Setting out of Alignment and achieving design embankment profile

Guidelines for setting out of alignment, construction  limits and achieving design embankment profile for highways.

yogendra borse

1 introduction

    Sometimes in case of Expressways, National Highways and New Village roads specially in case of road sanctioned under T6 head where project can be greenfield project, ( No wonder whatever type of road it may be, Basics remains the same 😌) in such cases setting out of alignment, construction limits and achieving design embankment profile is necessary.

Site Visit befor Estimate Preparation for village road

Current Site situation

    Detailed guidelines already exist in different publications of the Indian Road Congress for setting out of alignment and construction limits in the field. The manual for survey, investigation and preparation of road projects ( IRC special Publication number 19)  and the recommended practice for the construction of earth embankment  for road works (IRC:36)  may be referred to in this connection. The book of  ministry specifications  for road and Bridge works also contains a separate subsection  devoted to construction operation. for the convenience of the field officer some of the relevant portions of these quoted below for guidance. The instructions cannot substitute for the detailed instructions contained in these publications and the field engineer is advised to always refer to the original publications in case of additional information and for guidance regarding other concerned points.

2 Setting out of alignment

2.1 staking the final Centre line.

2.1.1 First the working drawings are prepared on the basis of what is called the final location survey to layout the final Centre line of the road in the field based on the alignment selected in the design of Ace and to collect necessary data for the preparation of working drawings.

2.1.2  The two main operations involved in the survey are the taking out of the final Centre line of the road by means of continuous transit theodolite survey and detailed levelling.

2.1.3 The central line of the road as determined in the design office is translated on the ground by means of a continuous Trance it survey and taking off the centre line as the survey proceeds. all angles should be measured with transit theodolite double reversal method should be adopted to all horizontal intersection points (HIP)  and at the intermediate points of the transit (P.O.T.) on long tangents. The HIPs  should be fixed on the hub's driven flash with the ground and suitable reference so that they may be relocated readily. did should be a serious number for easy identification.On long tangents intermediate transit points should also be fixed on hubs  in the case of new roads and by means of Spikes or nails driven into the pavement in the case of existing roads after proper referencing. The method of referring to HIPs and POTs will be more or less the same as indicated in the sketches (figure 1) For setting out construction limits appended to those guidelines.

Fig 1 (a) Reference peges at horizonatal intersection points (H.I.P) and at curves

Fig 1 (b) Reference pegs at Point of Transit (P.O.T.)

Fig 1 (a) & Fig 1 (b) Explains the method of referencing horizontal intersection points (H.I.P.) at curve and at point of Transit (P.O.T.)

2.1.4 The reference points should be so located that these will not be disturbed during construction. description and location of the reference points should be noted for reproduction on the final plan drawings. The distance of the reference points should be measured along the horizontal plane, where the train is undulating the distance is measured along the slope,  the slope angle determined with theodolite and the actual horizontal projection calculated.

2.1.5 all the curve points, Namely the beginning of the spiral transition curve (BS),  beginning of circular curve (BC) end of circular curve(BC)  and the end of spiral transition (ES)  should be fixed and referenced in the same manner as for POTs describe earlier ( for the procedure of setting curves,Reference maybe made to IRC:38 design tables for horizontal curve for highways. 

2.1.5 The final Central Line of the road should be suitably staked. stakes should be fixed at 50 M intervals in plane and rolling Terrain. The stakes are intended for only a short period for taking levels of the ground along the centre line and cross section with reference There to. In the case of existing roads, paint marks may be used instead of stakes.

2.1.7 Distance measurements along the final Centre line should be continuous following the horizontal curves where these occur.

2.1.8 AT Road Crossings, the angles which the intersecting roads make with the final Centre line should be measured with the help of a transit theodolite similar measurements should be made at railway level crossings.

2.2 Proper protection of points of reference

2.2.1  Among other things, field notes should give a clear description and location of all the Benchmarks and reference points. This information should be transferred to the plan drawings  and to the Computer memory also where facilities for such storage exist, so that at the time of construction the central line and the Benchmarks could be located in the field without any difficulty.

2.2.2 At the time of execution, Alwork operations. it may also ensure that the reference points do not fall within borrow areas, service roads for construction traffic and their location so clearly mark that construction traffic or Earth moving machinery do not foul with them. reference text of angle irons embedded in concrete or permanent benchmark pillars of design approved by the engineer.

2.3.0  reference pegs  shall be established on curd sections as well where these may be located at the distance of 10 to 20 m away from the apex points of the Curse in the extension of the tangent lines.

2.4.0 Intermediate pegs

 intermediate pigs shall be fixed along the centerline of the road once the points of the transit have been established. The intel Construction lines will be set out and checked with reference to the final Centre line established during the final location survey. It is important therefore that not only all the points referencing the centre line should be protected and the possibility of their being disturbed or removed till the construction is completed.

