subbase methodology

7/28/2019 Subbase Methodology

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UPGRADING OF APPROXIMATELY 104KM OF ROAD D104/D791 CHIPATATO MFUWE IN EASTERN PROVINCE

SUBBASE CONSTRUCTION METHODOLOFY

1.0 SCOPEThis method statement covers the construction of Sub-bases from

approved natural gravel or partially crushed gravels.

2.0 MATERIALS

2.1 General

Gravel material shall be obtained from approved sources in borrow

areas or cuts. Tests will have been made on potential borrow pits,

cuts and on the roadbed along the centre line of the route

envisaged, and the results will be reflected on the Drawings, which

also show the use for which the material could possibly be suitable.

The requirements to be complied with by the material are set out on

the Drawings and in the Project Specifications for the subbase. It

shall be ensured that only material which complies with the

prescribed requirements is used for subbase construction. Where

natural gravel does not fully comply with the requirements in

respect of maximum size, after having been broken down as

determined in Clause 3204, or after light crushing, oversize material

shall be removed as specified in Clause 3210.

2.2 Material

Sub-base materials shall be obtained from approved sources in

borrow or cut or such other sources as may be specified or

approved from time to time. The completed sub-base shall contain

no material having a maximum dimension larger than 63 mm or

larger than two thirds of the compacted layer thickness when

approved by the Engineer. Sub-base material shall, unless otherwise

authorised, conform to the following requirements when finally

placed:

(i) Grading Modulus


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The minimum grading modulus shall be 1.5 unless authorised by

the Engineer, in which case a minimum of 1.2 may be permitted.

The grading modulus may be relaxed to below 1.2 at the

discretion of the Engineer when no alternative sub-base materialexists provided that the specified CBR strength is obtained.

(ii)Plasticity Index

The maximum plasticity index of the natural material shall be

10% and that for material to be stabilised shall not exceed 6%

after treatment with the selected stabilising agent.

(iii) California Bearing Ratio

The minimum soaked CBR of the natural material shall be 30% at

the specified in situ density (not less than 95% Modified

AASHTO).

2.2 Compaction requirements

The minimum in situ compaction of gravel material shall be 95%

Mod AASHTO

2.3 Electrical conductivity

The maximum electrical conductivity of the dumped material on theroad shall not exceed 0.15 Sm-1 at 25C when tested in accordancewith TMH 1 Method A-21T.

3.0 EQUIPMENT

The following equipment shall be available for the work:-

i. 2 No 140G Grader

ii. 1 No. Bull dozeriii. 1 No. Front End Loaderiv. 6 No 15 ton Tipper Trucksv. 4 No Rollers (1No. grid roller, 2 No. Vibrating Steel Drum Rollers,

1 No. Pneumatic Roller)vi. 2 No Water Bowser (each of capacity 21,000 litres)

4.0 PERSONNEL

4.1 Working Force

1 Site Engineer


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1 Earthworks Foreman on site1 Soils Technician2 Laboratory Assistants3 General Workers for checking levels4 General Workers for picking rejected material

1 Machine Attendant for each item of plant and equipment2 Mechanics for daily servicing and minor repairs

4.2 Plant Operators and Drivers

1 Bulldozer Operator1 Loader Operator1 Operator for each Grader1 Operator for each Roller1 Driver for each Water tanker1 Driver for each Tipper Truck1 driver Tractor (disk harrow)

5.0 CONSTRUCTION

5.1 Requirements applying prior to the construction of the layer

The subbase shall be constructed only where the underlying

subgrade layer meets all the specified requirements and has been

approved by the Engineer. Before the construction of the subbase

layer, and also before any material is dumped on the road, the

underlying layer shall be checked to establish whether there is any

damage, wet spots or other defects, which shall be rectified in

accordance with the instructions of the Engineer.

5.2 Placing and compacting

The material for a pavement layer shall be placed, spread, broken

down, watered if necessary and mixed, and oversize material shallbe removed and the layer compacted, all in accordance with therequirements of Section 3200.

6.0 PROTECTION AND MAINTENANCE

It shall be ensured that the completed subbase layers are protected

against rain and flood water and against any undue wear and tear of

or damage to unsealed layers by construction or other traffic.

Maintenance shall include, the immediate repair of any damage ordefects that may occur.


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7.0 CONSTRUCTION TOLERANCES

The completed pavement layers shall comply with the construction

tolerances given below:

7.1 Level

The level tolerances referred to in Clause 7205 shall be as follows:

H90 Hmax

Selected layer................................. 25 mm 33 mm

Sub-base........................................ 20 mm 25 mm

Base............................................... . 15 mm 20 mm

7.2 Layer thicknesses

The thickness tolerances referred to in Clause 7205 shall be

as follows:

D90 Dmax

Daverage

Selected layer.......................... 30 mm 40 mm 10 mm

Sub-base.................................. 21 mm 27 mm 5 mm

Base........................................ 21 mm 27 mm 5 mm

Wearing course....................... - 30 mm 0 mm

Shoulders................................ - 30 mm 0 mm

7.3 Width

The average width of the layer shall not be less than the specified

width, and nowhere shall the outer edge deviate by more than the

following from the edge lines shown on the Drawings: 3400-4

Selected layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 mm

Sub-base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 mm

Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 mm

Shoulders and wearing course . . . . . . . . . . . . . . 75 mm

7.4 Cross-section


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When tested with a 3 m straight-edge laid at right angles to the

road centre line, the surface shall not deviate from the bottom of

the straight-edge by more than 6 mm. At any cross-section the

difference in level between any two points shall not vary from theirdifference in level computed from the cross-sections shown on the

Drawings by more than 15 mm.

8.0 ROUTINE INSPECTION AND TESTS

The Engineer will do routine inspections and conduct routine tests to

determine whether the quality of material and workmanship

provided comply with the requirements of this Section. The test

results and measurements shall be assessed in accordance with the

provisions of Section 7200.

9.0 SAFETY, WORKERS AND PROTECTIVE CLOTHING

All works shall overally be supervised by a Site Engineer, who shall

ensure that a safe working environment is always maintained, and

that on-site safety procedures are followed at all times.

The Site Engineer shall see to it that adequate measures are taken

to warn and protect road users and construction workers. He shall

instruct all supervisors and foremen in safety measures including

traffic control and use of clean standard temporary road signs

The Safety Officer shall conduct regular on-site inspections of works,

equipment, tools and personnel. He shall also hold regular safety

meetings with all work groups to discuss and instruct in safety, and

shall produce weekly reports to Management.

Good working and handling practices shall be ensured on site

All workers shall be provided with such protective clothing such as

hand gloves, overall suits, boots, reflective jackets and dust masks.

Only suitable hand tools and devices, free from any defect, shall be

used.

Only experience operators shall be used to operate plant andequipment on site.


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All detours shall be maintained in serviceable condition.

subbase methodology

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