This document covers excavation and ground preparation for the roadway and service trenches and shall include all excavation, shaping and sloping necessary for the construction, preparation and completion of all subgrade, shoulders, slopes, gutters, intersections, approaches, entrances and trenches in conformity to the alignment, grades, and cross-sections shown on the drawings or established by the Consultant.
Excavation will generally be in hydraulically placed reclaimed sandy material. However in the small area on project site where connection is to be made to existing infrastructure some limestone or hard material may be present.
The British Standards applicable for this activity are listed below together with any other references.
- BS 1377 : 1990: Methods of test for soils for civil engineering purposes.
- AASHTO T89: Liquid limit
- AASHTO T90: Plasticity index
- BRE Digest 250: Concrete in sulphate bearing soils and ground water.
Soil/Grade Testing Requirements
For the Contractor’s general guidance, the following tests will be required prior to use, and during the construction of the subgrade.
- Samples for AASHTO classification, laboratory moisture-density relation tests and CBR tests will be taken prior to use, from any proposed source of material. Each sample shall represent not more than 1,500 m3.
- For every completed layer of subgrade, one field density test shall be run on each 75 linear meters per carriageway.
Compaction Tests: During earthworks construction the Engineer shall instruct control tests to determine the degree of compaction achieved. Tests shall include:
Moisture-density Test: A Moisture-density Test (AASHTO T 180 Method D) and preliminary study will be made of each type of soil to be used in the construction of the Work to determine the Maximum Density, the Optimum Moisture Content and the Moisture Range required of the soil for satisfactory compaction. The Field Density and actual Moisture content of the compacted embankment shall be determined by field tests according to AASHTO T 191.
Relative Density Test: For cohesionless free-draining soils for which impact compaction will not produce a well-defined moisture density relationship curve and the maximum density, the Test for the relative Density of Cohesionless Soils (ASTM D 2049) shall be used to determine the relative density.
Relative density is defined as the state of compactness of a soil with respect to the loosest and densest states at which it can be placed by the laboratory procedures described in ASTM D 2049. The Field Density and actual Moisture Content of the compacted embankment shall be determined by field tests according to AASHTO T 191.
The Contractor shall allow for the cost of all required testing by a laboratory approved by the CONSULTANT. Testing will be undertaken by an independent testing laboratory proposed by the Contractor and approved by the CONSULTANT.
Definition and classification earthworks material
The Contractor shall supply sweet sand from places permitted by the Consultant, provided that the Contractor conducts prior analysis with the knowledge of and as directed by the Consultant and suitability for agricultural purposes is established. Sweet sand shall be obtained from well-drained inland dunes and shall be reasonably free from salts, refuse, roots, heavy or stiff clay, stones or rocks larger than 3cm in size, noxious seeds, sticks, brush, litter, and debris and other deleterious materials or substances.
Sweet sand shall be placed only after the Consultant’s approval and it has ascertained their readiness to proceed with the planting or grassing of the areas where sweet sand will be placed.
Equipment Requirements
Compaction equipment shall conform to the number and type outlined in the Contractor’s detailed Program of Work as approved by the Consultant. Other types of rollers may be used if approved by the Consultant and satisfactory performance shall be the basis for such approval.
Excavation levels: All excavations shall be carried out to the lines and levels shown on the Drawings or to such lines and levels as the consultant may direct. The Contractor shall trim all permanent excavations to the slopes and batters shown on the Drawings and shall leave the surfaces smooth and even, unless ordered otherwise by the consultant. Sides of temporary and soft excavation, which, at the Contractor’s choice, are excavated to unstable slopes, shall be adequately supported. The maximum gradient of an unprotected slope or batter shall be 1:3. Any excavation deeper than 1.0m shall be adequately protected.
Excavations beyond the true line and level
If, from any cause whatsoever, excavations other than for concrete work are carried out beyond their true line and level other than at the direction of the consultant, the Contractor shall at his own cost make good to the required line and level with approved material and in such a manner as the consultant may direct.
If from any cause whatsoever excavations for concrete works are carried out beyond their true line and level other than at the direction of the consultant, the Contractor shall, when directed by the consultant and at his own cost, fill to the required line and level with concrete similar in grade to that intended to be used in the true excavation unless otherwise directed.
Moisture Control Requirements
When the moisture content of the soil does not fall within the required moisture range, water shall be added and thoroughly mixed into the soil, by approved methods or the material shall be aerated, whichever is needed to adjust the soil to the proper moisture content.
The amount of water to be added shall be only that amount that will, as determined by the Consultant by field tests, provide moisture content in the soil within the required range plus a reasonable amount to compensate for evaporation and other unavoidable losses. Water added in excess of this amount shall be considered as excess water and must be removed by aeration or other suitable means directed by the Consultant. Satisfactory methods and sufficient equipment shall be used for the furnishing and handling of the water so that there will be no undue loss due to evaporating or waste.
If water is added to cut areas or borrow pits, the surface of the areas or pits shall be maintained in such a manner that will prevent undue loss of moisture.
De‑watering: During the construction of the roadway, the roadbed shall be maintained in such a condition that it will be well drained at all times. Side ditches or gutters shall be so constructed to avoid damage by erosion.
