Contents.Datum used The most common and convenient datum which is internationally accepted is. Countries take their nearby sea levels as datum planes for calculations of Reduced levels. For example, takes sea near as its datum while takes sea near as its datum for calculation of Reduced levels of different places in their respective countries.
The term Reduced Level is denoted shortly by ‘RL’. National survey departments of each country determines RL’s of significantly important locations or points. These points are called as permanent and this survey process is known as Great Trigonometrical Surveying (GTS). The permanent bench marks act as reference points for determining RL’s of other locations in a particular country. Instruments The instruments used to determine reduced level include:-.
Optical Levelling instruments like Automatic Level, Y-Level, Dumpy leveland Coke’s reversible level. Anyone of these can be used at a time for the purpose. Levelling Staff. Tripod standRL calculation RL of a survey point can be determined by two methods:-. Height of Instrument method. Rise and Fall methodSignificance.
For drainage of under a suitable slope is required. Thus roads are built in the fashion that their RL’s on sides are comparatively smaller than the RL at the mid-span of the road. This ensures proper drainage of water from roads. To construct a buildings, roads, a horizontal levelled surface is required. So, at construction sites, RLs of different points are obtained.
The ground surface is then being levelled to the RL which is obtained by taking of RLs of different points.References.
Through-cut Ditchon both Sidesmeters0.304.25.16.00.454.85.76.00.605.46.37.20.756.06.97.8Fill widening is another factor which modifies the subgradeor template width independent of traveled road width or ballast depth.Fill widening should be considered in cases where fills cannot be compactedwith proper equipment and where no compaction control is performed. Insuch cases fill widening of 0.30 m are recommended where fill slope heightis less than 2.00 m. Fill slope height in excess of 2.00 m should have0,60 m of fill widening (see Figure 53). Fill slope height in excess of6.00 m should be avoided altogether because of potential stability problems.Fill slope heightHf = 2m add 0.60 m.Maximum Fill slope heightHf 28.An alternative to the cost of a heavier pavement structureis the use of geotextile fabrics.
Fabrics have been found to be an economicallyacceptable alternative to conventional construction practices when dealingwith less than desirable soil material. Forest Service has successfullyused fabrics as filters for surface drainage, as separatory features toprevent subgrade soil contamination of base layers, and as subgrade restraininglayers for weak subgrades. A useful guide for the selection and utilizationof fabrics in constructing and maintaining low volume roads is presentedby Steward, et al. This report discusses the current knowledgeregarding the use of fabrics in road construction and contains a wealthof information regarding physical properties and costs of several brandsof fabric currently marketed in the United States and abroad.Proper thickness design of ballast layer not only helpsto reduce erosion but also reduces costs by requiring only so much rockas is actually required by traffic volume (number of axles) and vehicleweight (axle weight-wheel loads). The principle of thickness design isbased on the system developed by AASHO (American Association of StateHighway Officials) and adapted by Barenberg et al. (1975) to soft soils.Barenberg developed a relationship between required ballast thicknessand wheel or axle loads.
Soil strength can be simply measured either witha cone penetrometer or vane shear device, such as a Torvane.Thickness design for soft soil is based on the assumptionof foundation shear failure where the bearing capacity of the soil isexceeded. For rapid loading, such as the passage of a wheel, the bearingcapacity q is assumed to depend on cohesion only.q = Nc. Cwhereq = Bearing capacity of a soil (kg/cm²)C = Cohesive strength of soil (kg/cm²).Nc = Dimensionless bearing capacity factorBased on Barenberg's work, Steward, et al.
(1977) proposeda value of 2.8 to 3.3 and 5.0 to 6.0 for Nc. The significance of the bearingcapacity q is as follows:A. Q = 2.8 C is the stress level on the subgrade at whichvery little rutting will occur under heavy traffic (more than 1000 tripsof 8,160 kg axle equivalencies) without fabric.B. Q = 3.3 C is the stress level at which heavy ruttingwill occur under light axle loadings (less than 100 trips of 8,160 kgaxle equivalencies) without fabric.C.
