trails manual section10

8/14/2019 Trails Manual Section10

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Crusher Fines Surface Trails

Extreme Hazard ZonesExtreme hazard zones are specific instances from the above list where trail usage is either very high orconditions are particularly adverse. These zones should be widened even more. See the table below for theincreased trail widths in hazard zones.

Width Change ZoneUpon entering and leaving hazard zones, the changes in trail width should not be sudden or even noticeableby users. The width can change over any distance that makes sense at that location; typically this will be

from 3 to 15 feet.

Side Clearance for Pedestrian Only

Standard side clearance for pedestrian trails is 3 on each side.

Side clearance exceptionsThe edge of the crusher fines can, if necessary, extend to the edge of significant objects such as occasionaltree trunks, slow-growing trees and shrubs, boulders, and retaining walls. These occasional exceptions to

side clearance tend to make the trail more interesting as long as the exception is clearly necessary andappropriate. Clearance exceptions should not pose a safety hazard.

Trail width changes with extended low side clearanceIf side clearance is limited on one or both sides for 5 or more running feet, the section with low clearanceshould be treated as a hazard zone. The trail need not change width in passing discrete and limited lowclearance points as long as the length of the clearance exception is less than 5 continuous feet. See thetable below for the increases in trail width.

Hazard zones and low side clearance zonesThe values given are the number of feet to add to the normal trail width in the given cases. Low clearance on one side and Lowclearance on both sides are for cases where side clearance is restricted for 10 or more continuous feet.

Path type Hazard ZoneExtreme

Hazard ZoneLow Clearanceon One Side*

Low Clearanceon Both sides*

Length of WidtChange Zone

Pedestrianwith bike orwheelchair

+1-2 +2-4 +1 +2 3-15

See definition of hazard zone, above.*Applies only where clearance is restricted for 10 or more continuous feet.

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Vertical Clearance for Pedestrian Only

Vertical clearance should be 10 if equestrians are not permitted or 12 if the trail is open to equestrians.Trails used for cross-country skiing may require additional clearance (up to 16 depending on expected snowdepth).

Crusher Fines Trail Cross Sections

The cross section to use depends on the width of the crusher fines surface, the cross slope, and (in somecases) the trail grade. In all cross sections, the goal is to blend the trail into the site as well as possible whileprotecting the crusher fines from erosive drainage. Also see Crusher Fines Trail Drainage, p. 3-28.

Minimal Cross Slope

To provide for drainage on minimal cross slopes, the trail surface should never be the lowest place around. Ifthere is absolutely no cross slope, raise the trail surface slightly above ground level. When there is any crossslope, the surface pitch of the trail should be in the same direction as the slope so that sheet drainage fromuphill can continue unobstructed downhill across the trail. This preserves the natural drainage of the site andeliminates the need for side swales. On bike trails, note that superelevation (banking of curves) takesprecedence over the normal surface pitch where applicable. See Superelevation and Crusher Fines TrailDrainage, p. 3-28. Side swales are required if a moderate to large amount of water is likely to wash downfrom above. Side swales should also be used where runoff from above is likely to deposit silt and debris onthe trail surface.



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Cut/Fill ConstructionCut and fill construction can be used on cross slopes up to 30% in areas where all three following conditionsare met:

Soils are stable enough to use as fill and the fill can be adequately compacted The prospects of revegetation are good Vegetation disturbed by the cut and fill is easily replaced

Cut and fill sections can be alternated with full bench sections where trailside vegetation needs to bepreserved (see Full Bench Construction, following).

Cut and fill slopes should mimic natural landforms along the trail and should vary wheneverit makes sense tovary.

Where superelevation pitches the trail surface inward or moderate runoff is expected from above, provide aside swale on the inside edge. See Superelevation and Crusher Fines Trail Drainage, p. 3-28.

Grading Specifications

Topsoil, which covers areas to be cut or filled, shall be removed and stockpiled for site restoration.

Suitable material removed from the excavations shall be used as far as practicable for embankments andbackfilling. Excavated materials that are considered unsuitable and any surplus of excavated material notrequired for embankments or the Contractor shall dispose of backfill.

Fills and EmbankmentsFills and embankments shall be constructed by depositing, placing and compacting materials of acceptablequality above the natural ground in accordance with the specifications below. Clearing, tree removal, andtopsoil removal shall be performed before any embankment is placed.



