UNDERSTANDING THE EFFECTS OF SUBGRADES & SUBBASES ON PAVEMENT SERVICE LIFE. David J. White, Ph.D., P.E. Jerod P. Gross, P.E.

Transcription

1 UNDERSTANDING THE EFFECTS OF SUBGRADES & SUBBASES ON PAVEMENT SERVICE LIFE David J. White, Ph.D., P.E. Jerod P. Gross, P.E., LEED AP

2 Optimizing Pavement Base, Subbase, and Subgrade Layers for Cost and Performance of Local Roads TR640 David J. White, Ph.D., P.E. Associate Professor Director, CEER Pavana KR. Vennapusa, Ph.D. Research Assistant Professor Asst. Director, CEER

3 Acknowledgements Iowa Highway Research Board (TR-640) Snyder and Associates City of Ankeny City of Ames City of Knoxville City of Council Bluffs City of Burlington Story County Winneshiek County CPTech Center, Iowa State University CEER, Iowa State University

4 Summary of Pavements Tested Test sites located in - Central Iowa - South East Iowa - Western Iowa, and - North East Iowa Pavement Age: 30 days to 42 years Surface Distress Conditions: Poor to Excellent (PCI 35 to 100) Support Conditions: - Natural Subgrade - Fly Ash Stabilized Subgrade - 6 in. to 12 in. Granular Subbase Pavement Thickness: 6 to 11 in. Traffic (AADT): 110 to 8900

5 Design Input Parameters Modulus of Subgrade Reaction (k) Composite Modulus of Subgrade Reaction (k c ) Loss of Support (LS) Coefficient of Drainage (C d ) FWD, DCP CHP

6 Section with highest k-values was with 12 in. granular base (9 in. macadam) over subgrade Depth (in) CBR (%) PCC Crushed Limestone (Macadam) A-1-b, GM Subgrade Crushed Limestone (Choke Stone), A-1-b, GM Cumulative Blows Avg. CBR SG = 55.5 COV CBR SG = 30% Avg. CBR SB = COV CBR SB = 24% FWD Static k-value k c-fwd (pci) at Mid Panel FWD Intercept (mils) at Joints and Mid Panel LTE at Joints (%) PCC: 7 to 7.5 in. Age: 16 years PCI: 92 AADT: 660 with 6% trucks Intercept > 2 mils indicate void DCP2 DCP3 DCP4 DCP5 DCP6 DCP7 0DCP Low LTE due to no dowels at joints Distance (ft)

7 Section with lowest k-values was with PCC directly over subgrade PANEL# x x x x x x x x x x x x x x x x x x Cracks on pavement Asphalt Patching x FWD Test CHP Test DCP Test PCC Patching PANEL# x x x x x x x x x x x x x x x x CBR (%) Cumulative Blows Depth (in) PCC Subbase under PCC Patch at DCP8 Subgrade A-4(4), CL CBR SB = 21.0 at DCP8 Avg. CBR SG = 6.8 COV CBR SG = 57% DCP1 DCP2 DCP3 DCP4 DCP5 DCP6 DCP7 DCP PCC: 6 in., Age: 42 years PCI: 35, AADT: 560 with 3% trucks

8 CHP Tests to measure permeability of materials under pavements CHP#1 CHP#2 In-Situ Hydraulic Conductivity, K CHP (ft/day) CHP#2 - Cracked Panel CHP#1 - Good Panel Time (minutes)

9 CHP Tests under existing pavements showed evidence of erosion at the interface In-Situ Hydraulic Conductivity, K CHP (ft/day) Increase in K attributed to erosion at subgrade/ pavement interface Time (minutes) White and Vennapusa (2013)

