9.4 Sub tapak a) Di dalam pembinaan jalan, sub-tapak terdiri daripada bahan agregat terhancur yang mempunyai kualiti dan gred yang baik dengan saiz maksimum 75mm serta bebas daripada sifat tanah liat atau bahan berorganik.

b) Dalam keadaan tertentu, sub-tapak boleh juga terdiri daripada 2 lapisan yang dipadatkan dengan jumlah ketebalan 200mm. Untuk lapisan yang pertama, agregat terhancur perlu disebarkan dengan seragamnya hinggamembentuk lapisan dengan ketebalan 125mm dan perlu dipadatkan selepas itu dengan 4 kali laluan bagi 8 tan menggunakan jentera penggelek roda licin.

c) Kemudian permukaan yang telah dipadatkan akan disembur dengan debu- debu kuari yang nipis pada celahan-celahan dan dipadatkan semula dengan 6 kali laluan menggunakan jentera yang sama. Proses yang sama akan dilakukan kepada lapisan yang kedua.

d) Sebelum lapisan kedua dibina, kontraktor akan membina bonggol dengan kualiti tanah yang baik pada pinggir jalan untuk menyokong pinggir sub-tapak dan lapisan salut.

Do I really need a Sub-base?

The short answer is: if the pavement will be trafficked by vehicles, then yes, but if it will be foot-traffic only, then not necessarily.

The type of paving or surfacing that will be used is one of the key factors in determining whether a sub-base will be required. Some paving types, block paving for example, nearly always need a sub-base, while others, such as pattern imprinted concrete, may not need one, but wouldn't be any less effective if one was to be included. See Table 1 below for a summary.

Bear in mind the comments made on the Introduction to Sub-bases regarding their role within a paved area, how they help spread loads and provide an 'underlay' to the 'carpet' of paving. You should only exclude a sub-base if there is good reason to do so. Compacting a sub-base using a Plate Compactor

Type of paving Sub-base? Thickness(mm)

Comments

Flexible Block Paving

Always 75-15075mm min for foot traffic only,100-150mm for driveways,150mm+ for commercial applications

Rigid Block Usually 100-225 May be used beneath base

Paving

Patio Flags Occasionally 75-100 Not necessary, but useful if levels need to be elevated

Indian Sandstones

Occasionally 75-100 Not necessary, but useful if levels need to be elevated

Yorkstone flags Occasionally 75-150Not necessary for foot traffic, but may be used when constructing a driveway

BS Flags Usually 75-150Not necessary for patios, but may be used when constructing a driveway, and often specified for commercial works

Gravel Usually 75-150May be omitted in decorative areas, but must be used for trafficked areas

Setts/Cubes Usually 75-150Optional for rigid construction; essential for flexible construction

Cobbles Occasionally 75-150 May be used beneath a base layer

Tarmac Always 100-150 100mm for footways and light driveways

Concrete - PIC Usually 100-150Often omitted beneath PIC on driveways, which is not always a good idea

Resin Systems Usually 75-150 Depends on substrate type

Table 1: Requirements for Sub-base and recommended thicknesses

How deep should my sub-base be?

Generally, a flexible sub-base of unbound material needs to be at least 50mm deep, while a bound sub-base of CBM will usually be at least 100mm in depth. For Bitumen-bound material, then 50mm is a typical minimum thickness.

There's a 'rule of thumb' that we use with fill layers - they should always be at least twice the thickness of the largest particle size. This is done to ensure that no 'points' are created within a layer, that is, no particle or lump of stone can be in contact with both upper and lower surfaces at any time, but by definition must be 'cushioned' by finer material either above, below, or both above and below. This ensures that the layer acts to spread the loads it carries, but also ensures there are no 'rocking points' within the layer.

So, if maximum particle size is "x", then minimum layer thickness is "2x", as shown opposite.

DTp1 has, in theory, a maximum particle size of 37½mm, so using this 'rule of thumb', the min layer thickness would be 3½ × 2 = 75mm. If a fill layer or sub-base layer was using a 50mm crusher run material, then the minimum layer thickness/depth would be 50 × 2 = 100mm, and so on and so forth.

Conversely, if we know that we have space for only 50mm of fill material, then the maximum particle size would be 50 ÷ 2 = 25mm.

So, assuming a sub-base is going to be used, just how deep should it be? This is revealed in Table 2 below.

Project Typical depth

Patios, Garden Paths 75-100mm

Driveways, public footpaths 100-150mm

Heavy uses 150-225mm

Highways 150mm +

Table 2: recommended sub-base depths using DTp1

 

Can I use old housebricks as a sub-base?

Sub-bases aren't included in a pavement just for fun - they are often the key load-bearing layer, and trying to save a few bob by using old house bricks or other rubbish you have lying around is often a false economy. How would you feel if a carpet fitter suggested using a few old tea towels under your living room carpet, instead of proper underlay, as it would save a bit on cost??

The trouble with using old bricks is that folk tend to chuck them in 'as is', whole bricks and half-bricks dumped haphazardly over the sub-grade, with

Here’s a picture of what I mean about folk using full bricks as a sub-base….

hundreds of gaps and voids between them. The same goes for other so-called 'hardcore' such as old flagstones, edging kerbs, broken lumps of concrete and the like.

After a few weeks, any finer material used as additional fill, and/or the bedding material, starts to trickle down and settle into all those voids. And that's not an end to it - following this initial settlement, the outline of the bricks starts to show through, and so you find that, for instance, bitmac follows the ups and downs of the bricks, and flags start to 'rock' on the high points.

