Highway Engineering C3010 / UNIT3
THE TECHNIQUE OF PLANNING AND PRE-CONSTRUCTION ROAD WORK
( Part 2 )
OBJECTIVES
General Objective
To understand the earth work operation and its problems occurred.
Specific Objectives
At the end of the unit you should be able to :-
· describe the earth work operation.
· state the highway machinery
· state the types of slope.
· state the problem and its solutions.
· describe the method of slope protection.
PLANNING TECHNIQUE AND PRE-CONSTRUCTION
ROAD WORK – ( Part 2 )
3.0 INTRODUCTION
The sub-grade soil is prepared by bringing it to the desired grade and camber and by compacting adequately. The sub-grade may be either in embankment or in excavation, depending on the topography and the finalized vertical alignment of the road to be constructed.
3.1 EARTH WORH OPERATION
The earth work quantities are estimated based on longitudinal and transverse section along the alignment of the road. In order to reduce the cost of construction, it is necessary to plan the movement of materials from cuts to the nearest fills. It is necessary to decide the limits of economical haul and lift. It is advantageous to plot a mass haul diagram to compute the haulage details. The swelling and shrinkage factor may also be considered in the excavation and compaction of earth.
3.1.1 Excavation
Excavation is the process of cutting or loosening and removing earth including rock from its original position, transporting and dumping it as a fill or spoil bank. The excavation or cutting may be needed in soil, soft rock or even in hard rock, before preparing the sub-grade. The selection of excavation equipment and the cost analysis is made based on the stiffness of the materials to be excavated.
Earth excavation work may be divided as excavation or cutting, grading and compaction. The depth of the excavation is decided, among other factors, on requirement of vertical profile of the road. The slope to be provided is governed by the type of soil including stratification, if any, and the depth of the cutting. The stability computations may help in arriving at maximum permissible slope for the complex problems. However highway cuts much flatter slopes are preferred from other considerations including aesthetics construction of side drains also require excavations along road side.
a. Excavation equipment
The excavation equipment commonly used in highway projects includes bull dozers, scrapers, power shovels, draglines, clamshells and hoes. However, in small projects excavation is carried out manually using hand tools.
The machines that used are:-
i. Bull dozer and scraper
Bull dozer and scraper may be used for shallow excavation work and for hauling the earth for relatively short distances. Bull dozer is considered to be versatile machine for many construction projects as it may be used for clearing site. It can used for opening up pilot roads, moving earth for short haul distances of about 100m and also in several other jobs. Scraper is considered as one of the useful earth-moving equipment as it is self operating - it can dig, haul and discharge the material in uniformly thick layers. However scrapers are not capable of digging very stiff material.
Figure 3.1: Bull dozer
ii. Power shovel
Power shovel used primarily to excavate earth of all classes except rock and to load it into wagons. Power shovels may be mounted on crawler tracks and so they can move at low speeds. Figure 3.2 shows the operation and basic parts of a power shovel, these including the mounting, cab, boom, dipper stick, dipper and hoist line. The power shovel can effectively operate the excavate earth from a lower level where it stands and when the depth of the face to be excavated is not too shallow.
iii. Dragline
Dragline is used to excavate soft earth and to deposit in nearly banks or to load into wagons. Dragline may also be mounted on crawler. It can operate on natural ground while excavating from a pit with the bucket, thus it is not necessary for the dragline to go into the pit in order to excavate. The basic parts and operation of a dragline are shown in figure 3.3. The bucket is thrown out from the dragline on the top of the earth to be excavated and then pulled back towards the base of the machine.
iv. Clamshell
Clamshell consists of a bucket of two halves or shell which is hinged together at top. The shells may be attached to the shovel-crane units or at the boom of a drag line. The open clam-shell bucket is thrown on the top of the loose material to be dug and as the bucket is lift, the two halves close en trapping the material into the bucket. Figure 3.4 shows the clamshell bucket. This equipment is useful for excavation of soft to medium materials and loose material at or below existing ground surface.
v. Hoe
Hoe is an excavating equipment of the power-shovel family. Hoe is meant to excavate below the natural surface where the machine is stationed and is capable of having precise control of depth of excavation at close range work (see figure 3.5). Hoe can exert high tooth pressures and hence can excavate stiff material which normally can not be excavated by dragline.
The excavation equipment is selected depending on the natural of the material, the distance, to be hauled and the method of disposal. At times the selection may be made based on the availability also.
3.1.2 Embankment
When it is required to raise line of a highway above the existing ground level it becomes necessary to construct embankments. The grade line may raised due to any of the following reasons:
i. to keep the sub grade above the high ground water table.
ii. to prevent damage to pavement due to surface water and capillary water.
iii. to maintain the design standards of the highway with respect to the vertical alignment.
The design elements in highway embankments are:
i. height
ii. fill material
iii. settlements.
iv. stability of foundation, and
v. stability of slopes
a. Height
The height of the embankments depends on the desired grade line of the highway and the soil profile or topography. Also the height of the fill is some times governed by stability of foundation, particularly when the foundation soil is week.
b. Fill material
Granular soil is generally preferred as highway embankment material. Silts and clays are considered less desirable. Organic soils, particularly peat are unsuitable. The best of the soils available locally is often selected with a view to keep the lead and lift as lowers possible. At times light-weight fill material like cinder may be used to reduce the weight when foundation soil is weak.