3.0  setting out of construction limits

3.1.0 As already described in the previous paragraph, as the first step to the start of construction the alignment of the road finalisation shall be marked on the ground.

3.2.0 Central Line and reference pegs

On long tangent sections, points on transit shall be fixed At least every 500 m in plain terrain and 250 m in rolling and hilly terrain.The centre line pegs in case of points of transit may preferably consist of concrete hubs driven flush with the ground. These points shall be referenced by means of what are called “reference pegs”  fixed normal to the centerline of the road and either side at a distance of 20 to 25 m  from the centre line peg so as to be in a position safe from interference by clearing or other earthwork Operations.It may also be ensured that the reference points do not fall within borrow areas service roads for construction traffic and their location so clearly mark that construction traffic or Earth moving machinery do not foul with them.  reference bags Consist of angle irons embedded in concrete or permanent benchmark pillars of design approved by the engineer.It may also be ensured that the reference points do not fall within borrow areas service roads for construction traffic and their location so clearly mark that construction traffic or Earth moving machinery do not foul with them.  reference bags Celkon sist of angle irons embedded in concrete or permanent benchmark pillars of design approved by the engineer.

3.3.0  reference tags shall be established on curve sections as well where these may be located at a distance of 10 to 12 m away from the apex point of the curse in exchange of the tangent lines. 

3.4.0 Intermediate Pegs :-

Intermediate pegss shall be fixed along the Centre line of the road once the points of transit have been established the intervalbetween the intermediate pegs may be 50 m on straight sections and 20 m on curved sections, transition spirals in case of roads in Plain and rolling terrain. Interval between intermediate packs should be reduced to 20 m on straight sections and 10 m on curves in Case of Hilly terrain.these pegs may consist simply of stakes driven in the ground. 

3.5.0 Batter pegs:-

Fig .2 Batter pegs for defining the limits of Embankment

 After intermediate pegs have been established,  Batter pegs  marking the limits of the embankment shall be fixed  on both sides. Batter pegs Are meant as guides for the plant when commencing the earthworks. To ensure their safety it will be desirable to fix the pegs about  to  m.  back from the actual limit of the field and to paint them in a distinctive colour. 

3.6.0    Sketches explaining  the setting out of the control pigs for a bank construction are given in figure to appended to these guidelines.

 4.0.0     Staking and profile making:-

4.1.0    for most situations commonly made with the conventional method of taking and profile making shall prove to be adequate. The arrangement of Tax followed in 8 also ensures the level of loose fail and that of the compacted layer to a reasonable degree of accuracy along the entire cross section and the slopes. The procedure is simple but a bit time consuming and can be followed easily by the field staff with good results.

 4.1.1    marking of alignment

 Fixing of the central line as per the approved drawing by means of a continuous transit theodolite survey has already been mentioned in the previous paragraph. Sometimes small break pillars (20 CM x  20 CM in cross section and 1 m high on ground) Are constructed in semi permanent specifications. These are placed only on one side of the alignment at 20 m to 25 m away from the centre line at 200 m intervals. The pillars are so located that they do not foul with construction traffic, Earth moving machinery or fall in areas near marked for borrowing or excavation. the road side face of the pillar contains in legibly painted  letters  n the informations per point, the final levels of the subgrade, the road surface etc. and also details of crust composition in case comprehensive working drawing with details of cross sections are available separately in a book form for each section construction of brick pillars are not essential.

4.1.2    Staking and marking the profile:-

 the profile marking work can be carried out at the points along the alignment at an interval not exceeding 20 m for hilly terrain to 50 m  for plane and rolling terrain.2 thin Sal Balla poles aur bamboos may be temporary embedded into the ground at the top points of the embankment with the help of the batter pegs on both sides.The poles shall be painted in alternate yellow and black shades of 15 cm height for convenience of working the top of the poles may be kept at the same level with the help of levelling instrument.

4.1.3    Sequence of operation for laying of layers:-

 4.1.3.1    Sequence of operation for laying the first and the subsequent layers have been indicated in the sketches figure 3.1 to 3.4. The figures shown are illustrative for a side slope of one is to 1:2. Sketches for different side slopes may be prepared on the same lines.

Fig. 1 Expected final structure

 4.1.3.2    First layer

Fig 3.2 First Layer

 horizontal string S1 – S2 may be tied at both ends to the poles P1 – P2 at height of 1 and a half rung (225 mm) Of the poles measured in the pole with the lower ground level at the point of embedment. lose Earth may be late in the full width between the poles up to the level of the string for the purpose of compaction. Compaction of earth is to be carried out as far as possible up to the ages of the laid Earth.

