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Contents
CUT AND FILL: ......................................................................................................................................... 2
ROCK EXCAVATION ............................................................................................................................... 13
CLEARING AND GRUBBING ................................................................................................................... 16
WASTE MATERIAL ............................................................................................................................. 16
BORROW EXCAVATION ......................................................................................................................... 17
COMPACTION ........................................................................................................................................ 18
REFERENCE ............................................................................................................................................ 20
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CUT AND FILL:
The objectives of routine road cuts and fills are:
To create space for the road template and driving surface; To balance material between the cut and fill; To remain stable over time; To not be a source of sediment; and To minimize long-term costs.
Landslides and failed road cuts and fills can be a major source of sediment, they can close the
road or require major repairs, and they can greatly increase road maintenance costs (Photo
1.1). Vertical cut slopes should not be used unless the cut is in rock or very well cemented
soil. Long-term stable cut slopes in most soils and geographic areas are typically made with
about a 1:1 or :1 (horizontal: vertical) slope (Photo 1.2). Ideally, both cut and fill slopes
should be constructed so that they can be vegetated (Photo 1.3), but cut slopes in dense,
sterile soils or rocky material are often difficult to vegetate.
Photo 1.1 Over-steep slopes, wet areas, or existing slide areas can cause instability problems
for a road and increase repair and maintenance costs, as well as sediment production.
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Photo 1.2 Construct cut slopes at a 3/4:1 or flatter slope in most soils for long-term stability.
In well-cemented soils and rock, a 1/4:1 cut slope will usually be stable.
Photo 1.3 A well-stabilized cut slope, with about a 1:1 slope, that is well covered with
vegetation.
Fill slopes should be constructed with a 1 1/2:1 or flatter slope. Over-steep fill slopes (steeper
than a 1 :1 slope), commonly formed by side-casting loose fill material, may continue toravel with time, are difficult to stabilize, and are subject to sliver fill failures (Photo 1.4). A
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rock fill can be stable with a 1 1/3:1 slope. Ideally, fills should be constructed with a 2:1 or
flatter slope to promote growth of vegetation and slope stability (Photo 1.5). Terraces or
benches are desirable on large fill slopes to break up the flow of surface water. Table1.1
presents a range of commonly used cut and fills slope ratios appropriate for the soil and rock
types described. Also Figure 1.1 and Figure 1.2 show typical cut slope and fill slope design
options, respectively, for varying slope and site conditions. Note, however, that local
conditions can vary greatly, so determination of stable slopes should be based upon local
experience and judgment. Groundwater is the major cause of slope failures.
Photo 1.4 Avoid loose, overstep fill slopes (steeper than 1 1/5:1), particularly along streams
and at drainage crossings.
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Photo 1.5 Construct fill slopes with a 1 1/2:1 or flatter slope (to promote vegetation growth)
and stabilize the fill slope surface. Use benches (terraces) on large fill slopes to intercept any
flow of surface water.
Table 1.1
COMMON STABLE SLOPE RATIOS FOR VARYING SOIL/ROCK CONDITIONS
Soil/Rock Condition Slope Ratio (Hor:Vert)
Most rock :1 to :1
Very well cemented soils :1 to :1
Most in-place soils :1 to 1:1
Very fractured rock 1:1 to 1 :1
Loose coarse granular soils 1 :1
Heavy clay soils 2:1 to 3:1
Soft clay rich zones or wet seepage areas 2:1 to 3:1
Fills of most soils 1 :1 to 2:1
Fills of hard, angular rock 1 1/3:1
Low cuts and fills (
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Figure 1.1 Cut slope design options.
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Figure 1.2 Fill slope design options
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Figure 1.3 Slope problems and solutions with stabilization measures.
A wide range of slope stabilization measures is available to the engineer to solve slope
stability problems and cross an unstable area. In most excavation and embankment work,
relatively flat slopes, good compaction, and adding needed drainage will typically eliminate
routine instability problems (Photo 1.6). Once a failure has occurred, the most appropriate
stabilization measure will depend on site-specific conditions such as the size of the slide, soil
type, road use, alignment constraints, and the cause of the failure. Here are a range of
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common slope stabilization options appropriate for low-volume roads, presented roughly
from simplest and least expensive, to the most complex and expensive:
Simply remove the slide material. Ramp over or align the road around the slide. Replant the slope and add spot stabilization. Flatten or reconstruct the slope. Raise or lower the road level to buttress the cut or remove weight from the slide,
respectively.
