203 Roadway Excavation and Embankment

203 Roadway Excavation and Embankment

Introduction

Importance of Proper Embankment Construction

Importance of Proper Excavation

Materials (203.02)

Restrictions (203.03)

General Construction (203.04)

Embankment Construction Methods (203.05)

Benching

Spreading and Compacting (203.06)

Compaction and Moisture Requirements (203.07)

Method of Measurement (203.09)

Basis of Payment (203.10)

Documentation Requirements – 203 Roadway Excavation and Embankment

Introduction

After many years of solving soil and rock problems throughout the state, the author of this section can assure the reader of ‘One Constant’.

“Soil and Rock Conditions Vary, Vary and will Vary Again.”

The author could repeat this statement a hundred times throughout this manual and it would be a hundred times too few.

Earthwork consists of roadway excavations (cuts) and roadway embankments (fills) for highways and associated items of work. Earthwork includes all types of materials excavated and placed in embankment, including soil, granular material, rock, shale, and random material. Associated items of work considered to be in the broad range of earthwork that includes: preparation of foundations for embankment, disposal of excavated material, borrow, preparation of the subgrade, proof rolling, rock blasting, base construction, and berm aggregate construction.

If pavement is to remain smooth and stable during years of service under traffic, the earthwork on which it is built must be stable and must furnish uniform support. Where roughness, settlements and other distress develop in pavement during service under traffic, the cause often is a deficiency in the stability of earthwork, which supports the pavement.

Uniformity of earthwork is necessary and important to obtain high stability and long-term performance at all locations throughout the length and width of the project. Consider, for example, a highway project where 95 percent of the earthwork was performed according to the specifications. But 5 percent was non-specification and low-stability material, which appeared in many small areas throughout the project. Pavement roughness and distress developed in these areas during service under traffic loading. Such a project probably would be evaluated by the traveling public as a “rough job” or a “poorly constructed” project. No notice or credit would be given to the 95 percent of the work, which was constructed properly. The entire project might be discredited and be considered poor because a relatively small proportion of the project was constructed with poor earthwork construction procedures or practices.

The foregoing assumed example is intended to illustrate the need for consistent compliance with earthwork specifications in all areas, both large and small, throughout the length of the project, and throughout the time from the beginning to the end of earthwork construction.

Importance of Proper Embankment Construction

The embankments that ODOT constructs are structures. The success of these structures is directly proportional to the project’s emphasis on correct embankment techniques.

The importance of proper construction practices during construction cannot be overemphasized. The results of improper construction practices may or may not show up during construction. But, these improper practices will eventually become evident at some point during the life of the embankment structure.

The construction requirements in the specifications are written to maximize the embankment structure’s life. When the specifications are not followed, the life expectancy will decrease and the future maintenance cost will increase.

The embankment structure is shown in Figure 203.A. The structure consists of three main components:

1. Foundation
2. Embankment
3. Pavement

A geotechnical engineer ensures that the embankment will be stable as designed. The pavement is constructed on top of the embankment.

Figure 203.A – Embankment Structure

The embankment that is shown in the plans structurally bridges the foundation and supports the pavement. The embankment is built by compacting layers of materials in horizontal lifts, as shown in Figure 203.B. These lifts consist of soil, granular material, rock, shale, asphalt, concrete, or recycled materials. The embankment’s resistance to movement relies on the proper construction of these lifts. These lifts work together as a unit to resist the loads.

Figure 203.B – Embankment Layers

A condition such as the one in Figure 203.C can occur if an embankment is not properly constructed. When this condition occurs the Factor of Safety is less than 1.0 and the embankment fails.

Figure 203.C – Embankment Failure

A factor of safety is the ratio of the Resisting Forces divided by the Driving Forces, as shown in the following equation.

Minimum factors of safety for embankment structures are low, on the order of 1.2 to 1.5. Figure 203.D illustrates the resisting and driving forces. The weight of the fill works to move the foundation and the embankment counter clockwise to the right. The internal strength of the embankment layers and the foundation work together to support the pavement. Failure may occur in a circular fashion as shown, in a semi-circle, in a block mode or wedge. The basic principles are the same in all three modes of failure.

