Nebraska Concrete Paving WorkshopLincoln, Nebraska – January 19-20, 2016

SUBGRADE STABILIZATIONMaterials & Methods

Shiraz Tayabji, Ph.D., PEAdvanced Concrete Pavement

Consultancy LLCEllicott City, Maryland

Outlineo Subgrade Characteristics 101o Role of Subgrade in a Concrete Pavement System

o Bearing capacity/Structural supporto Working platform

o Marginal Subgrade Treatmento Removal/replacement & fillso Traditional compaction (&

mechanical stabilization)o Geotextile/geogrid useo Chemical Stabilizationo Other (deep stabilization, etc.)

o A little bit on Intelligent Compaction

Low volume roadways Residential streets State routes Interstate highways Airport runways and taxiways Parking lots Industrial storage facilities Port facilities Truck terminals Commercial sites

Where is Soil Stabilization Used?

In other words… In any pavement structure!

The Concrete Pavement System

Natural soil(Original Subgrade)

(Fill - New Subgrade)

SubgradeA critical construction item

– For long-term pavement performance– As a construction platform

Overall uniformity is also important– Reduce variability in pavement design– Ensure adequate compaction– Minimize instability

TRUE OR FALSE: Concrete pavement performance is insensitive to base and

subgrade supportOf course, it is FALSE, even though

design procedures may indicate otherwise!

Role of Subgrade in a Pavement System

Critical role/structural – if the support system is marginal, pavement will not perform well– Overall bearing capacity very important

Construction platform - if subgrade is not stiff/well-compacted, upper base/subbase layers cannot be compacted effectively

Long-term performance - poor subgrades are erodible under repeated traffic & presence of heavy truck traffic

Managing Subgrade in a Pavement System

Increasing slab thickness is not an effective method to account for marginal subgrades

European approach – good support, starting with an improved subgrade, if necessary, leads to longer lasting concrete pavements (less thick PCC)

Subgrade treatment/stabilization “uniformizes” the support condition along the project length, minimizing variability in design and performance

8

Pavement Structural Design Basics

(Yoder and Witczak, 1974)

Design Objective: Protect the subgrade!

If subgrade soils are strong enough to carry the millions of trucks over many years under

all types of climatic events, we would not need a pavement!

Asphalt Layer

How Pavements Carry Loads?- Conventional PCC & AC Pavements

Concrete’s rigidness spreads the load over a large areaand keeps pressures on the subgrade low.

120 psi120 psi

Characterizing Subgrade SoilsAASHTO Soil Classification

System

Granular Soils w/ <35% passing No. 200 (0.075 mm) sieve Silt-Clay Soils w/ >35% passing No. 200 (0.075 mm) sieve

A-1, A-2 & A-3 A-4, A-5, A-6, & A-7

GI = (F-35)[0.2+0.005(LL-40)] + 0.01(F-15)(PI-10) where, GI = Group Index - follows symbol in ( ). F = % < 0.075 mm

Plasticity Chart for AASHTO Soil Classification SystemCharacterizing Subgrade

Soils

Characterizing Subgrade Soils The Unified Soil Classification System

November 3, 2001

1100 0.2 Top soil, grass, and roots Groundwater

6.0 18 3

1090 12.0 18 8

20.0 18 281080

28.5 10 2230.0

1070 30.0

G. BensonAGB-4

Top Soil Oct/30/2001CL 32335MH TampaCH FloridaSP ASTM D 1586

Notes: Drive (split-barrel)

Hollow Stem Augers

ER = energy ratio per ASTM D-4633

at 8.9 ft on 11/02/01

Boring Terminated at 30'

Sample Recovery

(in)

Soil Sym.

K

Penetration N 60

(blows/ft) Remarks

Elevation (ft-msl)

Sample Depth

(ft)

Stratum Depth

(ft)Visual Soil Description

8.0

Soft red-brown fine to medium sandy CLAY (CL)

14.0

22.5

Loose-Firm gray-blue silty medium SAND (SM)

Firm yellow-tan slightly silty fine SAND (SP-SM to SP)

Firm yellow-white fine to medium SAND, trace silt (SP)

ENGI NEERING SOIL TEST BORI NG RECORD

Boring Number:Date Drilled:

Ef = Energy Efficiency of Hammer Used

Site Location:

Drilling Method:

Soil Symbols K (Unified Soil Classification System)

N = Penetration in blows per foot (ASTM D-1586)

Job Number

Other SymbolsWater Level

N60 = (Ef/60) * Nmeasured = Energy-Corrected N-valueCME-850

(truck mounted)

Driller:

Hammer Type:

Drilling Rig:

Diedrich Automatic (ER =82%)

Sampler:

Test Method:

Subgrade Soil Sampling – A Boring Log

(New Roadway Alignment)

Pavement Related Subgrade Issues

Need for improvement (presentation focus)– As a construction platform– As a structural layer for long-term performance

Frost heave mitigation– Typical treatment – use of select fill material

Swelling soils mitigation– Site specific treatment – lime-treatment,

encapsulation, etc.

