6. REGRADING

This is a term loosely applied to all operations involved in adjusting ground levels and gradients. All work is required to be in accordance with the drawings and specification but any changes which site management think may be appropriate should be brought to the attention of the Engineer for his consideration at the earliest opportunity,

THE ENGINEER MUST APPROVE ANY PROPOSED CHANGES BEFORE THEY ARE CARRIED OUT.


6.1 General

Regrading entails the movement of considerable quantities of earth before construction of roads or buildings commences and can be categorised into two sections:

    (a) Earthworks involved in major civil engineering contracts, e.g. motorways, open cast mining.

    (b) On site regrade for estate roads, housing and commercial developments.

The requirements for (a) are so wide ranging and subject to specific requirements regarding method of working, plant operation and outputs that they are not covered specifically here, although the comments do apply in general terms. The following information is for the assistance of the engineer performing the necessary regrade for (b).

6.2 Embankments and filling

The checks that need to be made before work starts include the stability and levels of the original ground or foundations at the base or top of embankments, selection of suitable fill material and the stability and profiles of side slopes.

THE STABILITY OF SLOPES TO CUTTINGS AND EMBANKMENTS (IN ALL WEATHER CONDITIONS) IS CRITICAL TO SAFETY ON SITE. IF IN DOUBT ASK.


    If a full soils report was not available at the time of design, certain assumptions may have been made as to the ground conditions. It is essential to verify these before commencing regrade and, if proved, to be incorrect, inform the Engineer immediately.

KNOW YOUR SOIL CONDITIONS BEFORE STARTING ANY LARGE SCALE EARTH MOVING OPERATIONS.


6.3 Existing services

Underground services can cause major problems on site especially when their location is in doubt. The correct procedure for dealing with existing services is detailed in Section 6-Drains, Sewers and Services. Overhead services can also restrict the operation of crane jibs, tipper lorries, mechanical excavators etc. Follow your Company's Safety Instructions and consult the appropriate service authority before any operations commence.

6.4 Filling material


Suitability depends on composition, moisture content and what the material is to be used for, therefore it is not easily defined.

    It is easier to define materials which are generally unsuitable for reuse,

such as:

    (a) Material from swamps, marshes and bogs.

    (b) Peat, logs, stumps and perishable materials.

    (c) Materials susceptible to spontaneous combustion.

    (d) Material in" a frozen condition ~

    (e) Soft clays.

    (f) Material having a moisture content greater than its optimum value.

    (g) Some rock material when subject to weathering.

    (h) Chemically contaminated material.

Other factors which need investigation:

    (a) The chemical characteristics and compatibility of the soils when they are in contact with foundations of buildings, roads, sewers and services so that the appropriate            precautions can be taken.

    (b) Sulphates. See BS : Code of practice for foundations.

    (c) Frost susceptibility, especially related to sub-grade for road construction.

6.5 Stripping

Keep stripping of topsoil to a minimum and only remove as necessary, taking into account the nature of the sub soil and the use of special earth moving plant. For example, clay sub soil deteriorates when exposed to wet weather and easily becomes unworkable.

    The location and size of spoil heaps should be carefully planned considering:

    (a) the need to keep the topsoil on site

    (b) is the topsoil re-usable?

    (c) where the topsoil is to be replaced

    (d) period of time before topsoil is to be re-used

    (e) working areas for. construction

    (f) stability of slopes.

Note: Item (d) -Topsoil may be suitable when stripped but can deteriorate if kept in a heap for a long time, weathering causing segregation, or excessive height of the heap causing changes in the internal water pressures in the soil. Also remember that double handling of soil is expensive.

6.6 Stability of slopes


The safe angle which is selected for a slope is dependent on the nature of the material under the worst conditions. The Engineer should have indicated on drawings the position of the top and bottom of any permanent slope after taking into consideration the necessary slope angle. Where softer material overlies rock, excavate the former at a flatter slope than the latter and leave a berm or horizontal ledge at the junction of the two materials. With cuttings in chalk and other friable rocks, allow for regular surface disintegration caused by weathering. Where necessary, safeguard the public and work-force from the falling, or accumulation of, debris.

