LTRC QUALITY CONTROL OF TRENCH BACKFILL AT HIGHWAY CROSS-DRAIN PIPES. Zhongjie Doc Zhang, PhD, P.E. October 2004

Transcription

1 QUALITY CONTROL OF TRENCH BACKFILL AT HIGHWAY CROSS-DRAIN PIPES Zhongjie Doc Zhang, PhD, P.E. LTRC October 24 Louisiana Transportation Research Center

2 Purposes of Quality Control Guarantee sufficient surrounding support to pipes for their stability and integrity Provide adequate support to pavement structures at the locations of highway cross-drain pipes

3 Rideability Problem The unexpected deterioration of pavement ride comfort due to dips in pavement surfaces at highway cross- drain locations

4 Field Investigation Indicates Under the same traffic and environmental conditions, pavement dips occurred at some cross-drain pipe locations but not at others Pavement dips occurred mainly due to weaker trench backfill than adjacent subgrade soils

5 Pavement dips and PRR PRR is the ratio of average penetration resistances within and outside of a trench area Penetration Resistance Ratio with dip without dip Case Number

6 Typical DCP Profiles N DCP, Blows Per 1 cm Penetration Asphalt: 9 cm (3.5") Soil Cement: 21.5 cm (8.5") N DCP, blows/1 cm AC (2 cm or 8'' ) 2 Soil cement (15 cm or 6'') 4 concrete pipe 6 Depth, cm cm (36") out of trench in trench 1 in trench 2 Depth, cm Concrete pipe dia. = 66cm Concrete pipe dia. = 66 cm Backfill Backfill Subgrade N DCP, Blows Per 1 cm Penetration asphalt 4 cm (1.5 in). soil cement 2 cm (8 in). N DCP, Blows Per 1 cm Penetration Asphalt: 2 cm (8") Soil Cement: 15 cm (6") 4 4 in trench 1 in trench 2 out of trench 8 metal pipe: 112 cm (44 in.) 8 Depth, cm 12 Depth, cm 12 Concrete pipe: 112 cm (44 in) Site 1: in trench Site 2: in trench Site 3: ou t of tre nc h

7 Field Testing

8 DCP Data Probability Histogram Subgrade Soils Sand Backfill Average N DCP = 7. Median N DCP = Average N DCP = 4. Median N DCP = Probability (%) N DCP (blows / 1 cm) N DCP (blows / 1 cm)

9 Prevention of Pavement Dips To prevent pavement surface dips, trench backfill should be compatible with the adjacent subgrade soils in stiffness, or have a penetration blow count, N DCP, equal to or larger than 1 hammer blows per 4 inch (1cm) penetration in Louisiana

10 Objective of Field Testing Investigate How the criterion of backfill quality could be missed in the field construction What kind of measures should be taken to prevent it from occurring

11 Summary of Backfill Testing Sections Construction Project No. of Trenches Backfill Material Status of Testing In Trench On Subgrade (LA 964) 4 Sand (Compaction) Finished Finished (US 61) 4 Kentucky crushed limestone (Compaction) Finished Finished 1 Sand (flooding) Finished Not finished 1 Bedding material (3% sand, 7% gravel, compaction) Finished Not finished (LA 73/LA 74) 3 Selected soil PI < 1 Finished Finished 1 Mexican crushed limestone Finished Finished 1 Sand (compaction) Finished Not finished (LA 1) 4 RAP Finished Not finished

12 Major Influence Factors on Trench Backfill Quality Contractors Behavior Backfill Material Pipe Cover Layer Configuration of Cross-Drain Trench

13 Contractors Behavior The most important factor in quality control Most are experienced Tension can build in the field when under pressure due to construction delays, under traffic, bad weather, wrong moisture content of backfill, poor drainage conditions They should be a team player and share the consequences of the quality of their construction products, good or bad

14 Major Influence Factors on Trench Backfill Quality Contractors Behavior Backfill Material Pipe Cover Layer Configuration of Cross-Drain Trench