In order that the works may not be subject to wetting, during or after construction, the Contractor shall at all times, and especially at an early stage of the Work, be required to provide adequate drainage by scheduling ditch work so as to prevent such wetting. The Contractor shall clean and trim all such drainage ditches from time to time during the work and/or when directed by the Consultant, so that there may be a free water flow throughout the whole time of the Work.
Damage to the work attributable to wetting through failure to provide such adequate drainage shall be immediately repaired by the Contractor at the Contractor’s expense.
Removal of unsuitable material
Rock, shale, and other unsuitable material encountered in cuts shall be excavated to the lateral limits and depth indicated on the plans or as otherwise directed. Any over breakage below the depth shown on the plans or as otherwise directed will not be paid for. Backfill of the over breakage shall be approved material and shall have the same density requirements as specified on the plans and shall be at the expense of the Contractor.
If the backfill of the over breakage is designated on the plans to be a crushed aggregate or other special aggregate, the entire cost of the backfill of the over breakage shall be at the expense of the Contractor. Undrained pockets shall not be left in the surface of rock.
All slopes shall be trimmed accurately to the slopes shown on the cross-section drawings, and care must be exercised that no material shall be loosened beyond the required slopes.
Removal of old roadways shall include filling of all pits etc and rough grading of the old roadway to restore approximately the original contour of the ground or to produce a pleasing appearance by forming natural slopes.
Where excavation to the finished graded section results in a subgrade or slopes of unsuitable soil, the Consultant may require the Contractor to remove the unstable materials and backfill to the finished graded section with approved material. The Contractor shall conduct his operations in such a way that the necessary cross-sectional measurements can be taken before the backfill is placed.
Soil disposal
All materials removed from the excavation shall be used in the works and at such other places as directed, unless it is declared unsuitable and ordered to be wasted by the Consultant. No excavation material shall be wasted without written permission from the Consultant, and when such material is to be wasted, it shall be so placed that it will present a pleasing appearance and not be a risk to people or facilities.
The plans may designate certain materials, such as specific quantities and/or ledges of rock or quantities of existing concrete, bituminous or other materials, to be excavated and stockpiled for a specific purpose or for future use. Such materials shall be carefully excavated and handled in a manner that will exclude foreign or undesirable material. The stockpiles shall be neatly and compactly constructed in an approved manner.
Compacting of fill material
Prior to the commencement of Earthworks the Contractor shall construct trial compaction lengths as directed by the Consultant. The soils used in the trials shall be those that will be used for the construction of road and the compaction equipment to be used shall be that specified in the Contractor’s detailed programme of Work and approved by the Consultant. The object of these trials is to determine the best field moisture content of the materials and the relationship between the number of compaction equipment passes and density attained.
Compaction Requirements: This Work shall consist of the compaction of earthwork by rolling or tamping or any combination of these methods in accordance with the requirements specified on the plans or ordered by the Consultant.
Suitable fill material shall be compacted to 95 percent maximum dry density. Suitable fill material placed within 300mm of formation level shall in addition have a minimum soaked CBR Value of 15 percent (or as shown on the drawings) at 95 percent maximum dry density. Maximum dry density is determined by the modified AASHTO tests carried out in accordance with BS 1377 (1975) Test No.13.
Tamping: Whenever materials are placed adjacent to structures or at locations where it is not practicable to use a roller, the materials shall be tamped by use of mechanical rammers or tampers. Each layer shall be compacted to a density equal to or greater than obtained under the above rolling procedure for the type of compaction designated.
Each successive layer shall contain only that amount of material which will insure proper compaction but in no instance shall any layer be greater than twenty (20) centimetres (loose measurement) in depth. Each layer must be approved by the Consultant before the next layer is placed. When the quantity of Work is small, a hand tamper may be used with the permission of the Consultant.
Special Provisions for Other Rollers: When special heavy rollers are used, the compacted thickness of the layer may be increased when approved by the Consultant as long as satisfactory compaction is obtained. Satisfactory compaction is defined as compaction which results in a uniform density throughout the entire depth of the layer equal to or in excess of the specified density. The maximum compacted thickness of the layer shall be established by the Consultant of each type of heavy roller used and for the various types of soil encountered. The Consultant reserves the right to vary the compacted thickness of the layer as the Work progresses to insure adequate compaction or to rescind approval of the heavy rollers.
Borderline Materials: In cases where borderline materials are encountered, both methods will be utilized and the method which results in the higher laboratory maximum density shall be used as the standard to which the field density is compared.
Subgrade foundation: All vegetable matter shall be removed from the surface upon which the Subgrade is to be placed and the cleared surface shall be completely broken up by ploughing, scarifying or stepping to a minimum depth of twenty (20) centimetres.
The existing material shall be adjusted to a moisture range and compacted to an average density of not less than ninety five (95) percent of M.D.D specified by the curve of Modified Proctor Density with no single density value below ninety-two (92) percent. No direct payment shall be made for subgrade foundation but their cost shall be considered subsidiary to the various Earthwork pay items.
Subgrade: The subgrade layers will always be constructed by laying and compacting materials in layers that should not exceed 15 cm in thickness (compacted measurement). The compacted density of the material forming the Subgrade (both in fill and in cut) shall be equal to or greater than an average value of hundred (100) percent of M.D.D. with no single value below ninety eight (98) percent except for Subgrade in cycle paths and sidewalks for which the compacted density shall be equal to or greater than an average value of ninety-eight (98) percent of M.D.D with no single value below ninety-six (96) percent.