10 Section with poor utility backfill compaction showed evidence of loss of support and nonuniformity In Situ Test Locations and Crack Map 22 Panels tested on SW Westlawn Drive Just South of SW 4th St., Ankeny PANEL# PCC: 9 in., Age: 4 years PCI: 85, AADT: 1000, 1% trucks x x x x x x x x x x x x x x x x x x x x 11 Cracks On Pavement Utility Gasline x FWD Test CHP Test DCP Test DCP Test from 2008 x x x x x x x x x x x x x x x x x x x x x x Utility Gasline Woven Geosynthetic under Panels PANEL# Fire Hydrant

11 Section with poor utility backfill compaction showed evidence of loss of support and nonuniformity Water draining through void at the pavement/base interface FWD D 0 (mils) for 9,000 lbs at Joints and Mid Panel FWD Static k-value k c-fwd (pci) at Mid Panel FWD Intercept (mils) at Joints and Mid Panel Intercept > 2 mils indicate void Old DCP5 DCP8 CHP1 DCP6 Old 10 DCP4 DCP3 DCP7 CHP Longitudinal Crack 150 Transverse Woven Geosynthetic at Crack on Panel Subgrade/Subbase Interface Utility Gas Line Old DCP4 Utility Gas Line Old DCP In-Situ Hydraulic Conductivity, K CHP (ft/day) CHP#1 - No Geosynthetic CHP#2 - Geosynthetic Section Time (minutes)

12 Comparison of DCP profiles during and 4 years after construction Indicate increase in CBR due to potential postconstruction settlement and moisture content variations CBR (%) CBR (%) PCC Crushed Limestone Subbase 12 PCC Crushed Limestone Subbase Woven Geosynthetic Subgrade Subgrade Depth (in) DCP2 DCP3 DCP4 DCP5 DCP6 Old DCP2 Old DCP3 Old DCP DCP7 DCP8 Old DCP

13 k-values from FWD and DCP Static k c-fwd (pci) Sites 82 tests 1 Site 12 tests 5 Sites 100 tests 1 Site 20 tests 2 Sites 41 tests W38 Locust Rd 9 in. Macadam Subbase 1 Site 18 tests 1 Site 20 tests Westlawn Dr., Poorly compacted utility backfill in subgrade k c-dcp (pci) SG 5 Sites 35 tests FA-SG 3 to 6 in. SB + SG 1 Site 5 tests 3.5 in. SB + FA-SG 5 Sites 36 tests 1 Site 6 tests 2 Sites 16 tests 12 in. SB + FA-SG 8.5 to 10 in. SB + SG W38 Locust Rd 9 in. Macadam Subbase 12 in. SB + SG 1 Site 7 tests 1 Site 7 tests Westlawn Dr., Poorly compacted utility backfill in subgrade 0 SG FA-SG 3 to 6 in. SB + SG 3.5 in. SB + FA-SG 12 in. SB + FA-SG 8.5 to 10 in. SB + SG 12 in. SB + SG

14 Comparing k-values from FWD and DCP provides an indication of LOS Average Static k c-fwd (pci) AASHTO (1993) Recommended LOS Values: Untreated Granular Materials to 3.0 Fine-Grained Materials to 3.0 LOS = 0 SUDAS LOS = 1 LOS = 1 LOS = 2 LOS = Average k c-dcp (pci) NW Greenwood & 3rd St. - Subgrade NW Greenwood & 5th St. - Subgrade E63 - Subgrade Riverside Rd.- Subbase E23 - Subgrade Westlawn Dr.- Subbase SW Logan St. - Subbase W. Main St.- Subbase S. 5th St.- Subbase Valley View Dr.- Subbase 9th Ave.- Subgrade Cliff Rd (Site 1) - Subbase Cliff Rd (Site 2) - Subbase Meadowbrook Dr.- Subbase W38 Locust - Subbase 175th St.- Subgrade