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Remove and dispose of obstructions and rubbish to a minimum depth of 12 below subgradeelevation Remove trees and stumps to a minimum depth of 8 below subgrade elevation Remove sod to a minimum depth of 6

Geotextile should be used between the crusher fines layer and the top of any fill or embankment.

Site RestorationTopsoil shall be replaced on all fill surfaces and on cut surfaces where possible. See Site Preservation,Restoration, and Revegetation, p. 2-103.



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Full Bench Construction

Full bench construction is to be used on any cross slope with one or more of the following conditions:

Cross slope exceeds 30% Soils are not stable enough to use as fill The prospects of revegetation are poor Vegetation which would be disturbed by a cut and fill is difficult to replace (i.e., mature trees,

fragile or unresilient vegetation)

Full bench sections can be alternated with cut and fill sections where conditions permit (see previous page).Remove and stockpile at least the top 6 of topsoil before excavating.Provide a minimum 12 shoulder at the outside edge of the crusher fines surface (24 on bike paths).

Where superelevation pitches the trail surface inward or moderate runoff is expected from above, provide aside swale on the inside edge. See Superelevation and Crusher Fines Trail Drainage, p. 3-28.



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Crusher Fines Trail Drainage

For overall drainage guidelines, see also Crusher Fines Type 1: Bike Paths, p. 3-9, Crusher Fines Type 2:Primarily Pedestrian with Bicycle and/or Wheelchair Access, p. 3-14, and Crusher Fines Type 3: PedestrianOnly, p. 3-19.

Concentrated water and insufficient drainage can quickly destroy a crusher fines trail - if the surface washesout, the fines are carried into nearby water channels (leaving a clear signature of deposited fines anddiscolored water), and the fines then have to be replaced in the trail only to be washed out again by the nextheavy runoff. All of this can be prevented through design and construction that takes drainage into account atevery step.

Three goals and one key to drainage

Crusher fines trail drainage consists of a variety of methods that work together to:

Keep crusher fines from becoming or remaining saturated with water,

Prevent concentrated flows of site runoff from reaching crusher fines surfaces, and

Quickly and efficiently drain crusher fines surfaces before water can form a concentrated flowacross the fines.

Those are the goals, and the key to attaining them is very simple:

All trail drainage is designed to give water a lower place to flow than the trail itself.

The following pages specify various methods of giving water a lower place to go. These methods aredesigned for use singly and in combination on the trail types as given.

Outslope

Outslope, also called pitch, is slanting the trail surface such that water on the surface flows off to the side.The term outslope is used because trails are usually traversing a cross slope and the water should drain

toward the outer, or down slope, side of the trail.



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Where to use outslopeEvery inch of a trail should use either outslope, crowning, or superelevation to drain the trail surface. On trailsgraded mechanically, outslope is easier to form than crowning, but crowning should be used instead ofoutslope on crusher fines surfaces with a grade of 6% or more. For trails designed for wheelchair users, theamount of outslope has to be minimized to 2% to help prevent wheelchairs from rolling toward the side of thetrail.

If superelevation is to be used in a given situation, it replaces outslope.

Crowning

Crowning is raising the center of the trail so that water is continually drained toward both sides. Crowningcan only be used when both sides have someplace to drain.

Where to use crowningCrowning should be used on all trails when the grade on crusher fines surface is 6% or more. For trailsdesigned for wheelchair users, the amount of crowning has to be minimized to 2% to help preventwheelchairs from rolling toward the sides of the trail. If superelevation is to be used in a given situation, itreplaces crowning.



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Superelevation

Where to use superelevationSuperelevation (banking of curves) is used only on bike paths on curves of less than a specified radius inmoderate to high speed zones (see Superelevation in Crusher Fines Type 1: Bike Paths, p. 3-12).

Where superelevation is specified, it replaces both outslope and crowning. If superelevation causes the trail

to be insloped (opposite of outslope), an inside swale or other means of adequate drainage must be providedto drain the trail.

Side Swales and Culverts

Side swales (broad shallow ditches parallel to a trail) protect trails from runoff water from elsewhere on thesite and give water on the trail surface a lower place to drain. Swales can either empty into natural drainages(preferred) or be drained at intervals by culverts under the trail.

Where to use an inside swaleA swale on the uphill side of a trail protects the trail from concentrated water flowing from above. On asuperelevated or crowned path, an inside swale may also drain water draining from the trail surface itself.

