Method Statement for Placement of Sub Base and Road Base

The purpose of this roadworks method statement is to describe the activities, sequence and methods that are mandatory for carrying out the standard placement of subbase and road base for a project. The sequence of the road work activities shall remain in three stages i.e. placing of lower sub base, placing of upper sub base and finally placement of road base and wet mix macadam.

Step 1: Method of Subbase Placement

Prior to placing any lower subbase material, the under laying subgrade shall be shaped and compacted in accordance to the project specifications.

The material shall be placed over the full width of the formation to the required thickness as shown in the drawings in one layer or more, each layer not exceeding compacted thickness of 200 mm.

Each layer of lower sub base shall be processed as necessary to bring its moisture content to a uniform level throughout the material suitable for compaction.

The lower subbase shall be compacted using suitable compaction equipment approved by the consulting engineer to not less than 95% of the maximum dry density determined in the B.S. 1377 Compaction Test (4.5 kg rammer method).

The top of lower subbase shall have the required thickness, shape, super elevation, levels and grades as required in the drawings and shall be within the tolerances as specified in the roadworks specifications.

In the course of the road construction, the level shall be checked using control/dipping pegs, set out at regular interval along both sides of the pavement.

Step 2: Placement of Upper Sub base

Prior to placing any upper sub base material, the underlaying subgrade or subbase shall be shaped and compacted in accordance to the specifications.

The crushed aggregate from approved sources shall be placed over the full width and to the required thickness as shown in the drawings in one layer or more, each layer not exceeding 200mm compacted thickness.

Each layer of upper subbase shall be processed as necessary to bring its moisture content to a uniform level throughout the material suitable for compaction. The correct moisture content of the material shall be maintained by the sprinkling water with a water truck.

The upper subbase shall be compacted using suitable compaction equipment approved by the Engineer to not less than 95% of the maximum dry density determined in the B.S. 1377 Compaction Test (4.5 kg rammer method).

The motorgrader shall be taken care in maintaining uniform gradation of the materials and prevent its separation into coarse and fine parts.

The top of upper sub base shall have the required thickness, shape, super elevation, levels and grades as required in the drawings and shall be within the tolerances as specified in the specifications.

In the course of the construction, the level shall be checked using control/dipping pegs, set out at regular interval along both sides of the pavement.Joint survey works on the existing road profile level shall be carried out to establish the original profile of the location being overlaid.

Step 3: Placing of Road Base or Wet Mix Macadam

The material shall be laid using approved machinery and compacted in layers in accordance with specification. Any areas of compacted material having a loose surface deficient in fines due to segregation or otherwise shall be made good by being removed and replaced with properly graded material.

Preparation of Road base and Wet Mix Macadam shall follow previous layer procedure.

Prior to laying, the stringline for the gradeline sensor shall be set out by the survey team at regular intervals along the sides of the carriageway and shall be set to the required levels.

After the stringline level is jointly surveyed, the laying operations may commence. Prior laying, foreman shall ensure that all gradeline and slope sensors are set and working properly.

Field Compaction Trial

At the start of constructing the improved sub grade and subbase layers a field compaction trial should be carried out. This is to determine the optimum moisture content and the required number of passes of the compaction equipment to comply with the Specification.

This trial should also determine the relationship between the loose and compacted thickness to ensure the material placed is adequately compacted to the specified density and the finished surface profile complies with the design level and permitted tolerances. Provided the method produces satisfactory results it should then be used for all subsequent compaction of that material. In this case this is well understood that contractor follows the agreed procedure, the frequency of checking the compacted density of the layer may be reduced with the consultant engineer’s approval.