Q = 5.0 C is the stress level at which very littlerafting would be expected to occur at high traffic volumes (more than1000 trips of 8,160 kg equivalency axles) using fabric.D. Q = 6.0 C is the stress level at which heavy ruttingwill occur under light axle loadings (less than 100 trips of 8,160 kgaxle equivalencies) using fabric.(Heavy rutting is defined as ruts havinga depth of 10 cm or greater. Very little rutting is defined as ruts havinga depth of less than 5 cm extending into the subgrade.)Charts relating soil strength (as measured with a vaneshear device) to axle load and ballast thickness are shown in Figure 56through 58. Figure 56 is based on a single axle, single wheel load.
Figure57 is based on a double wheel, single axle toad, and Figure 58 is basedon a tandem wheel configuration typical of 3 axle dump trucks or stingertype log-trucks.It should be noted that axle and wheel configurationhave a tremendous impact on the load bearing capacity of a road. The relationshipbetween axle load and subgrade failure is not linear.
Allowing 16,000kg axle load vehicle to use a road designed for a standard axle load of8,200 kg, is equivalent to 15 trips with the 8,200 kg axle load vehicle.Premature rut formation and its prevention depends on the selection ofthe proper axle load and strict enforcement of the selected load standard.Some typical truck configurations, gross vehicle weights(GVW), and axle or wheel loads are given in Table 24. Vehicles under 3tonnes GVW have no measurable effect on subgrade stress and deterioration.D esign ExampleA road is to be constructed to access a watershed. Becauseof erosive conditions and traffic volumes, only 5 cm of rutting can betolerated. Expected traffic volume is high (greater than 1,000 axle loads).Three vehicle types are using the road:1. Utility truck - 10 tonnes GVW; 4,500 kg single wheel load (9,000 kgaxle load on rear axle, loaded).2.
Dump truck -15 tonnes GVW with two axles; (11 tonnes rear axle loador 5.5 tonnes per dual wheel).3. Log truck - 36 tonnes GVW with 5 axles, rear tandem axle load equals15.9 tonnes or 7.95 tonnes per tandem wheel set.Soil tests: Visually segmentthe road into logical construction segments based on soil type. Take soilstrength measurements with vane shear device. Measurements should be takenduring wet soil conditions, its weakest state. Take at least 10 vane shearreadings at approximately 10 cm and 40 cm below the surface (in mineralsoil).
Tabulate readings in descending order from largest to smallestvalue. Your design shear strength is the 25th percentile shear strength-thevalue at which 75 percent of the soil strength readings are higher.Vane Shear Readings1.0.58 kg/cm²2.0.583.0.504.0.465.0.45. 7.0.408.0.379.0.36- 0.36 (25 percentile) - Design strength to be used in calculation.10.0.3211.0.3212.0.30Subgrade strength for design purpose is taken as 0.36 kg/cm².Ballast Depth Calculation: Calculate thesoil stress value without fabric for little rutting (less than 5 cm formore than 1,000 axle loads).q = 2.8. 0.36 = 1.01 kg/cm²(Conversion factor: Multiply kg/cm² by 14.22 to get psi.
This gives avalue of 14.33 psi in this example).In the case of the utility truck with 4,500 kg (10,000Ibs) wheel load, enter Figure 56 at 14.33 on the bottom line and readupwards to the 4,500 kg (10,000 Ibs) single wheel load. A reading of 42cm (16.5 in.) is obtained.
Since 7 - 12 cm additional ballast is neededto compensate for intrusion from the soft subgrade, a total of 49 - 54cm of ballast is required. When fabric is used, a factor of 5.0 (littlerutting for high traffic volumes) is applied to determine the ballastdepth.q = (5.0. 0.36. 14.22) + 25.6 psiA reading of 28 cm (11 in.) is obtained from Figure 56 indicating a savingof 21-26 cm of rock when fabric is used.The same analysis is carried out for the other vehicles. Ballast depthrequired to support the other vehicles is shown in Table 24.Table 24. Required depth of ballastfor three design vehicles.