15 CBR values of subgrade and base layers 1000 CBR SG (%) Sites 35 tests 1 Site 5 tests 5 Sites 36 tests 1 Site 6 tests 2 Sites 16 tests 1 Site 7 tests 1 Site 7 tests VERY GOOD FAIR-GOOD POOR-FAIR VERY POOR 1 Ratings provided in SUDAS (2013a) CBR Sb (%) SG EXCELLENT VERY GOOD GOOD FA-SG Ratings provided in SUDAS (2013a) 5 Sites 36 tests 3 to 6 in. SB + SG 1 Site 6 tests 3.5 in. SB + FA-SG 12 in. SB + FA-SG 2 Sites 16 tests 1 Site 7 tests 8.5 to 10 in. SB + SG 1 Site 7 tests 12 in. SB + SG 1 SG FA-SG 3 to 6 in. SB + SG 3.5 in. SB + FA-SG 12 in. SB + FA-SG 8.5 to 10 in. SB + SG 12 in. SB + SG

16 In Situ Permeability and C d values Sites 8 tests Westlawn Dr., Void Underneath Pavement 1 Site 2 tests 10 K CHP (ft/day) 1 5 Sites 8 tests 1 Site 1 test 1 Site 1 test 2 Sites 4 tests 1 Site 2 tests SG EXCELLENT Plot 1 GOOD 4 Sites 8 tests FA-SG 5 Sites 8 tests 3 to 6 in. SB + SG 3.5 in. SB + FA-SG Westlawn Dr., Void Underneath Pavement 12 in. SB + FA-SG 1 Site 2 tests 8.5 to 10 in. SB + SG 12 in. SB + SG C d 0.9 FAIR 1 Site 2 tests SUDAS C d = POOR VERY POOR 1 Site 1 test 1 Site 1 test 2 Sites 4 tests SUDAS C d = SG FA-SG 3 to 6 in. SB + SG 3.5 in. SB + FA-SG 12 in. SB + FA-SG 8.5 to 10 in. SB + SG 12 in. SB + SG

17 Field results indicate that the following are key factors affecting performance Poor support (due to low stiffness or CBR) Poor drainage Seasonal variations (freeze-thaw and frost-heave) Shrink-swell due to moisture variations Loss of support (due to erosion, non-uniform settlement, curling/warping) Poorly compacted utility trench backfill Differential settlement of foundation layers Overall non-uniformity

18 Recommendations for typical foundation treatment options to improve performance Foundation treatment Engineered Subgrade and Backfill Compaction with Moisture, Density, and Lift Thickness Control Portland Cement Stabilization of Subgrade Fly Ash Stabilization of Subgrade (Self-Cementing) Lime Stabilization of Subgrade Granular Subbase (Untreated) Cement or Asphalt Stabilization of Subbase Cement + Fiber Stabilization of Subbase Geotextile Separation Layer at Subbase/Subgrade Interface Geogrid Reinforcement at Subbase/Subgrade Interface Geocomposite Drainage System at Subbase/Subgrade Interface Granular Macadam Subbase with Choke Stone Cover Emulsified Asphalt Stabilized Granular Macadam Subbase Issues that can be mitigated Poorly compacted utility trench backfill Differential settlement of foundation layers Loss of support (due to non-uniform settlement) Shrink-swell potential due to moisture variations (if high plasticity clays are excavated and replaced with engineered fill) Frost-heave and thaw-softening Shrink-swell potential (applicable for high plasticity clays) Wet/soft subgrade conditions during construction (to serve as construction platform) Non-uniformity of stiffness 1 Wet/soft subgrade conditions during construction (to serve as construction platform) Shrink-swell potential (applicable for high plasticity clays) Non-uniformity of stiffness 1 Shrink-swell potential (applicable for high plasticity clays) Non-uniformity of stiffness 1 Poor drainage 2 Frost-heave 3 and thaw-softening Poor support (low stiffness) 4 Poor drainage 5 Poor support (low stiffness) Frost-heave and thaw-softening Poor drainage 5 Poor support (low stiffness) Frost-heave and thaw-softening Poor drainage 6 Poor support (low CBR) Poor support (low CBR) Poor drainage Poor drainage 7 Poor Support (low stiffness and CBR) 8 Frost-heave and thaw-softening Poor drainage 9 Poor Support (low stiffness and CBR) 9 Frost-heave and thaw-softening 1 N if i b i ll d d if i h i f ll d (i f if i i i

