Earthworks and Layerworks Theory and Construction Manual

Objectives and Principles of Roadworks and Earthworks

• Chapter Objectives:

  • Introduce terminology and basic principles for the design and construction of roadworks.

  • Establish principles for road cuttings and embankments.

  • Provide methods for the measurement and payment of earthworks.

  • Introduce and analyze mass-haul diagrams.

• The Pavement Structure:

  • A pavement is composed of layers designed to distribute traffic loads to the in-situ soils beneath the roadway.

  • Each constituent layer must possess sufficient strength to resist imposed stresses.

  • The greatest strength is required at the surface to resist lateral and vertical stresses from vehicle wheels.

  • Layer thickness must be sufficient to ensure that the layer immediately below is not overstressed.

• Earthworks Categories:

  1. Below Formation Level: Consists of excavations (cut) or embankments (fill) required to reach the platform level.

  2. Above Formation Level: Generally consistent structural pavement layers of specified thickness (selected sub-grade, sub-base, and base).

  • Formation Level: Defined as the underside of the structural pavement layers (the bottom of the selected sub-grade).

Design and Construction of Road Cuttings

• Guidelines: Refer to TRH 18: The investigation, design, construction and maintenance of road cuttings.

• Site Investigation Requirements:

  • Essential for long-term stability and identifying construction methods (blasting, ripping, or scraping).

  • Methods include trial pits, auguring, and drilling.

  • Identification of discontinuities (joints, bedding planes, lenses, faults, and slip planes) is critical.

• Problem Area Identification:

  • Landslides/Slips: Due to removal of lateral support.

  • Geological Factors: Sedimentary rocks with planes sloping toward the road; boulder formations in erodible matrices.

  • External Forces: Seismic forces and sub-surface water seepage.

  • Early Indicators:

    • Topography: Scars, terracing, hummocky ground, bent trees, or displaced fences.

    • Geology: Local soil and geological maps.

    • Water: Most slides are triggered by water; surplus moisture zones require assessment.

• Slopes of Cuts (Batters):

  • Defined as $1:X$ ($1\text{ Vertical to } X\text{ Horizontal}$).

  • Standard Minimum Slopes:

    • Soft Material: $1:1.5$ (or $1:2$ if vegetation is required).

    • Intermediate Material: $1:1$.

    • Hard Material: $1:0.25$.

• Slope Stability Factors of Safety (FOS):

  • Freeways/Major Interurban Roads: $1.50$.

  • Major Rural Roads: $1.25$.

  • Lightly Trafficked Roads: $1.10$.

• Analysis Methods:

  • Slip circle analysis.

  • Planar slip failure.

  • Non-circular slip surfaces.

  • Wedge failures.

  • Finite element methods.

• Construction Procedures:

  • Topsoil Stripping: Usually $150\text{--}200\text{ mm}$ depth; stockpiled for restoration.

  • Excavation: Scrapers for soft; ripping or blasting for intermediate/hard rock.

  • Rock Protection: "Barring down" loose rocks using crowbars; ditches at the slope foot; fencing.

  • Retaining Structures: Reinforced concrete or gabion walls.

• Drainage Systems:

  • Catchwater drains (at the top or on benches).

  • Side drains (through the cutting).

  • Interceptor drains (for springs/seepage).

  • Subsurface drains (for groundwater).

Embankments (Fills)

• Guidelines: Refer to TRH 10: The design of road embankments.

• Design Considerations:

  • Effect of fill weight on in-situ sub-grade (settlement and instability).

  • Stability of fill slopes (frequently using slip circle analysis).

  • Settlement predictions (differential settlement is more critical than even settlement).

• Construction Steps:

  • Clearing: Removal of trees, bush, rubbish, and structures.

  • Grubbing: Removal of stumps and roots; holes backfilled and compacted.

  • Roadbed Preparation: Removal of unsuitable/wet materials; replacement with a "pioneer layer" (dump rock) if necessary.

• Roadbed Compaction Methods:

  • Rolling (pneumatic, vibrating, or impact).

  • Impact Rollers: Three or four-sided; effective for collapsing sands; can produce settlements up to $0.5\text{ m}$.