15 Backfill Material Compaction Moisture Adjustment Seepage Stability

16 Gradation of Different Backfills Gradation Curve DOTD Granular upbound 1 9 DOTD Granular lowbound Percent Passing, % sand, LA 964 Sand gravel, LA 964 Kentucky limestone Mexican limestone 1 Selected soil Grain Size, mm RAP

17 Gradation Characteristics of Backfill Material Material Effective size D 1 (mm) D 3 (mm) D 6 (mm) Uniformity Coefficient C u Coefficient of Gradation C c Sand Kentucky limestone Mexican limestone Bedding (Sand gravel) RAP Selected soil.14.48

18 Field Compaction Equipment Wacker Packer Vibratory Roller Vibratory Plate

19 Effects of Compaction NDCP, blows/1 cm Backfill Thickness, cm RAP, light RAP, medium RAP, heavy sand, light sand, medium sand, heavy crushed stone, light crushed stone, medium crushed stone, heavy

20 Field Compaction - Sand Depth (cm) N-DCP (blows/1cm) DCP 1 DCP 2 DCP N-DCP (blows/1cm) N-DCP (blows/1cm) Pipe Depth = 2.5"(52cm) "(225cm) Pipe Depth = "(cm) - 52"(133cm) Depth (cm) N-DCP (blows/1cm) DCP 5 Depth (cm) N-DCP (blows/1cm) DCP (a) Sand Backfill with Good Compaction (b) Sand Backfill with Poor Compaction

21 Field Compaction Kentucky Limestone N-DCP (blows/1cm) N-DCP (blows/1cm) Depth(cm) DCP 1 DCP 2 DCP 3 >15 >15 >15 >15 >15 >15 >15 >15 Pipe Depth=6"(15cm)-48"(122cm) Depth(cm) >15 DCP 1 DCP 2 DCP 3 >15 >15 >15 >15 >15 >15 Pipe Depth=4"(1cm)-52"(132cm) (a) 1-ft Lift (b) 2-ft Lift

22 Field Compaction RAP N-DCP (blows/1cm) N-DCP (blows/1cm) N-DCP (blows/1cm) N-DCP (blows/1cm) DCP 1 2 DCP 2 2 DCP 1 2 DCP 3 Pipe Depth = 16"(41cm) - 66"(168cm) Depth (cm) Depth (cm) Pipe Depth = 6"(15cm) - 56"(142cm) Depth (cm) Depth (cm) (a) 12-inch Lift (b) 18-inch Lift

23 Field Compaction Selected Soil N-DCP (blows/1cm) N-DCP (blows/1cm) DCP 1 2 DCP 4 Pipe Depth = 17"(43cm) - 71"(18cm) Depth (cm) Depth (cm)

24 Compaction Characteristic Summary Material Maximum Dry Density pcf Working Moisture Range, % Compaction in field Standard Proctor Modified Proctor Standard Proctor Modified Proctor Sand Difficult Kentucky limestone Very easy Mexican limestone Very easy Bedding (Sand gravel) Easy RAP Easy Selected soil Not easy

25 Backfill Material Compaction Moisture Adjustment Seepage Stability

26 Moisture Adjustment How fast a backfill material can be dried out in the field Laboratory dry-out test

27 Laboratory Dry-Out Test Results Hours to Optimum Moisture, hour % 5% 1% 15% 2% 25% 3% 35% Moisture Content, % Silt Sand Kentucky limestone Mexican Limestone RAP Bedding Material

28 Typical Moisture Content Values Material Optimum Moisture Content, % Maximum Moisture without water flowout, % Possible Field Moisture, % Field Working Moisture Range, % Sand 5 17 < Kentucky limestone < Mexican limestone < Bedding (Sand gravel) < RAP 6 15 < 15 9 Selected soil < 31 15