19 Additional information: David J. White, Ph.D., P.E. Associate Professor and holder of Richard L. Handy Professorship Director, Center for Earthworks Engineering Research (CEER) Department of Civil, Construction and Environmental Engineering Iowa State University Phone:

20 Subbase Design Guidelines

21 Subbase & Pavement Design Good design applies these parameters: Actual soil characteristics (subgrade) Soil preparation Use of subbase/stabilization Traffic volumes including % trucks Design Life 20 to 50 years Expected traffic growth per year over design life Natural Subgrades

22 Why consider subbase? + Construction platform + Uniform support + Reduce delays + Drainability + Capillary cutoff - Upfront cost

23 Subbases PCC thickness is not highly sensitive to support stiffness (k) Not always cost effective to increase subbase for structure

24 PCC Pavement Design Parameters AASHTO 93 Coefficient of Drainage, Cd Modulus of Subgrade Reaction, k Loss of Support, LOS Truck Traffic (quantity and type)

25 PCC Pavement Design Parameters

26 PCC Pavement Design Parameters Serviceability Ability to serve traffic (condition rating from 5 to 0) P o Initial Serviceability P t Terminal Serviceability

27 PCC Pavement Design Parameters Reliability is the probability that the design will succeed for the life of the pavement.

28 Pavement Thickness Design Use Modulus of Subgrade Reaction k - Concrete needs uniformity of support - Modulus of Subgrade reaction, k = M R / Composite Modulus of Subgrade Reaction = (k c ) Strength corrected from subbase material Reaction Hydraulic Jack Stacked Plates Pressure Gauge Deflection Dial at 1/3 Points k (psi/in) = unit load on plate / plate deflection

29 Pavement Thickness Design Soil Resilient Modulus M R - Stiffness or elasticity of soil under dynamic loading - Calculated based on California Bearing Ratio (CBR) CBR Value M R Value MR= x CBR Simple strength test comparing a given soil with wellgraded crushed stone

30 Pavement Thickness Design Vehicle Type Percent of Total Trucks Loading Percent of Truck Type Vehicle Axle Type Axle ESAL Factor LEF Weight S - Single Load (per axle) (by Vehicle) (lbs) TA - Tandem (lbs) Rigid Flexible Rigid Flexible Single Unit Front - S 7, Empty 30% 14,500 (2 axles) Rear - S 7, (Class 5/6 Truck) Partial Load Front - S 8, % 50% 20,500 (50% Capacity) Rear - S 12, Fully Loaded 20% 26,000 Front - S 9, Rear - S 17, Dump Trucks - 3 axles Front - S 10, Empty 50% 22,000 (Class 7/8 truck) Rear - TA 12, % (doesn't address Front - S 20, cheater axles) Fully Loaded 50% 54,000 Rear - TA 34, Semis Front - S 12, (5 axles) Empty 20% 26,000 Rear - TA 7, % Partial Load (50% Capacity) Fully Loaded 60% 20% 53,000 80,000 Trailer - TA 7, Front - S 13, Rear - TA 20, Trailer - TA 20, Front - S 20, Rear - TA 34, Trailer - TA 34, Composite Load Equivalency Factor (LEF) for "Trucks" SUDAS Ch. 5F-1 (High Volume) ESAL factors for individual axles were determined from AASHTO Design Guide. ESAL factors do not account for directional split. If 50/50 split, then divide ESAL factor in half Iowa DOT traffic counts were used to develop this table.