Subgrade ImprovementPurpose:

– Improve low strength soil (CBR < 6)– Improve construction conditions

Methods:– Excavation/replacement with select fill – Compaction/mechanical improvement (mixing

in coarser material)– Reinforcement with geosynthetics/geogrids– Chemical stabilization (e.g., lime, cement)

Subgrade “Strength” Parameters

Old (Historical)– California Bearing Ratio (CBR)– R-value– DCP– Modulus of subgrade reaction (k-

value)

New (Better mechanistic representation)– Resilient modulus (as used in the MEPDG)

Historical - Modulus of Subgrade Reaction (k-value)

Primary subgrade design variable for concrete pavement design in the “good old days”– Westergaard models– PCA design procedure, etc.

Measure of soil resistance to vertical pressure

PressureDeflectionk =

Modern - FWD Deflection Testing

(k-value and Resilient Modulus)

Weight

Buffering System

Drop Height

Pavement

{Seven Sensors Spaced at 12 in

Load Plate

Correlation between Soil Type & Strength Properties

• Backcalculation from deflection testing

Soil Type

Silts / Clays

Fine grained

Sands

Gravely soils

Strength

V. Low

Low

Med

High

k-value(psi / in)

50-100

100-150

150-250

250+

ResilientModulus (psi)

1000-2000

2000-2000

3000-4500

4300-4850

CBR

<3

3-6

6-12

>12

Marginal Subgrade Treatmento Removal/replacement & fillso Traditional compaction

(& mechanical stabilization)o Geotextile/geogrid use

o Chemical stabilization (lime & cement)

Removal & Replacement of Weak Soils

Removal of unsuitable/unstable soils (variable depth) within, typically, localized areas and replace with good quality material

Simple but can be expensive

Mechanical Modification of Soil

Use of a thick granular material layer over poor soils or mixing in better graded granular or recycled material with the poor soil

• Geogrids & geotextiles may also be used

Traditional Compaction of SoilLow-cost standard first-

choice optionVarious types rollers

used to densify the subgrade soil & improve the soil properties & provide stable foundation Need moisture-density

management Field compaction testing

(Proctor testing)

Sheepsfoot, rubber-tired

rollers, and static or vibratory steel drum rollers used depending on soil

type

Compaction (Moisture Density relationship)

1t

d w

Soils & granular materials should be compacted to maximum dry density; otherwise poor pavement performance will result

Effect of Compaction Energy

Standard Proctor: 12,300 ft-lb/ft3

Modified Proctor: 56,000 ft-lb/ft3 for truck trafficS = Degree of saturation, %

Geo-Grid Use in Roadway Applications

Photos: Mark H. Wayne, Tensar International

Soil Stabilization Using Admixtures Admixtures

– Lime (quick lime/CaO, hydrated lime/Ca (OH)2)– Cement– Flyash, cement kiln dust (CKD)

Source: National Lime Association & NCHRP Web-only Document 144 – Recommended Practice for Stab. of

Subgrade Soils & Bases

• To improve mechanical/mechanistic (strength & deformation) properties of soils

• Also, to control frost-heave and swelling• Reduces Plasticity Index & increases

strength over time

Soil Modification vs. Soil Stabilization

Soil modification is a less intensive version of soil stabilization to facilitate construction operations

– Usually performed on clay soils– Drying effect & lower PI & volume stability– Some strength gain

Soil stabilization is performed to achieve a target strength & meet durability requirements

– Structural contribution to the pavement system– Mix design need to be developed

Soil Stabilization Decision Process

Soil Stabilization Using Admixtures

Applications– Lime for highly plastic soils– Cement for non-plastic soils, typically– Flyash for soils with little or no plastic fines– Lime and CKD also for treating expansive soils– Typically to a depth of 6 to 12 in.– For deeper stabilization, use of pugmill

Soil Stabilization using Lime(Typically using high calcium lime)

Source: National Lime Association: Lime Treated Soil Construction Manual

Lime Stabilization2 to 8 % lime mixed with soil

– Low level for soil modification/platform; dries wet soils quickly (flocculation)

– Higher level for pavement structural benefits

– Lab testing to determine optimum lime content

Can significantly improve strength of fine grained soils – UC Strength 50 psi in 28 days

Moderate improvement in granular soilsAlso, reduces swelling in highly plastic soils

Lime Stabilization – Mix Design

Initial evaluation – good soil candidates– >25% passing 75 micron sieve & PI>10

Eades & Grim test– Minimum lime to bring PH to 12.4

Proctor test – Optimum mc & max dry density

Unconfined compressive strength tests– Adjust lime % to achieve desired strength

Lime Stabilization: Durability Issues

With some soils fatigue cracking is an issue Water reduces strength by 15 to 25%

– test soaked Freeze-thaw reduces strength (may reverse during warm

periods) Leaching Carbonation may revert lime and make it unstable Sulfate attack may cause swelling

– swell test should be performed

Initial Subgrade Preparation(Common step for any admixture

stabilization)Shape area to crown and gradeCorrect unstable subgrade areasInitial soil moisture content

– Pre-wet if too dry– Aerate/dry if too wet

Reshape to crown and grade

Lime Stabilization: Construction Steps

Scarifying/pulverizing soil to specified deptha. Provides more surface area for lime to interact

b. Remove non-soil material larger than 3 in.