29 Backfill Material Compaction Moisture Adjustment Seepage Stability

30 Seepage Stability Resistance to erosion caused by underground water that goes through backfill, or water leaks to the backfill from the joints of cross-drain pipes. Backfill dry density Backfill cohesion or particle interlock Material comparison Density Strength Kentucky limestone: high high Mexican limestone: medium medium RAP: low high Bedding: medium medium Selected soil: low low Sand: low low

31 Summary for Backfill Material Kentucky limestone: The best choice Easy to compact to a high dry density and stiffness Narrow moisture range in the field Strong particle interlock with good seepage stability Followed by Mexican limestone, RAP, bedding material (sand gravel mixture), selected soil, and sand

32 Major Influence Factors on Trench Backfill Quality Contractors Behavior Backfill Material Pipe Cover Layer Configuration of Cross-Drain Trench

33 Pipe Cover Layer Refers to the subgrade soil over pipes It is a buffer zone between a pavement structure and pipes for construction and traffic loading A thick and well compacted cover layer will spread traffic loading well out of trench area

34 Pipe Cover Layer 4.4 feet N-DCP (blows/1cm) Cover Layer 1 = >15 47"(119cm) 53"(135cm) 15 Subgrade Subgrade Crushed Stone Crushed Stone Subgrade Subgrade Pipe 2 DCP 1 DCP 2 DCP 3 DCP 4 DCP 5 DCP 6 Limestone backfill

35 Construction Loading Influence Embedded pressure cells 8" Asphalt Pavement Pressure cells 8.5" Mexican Limestone Subgrade 42" 26.4" layer 1 layer 1 12" Pressure cell 4" 12" Selected Soil

36 Construction Loading on Binder Pressure, psi depth Top Horizontal psi Recorded Data Point depth Top Inclined Bottom Horizontal Pressure, psi psi Recorded Data Point Pressure, psi depth psi Recorded Data Point

37 Traffic loading influence - Stresses at various construction stages 36.8 psi 36.8 psi 36.8 psi 2" Wearing Course 36.8 psi 5" Binder Course 8.5" Soil Cement 25.5" Lime Treated 12" 4.56 psi 3.47 psi.88 psi.86 psi Bottom Guage psi 4.15 psi 1.19 psi 1.17 psi Subgrade S1 S2 S3 S4

38 Major Influence Factors on Trench Backfill Quality Contractors Behavior Backfill Material Pipe Cover Layer Configuration of Cross-Drain Trench

39 Configuration of Cross-Drain Trench Compaction in subgrade backfill areas is important If properly compacted, it can provide a smooth transition in stiffness Otherwise, it can be a weak area if poorly compacted Su bgrade Subgra de Backfill Are a Trench Backfill Are a Sub grade Backfill Area

40 Field Example of Subgrade Backfill Area

41 Unit Cost Comparison Material Material Unit Cost $/cubic yd Sand 5 6 Kentucky limestone 29 Mexican limestone 25 Bedding (sand gravel) 8 15 RAP Selected soil 3 8 Flowable Fill 7 1

42 Conclusions Material: Crushed Limestone is recommended as cross-drain trench backfill if their gradations meet LA Specification 13.3(d) Flowable fill can be used when cost is justified Sand and Selected soil that meet current LA DOTD specification can be used if proper compaction can be reached. More field testing work using RAP and bedding material as backfill should be conducted

43 Conclusions (cont.) Compaction control: Contractors share the consequences of their products by deducting payment for poor construction quality DCP test can be used for final acceptance Average DCP values over the depth of 4 ft within backfill shall at least be 1 blows per 4 penetration (smaller than 1 mm/blow), or Average DCP values over the 4 ft depth within and out of trench areas are compatible with each other

44 Implementation LTRC research project: Alternative Methods to Trench Backfill Project Review Committee recommends a change in the current specifications for Trench backfill Stone aggregate or recycled Portland cement concrete be required for backfill materials for all cross drain pipes and side drain pipes under paved areas of travel lanes, shoulders, and turnouts

45 Implementation (cont.) Each district evaluates the option of using RAP as trench backfill before RAP is officially specified as backfill material The modification of Section 71 is coming THANKS!