Relocate the road to a new stable location. Install slope drainage such as deep cut off trenches or dewater with horizontal drains. Design and construct buttresses (Photo 1.7), retaining structures, or rock anchors.
Retaining structures are relatively expensive but necessary in steep areas to gain roadway
space or to support the roadbed on a steep slope, rather than make a large cut into the hillside.
They can also be used for slope stabilization. Figure 1.4 (a and b) presents information on
common types of retaining walls and simple design criteria for rock walls, where the base
width is commonly 0.7 times the wall height (Photo 1.8). Figure 1.4c presents common
gabion gravity wall designs and basket configurations for varying wall heights. Gabion
structures are very commonly used for walls up to 6 meters high, particularly because they
use locally available rock and are labour intensive (Photo 1.9).
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Photo 1.6 Simple hand compaction behind a low rock wall. Compaction is important behind
any retaining structure or fill. It can be achieved by hand or, preferably, using equipment such
as a wacker or small compactor.
Photo 1.7 A drained rock buttress can be used to stabilize a cut slope failure area.
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Figure 1.4 Construction of various types of retaining structures. (Adapted from Gray &
Leiser, 1982)
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Figure 1.4 Continued. (Adapted from Gray & Leiser, 1982)
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ROCK EXCAVATION
Reviews standard excavation practices used to construct and modify rock slopes and provides
current design and construction guidelines for their use in context sensitive areas. Several of
these practices have been used for many years, while others are new techniques or recent
modifications of established methods. The most common are blasting (which includes drilling
the holes to be filled with explosives), ripping, and breaking. Table 1 provides a brief
description of these procedures, along with the advantages and limitations of each. Each
procedure is discussed in detail below.
BLASTING
Blastingthe controlled use of explosives to excavate rockhas been part of construction
engineering for hundreds of years. In any blasting situation, the geologic structure of the rock
mass will be the most important consideration. Other considerations include the degree of
scarring that would be acceptable (some areas can tolerate more blasting scars than others),
cost, and safety (blasting cannot be performed in close proximity to populated areas).
Effect of Geologic Structure on Blasting Procedure
The first consideration when designing a blasting operation should be the local geologic
conditions. Rock competency and fracture patterns can have a significant impact on the
success of a blasting operation.
Discontinuity Sets
When discussing blasting, the single most important geologic factor is fracture: the spacing
and orientation of any breaks, or discontinuity sets, in the rock. In particular, the orientation
of the discontinuity sets with respect to the cut slope angle will influence any slope failures
that may occur along the slope face. The modes of failure can be grouped into four primary
mechanisms, shown left to right in Figure 1
Planar failure (a), Wedge failure (b), Circular failure (c), and Toppling failure (d).
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Figure 1. Illustration. The four primary mechanisms of slope failure.
Table 1. Description, advantages, and limitations of common excavation practices.
PROCEDURE DESCRIPTION ADVANTAGES LIMITATIONS
Presplit
Blasting
Presplit holes are blasted
before production blasts.
Procedure uses smalldiameter holes at close
spacing and lightly
loaded with distributed
charges.
Protects the final cut by
producing a fracture plane
along the final slope face
that fractures fromproduction blasts cannot
pass. Can produce steeper
cuts with less maintenance
issues.
Performs well in hard
competent rock.
The small diameter
borings limit the
blasting depth to 15 m
(50 ft). Borehole tracesare present for entire
length of boring. Does
not perform well in
highly fractured, weak
rock.
Smooth
Blasting
Smooth blast holes are
blasted after production
blasts. Procedure uses
small diameter holes at
close spacing and lightly
loaded with distributed
charges.
Produces a cosmetically
appealing, stable perimeter.
Can be done on slopes yearsafter initial construction.
Drill hole traces are less
apparent than presplitting.
Performs best is hard,
competent rock.
The small boring
diameter limits blasting
depth to 15 m (50 ft).
Borehole traces are
present for much of theboring length. Does not
protect the slope from
damage caused by
production blasting.
Does not perform well
in highly fractured,
weak rock.
Cushion
Blasting
Cushion blasting is done
after production blasts.
Larger drill holes areused with small diameter,
lightly loaded distributed
loads. Space around the
explosive is filled with
crushed rock to cushion
the explosive force.
Reduces the amount of
radial fracturing around the
borehole and also reducesborehole traces. The large
diameter holes allow
blasting depths up to 30 m
(100 ft). Produces a ragged
final slope face. Performs
well in all rock types.
Radial fractures are
more abundant than
presplit and smooth
blasting. Slope face ismore prone to raveling.