IF IN DOUBT SEEK' INSTRUCTIONS FROM THE ENGINEER.


6.7 Slippages

These can occur on slopes without any apparent warning. Some causes are:

    (a)   Excavation or erosion at or near the toe of a slope.

    (b)   Additional loading at the top of a slope caused by traffic, building,

          plant tipping etc.

    (c)   Increase in weight of soil by taking up water.

    (d)   Steepening of the slope.

    (e)   Increase in internal water pressure.

    (f)   Softening of clay soils.

    (g)   Surface drying of clay soils causing shrinkage cracking with

          consequent water penetration.

    (h)   Increase in water table increases buoyancy of granular soils.

CHANGES IN WEATHER CONDITIONS CAN CAUSE SLIPS TO OCCUR.


6.8 Filling

Before starting, consider the stability of the material on which the filling is to be founded -the additional weight may cause problems. In general, topsoil must be removed before filling commences.

    The choice of suitable fill materials depends on:

    (a)   the purpose of the embankment

    (b)   availability of local material

    (c)   consolidation and settlement properties of the material

    (d)   wet weather working

    (e)   type of plant to be used.

Ditches, land drains or french drains are generally located at the toe of finished embankment slopes depending on the topography of the site. Ditches may also be required to drain the working area as the filling proceeds especially if an existing watercourse/drain is disrupted when a permanent diversion has to take place.

6.9 Compaction and testing

Correct compaction of embankments is essential and must be strictly in accordance with drawings and specification. Choice of the most suitable compaction plant and careful planning are essential for the most economic results. Refer to the Department of Transport Specification for Highway Works and the Contract specified requirements.

For general fill areas, the specification will usually require that the in situ density is a percentage of maximum dry density, usually 95 %. The strength of the formation for road construction will usually be specified as a percentage California Bearing Ratio. The CBR tests can be carried out in the laboratory but some Highway Authorities may require insitu CBR tests. See BS 1377- "Methods of test for soils for civil engineering purposes".

6.10 Quantities and planning

The design should have ensured the most economical balance between cut and fill, but this may have been based on a small amount of information of the soil types to be encountered and also there may have been only little knowledge of the local cost and source of fill material to be used. Therefore if site conditions vary greatly from the information available at the design stage, advise the Engineer of these changes so that the need for any redesign can be considered.

If the designed levels are the most economical, the locations of the cut and fill areas should be available and careful planning of haul routes and working methods should be completed before starting work.

DON'T RUSH -CAREFUL PLANNING CAN SAVE TIME AND MONEY.

Check any quantities provided and check whether or not they include construction thicknesses of any surfacing or sub-base. Full details of the quantities are required to plan the most economic operations. Just the total quantities of cut and fill in cubic metres are not of great use without knowing the average depths and areas involved. This especially applies to topsoil quantities over large areas - a small increase in depth can greatly increase total volume. This is important when considering how much site control is required in supervising topsoil strip. In handling any quantity of soil, allowance must be made for bulking which is usually in the order of 10%.

Before commencing work on site, record a grid of existing ground levels and compare with the original survey. A long period might have elapsed between original survey and start on site and any discrepancies between the two sets of levels must be brought to the attention of the Engineer before work commences.

6.11 Drawings

The regrade information will generally be shown on the following drawings:

    (a) Road longitudinal sections augmented by the road construction details showing the construction depths.

    (b) Regrade layout which normally shows the finished levels as contours superimposed on the existing ground contours. Spot heights may clarify areas not covered by regrade contours. The layout will also show any features closely linked with regrade, e.g.

        retaining walls, banks or steps.
    (c) Site cross sections showing proposed and existing levels.

    (d) Cut and fill drawing with areas of cut coloured red, and fill areas green. If this drawing is hand drawn, it merely denotes areas of cut and fill with no consideration given to the depth. However, computer plotting can show a range of depths denoted by density of shading.

    (e) Quantities produced by computers.

BEFORE STARTING REGRADE, STUDY ALL THE DRAWINGS AND INFORMATION CAREFULLY.