Road designed to withstand large traffic volumes( 1,000 axle loads) with less than 5 cm of rutting.Bearing CapacityqC.Nc.14.22Utility truck10 t GVW4 500 kg (10,000 Ibs) Single wheelDump truck15 t GVW5,500 kg (12,000 Ibs) Dual wheelLog truck36 t GVW7,850 kg (17,500 Ibs) Tandem wheelBallast ThicknessWithout. Fabric14.33 psi41 + 10 = 51 cm(1.01 kg/cm²)42 + 10 = 52 cm40 + 10 = 50 cmWith.Fabric25.6 psi(1.80 kg/cm²)31 cm28 cm24 cm. 7 -12 cm additional rock is needed to compensate for contaminationfrom the soft subgrade.
The fabric separates the subgrade from the ballast. No intrusionof fines into subgrade.The dump truck (15 t GVW) represents the most criticalload and requires 52 cm of rock over the subgrade to provide an adequateroad surface. If fabric were to be used, the utility truck (10 t GVW)would be the critical vehicle.
The rock requirement would be reduced by21 cm to 31 cm. A cost analysis would determine if the cost of fabricis justified.The above example shows that a simple, 2 axle truck canstress the subgrade more than a 36 tonne log truck. The engineer shouldconsider the possibility and frequency of overloading single-axle, singlewheel trucks.
Overloading a 4,500 kg (10,000 Ibs) single wheel load truckto 6,750 kg (15,000 lb) increases the the rock requirements from 51 to62 cm.Figure 56. Ballast thickness curves for single wheelloads (from Steward, et al., 1977).Conversion factors: 1 inch = 2.5 cm; 1 kg/cm² = 14.22 psi.Multiply (kg / cm²) with 14.22 to get (PSI)Figure 57. Ballast thickness curves for dual wheel loads(from Steward, et al., 1977).Conversion factors: 1 inch = 2.5 cm; 1 kg/cm² = 14.22 psi.Multiply (kg / cm²) with 14.22 to get (PSI)Figure 58. Ballast thickness curves for tandem wheelloads (form Steward et al 1977).Conversion factors: 1 inch = 2.5 cm; 1 kg/cm² = 14.22 psi.Multiply (kg / cm²) with 14.22 to get (PSI)LITERATURE CITEDArmstrong, C.
A method of measuring road surface wear. 27-32.Atkins, H. Highway materials, soils, and concretes. Prentice-HallCo., Reston, VA. 325 p.Barenberg, E. Evaluationof Soil Aggregate Systems with Mirafy Fabrics.
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Surfacing toreduce erosion of forest roads built in granitic soils. In: Symposiumon effects of forest landslides on erosion and slope stability. May 1984.University of Hawaii, p. 255 - 264.Cain, C. Maximum grades for log trucks on forest roads. USDA Forest Service, Eng. Syst.,Washington D.C.Cain, C.
1982: A guide for determining road widthon curves for single-lane forest roads. 14, No.4-6, 1982. USDA Forest Service, Eng. Syst., Washington D.C.Chen, W. Limit analysis of stability of slopes.J. Soil Mechanics and Foundations Div., Proc. Paper 7828, Jan.
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44 pp.Kramer, B. Vehicle tracking simulation techniques for low speedforest roads in the Pacific Northwest. PNR - 82 - 508. Joseph,Michigan.Kuonen, V. Wald-und Gueterstrassen Eigenverlag, Pfaffhausen, Switzerland.743 pp.Ohmstede, R. Vertical curves and their influence on the performanceof log trucks. USDA Forest Service,Eng.
Syst., Washington D.C.Pearce, J. Forest Engineering Handbook. Prepared for U. Departmentof the Interior, Bureau of Land Management.
220 pp.Prellwitz,R. Simplified slope design for low volume roads inmountaineous areas. In: Low volume roads. Proceedings, June1975.
Special Report 160. Research Board, National Academy ofSciences, Washington, D.C., pp. 65-74.Reid, L. Sediment production from gravel-surfaced forestroads, Clearwater Basin, Washington. FRI-UW-8108, Univ. Of Washington,Seattle. 247 p.Reid, L.
Sediment production from forest roadsurfaces. Water Resources Research 20:1753-61.Steward, J. Williamson, and J. Guidelines for useof fabrics in construction and maintenance of low volume roads. Springfield, VA 22161. 172 pp.USDA, Forest Service. Transportation Engineering Handbook, Slopedesign guide.