  • Scarifying, watering, and recompacting to usually $90\text{--}100\text{ percent}$ Mod AASHTO.

• Fill Layer Construction:

  • Layers typically $150\text{--}200\text{ mm}$ thick.

  • Strict moisture control at Optimum Moisture Content (OMC).

  • Special measures for high fills: sand filter blankets, vertical drains for pore water pressure relief, and surcharging.

Classification and Measurement of Earthworks

• Material Classifications:

  • Soft: Excavated by scrapers/loaders without prior loosening.

  • Intermediate: Weathered rock requiring ripping but not blasting.

  • Hard: Hard rock requiring blasting or heavy ripping.

• Measurement for Payment:

  • Based on fill volumes shown on drawings "in-situ" after compaction to specified density.

  • Shrinkage factors are critical because in-situ density ($75\text{--}88\text{ percent}$ Mod AASHTO) is lower than fill density ($90\text{--}100\text{ percent}$ Mod AASHTO).

• Volumetric Terminology:

  • Bank Volume ($B$): In-situ, undisturbed material.

  • Loose Volume ($L$): Transported material that has swelled/bulked.

  • Compacted Volume ($C$): Final volume after compaction.

• Conversion Factors:

  • $\text{Shrinkage Factor} = \frac{\text{Compacted Volume}}{\text{Bank Volume}}$ (typically < 1.0 for soils).

  • $\text{Swell Factor} = \frac{\text{Compacted Volume}}{\text{Bank Volume}}$ (typically > 1.0 for rock).

• Method of End Areas:

  • $V_{s \rightarrow e} = \frac{A_s + A_e}{2} \times L$

  • Where $A_s$ is area at start, $A_e$ is area at end, and $L$ is distance.

Mass-Haul Diagrams (MHD)

• Definitions:

  • Haul: Distance material is moved along the centreline.

  • Free Haul: Specified distance (e.g., $150\text{--}500\text{ m}$) where haul cost is included in excavation price.

  • Overhaul: Distance beyond free haul; paid in $m^3 \times m$ or $m^3 \times km$.

  • Economic Haul: Point where cost of hauling equals cost of borrow and spoil.

• MHD Characteristics:

  • Ordinate represents accumulated volume at a station.

  • Maximum Ordinate: Change from cut to fill.

  • Minimum Ordinate: Change from fill to cut.

  • Rising Curve: Excess excavation (cut).

  • Falling Curve: Excess embankment (fill).

  • Convex Loop: Haul direction is left to right.

  • Concave Loop: Haul direction is right to left.

  • Balancing Line: A line parallel to the base; intersections are "balancing points" where cut equals fill.

• Average Haul Distance Estimation:

  • Estimated by dividing the area between the balance line and curve by the maximum ordinate.

  • Approximated by drawing a horizontal line through the midpoint of the maximum ordinate.

• Limitations:

  • Limited to linear construction (longitudinal movement).

  • Does not handle multiple adjacent projects.

  • Assumes all excavated material is suitable for fill.

Layerworks

• Components and Thicknesses:

  • Selected Sub-grade: $1\text{ or } 2\text{ layers}$, $100\text{--}150\text{ mm}$ thick; fair quality natural soils/sands.

  • Sub-base: $1\text{ or } 2\text{ layers}$, $100\text{--}150\text{ mm}$ thick; good gravels, sometimes stabilized.

  • Base: $1\text{ layer}$, $100\text{--}150\text{ mm}$ thick; crushed rock or high-quality gravel.

• Construction Process:

  1. Place correct volume for final compacted thickness.

  2. Apply water according to Dry Density/OMC tests (Method A7).

  3. Mix using a motor grader or disc harrow.

  4. Compact with suitable rollers; finish to specified camber and level.

Soil Compaction Principles

• Objectives:

  • Reduce void ratio and permeability.

  • Increase shear strength and bearing capacity.

  • Reduce settlement under traffic vibration.

• Optimum Moisture Content (OMC):

  • The water content at which maximum dry density is achieved for a given compactive effort.