 4.1.3.3    Second layer

Fig 3.2 Second Layer

Subsequent layers of embankment shall not be placed until the layer under construction has been compacted to the specified density. After the compaction of the first layer has been completed,  two small wooden bags about 50 cm long  may be fixed in the ground in such a manner that they can be taken out easily at a distance of 30 cm. inside from pole P1 and P2. As in case of the first layer the height of the horizontal string S1 and S2 may now be increased by 225 MM above the compacted first layer and loose Earth may be laid for compaction in the width between the two small wooden pegs  Up to the level of string. The projection in the ends of the compacted layers may be cut by 30 cm at the ages for making the sides slopes after the construction of the embankment is completed. The extra quantity of earthwork involved shall not be paid as it is a construction requirement to ensure the desired degree of compaction which otherwise it is not possible to achieve by normal means of compaction.

4.1.3.4    Subsequent layers 

Fig. 3.4 Subsequent Layers

in subsequent layers the entire operation shall be repeated in a similar manner by fixing string laying loose layers and compacting.

 The achievement of uniformity in level in spreading of earth in the longitudinal direction can be checked at regular intervals of time by longitudinal strings at different points between string profiles described above.The method has been shown in the sketches in figures 3.1 to 3.4 appended to this guidelines where the broken lines show the proposed final embankment and the firm lines show the layer of loose compacted earthwork.

 necessary modifications has to be made in the procedure for the work in which depending up on the nature of material and compaction equipment available a different layer thickness has been allowed by the engineer.

4.1.3.5 Final embankment and subgrade layers

In the final  embankment layer i.e.  the layer just below the subgrade and in the subgrade layer earthwork shall be carried out very carefully. In such cases the help of an additional small pig may be taken fixed at the centre of the embankment for achieving the specified crossfall (camber) requirement.

In case of construction of the final embankment mint layer with superelevation the calculated difference in level between the two ages of the road has to be taken into account and suitable modification has to be made in string profile for the same.

Both the compaction and the finish of the layer shall be strictly checked as per the specified requirement of subgrade and the profile shall be checked with the template and shall be passed if within the specified tolerances.

4.2    However for certain situations where a large fleet of heavy equipment are in operation in clothes succession it may not be possible to keep the string in a dot condition for long and its use may be restricted to intermittent checking only.

 4.2.1    In such a situation the usual method is to make a temporary marking on the stake itself with the help of the levelling instrument and the staff indicating the level of the loose and the compacted layer and control the earthquake as per the same. In search cases the stakes may be put up at critical points like the meeting point of the back slope of the adjacent high ground,  the point where the design prism of the embankment intersects the natural ground or at 10 M intervals all along the cross section.

4.2.2    Depending upon their location and purpose such takes are generally called works stakes, slope stakes or grade stakes.

The work stakes are laid first in the order of precedence; these indicate the location and difference in elevation either by ‘+’ or ‘-’ sign by ‘P’ or ‘C’ prefix of the design grade of the embankment at that location with respect to the ground level.These are essential for commencing construction.

In case of large embankments or excavations it becomes difficult for the machinery to accurately feel or cut maintaining the desired slope without a guide. Slope staking in walls the placement of stakes along the slope at major the distance is from the baseline and recording on them the required cut and feel amount for achieving the design profile.

 The grade stakes are put up last after rough design prism has been constructed by means of the work and slope stakes the grade Stakes indicate the final profile and are placed at centre on Shoulders/verge  at intermediate points if the cross section width warrants such a provision and also at side slope ends and ditch points which mark the change in the slope.

4.2.3    For staking along the cross section sometimes the top width of the proposed cross section is extended by a couple of metres depending upon the height of the field at both ends to cover the area of the sloped sides also. After the compaction tests  are carried out by the prescribed method the slopes are cut on the sides. It is reported that it ensures better compaction of edges of the embankment though it involves Extra earthwork which is not to be paid extra as it is a methodology to ensure desired degree of compaction at ages of the embankment.

4.2.4    In such cases for achieving accurate side slow profile the slope is cut with the help of Machines. If strings are not being used for obtaining correct side slopes, a slope template may be prepared for the required slope profile And used for the purpose of checking. 

4.2.5    For works involving very large quantities of earthwork or design in embankments with cut and feel balancing proposals an additional category of stakes called haul stakes are provided.These are strips of small wooden bound placed at the top of stakes with narrow marks indicating the haul points, The direction of haul and fill.The haul stakes are quite helpful for projects involving large areas of earthwork or multiple certain field for economy in material utilisation and completion of projects as per design provision. 

4.3    For most purposes the conventional method described in paragraph 4.1 will be adequate and full proof however for large work of more dynamic nature deploying heavy mechanised fleet the method described in paragraph 4.2 may be adopted this will have ever require more of attentiveness and sincerity on the part of the field staff who have to calculate, control and check the level achieved with instructions constantly during the progress of the work.