Ground Improvement Why? Which? When? Ray Franz, PE, D.GE, Vice - PowerPoint PPT Presentation

Ground Improvement Why? Which? When? Ray Franz, PE, D.GE, Vice President 1 haywardbaker.com

Outline of Presentation Address Topics of Interest solicited from you by Mr. Merklin • Introduce project conditions leading to consideration of Ground Improvement (Why?) • Summarize currently available improvement techniques (Which?) • Discuss applicability of techniques to subsurface conditions (When?)  More detail to be provided on Aggregate Columns and Rigid Inclusions by Mr. Griffin and Mr. Simonton • Review important geotechnical information • Discuss framework for specifications 2

Why use Ground Improvement? Ground improvement technologies are geotechnical construction methods used to modify and improve poor and marginal soil and rock conditions to meet project requirements . [FHWA NHI-04-001] Ground modification is defined as the alteration of site foundation conditions or project earth structures to provide better performance under design and/or operational loading conditions [FHWA-NHI-16-027 / FHWA GEC 013] 3

Why use Ground Improvement? • If “conventional” construction methods will not provide the required performance. • If “conventional” construction methods are not cost-effective.  Ground improvement may ADD VALUE 4

Why use Ground Improvement? • What is included when we define Required Performance ? − Adequate safety against or low probability of instability or strength failure − Serviceable deformations • Settlement • Lateral movements − Adequate resistance to extreme or natural hazard loading conditions − Constructable 5

Why use Ground Improvement? • How do we assess Cost-Effectiveness ? − Acceptable initial construction cost − Acceptable construction schedule − Predictable and acceptable life-cycle costs • Operating and monitoring • Maintenance, repair and replacement 6

Why use Ground Improvement? • Each of your states has multiple physiographic regions with distinct surficial geology and the associated challenges. • Not every state includes all of these issues, but several of these subsurface conditions are prevalent across the Midwest: • Loose, granular soils • Compressible and low-strength, fine-grained soils • Organic materials • Karstic formations • Abandoned mine lands 7

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Loose, Granular Soils • Performance Concerns are −Excessive Settlement −Liquefaction Potential • How to modify / improve / alter? − Densification 9

Densification techniques apply energy to rearrange particles -- decreasing void space, increasing grain to grain contact 10

Densification Techniques Vibro Compaction Dynamic Compaction 11

But, what if there are fines in the soil matrix? The realignment of the sand grains and, therefore, proper densification generally cannot be achieved when the granular soil contains more than 12 to 15 percent silt OR more than 2 percent clay. 12

Compressible and Low-strength Soils • Performance Concerns are −Bearing Capacity Failure −Global Instability −Slope Instability −Excessive Settlement −Differential Settlement −Creep / Long-term Settlement 13

Compressible and Low-strength Soils • How to modify / improve / alter? − Remove and Replace (Soil “ Correction” MN ) − Accelerate consolidation − Reinforce 14

Compressible and Low-strength Soils Remove and Replace − Usually, 20 ft. or less in depth  Controlled by equipment, available area (safe slopes), depth to groundwater − How to determine the limits of work?  To what extent will conditions influence performance? • Probably no further  Use probing • Calibrated probe (can be relatively simple tools) • Cone Penetration Testing 15

Compressible and Low-strength Soils Accelerate Consolidation (and strength gain ±) −Pre-fabricated Vertical (wick) Drains  With pre-load / surcharge  Staged construction −Without other measures, structures must be able to accommodate/tolerate magnitude of deformations 16

Compressible and Low-strength Soils Accelerate Consolidation (and strength gain) −One question was related to estimation of strength gain  Ladd’s SHANSEP model ( S tress H istory and N ormalized S oil E ngineering P roperties) can be used • Requires “undisturbed” samples, oedometer and triaxial testing • Requires accurate field measurements of induced/residual pore pressures to compute effective stress with time 17

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Compressible and Low-strength Soils Reinforce −More to follow by subsequent speakers … −Generally,  Creating stiff elements below grade • Attract stresses to reduce deformation of compressible soils • Transfer portion of applied stress to more competent strata 19

Compressible and Low-strength Soils Reinforcement by: − Aggregate-based (granular) elements  Requires confinement by / support from surrounding ground • Unsuitable for strata that are too soft or susceptible to degradation or decomposition − Cemented or confined elements  Better suited to very soft and organic deposits • Could include Rigid Inclusions and Soil Mixed elements 20

The reinforcing concept… 21

Forms of Reinforcement • From Han, Principles and Practice of Ground Improvement, Wiley, 2015 22

Options and Selection: Suggested References • FHWA-NHI-16-027 / FHWA GEC 013 / NHI Course No. 132034 Ground Modification Methods Reference Manual – Vol. I , April 2017 • FHWA-NHI-16-028 / FHWA GEC 013 / NHI Course No. 132034 Ground Modification Methods Reference Manual – Vol. II , April 2017 • https://geotechtools.org Developed for the second Strategic Highway Research Program Project Number R02 (SHRP 2 R02) Geotechnical Solutions for Soil Improvement, Rapid Embankment Construction, and Stabilization of the Pavement Working Platform + http://www.trb.org/SHRP2 23

Geotechnical Information • Which data are most valuable? • In what format do specialist contractors prefer to see the data? − All of it! • Not really kidding… • You must have enough to really understand the expected poor performance to engineer the improved behavior 24

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Information to be provided in contract documents • Complete geotechnical report  Not just logs and test result tables  Scaled profiles are good • Existing and planned grades • Existing and planned utilities [below grade and above grade/overhead] • Groundwater regime and planned surface drainage features • Information regarding site history , reported past activities • Known contamination • Planned construction sequence 26

• GeoConstructability – An Owner’s Guide to Obtaining Essential Geotechnical Information for Construction, Report of the Geotechnical Constructability Task Force, Geo-Institute of ASCE, American Society of Civil Engineers, 2011. [especially Appendices Nos. 1 and 7] 27

Preparing Specifications and Plans • “Discussion of how to write specifications (method based, performance based, etc.) while avoiding proprietary aspects…” Contracting Framework = Design – Bid – Build 28

Preparing Specifications and Plans • Avoid the use of “method” specifications  Unlikely to accommodate available tools and approaches offered by the various specialist contractors  May preclude certain methods that might otherwise provide desired performance more economically • Performance is the ultimate goal 29

Do’s and Don’ts: Suggested References • Guidance for Drafting Specifications for Ground Improvement Deep Foundations Magazine, April 2016 30

Performance Expectations and Requirements • Regardless of Design - Bid -Build or Design-Build ,  Understand the performance requirements for the new construction  Communicate them clearly, and completely, in the contract documents  Anticipate natural variability and appropriate means of compensation • Unit price by “area” units is unlikely to fairly share risk 31

Performance Expectations and Requirements • Specify, not only the required behavior, but also the acceptable design/analysis methods to be used by the specialist Ground Improvement engineer • Define review and acceptance responsibilities among the parties 32

The recurring expectation… • Provide clarity  For the specialist engineer and construction team  For the reviewer(s)  For the Owner 33

When considering Ground Improvement … • Evaluate project setting and risks • Have the end in mind • Don’t forget the fundamental soil mechanics 34

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