Figure 203.D – Resisting and Driving Forces for Embankment Failure

Importance of Proper Excavation

Proper excavation techniques in cut sections are just as important as embankment construction; the only difference is that in the failure mode the rock or soil falls onto the roadway instead of the roadway failing.

This is illustrated in Figures 203.E-1 and 203.E-2. If a soil cut is cut too steep then the soil can flow on to the roadway as illustrated in 203.E-1. This figure shows a deep-seated wedge failure. This failure can occur in an embankment condition also.

Figure 203.E-1 – Cut Slope Failure (deep seated wedge)

Figure 203.E-2 – Cut Slope Failure (rotational failed condition)

Figure 203.E-2 details a rotational failed condition on the left. The right side shows a design that is properly benching so that it reduces the driving forces. If a rock cut is cut too steep, the rock can fall on the roadway.

Figure 203.F – Falling Debris from Vertical or Nearly Vertical Faces near Roadway

The above rock and soil conditions can be avoided during the design or construction of a project. Ensure that the plan intent is followed in these cut locations on the project. Rock and shale excavations will be detailed under Section 208 Rock Blasting.

Materials (203.02)

The biggest changes in the 2002 version of the specification were the definitions and material requirements of the type of material allowed under the specifications.

In order to properly detail the requirements, it was necessary to divide up natural and recycled material requirements. Too many times in the past Contractors would try to obtain approval for materials that were not intended under the specifications.

A natural material is a material that was created by nature; a material that is mined or excavated and graded is a natural material. A material that is chemically altered by a manufacturing process such as concrete, fly ash, foundry sand, or slag is a recycled material.

Materials are defined in 203.02. As the materials are defined in 203.02, all of the allowed materials are detailed in 203.02.R as “Suitable Materials.” Specific, more detailed material requirements are located in 703.16.

In the following sections, the materials will be detailed in the specific 203.02 sections for clarity.

If there is any doubt on the condition, status, acceptability, or approval of the materials throughout the following sections, then the project should contact one of the following: the District Engineer of Tests, the District Geotechnical Engineer, the Aggregate Section of the Office of Materials Management, or the Office of Geotechnical Engineering.

Natural Soil (203.02.I)

The definition for natural materials in 203.02.I is as follows: “All natural earth materials, organic or inorganic, resulting from natural processes such as weathering, decay, and chemical action.”

Allowable materials are materials such as clay, silt, sand or gravel. These are allowed as suitable materials and are further defined in 703.16.A.

Department Group Classifications A-4-a, A-4-b, A-6-a, A-6-b, and A-7-6 are allowed. All of these materials are fine graded and have more than 35 percent of the particles passing the No. 200 sieve. More detail can be found by examining Figure 203.G. These classifications are further defined on the right side of the chart under Silt-Clay Materials.

Materials must have a maximum laboratory dry weight of at least 90 pounds per cubic foot (1450 kg/m3). Materials that are less than this weight usually have too much organic matter or clay materials.

Soils that have a liquid limit in excess of 65 or identified as Department Group Classifications A-5, or A-7-5 are not allowed. The A-5 material is highly elastic by virtue of its high liquid limit. The A-7-5 material is highly elastic and subject to volume change.

Natural Granular Materials (203.02.H)

These materials are defined in 203.03.H as follows: “Natural granular materials includes broken or crushed rock, gravel, sand, durable siltstone, and durable sandstone that can be placed in an 8-inch (200 mm) loose lift.”

These materials are allowed in 203.02.R, Suitable Materials. The material requirements are further detailed in 703.16.B and 703.16.C.

Under 703.16.B, Department Group Classifications A-1-a, A-1-b, A-3, A-3-a, A-2-4, A-2-6, or A-2-7 are allowed. All of these materials generally are mixtures of coarse and fine graded materials. These materials have less than 35 percent of the particles passing the No. 200 sieve. More detail can be found by examining Figure 203.G. These classifications are further defined on the left side of the chart under Granular Materials.

Granular material classified as A-2-5 is not allowed because of its low weight, high optimum moisture, high LL, low PI, and its propensity to slough.

