GRANULAR SUB BASE CONSTRUCTION – GSB CONSTRUCTION

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GRANULAR SUB BASE CONSTRUCTION - GSB CONSTRUCTION
GSB Construction


GRANULAR SUB BASE CONSTRUCTION

FUNCTION:➱

             It Acts as Drainage layer for The pavements to avoid excessive wetting and weakening of Sub grade.

             In Strength it is More superior as compared to the sub grade, various materials and techniques are used for construction of sub base course.

             It is an intermediate layer between sub grade and granular base course.

What is granular sub base?
What is granular sub base in road construction?

        If The Thickness of sub 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 sub base functions as a drainage layer to drain away the that enters through surface cracks.
➤The lower layer of sub base should function as a filter/separation layer to prevent intrusion of sub grade soil into the pavements.
GRANULAR SUB BASE CONSTRUCTION - GSB CONSTRUCTION
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,
GRANULAR SUB BASE CONSTRUCTION - GSB CONSTRUCTION
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GRANULAR SUB BASE CONSTRUCTION - GSB CONSTRUCTION
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GRANULAR SUB BASE CONSTRUCTION - GSB CONSTRUCTION
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GRANULAR SUB BASE CONSTRUCTION OPERATION

TOE LINE FIXING:➩

➤The Toe line/fixing of pegs to mark the limits of Granular sub base shall be done before the commencement of work.
➤The width of pavements for toe line shall be calculated by referring the drawing with due considerations to formation width plus  the offsets and slopes of different layers depth the current layer from the formation road level( FRL).
GRANULAR SUB BASE CONSTRUCTION - GSB CONSTRUCTION
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 site,
➤Dumping Pattern Such as to ensure uniform Spread shall be determined and explained to dumping supervisor or staff,
➤The materials for GSB shall be hauled to by tippers and dumped on bed in predetermined pattern,
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GRANULAR SUB BASE CONSTRUCTION OPERATION 

SPREADING AND GRADING:➯

➤The materials shall be up to the required thickness,
➤Control pegs at every 10 meter along center line shall be fixed on both sides of bed in such a manner That they are not disturbed by movements of 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 at both sides,
➤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 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 observed, instruct 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 the 2/3rd the thickness of layer shall be removed manually,
➤The compaction shall be done mechanically using vibratory rollers of 80 to 100 KN static weight for entire width of pavement,
➤The rolling shall be commenced 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 same properties and high spots shall be excavated and removed manually,
➤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 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 scarified,fresh material shall be placed and compacted,

BUMPING:➱

➤If The surface is exhibiting bumping then material in 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 defective area shall be removed to full depth and either made good and replaced or replaced by fresh material,
GRANULAR SUB BASE CONSTRUCTION - GSB CONSTRUCTION
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

PROBLEMS REMEDIES 
➤ GSB bed compaction less than required

➤Variation in surface level

➤Over burden materials mixed during preparation of GSB material,

➤Approved GSB bed

➤GSB Material Dumped Over moist or Wet Surface

➤Required Compaction should not be less than 98 %,
poor Compaction may lead to settlement
➤surface level tolerance limits:➯
for Sub grade:- +/- 20 mm,
for GSB        :- +/- 10 mm,
The Surface level shall be maintained within the tolerance limits else may impact of required thickness of next layer,
Rework may occur due to unacceptable tolerance,
➤Gradation:➯ specified gradation as per approved design
Liquid limit and Plasticity Index:➯ Max 25 And max. 6,
In such case, specified gradation and plasticity index limit shall be ensured before loading the GSB materials or at least before spreading.
Chances of Impact on the drainage properties,
➤At actual completely disturbed bed due to the following reasons,
➯Poorly Graded ⟶ Leads To Segregation,
➯Poor Workmanship⟶after final grading it would have rectified,
➯Poor Quality Control⟶before testing it would have suggested to concerned site Engineer to rectify the bed which leads poor compaction and segregation,
100 % Rework Required
➤Moisture Content ⟶OMC +1% and OMC -2%
➱The bed Should Have moisture Content as per requirement before proceeding for next layer,
➱Rework may occur due to improper supervision
➱settlement of successive layer

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