c. Settlement
The embankment may settle after the completion of construction either due to consolidation and settlement of the foundation or due to settlement of the fill or due to both. If the embankments foundation consists of compressible soil with high moisture content, the consolidation can occur due to increase in the load. The settlement of the fill is generally due to inadequate compaction during construction, and hence by proper compaction this type of settlement may be almost eliminated. Whatever be the type of settlement, it is desired that the settlement is almost complete before the construction of saturated. Foundation clay, vertical sand drains are sometimes constructed. These are vertical columns of sand installed in the compressible foundation like marshy soils in order to decrease drainage path and thus accelerate the rate of consolidation. The vertical sand columns may be of 30 to 60 cm diameter and 2.5 to 6 meter spacing, arranged in hexagonal pattern. A horizontal sand blanket, 40 to 60 cm thick is placed at the top of the drains extending across the entire width of embankment at its bottom. This helps the water to flow out with ease.
d. Stability Of Foundation
When the embankment foundation consists of weak soil just beneath are at a certain depth below in the form of a weak stratum, it is essential to consider the stability of the foundation against a failure. This is more essential in the case of high embankment. The foundation stability is evaluated and the factor of safety is estimated by any of the following approaches.
i. Assuming a certain failure surface such as a circular arc or any other composite shape and analyzing it with Swedish circular arc analysis or method of wedges. As the case may be.
ii. Estimating the average shear stress and strength at the foundation layers by approximate methods and estimating the factor of safety.
iii. Using theoretical analysis base on elastic theory.
The factor of safety in the case of compressible soil foundation is likely to be minimum just after the completion of the embankment. Later due to consolidation of foundation and consequent gain in strength there will be an increase in the foundation factor of safety. Thus it is evident that in such compressible foundation soils, the vertical sand drains would be useful also to increase the rate of gain in strength. By proper design of vertical sand drains, it is possible to limit the decrease in foundation factor of safety due to the construction, within the allowable value.
e. Stability Of Slopes
The embankment slopes should be stable enough to eliminate the possibility of a failure under adverse moisture and other conditions. Hence the stability of the slope should be checked or the slope should be designed providing minimum factor of safety of 1.5. Often much flatter slopes are preferred in highway embankment due to aesthetic and other reasons.
3.1.3 Construction of embankment
The embankment may be constructed either by rolling in relatively thin layers or by hydraulic fills. The former is called rolled-earth method and is preferred in highway embankments. Each layer is compacted by rolling to a satisfactory degree or to a desired density before the next layer is placed. Compaction is carried out at optimum moisture content so as to take advantages of maximum dry density using a specified compacting effort and equipment. The thickness of the layers may vary between 10 and 30 cm depending on various factors such as soils type, equipment, specifications etc.
The practice of dumping the earth without compacting properly and allowing the fill to get consolidated under weather during few subsequent seasons should be avoided as the settlement will continue for a very long period. If pavement is constructed before the settlement of the fill is almost complete, the pavement is likely to become uneven and also fail later-on. Compaction of soil is discussed separately in topic Soil Compaction.
Let’s test your understanding before we continue to the next input…. GOOD LUCK….
3.1 Earth work operation can be divided into two categories. Name the work by filling in the chart below.
3.2 Match the excavation equipment listed below with its description. Then unite the words in the boxes provided.
3.3 Complete the chart of design elements in highway embankments below
3.13.2
3.3
HOW DO YOU FIND THE ACTIVITIES? FUN AREN’T THEY? LET’S PROCEED TO THE NEXT INPUT…..
3.2 SIDE SLOPE OPERATION
There are two types of side slope in Highway Engineering. Alignments of highways through hilly and rolling topography result in many cut and fill slopes of exposed soil. Highway construction until recently has been simply a matter of location and design. Very little consideration has been given to the possibility of erosion or sloughing on cut and fill.
3.2.1 CUT SLOPES
There have several types of strata in the soil and rock. When the soil and the rock are being cut and excavated, the stability of the content will decrease, meanwhile, the renovating have to be done to get the slope cutting ready and safe. The safety works are important to prevent erosion and sloughing. Slope excavation takes place when the original soil level higher then the level of new road formation to be built.
Figure 3.1 : Cut slope for equivalent soil strata.
Figure 3.2: Cut slope for variable soil strata.
Type of soil / Cutting depth / slopingSolid rock / 1:0.3 – 1:0.0
Rock / 1:0.5 – 1:1.2
Sand / 1:0.5 – 1:1.0
Sandy soil / 0 – 5 / 1:0.8 - 1:1.2
Compact earth / 5-10 / 1:1.0 – 1:1.2
Sand, loose / 5-10 / 1:1.2 – 1:1.5
Clay soil / 0-10 / 1:0.8 – 1:1.2
Clay-stone / 5-10 / 1:1.2 - 1:1.5
Table 3.1: Table for safety slope cutting
3.2.1.1 Problems in cut slope
There are several problems that will be stated in cut slope.
a. Landslide
Land slide is a problem that always happens in the work of slope cutting. It is because the cut of soil influence the stability of the soil. There are a few ways to overcome the problem.
i. Prepare the slope with suitable slanting degrees.
ii. Turfing the slope areas to maintain the soil stability
iii. Hydro ‘seeding’ is use to plant the plant on the alkali soil.
iv. By using Geofabric or geotextile to avoid the diffusion of ground water.
Rock slope is dangerous because the area of cutting does not have stable binding ability. If the failure of stone happen, it will cause the accident and dangerous for us. There are some methods which had being used in our country to overcome the problems.
i. Build the fencing along the slope areas.
ii. Use a net.
iii. Gabion block.
b. Water Infiltration
Water infiltration in soil also influences the soil stability. Storey and soil drainage system are important. Sub-soil drains are always used to control the flow of infiltration.