Where to use swales on both sidesSwales are specified for both sides of the trail when there is very little cross slope and one or more additionalconditions:

a moderate to heavy amount of water but little cross slope (rendering outslopeineffective)

trail grades are approaching or exceeding the cross slope (a situation that causeswater to readily flow down the trail).

Outslope and crowning are preferred over swalesBecause of the higher initial construction expense and continual maintenance expense of swales andculverts, outslope and crowning are the preferred methods of providing continual trail drainage. Locatingcrusher fines trails in drier sites with grades not exceeding 5% can minimize use of swales.



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Grade Breaks

Grade breaks are reversals in trail grade, which create low points along the trail. These low points are fail-safe drainage features - any water that has not been drained from the trail by other means is guaranteed toflow off the trail in these low points.

In bike paths (Type 1) where use of surface drainage features is limited, grade breaks are one of the primarymeans of ensuring that large sections of the trail do not wash out. On primarily pedestrian (Type 2) andpedestrian only (Type 3) trails, grade breaks are a last-resort drainage feature if all other features fail or areoverwhelmed by extraordinary amounts of water. On all trail types, grade breaks which are sensitivelydesigned into the site and alignment can add to user comfort by breaking long grades.

Grade breaks have to be designed into the alignment from the startGrade breaks are as much alignment features as drainage features. They can be elegantly and simplyincluded in a trail only if they are located in natural drainage crossings and other logical locations along thetrail. Each grade break should, as much as possible, look like a logical response to topography or to anobvious site constraint. The resulting trail will have a gently rolling profile that is comfortable for users andstable against large-scale washouts.

Where to use grade breaksThe standards for each trail type specify the spacing between grade breaks for different trail grades. Thegiven spacing is an approximation - use the topography, site conditions, and site drainage to make the finaldetermination. Whenever possible, use natural drainage as grade breaks.

On Type 2 and Type 3 trails, the use of grade breaks is optional. While grade breaks are preferred, they canbe replaced with drainage dips if additional intermediate drainage dips are used as specified.



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Grade breaks constructed in other than natural drainageIf site topography or alignment does not create the opportunity for a grade break where one is needed, aligning a shortand shallow grade reversal into the trail can create a grade break. The grade break should be long and gently rolling -not abrupt in any way and with no sudden or sharp edges. The critical difference between a grade break and a drainagedip is that, in a grade break, the trail, shoulders, and swales all reach a low point in the bottom of the break so that nowater can possibly continue down the trail past the break. This fail-safe quality is the main advantage of grade breaks.

Drainage grade breaksIf swales empty into the break, the water should enter drain under the trail via a culvert, across the trail surface via aconstructed swale crossing (see Swale Crossings) or, if the expected water is minimal or grades very slight, across thesurface of the crusher fines without any additional structures. If the grade break receives only water flowing down thetrail surface, a swale crossing can be used or, if expected water is minimal, water can flow across the fines in the bottomof the break.



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Drainage Dips

Drainage dips are small-scale grade breaks that create a small low point in the trail surface, which catcheswater flowing down the trail and diverts it off. Like grade breaks, drainage dips serve as a backup to outslopeand crowning - the dips catch water that may be flowing down the trail in a heavy flow or because of adversetrail surface conditions. Unlike grade breaks, however, drainage dips typically are a small break in the trailsurface only, not in the shoulders or swales (except for the outside shoulder on a significant cross slope).

Drainage dips are much smaller and easier to construct than grade breaks and can be used more frequently.While they do not have to be planned into the alignment like grade breaks, advance planning can be verybeneficial to overall user comfort and physical stability of the trail. The major disadvantage of drainage dips isthat they can be abrupt and severe for high speed users such as bicyclists, and locally steep for wheelchairusers.

On outsloped trails, drainage dips drain toward the outside. On trails with very little cross slope which arecrowned and/or have swales on either sides or neither side, dips drain toward both sides of the trail.

Where to use drainage dipsDrainage dips can be used on Type 2 and Type 3 trails. Frequency of use varies with trail type and grade -see the tables for these trail types for specifications. While these specifications are good approximations,actual site conditions dictate exact placement. Study trail topography to locate dips where they do the most

good, (i.e.) on grades where water is likely to be flowing down the trail. Drainage dips at the very top andbottoms of grades serve no purpose.

Use drainage dips above structures and steep gradesIn addition to the locations given in the above-mentioned tables, use drainage dips in the trail just above 1)significant structures such as steps, retaining walls, etc., and 2) just above the point where a grade becomessignificantly steeper. Placement at these locations protects structures and steep grades from water flowingdown the trail from above.