Spreading lime (uniformly across or in windrows)a. Dry quicklime, Dry hydrated lime

b. Slurry lime (~40% quicklime (hot) or hydrated lime)

Note: depending on mixing equipment, dry lime may be applied without scarifying the soil (but more lime loss due to wind)a. Slurry is always applied over scarified soil

Lime Stabilization: Steps

Dust Problem

Scarifying Lime spreading

Slurry application

Lime Stabilization: Construction Steps

Initial mixinga. To distribute the lime to required depth

b. For heavy clays, mixing may need to be done a second time after 24 to 48 hours

c. Shape and lightly compact the mixture

Mellowing perioda. Allows chemical reaction to take place (break down of

the soil)

b. Period: 1 to 7 days, based on soil type

Lime Stabilization: Construction Steps

Final mixing & pulverization• Mixing/pulverization should continue until 100% of

non-stone material passes the 1 in. sieve and >60% non-stone material passes the No. 4 sieve

Lime Stabilization: Steps

Scarifying after lime addition Water addition

Rotary mixer for initial mixingFinal mixing & pulverization after mellowing period

Lime Stabilization: Construction Steps

Compaction – Density ~95% of Standard Proctor (based on lab curve)

– Compaction begins immediately after final mixing

– Initial – using sheepsfoot type roller

– Final – steel wheel roller

Lime Stabilization: Construction Steps

Curing – Moist curing for up to 7 days or until covered up – light sprinkling and rolling as needed

– Membrane curing – sealing compacted surface with AC emulsion – prevent loss of moisture at the surface

Placing the next paving layer

Soil Stabilization using Cement(typically for non-plastic soils)

Steps very similar to those for lime stabilization

Cement Stabilization – Lab Testing

Misture Design1. Laboratory tests on soils

• Sieve Analysis (ASTM C136)• Atterberg Limits (ASTM D4318)

2. Determine admixture content • Conduct moisture density test with mix design• Prepare specimens at anticipated construction condition• Cure samples• Determine durability – F-T & Wet-dry• Determine compressive strength

Typical Cement Contents

Gravels A-1a A-1b

3-5% by weight 5-8%

Sands A-2 A-3

5-9% 7-11%

Silts A-4 A-5

7-12% 8-13%

Clays A-6 A-7

9-15% 10-16%

Mix Design Moisture/Density/Strength

Relationships

Standard Proctor ASTM D558

Modified ProctorASTM 1557

Mix Design Moisture/Density/Strength

Relationships

Soil-Cement Wet-Dry &Freeze-Thaw Tests

Wet-Dry– Soaked in water for 5 hrs,

dried for 42 hrs– Brushed– Repeat 12 times

Freeze-Thaw– Placed into freezer for 24

hrs, thawed for 23 hrs– Brushed– Repeat 12 times

PCA Mix Design CriteriaWeight Loss & Strength

Cement Stabilization - Mixing

Mixed-in-place– Spread portland cement and mix– Apply water and re-mix

Central mixing plant (pug mill)– Mix soil/aggregate, cement and water– Haul mixed material to placing area– Spread soil-cement uniformly

Soil-Cement Pug Mill Mixing

CuringMoisture curingMembrane curing

Curing compound

AC emulsion

QA/QC – Stabilized Subgrade Depth of stabilization check (pH)

– Phenolphthalein spray (clear to red) Typically, compaction tests

performed (density & mc) Strength testing Grade checked – Smooth, uniform

pavement starts at the subgrade– Subgrade uniformity is a key to

pavement performance Proof-rolling may be used for

acceptance and to identify marginal areas

Intelligent Compaction(a European technology being

implemented in the US) Roller Intelligent Compaction (IC) technologies

provide a record of soil compaction properties to the operator via an on-board display unit in the roller cabin in real-time with 100% coverage of compacted areas

Several IC monitoring

technologies are available. Most are vibratory

based systems applied to self

propelled vibratory smooth

drum rollers.

Intelligent Compaction(a European technology being

implemented in the US) Improves quality - allows the operator to identify areas of poor

compaction in real-time and ensures compaction requirements are met the first time

Reduces rework and maximizes productivity Ten roller-integrated compaction monitoring technologies available.

Most of these technologies are vibratory based systems applied to self propelled vibratory smooth drum rollers.

Some Good References1. NCHRP Web-Only Document 144 –

Recommended Practice for Stabilization of Subgrade Soils & Base Materials, 2009

2. SHRP2 R02 material (www.GeoTechTools.org)

3. FHWA Study: Geotechnical Aspects of Pavements• Reference Manual/Participant

Workbook Publication No. FHWA NHI-05-037, 2006

• www.fhwa.dot.gov/engineering/geotech/pubs/05037/index.cfm

4. PCA & National Lime Association publications

Thank You!

stayabji@gmail.com