Section 703.16.C allows durable sandstone and durable siltstone. If these materials meet the slake durability requirements in ASTM D 4644, then the material is considered equivalent in strength and durability to other natural granular materials.

Section 703.16.C allows slags and recycled Portland cement concrete to be used as granular material types.

Contact the Office of Geotechnical Engineering to arrange for the appropriate materials testing if sandstone or siltstone is used for this application.

Figure 203.G - Department Soils Classification Chart

Identifying Soil and Granular Materials in the Field

It is sometimes necessary to make field decisions based on very little (if any) laboratory soils information. Or, it may be necessary to verify the accuracy of plan soil borings in the field. In these two cases and on other occasions, it is important to have a basic understanding of how to identify types of soils and granular materials in the field. The following is some, but certainly not all, of the methods that can be used to identify these materials in the field.

Granular Soils

Granular soils are easily identified by their particle size in the field. A sample may be taken inside and spread on a table to dry. A rough estimate of the material retained or passing each sieve may be obtained by examining the material when dry: the finer materials such as clays and silts cannot be separated and can only be distinguished between one another by a settling technique. This can be accomplished by using a hydrometer or by performing a crude settling test. This technique is beyond the scope of this manual.

Fine Grained Soils (Clays and Silts)

It is more important, yet harder, to distinguish between a clay and silt material in the field. Clays and silts should be treated and used differently in the field because of their difference in engineering and compaction properties. See the properties of soils in the next section.

A clay material can be easily rolled into a thread at moisture contents at, near, or above the plastic limit of the material. Clays can often be rolled into 1/8 inch (3 mm) diameter threads (about half the diameter of a pencil). See the plastic limit test later in this manual for further information. The thread may be easier and may be rolled into smaller sizes as the clay content increases. You cannot roll a pure silt material into a 1/4 inch (6 mm) thread no matter what the soil content.

Clay forms hard pieces that cannot be broken by hand pressure when it is dry. Place an irregular piece of dry soil between the index finger and the thumb, and try to break the material. If the material is difficult or impossible to break, it is probably clay. A silt or sandy material will generally break easily with this amount of hand pressure.

Clay fines are generally greasy, soapy, and sticky. When wet, clay dries slowly, while silt dries faster than clay.

When performing these hand techniques, observe the soil residue found on your hands for further information. If the soil on your hands is difficult to remove and the hands need to be rubbed briskly together to remove the soil, the material is probably a clay. A silt material is generally easily removed from the hands when rubbed together.

A silt material will react to vibration or shaking. Place a small amount of pliable soil in your hand. Hold the material in one hand and drop that hand on the other hand or a hard surface. Water will form on the surface of a silt material. You can also put the soil in a bowl and tap it on a table to get the same result. Clay will not react to this test.

The above crude identification techniques should not replace classification by the laboratory but should be used as a supplement.

If there is any concern, send a sample to the District Engineer of Tests for further classification as soon as possible.

Engineering Properties of Soil and Granular Materials

The following are general statements regarding the engineering properties of soil and granular materials. Consider these properties when solving field problems.

Properties of Granular Soils

1. Good foundation and embankment material.
2. Not frost susceptible, if free draining.
3. May erode on embankment side slopes.
4. Identified by the particle size.
5. Easily compacted when well graded.

Properties of Fine Grained Soils

1. Often have low strengths.
2. Plastic and compressible.
3. Lose part of their shear strength when wet or if disturbed.
4. Practically impervious.
5. Slopes are prone to slides.

Properties of Silts

1. High capillary action and frost susceptible.
2. No cohesion and non-plastic when pure silt.
3. Highly erodible.
4. Difficult to compact.
5. Release water readily when vibrated.
6. Acts like an extremely fine sand during compaction.

Properties of Clay as They Relate to Silt

1. Better load-carrying qualities.
2. Less permeable than silt.
3. Easier to compact than silt. (Any soil is easier to compact than silt.)
4. More volume change potential.
5. Plastic or putty-like property.
6. Clays are weaker when compacted wet of optimum.

Moisture Effects on Soils

Granular soils are less affected by the moisture content than clays and silts, have larger voids, and are free draining. Granular materials have relatively larger particles as compared to silts and clays.