7organics / dense sand & limestone

Solution: Minipiles installed w/low overhead drill rig, doweled into existing column

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Underpinning

Example: Minipiles for Repair of Settlement, Contd

Hayward Baker, Inc.


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Underpinning

Example: Minipiles for Repair of Settlement, Contd

Hayward Baker, Inc.

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Underpinning

Using Proprietary Steel Anchors Helical PulldownTM Anchors (AB CHANCE Anchors) Good sources of info:- AB Chance Co.

http://www.abchance.com

-- Solid Earth Technologies, Inc.Solid Earth Technologies, Inc.

www.solidearthtech.com

Solid Earth Technologies, Inc.


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Underpinning

Jacked Steel (Atlas) Piers End-bearing piers hydraulically

pushed to load bearing strata

ATLAS SYSTEMS, INC.1026-B South Powell Road, Independence, MO 64056Telephone: (816) 796-6800, web http://www.atlassys.com

Hayward Baker, Inc. and Atlas Systems, Inc.

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Enlarging Footings Enlarging Existing Footings

FEMA 547


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Enlarging Footings

Idealized model with uniform pressure

Some Assumptions in Widening Footings

After Poul Beckmann, Structural Aspects of Building Conservation, McGraw-Hill Intl, London, 1995

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Enlarging Footings

Drilled-in SS threaded rods or hooked bars Threaded SS PT bars in pressure-grouted holes

Assumptions in Widening Foundations, Contd



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Enlarging Footings

Real life: Unequal pressure => more settlement when loaded Using flatjacks over the pressure slab to preload soil Preload gradually in clay (weeks)

Widening Concrete Foundations, Contd


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Enlarging Footings

Widening Rubble

Foundations Pressure grout before

underpinning

Grout travels easier horizontally; vertical spacing of ~ 2 ft?

Remove finishes to expose joints


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Enlarging Footings

Widening Rubble Foundations, Contd Cut into existing foundation Alternate the hooks Corrosion-resistant coating?

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Enlarging Footings

Drill and PT after 28 days Use to widen the footing or make

a beam to span between new

deep foundations

Widening Rubble Foundations, Contd


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Other Strengthening Issues

Increasing Uplift Capacity of Column Footings Increase size by underpinning as above Add drilled piers or soil anchors (use drill bits from 4 to 6 dia.,

insert deformed rod, pump grout as bit is withdrawn). Can PT rods if anchored into grouted soil below casing.

Contractors for FEMA use a power drill to place 4ft long steel anchors into sand to tie down FEMA travel trailers in Pensacola, June 2005 (FEMA)

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Other Strengthening Issues Increasing Uplift Capacity of Footings, Contd

Connect to adjacent footings with deep tie beams May need a concrete overlay on top of footing Will concrete overlay on top of footing help?


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Other Strengthening Issues

Increasing Lateral Resistance of Column Footings Increase footing size to increase passive pressure Connect to adjacent footings with deep tie beams for same Improve soil behind the footing to increase passive pressure

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Soil Improvement Soil Improvement

Methods depend on type of soil get expert advice For coarse sand and gravel, cement grouting to bind particles For fine sands and coarse silts, chemical grout injection (better

penetration than cement but more $)

For other soils, compaction grouting Excellent source of info:

www.haywardbaker.com

FEMA 547


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Soil Improvement Chemical Grout Injection

Permeation of sands with fluid grouts to produce sandstone-like masses to carry loads. Grout mixes with sand, forms composite material with higher soil strength (200-300 psi possible)

Grouts: Sodium silicates, Acrylates, Acrylamides,

Polyurethanes

Gel sets in 1-3 hrs Usually, no heave

Hayward Baker, Inc.

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Soil Improvement Chemical Grouting, Contd

Grout injected in clean sand at regular patterns below footing May require holes through floor slab (disruption)

Hayward Baker, Inc.


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Soil Improvement Chemical Grouting, Contd

Proper confinement stress reqd to prevent heave (soil + found. DL may be OK).

Cannot be done near the surface may blow off the soil Take precautions not to fill adjacent cracked sewers, duct banks

Hayward Baker, Inc.

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Soil Improvement

Example: Chemical Grouting at Trane Company Manufacturing Facility, La Crosse, WI Problem: Addition to an existing manufacturing facility building would

significantly increase foundation bearing pressure

Solution: Chemical grouting (sodium silicate) to consolidate clean sandy soils beneath the footings to increase allowable bearing capacity and distribute new foundation loading

Hayward Baker, Inc.


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Soil Improvement

Example: Chemical Grouting, Contd

Hayward Baker, Inc. Section Showing Zone Of Stabilized Soil Beneath Footing

Installation of sleeve port (Tam) grout pipes (with holes in sides) using portable drilling equipment

(Another method of grouting: Pump and withdraw)

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Soil Improvement Example: Chemical Grouting, Contd

Hayward Baker, Inc.

Grouting Operations Underway Showing Grout Pipe Locations and Chemical Grout Storage Tanker


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Soil Improvement

Compaction Grouting Sim. to chemical, but grout displaces

soil rather than mixes with it. A very viscous and stiff (0-3 slump) site-mixed grout is pumped in stages, forming grout bulbs, which displace & densify the soil under.

Some overburden stress reqd Can act as both a column and a soil-

improvement system.

Hayward Baker, Inc.

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Soil Improvement Jet Grouting

High-velocity injection of fluids erodes soil, replaces it with interconnected soilcrete columns 3-4 dia. (strength > 1000 psi).

Widely used to underpin historic structures. Gravels and sands easiest to erode, clays more difficult

Hayward Baker, Inc.


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Soil Improvement

Jet Grouting Systems Single Fluid Jet Grouting (Soilcrete S) best for cohesionless

soils

Double Fluid Jet Grouting (Soilcrete D): Grout jet shrouded withair for more efficient erosion in cohesive soils.

Triple Fluid Jet Grouting (Soilcrete T): Grout, air and water are pumped through different lines, yielding higher quality soilcrete. Most effective system for cohesive soils.

Hayward Baker, Inc.

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Soil Improvement Soil Mixing (Deep Mixing Method)

Mechanical blending of the in situ soil with cementitious materials (reagent binder) using a hollow stem auger and paddle arrangement to achieve improved character, generally a design compressive strength or shear strength and/or permeability.

Hayward Baker, Inc.


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In Some Cases, Building Replacement is Best A case of Westin La Paloma parking garage in Phoenix. 400 x 120 ft 2-deck precast structure was built into the side of a hill

on uncompacted fill

A decade of problems: Subsidence, cracking, rotation of retaining walls on 3 sides~$300K of studies and repairs

Replaced with PT framing on caissons bypassing the fill

Source: ENR, August 4, 1997, p. 17.

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Alexander Newman, PEExponent Failure Analysis Associates,

Natick, MA(508) 652-8500

[email protected]

Q & A


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Course Dates Course TopicsTuesday, Aug. 24 Structural AnalysisThursday, Aug. 26 Strength of MaterialsTuesday, Aug. 31 Geometric DesignThursday, Sept. 2 Concrete DesignTuesday, Sept. 7 Soil MechanicsThursday, Sept. 9 Foundation Engineering Tuesday, Sept. 14 Hydraulics Thursday, Sept. 16 HydrologyTuesday, Sept. 21 Steel DesignThursday, Sept. 23 Waste & Water TreatmentTuesday, Sept. 28 Construction MaterialsThursday, Sept. 30 Construction Management



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Course Dates Course TopicsTue., Oct. 5 Traffic Engineering Wed., Oct. 6 Geotechnical

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Documents

Underpinning Foundations