GRANULAR SUB-BASE CONSTRUCTION – GSB CONSTRUCTION
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| GSB Construction |
GRANULAR SUB BASE CONSTRUCTION
FUNCTION:➱
It Acts as a Drainage layer for The pavements to avoid excessive wetting and weakening of the subgrade.
In Strength, it is superior as compared to the subgrade, various materials and techniques are used for the construction of the base course.
It is an intermediate layer between the subgrade and granular base course.
What is a granular sub-base?
What is a granular sub-base in road construction?
If The Thickness of the base layer provided in the design permits,
➤The sub-base layer shall have two sub-layers.⇩
➤Drainage layer and the filter layer.
➤The upper layer of the sub-base functions as a drainage layer to drain away the that enters through surface cracks.
➤The lower layer of the base should function as a filter/separation layer to prevent intrusion of sub grade soil into the pavements.
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| permeability testing of GSB |
If the thickness of the granular sub-base layer is less than or equal to 200 mm.
➤Both drainage and filter layers can not be provided separately ( considering the minimum thickness requirements as per 7.2.2).
➤ For such cases, a single drainage-cum-filter layer with GSB gradation Ⅴ or Ⅵ of MoRTH specification may be provided.
MATERIALS REQUIREMENTS:⇨
➤Natural sand,
➤Crushed gravel,
➤Crushed stone,
➤Crushed slag,
➤Brick metal,kankar and crushed concrete is permitted in the lower sub-base,
➤Shall be free from organic or other deleterious constituents,
➤Shall satisfy the grading requirements as is explained below,
➤shall satisfy the physical requirements as it is explained below,
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| materials required for GSB |
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| grading requirements |
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| physical requirements |
Is code for a granular sub-base?
What is the Type 1 granular sub-base?
What is the granular subbase used for
What is a granular base course
What is the size of GSB
GRANULAR SUB BASE CONSTRUCTION OPERATION
TOE LINE FIXING:➩
➤The Toe line/fixing of pegs to mark the limits of the Granular sub-base shall be done before the commencement of work.
➤The width of pavements for the toe line shall be calculated by referring to the drawing with due consideration to formation width plus the offsets and slopes of different layers’ depth the current layer from the formation road level( FRL).
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| grading and rolling of GSB |
HAULING AND DUMPING:➯
➤The Required Quantity of GSB shall be Calculated and informed to the GSB Dumping Supervisor or Staff at the site,
➤Dumping Patterns Such as to ensure uniform Spread shall be determined and explained to the dumping supervisor or staff,
➤The materials for GSB shall be hauled to by tippers and dumped on the bed in a predetermined pattern,
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| GSB Material Quantity Calculation |
How is base course aggregate calculated
What is base course material
What is GSB material
GRANULAR SUB BASE CONSTRUCTION OPERATION
SPREADING AND GRADING:➯
➤The materials shall be up to the required thickness,
➤Control pegs at every 10 meters along centre line shall be fixed on both sides of the bed in such a manner That they are not disturbed by the movements of the grader,
➤Required level shall be marked on the pegs with suitable vertical offsets,
➤After rough initial spreading, a cotton/nylon string line(line dori) shall be held at the marks on the control pegs on both sides,
➤ The height of String line from the bed top is measured at fixed horizontal offsets from the center line,
➤The grader operator shall be directed according to the measurement in the above step to cut and fill material at every point,
➤Each grader run shall overlap by at least 1/3rd of the bled length,
➤Most important if any suggestion is observed, instruct the operator to correct the same while grading,
WATERING:➱
➤The moisture content shall be checked for OMC and if it is less than required water may be added by sprinkling and if it is more than required, shall be allowed to dry by exposure to the sun,
➤The moisture content shall be adjusted with allowance for evaporation losses in a manner to achieve 1 % above to 2 % below OMC During compaction,
COMPACTING:➱
➤Before the commencement of compaction and while spreading the GSB all the oversize materials available more than 2/3rd the thickness of the layer shall be removed manually,
➤The compaction shall be done mechanically using vibratory rollers of 80 to 100 KN static weight for the entire width of the pavement,
➤The rolling shall commence from the lower edge and proceed towards the upper edge, each pass of the roller shall overlap 1/3rd of the proceeding pass,
➤During the compaction, any low spots shall be filled with the material of the same properties and high spots shall be excavated and removed manually,
➤ The number of passes shall be decided beforehand by conducting trial,
➤Each layer shall be tested by sand replacement method,
RECTIFICATION:➱
➤If The surface is higher than the required level the defective area shall be trimmed off and compacted,
➤If the surface is lower than the required level the defective area shall be sacrificed, fresh material shall be placed and compacted,
BUMPING:➱
➤If The surface is exhibiting bumping then the material in the defective area shall be removed to full depth and fresh material will be placed and compacted,
SEGREGATION:➱
➤If segregation is observed then the material from the defective area shall be removed to full depth and either made good and replaced or replaced by fresh material,
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| GSB Construction |
QUALITY CONTROL TEST AND THEIR FREQUENCY
➤Gradation at a frequency of 1 test for every 200 cum,
➤Moisture content test as per IS 2720 Part-2 at a frequency of 1 test for every 250 cum at the time of spreading and rolling,
➤MDD and OMC As per IS 2720 Part-8 at a frequency of 2 test per 3000 cum,
➤Laboratory determination of CBR as per IS 2720 Part-16 as required,
➤Atterberg’s limits at a frequency of 1 test per 200 cum,
➤Field Density test as per IS 2720 Part-28 at a frequency a set of test for every 1000 sqm to a minimum of 6 tests up to 3000 sqm, if nuclear density meter is used the frequency shall be doubled,
PROBLEMS AND REMEDIES IN GSB CONSTRUCTION
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