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Swale Crossings

Swale crossings are controlled means of allowing site runoff to cross the trail surface. Where water flows areminimal, swale crossings are an alternative to culverts and other channel crossings. Swale crossings canalso be used to cross flood-prone channels that rarely have water but need to flow unobstructed in case offlood.

The swale crossing usually involves constructing a reinforced surface such as concrete or stone in a lowpoint in a drainage crossing. Crusher fines trail Type 1 and Type 2 are limited in possible swale crossingdesigns because of the need to accommodate bicycles and/or wheelchairs. Type 3 (pedestrian only) trailscan have many designs, some of which create interesting and artful trail features for users.

Where to use swale crossingsThe main criteria for whether or not to use a swale crossing are 1) the amount of water and 2) how long thewater flows. Swale crossings should only be used for crossing drainage that carries a minimal flow onlyduring and immediately after a precipitation event or at a very low level during spring snowmelt. Usually theswales are very small and can be stepped across or at most jumped across. If water flows across the trail atmore than a depth of 1/4 for a distance too far to step or jump over, the trail is effectively closed topedestrians. Do not create swale crossings that exceed those parameters unless provision is made for a wayfor pedestrians to cross when wet.

Swale crossings can also be used across flood-prone channels that are usually dry. Usually used in desert-like conditions where flash floods dry up quickly, these swales are made of reinforced concrete to withstandflood damage. If water flow continues after precipitation ends, a low water crossing (a low causeway withmultiple smaller culverts) or other means should be used instead.



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Causeway and Channelization

Causeways can be used as a way to cross some occasionally wet or damp areas without installingboardwalk or concrete. The principal intended use for causeways is to elevate crusher fines aboveseasonally damp areas that become saturated with water in spring but remain dry for most of the year.

On pedestrian-only trails, channelization provides an aesthetic way for small amounts of slow-moving waterto flow through low causeways without the use of culverts or bridges.

Conditions for using causewaysA causeway should not alter water flows in ways that disturb nearby ecosystems, nor should it block orchannel water in floodplains. Also, the ground and soil beneath a causeway must be dry enough and stableenough to support the weight of the causeway. Causeways longer than 15 should be considered hazardzones and widened accordingly.

Where not to use causewaysCauseways should not be used to bridge areas with wetland ecology (regardless of their official wetlanddesignation). Crossed areas should not be subject to more flooding than the trail itself, nor to more thansmall amounts of openly-flowing water during major rainstorms or runoff periods.

Conditions for using channelizationChannelization is intended only for occasional use in pedestrian-only causeways with a trail surface level nomore than 8 above undisturbed ground. If the causeway is higher than 8, culverts, bridge, or a boardwalkshould be used instead.



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Stepping Stones

Where to use stepping stonesStepping stones can be used on low-traffic pedestrian-only paths as a way to cross wet yet stable areas.When correctly installed, stepping stones are an environmentally neutral way of crossing some low-flowdrainage and wet areas without culverts, bridges, or boardwalks.

In order to be useful, stepping stones should be evenly spaced one pace apart, and each stone should belarge enough (and flat enough on top) to be comfortable to step on. The ground beneath each stone must bestable enough to bear the weight of the stone and users. If the area is wet only occasionally, a bypass pathof coarse gravel can be installed alongside the stepping stones as an easier-to-use path in dry conditions.

For user comfort, the total running length of a set of stepping stones should not exceed 25.



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Steps

Steps can and should be used in pedestrian-only trails to reduce steeper grades and thereby help preventtrail washouts. A few steps can eliminate a whole series of awkward drainage dips and grade breaks onsteeper grades.

Steps can be made of either stone or timber. Stone is much preferred for its longevity and aesthetic qualitiesbut treated timbers can be used where stone is unavailable or inappropriate. If stone is appropriate butcannot be found onsite, consider importing it.

Where to use stepsOn pedestrian-only trails, use steps to avoid creating grades exceeding 6% on crusher fines surfaces. Stepsare also used to help prevent washouts on trails with alignments near the fall line of site drainage. SeeCrusher Fines Type 3: Pedestrian Only, p. 3-19, for details on where to use steps.

For trails with winter use, the use of steps that face north is discouraged because of snow and iceaccumulation. Steps that face south and west will often be dry even in winter, while steps that face east willdry more slowly.

See specifications for steps on the next page.



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