A catchment area is
recommended at slope
base. More demanding
on the driller. Borehole
traces still apparent in
hard, competent rock.
Step Drilling
Larger diameter drill
holes, drilled vertically
and used as production
blasting (although spaced
If properly designed the
final slope face shows
minimal signs of blasting.
Can be used when sloped
Can produce extensive
damage to slope or
inadequate base
fracturing if not
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closer and loaded lighter
to minimize radial
fractures). Slope face is
formed along base of
blast holes.
controlled blasting cannot.
Best used in moderately to
highly fractured rock.
designed properly.
Should only be used
with experienced driller
and blasting engineer.
Only applicable for
slopes between 0.7:1and 1:1 (H:V). Does not
perform well in hard
competent rock.
Horizontal
Drilling
Larger diameter, closely
spaced, lightly loaded
horizontal borings are
used for production style
blasting. Used in massive
rock to eliminate drillholes or in areas of poor
access.
Eliminates bore hole traces
when drilled perpendicular
to the slope face. Good in
massive rock where traces
are not acceptable.
Demanding on the
driller and explosives
engineer. Can produce
extensive radial
fractures or inadequate
base fracturing if not
loaded properly.
Requires complicatedloading and timing
procedures, and special
stemming procedures.
Ripping
Uses a tractor with an
attached tooth or teeth
that is lowered into the
rock and dragged to
break up material for
excavation.
Much cheaper and safer
than blasting. Can be done
in close proximity to
development without
disturbance. Is effective on
a variety of angled cuts and
an excavator can be used
after ripping to slope
sculpting.
The tooth of the ripper
can leave scars on the
rock surface. The
tractor cannot be used
on steep slopes because
of risk of overturning.
Ripping is limited to
relatively low density
rocks.
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CLEARING AND GRUBBING
The clearing and grubbing topics addresses clearing and grubbing, the first of several
operations that are required to bring the construction site to subgrade. Clearing and grubbing
is the satisfactory removal of materials that cannot be used in the work. These materials
include trees, stumps, shrubs, topsoil, buildings, fences, and other obstacles interfering with
the work.
Material & Equipment
There is no material required for this operation, but the contractor may be required to
stockpile topsoil that is stripped from the site for later use. There are various types of
equipment used in clearing and grubbing, including chain saws, log skidders, wood chippers
and grinders, harvesters, fellers and bunchers, delimbers, bulldozers, scrapers and pans,
excavators, and dump trucks.
Construction Methods
Clearing and grubbing is usually the first construction operation performed. The process for
clearing and grubbing involves cutting and removing trees (clearing) or other obstructions
within the project site, removing stumps and brush, and removing topsoil and organic
material (grubbing). The work also includes disposal of removed material, salvaging and
storing material, stockpiling topsoil, and chipping and stockpiling wood waste material. The
demolition of structures, such as buildings, garages, sheds, sign stone, fences, signs, markers,
and guide rails, is also performed during this phase. The contract may require that certain
items, such as guide rails, signs, and posts, be reserved for PennDOT reuse.
WASTE MATERIAL
This chapter addresses the satisfactory disposal of waste material from earthwork operations.
Material and Equipment
Equipment used in the disposal of waste material includes bulldozers, excavators, dump
trucks, rollers, compactors, and seeding and mulching equipment.
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Construction Methods
The contractor loads waste material from the project into dump trucks or other hauling
equipment and hauls the waste to a designated waste area. The material is placed and
compacted in the same manner as for embankment construction. Once all material is in the
waste area, it is graded for positive drainage. The appropriate seeding formula and mulch is
then applied. At on-site waste disposal areas, frequent moisture-density tests are performed to
ensure the soils placed meet specification requirements. The contractor may be required to
perform other items if required by DEP permits.
BORROW EXCAVATION
This chapter addresses excavation required to supply material needed on a project in addition
to the material available from project operations. These excavations are called borrow
excavations. Borrow excavation is classified as common, foreign, or selected, depending on
where the material is obtained.
Common borrow excavation occurs when extra material, measured before and after
excavation, is obtained from a location within the limits of the project or outside of the right
of way limits. Foreign borrow excavation occurs when extra material cannot be obtained
from within the project limits, must be obtained elsewhere, and cannot be measured before or
after excavation. Selected borrow excavation occurs when material for the work is obtained
from sources outside the project limits that cannot be measured before or after excavation and
is used for specific items of work due to quality or size requirements.