31 Pavement Thickness Design 1993 AASHTO Rigid Pavement Structural Design Or use SUDAS Chapter 5F-1

32 SUDAS Design Values Field Test Calculated

33 Parameter Values Design & Actual Loss of Support 1 (subgrade) 0 (subbase) SUDAS Subgrade Subbase CBR SG to to to 2.0 CBR SB 55* Modulus of Subgrade Reaction Coefficient of Drainage, C d k c (6 ) k ** 1.0 (subgrade) 1.1 (subbase) TR640 Field Tests k=50 to 133 k c =66 to to to 0.99 * Based on 30,000 M R ** Per SUDAS, k is adjusted by reducing 200 to 300 if no subbase is used (due to loss of support)

34 TR640 Summary What did we find out about Loss of Support with TR 640 testing? k c-fwd were on average of 4 to 10 times lower than k c-dcp k c-fwd values provide a direct measure of stiffness while k c-dcp is empirically estimated

35 TR640 Summary The k c-dcp values do not account for loss of support under pavements in situ while the k c-fwd values do Loss of support factors were within AASHTO recommended design, but higher than SUDAS suggested values (1 for natural subgrade, 0 for subbase)

36 What are the implications? Found variable parameter values Further analysis of TR640 test results How reliable are our pavements?

37 Consequence of using assumed design 7.75 parameters ,400, WinPAS12 Solve for Reliability Lower reliability = Lower performance Higher maintenance costs Reliability

38 TR640 Data Street Name Age PCC Thickness (in.) AADT Percentage of Trucks Number of Trucks (2way) PCI - Test Site Only PCI- from IPMP Reliability Reliability Design Value (using - SUDAS WinPAS 12 AASHTO 93) NW Greenwood St % *75 83% 80% NW Greenwood St % *40 82% 80% E , % % 88% Riverside Rd % % 88% E , % % 88% SW Westlawn Dr , % % 80% SW Logan St % % 80% West Main St , % % 80% South 5th St , % % 80% Valley View Dr , % % 88% 9th Ave % % 88% Cliff Rd % % 80% Cliff Rd , % % 80% Meadowbrook Dr % % 80% W38 Locust Rd , % % 88% 175th Street , % % 88%

39 Life Cycle Cost How can we predict when maintenance is needed? How much maintenance and what will it cost?

40 MEPDG (Mechanistic Empirical Pavement Design Guide) Life Cycle Cost Determines loading directly from axle configurations and weights (No ESALs) More precise characterization of traffic but relies on the same input data used to calculate ESALs Costly Program

41 MEPDG

42 Life Cycle Cost Study two equal pavements - one w\subbase, one without Run MEPDG Then complete a life cycle cost estimate And the winner is.

43 Life Cycle Cost New Construction Subgrade Preparation Subdrain Subbase (8 thickness) Fly-Ash Treated Subgrade Cement Treated Subgrade Geosynthetics PCC Pavement (7 ) Costs $2 per square yard $11 per lineal foot $10 per square yard $8 - $10 per square yard $8 - $10 per square yard $2 - $5 per square yard $40 per square yard

44 Life Cycle Cost Rehabilitation Partial Depth Patching** Full Depth Patching (M Mix) Joint & Crack Cleaning, Routing & Sealing* Joint & Crack Cleaning & Sealing* Diamond Grinding Dowel Bar Retrofit Costs $25-$30 per square foot $65-80 per square yard $2.00 per lineal foot $1.25 per lineal foot $ $6.70 per square yard $25 - $35 per dowel bar *If silicones are used, cost is approximately 2 times more than hot pour sealants. ** Cost on Major Quantities of Longitudinal patching can be $12-$20 per square foot. Cost on night work can be as high as $55 per square foot.

45 What have we learned? Soil modulus (M R ) varies during seasons and throughout life Distress related to Loss of Support Stabilization provides long term freeze thaw durability & stiffness Subbase isn t the only answer - treat the subgrade Factors critical in pavement design Truck Traffic, C d, k & k c

46 Where do we go from here? Consider range of LOS values Use realistic design parameters or consider mitigation Pay attention to the subgrade Continued testing ospring conditions ore-visit same sites TR 640 Design Guide

47 Thank You David J. White, Ph.D., P.E. Associate Professor Director CEER Jerod Gross, PE, LEED AP Project Manager