  • Hand Test:

    • Crumbles easily: Below OMC.

    • Firm ball: Near OMC.

    • Wet/Glistens: Above OMC.

• Compactive Efforts (Laboratory):

  • Mod AASHTO: $4.536\text{ kg}$ hammer, $457.2\text{ mm}$ drop, $5\text{ layers}$, $55\text{ blows/layer}$.

  • Proctor: $2.495\text{ kg}$ hammer, $304.8\text{ mm}$ drop, $3\text{ layers}$, $55\text{ blows/layer}$.

• Roller Selection Guide:

  • Fine-grained cohesive soils (Clay): Sheeps-foot, tamping-foot, or heavy pneumatic rollers.

  • Coarse-grained gravels (Sand): Vibrating rollers, grid rollers, or steel-wheel rollers.

  • Rock-fill: Heavy vibrating smooth-wheeled or impact rollers.

• Density and Water Calculations:

  • $\text{Mass of water lost} = \text{Mass of wet soil} - \text{Mass of dry soil}$

  • $\text{Moisture Content (\text{\%})} = \frac{\text{Mass of water}}{\text{Mass of dry soil}} \times 100$

  • $\text{Water added per } m^3 = \frac{\text{Maximum Density}}{100} \times \text{Percentage Difference}$

Construction Productivity and Logistics

• Haul Efficiency by Machine:

  • Wheel-loaders: < 50\text{ m}.

  • Bulldozers: Highly efficient $30\text{--}60\text{ m}$; inefficient > 100\text{ m}.

  • Scrapers: Efficient medium haul, up to $1\text{ km}$.

  • Loader-Truck combinations: > 1\text{ km}.

• Productivity Example (CAT 950F Loader):

  • Bucket capacity: $2.55\text{ } m^3$; Fill factor: $95\text{ percent}$.

  • Cycle time: $0.59\text{ minutes}$.

  • Efficiency ($50\text{ minute hour}$ or $80\text{ percent}$): $195\text{ } m^3/\text{hour}$.

  • Truck requirement: If truck cycle is $20\text{ min}$, trips/hr = $2.5$. Trucks needed = $\frac{\text{Loader Productivity}}{\text{Truck Productivity}}$.

Geotechnical Evaluation Tools

• California Bearing Ratio (CBR):

  • Measures soil strength through piston penetration resistance.

  • Standards: $13.344\text{ kN}$ at $2.54\text{ mm}$, $20.016\text{ kN}$ at $5.08\text{ mm}$, and $25.354\text{ kN}$ at $7.62\text{ mm}$.

• Dynamic Cone Penetrometer (DCP):

  • Portable tool for field strength measurement.

  • Formula for CBR Conversion:         $CBR = \frac{401.8}{(\text{DCP Number})^{1.284}}$

  • DCP Number is measured in $\text{mm/blow}$.

Self-Evaluation Questions

1. What is "formation level" and how is it useful in estimating quantities for roadworks?

2. Why are the steepest slopes of cuttings given as $1:1.5$ for soft soil, $1:1$ for intermediate materials and $1:0.25$ for hard rock?

3. What conclusion would you draw after a site inspection of hummocky, uneven ground with displaced fences?

4. Why is topsoil stripped and stockpiled?

5. Explain the difference between "clearing" and "grubbing".

6. Why are road-making soils wetted with water before compaction?

7. How do you deal with excessive pore water pressures below high embankments?

8. Give the main soil classifications used for payment.

9. Define "bank volume", "loose volume", and "compacted volume".

10. Define "shrinkage factor" and "swell factor".

11. If a material bulks $10\text{ percent}$ and loses $15\text{ percent}$ of loose volume on compaction, calculate shrinkage factor.

12. If a material bulks $50\text{ percent}$ and loses $30\text{ percent}$ of loose volume on compaction, calculate swell factor.

13. What are "haul", "freehaul", and "overhaul"?

14. Calculate the volume of earthworks for a roadway using the method of end areas (Standard units).

15. Plot a mass haul diagram for a $2\text{ km}$ roadway with a $96\text{ percent}$ shrinkage factor; comment on balance.