Material & Equipment
Equipment typically used to excavate and load material includes bulldozers, backhoes,
excavators, scrapers, drag lines, clamshell buckets, and front-end loaders. Dump trucks,
scrapers, and off-road trucks are used to transport and dump the excavated material. Rollers
and compactors are used to compact the material and to seal the surface of the excavation at
the end of the workday. Seeding and mulching equipment is used to see and mulch borrow
areas.
Construction Methods
All required permits and agreements must be obtained before the work begins. Borrow pits
should be identified early enough to allow sufficient time for sampling and laboratory testing.
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Test results must be forwarded to the District soils engineer before the material is
incorporated into the project. If more than one type or classification of material is obtained
from the same borrow pit, samples must be taken and submitted for each type of material.
These separate materials must be removed consecutively rather than concurrently should not
be mixed.
When borrow excavation is required, excavation cannot begin until the material and
placement sequence has been accepted in writing, and an erosion and sediment control plan
has been submitted and accepted by the county conservation district, the Pennsylvania DEP,
and by PennDOT. The Department also requires a signed agreement with the property owner.
The inspector should make certain the borrow pit is cleared of vegetation, debris, unsuitable
material, topsoil, and overburden before production work begins. The removed topsoil is
stockpiled for reuse in restoring the pit.
During excavation, the material is monitored for changes. The District soils engineer must be
notified of changes and if previously unsampled material is found during the course of
borrow pit excavation (in which case additional sampling and testing is required).
Final cross sections must be taken after the last of the material has been removed from the
borrow pit. After completion of the cross sections, the borrow pit should be restored
according to the approved restoration plan.
COMPACTION
This chapter addresses compaction methods and equipment, and applies to embankment,
benches, and subgrade compaction.
Compaction is the process of making particles of a given material fit together in the smallest
possible space. Proper compaction is important because it makes material more stable and
increases its supporting or load bearing capacity.
Materials are comprised of different grain sizes and physical compositions; hence, each
material has unique compaction characteristics. When a given material is fully compacted,
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the spaces or voids between particles are limited to their smallest possible configuration. The
greatest density obtainable through compaction is the maximum density. The moisture
content of a given material directly correlates to the materials ability to be compacted to
maximum density. The maximum water content needed to achieve maximum density is
known as optimum moisture content. Every material has its own optimum moisture content,
determined by conducting proctor tests. Too little or too much moisture inhibits adequate
compaction.
In general, there are five different methods used to compact materials.
The first method is to allow the material to settle and compact on its own with no mechanical
assistance. This method is time consuming and the results are unpredictable; therefore, it is
rarely used.
The second method, known as surcharging, involves placing of the excess material to a given
height above the plan elevation. This material is kept in place for a certain amount of time.
Compaction is obtained by the weight of the excess material, which is removed after the
required amount of time.
The third method involves forcing the particles together, much the same as a snowball is
formed by squeezing snow in the hands. Most non-vibratory rollers and compactors compact
material in a similar manner.
The fourth method of compaction involves applying weight and vibration. This method is
used for broken stone or sandy material that is difficult to compact. Generally speaking, these
materials are not compacted, they are consolidated. Vibratory rollers and plate tampers
achieve compaction through consolidation. The amount of compaction achieved depends
upon the amplitude (frequency) of machine vibration, how fast the machine is moved forward
and backward, and the weight of the machine.
The fifth compaction method incorporates tampers. Tampers are usually gasoline powered
and used in areas that are inaccessible to large rollers. Hand tampers are also used, but are
often not as effective as mechanical tampers.
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Material & Equipment
Materials used for compaction are those specified for embankment and/or backfill of the
various types.
Equipment typically used includes vibratory compactors, vibratory rollers, tandem rollers,
three-wheel rollers, rubber tired rollers, sheeps foot rollers, and tampers. Equipment
requirements vary by the type of material and its application.
The contractor also needs equipment, including sand cone and nuclear testing equipment, for
testing the compaction and moisture of the material used.
Construction Methods
Material used for embankment structure backfill or other work is placed in appropriate layers
per specifications and standards. Compaction is applied using the appropriate equipment,
depending on the material and its application.
After compaction, the contractors certified materials technician performs compaction testing
using either the sand cone or nuclear procedures. If appropriate (as noted in the contractors
approved quality control plan), the visual non-movement under equipment procedure may be
used.
Test results are obtained, evaluated, and accepted prior to placing the next layer.
REFERENCE
Document
Section 200Earthwork. PennDOT Publication 8. PDF Document. Slope Stabilization and Stability of Cuts and Fills. LOW-VOLUME ROADS BMPS.
PDF Document.
Chapter 3Rock Excavation Methods. PDF Document.