…it’s a big NO-NO!!  

Even breaking them in half isn’t ideal. Although this does tend to reduce the problems, it doesn’t get rid of them altogether. What you need to do is break up the bricks into quarters or smaller – the smaller the better – and then ‘blind’ them with grit sand, rock fines or crushed stone, so that all the voids between the smashed bricks are filled, and then compact the lot, repeatedly running the vib plate or roller over the area until all the ‘fines’ are rattled into the voids and you have a reasonable ‘capping layer’.

You can then regulate the top of this layer with 75mm or more of quality sub-base material – a DTp1, DTp2, or a crusher-run material.

Skimping on the sub-base is one of the surest ways of shortening the life of any driveway. For what it costs, spending a few quid extra on quality sub-base material is a sound investment that will pay for itself over the years of extended service it enables.

Introduction and General Notes

PrinciplesThe sub-base layer is often the main load-bearing layer of a pavement. It is designed to evenly spread the load of the paving, and any traffic thereon, to the sub-grade below. A well-constructed sub-base will prevent settlement and channelisation - the phenomenon common on cheap installations of block paving, where two 'ruts' develop in the paving. These 'ruts' are caused when a car travels over the same line of paving to the garage, every morning and every night. Channelisation is also apparent on carriageways, particularly at the approach to traffic lights and on upward gradients. The sub-base is intended to prevent channelisation and settlement.

All too often, cut-price residential block-paving installations omit the sub-base as a major cost-saver. Householders are unlikely to be aware of the need for such a layer, and, if out all day when the work is being done, may never notice its absence. A common tale I hear is....."such-and-such-abody said we didn't need any stone, because the dirt under the old flagstones will be firm enough!" - I usually get this tale once the paving has sunk, and the original contractor is long gone.

When discussing the construction of driveways with clients, I liken the sub-base to a carpet underlay...you can spend a fortune on the finest Axminster carpet, but if you don't use an underlay, it's never going to look or feel right. If your contractor tells you that a sub-base isn't required beneath your new drive, then ask how the sub-grade (soil beneath drive) is going to spread the load of the paving and what guarantee is offered with regard to settlement. Alternatively, get a different contractor to give his opinion.

Once a good sub-base has been installed, it can be re-used without any further work, if you decide to change the surfacing/paving of your driveway. Many newer houses have tarmac drives with a good sub-base beneath. In such cases, the tarmac can be stripped off, the sub-base checked and re-levelled if required, and the new paving can be laid over the old sub-base.

A finished sub-base should not deviate from the correct level by more than 10mm, and should reflect the final profile of the paving. The bedding layer above the sub-base ought to be a constant thickness to avoid differential settlement.

A Vibrating Plate Compactor often referred to as a 'Wacker Plate'

There is a separate page that considers Sub-base FAQs including...

Do I really need a sub-base? What depth of sub-base is required?

Suitable Materials

See also British Standards Page

The materials used to construct a sub-base are chosen for their inherent load-spreading capabilities when correctly laid. Two different types of material are considered here...

i. Unbound Granular Material ii. Cement Bound Material

Unbound materials are defined as those aggregates which are loose and do not bond or adhere to their neighbours when laid and compacted, but rely on the natural interlocking of adjacent particles. An unbound sub-

base material can be loosened and removed quite easily.

Bound materials are, therefore, those which do bond with their neighbouring particles by means of a "binder". The two most common binders are Bitumen (tar) and Cement. Hence, 'tarmac/bitmac' is actually a Bitumen Bound Material and 'concrete' is a Cement Bound Material (CBM). Bound materials 'set' and are consequently more difficult to loosen and remove.

Unbound Materials rely on natural interlock

Bound materials use a binder, which can be bitumen or cement

Unbound Granular Materials

DTp Type 1 Granular sub-baseThe name refers to Dept. of Transport specifications. The full title is "Type 1 granular sub-base to the Department of Transport Specification for Highway Works" It is still referred to as MOT1 (the old Ministry of Transport) or even the more inaccurate "40mm to dust". In Ireland it is usually known as "803", a name derived from the fact that the full specification is to be found in clause 803 of the National Roads Authority's Specification for Road Works - can you spot a trend here?

There is a slight difference between the Spec used in Britain and that used in Ireland that results in DTp1 not being quite the same as 804 Type A, but we are talking about hundredths of a millimetre. For anyone wanting to know more, a PowerPoint Training presentation comparing and contrasting is available on request.

This is a material from a certified source and should have the correct proportion of 'fines' (dusty material) and solids ('lumps'). The solids should not exceed 37.5mm in size. The mixture of solids and fines is designed to ensure that there are no voids in the sub-base material once compacted, that the aggregate forms an acceptable level of interlock between the angular particles, and that the compacted sub-base will allow any ground water to drain through.

This material is typically crushed stone, crushed slag, crushed concrete or non-plastic well-burnt shale. It can be obtained from most Builders' Merchants in multiples of 1 tonne loads or direct from quarries in full wagon loads (usually 16 or 20 tonne loads).

Grading range for DTp1 material

1 tonne of DTp 1 covers approx 5 m² at 100mm compacted thickness, and approx. 3.5m² at 150mm compacted thickness. See below for pop-up calculator.

DTp Type 2 Granular sub-baseThis is an unbound sub-base material that has less 'lumps' and more fines than Type 1, and overall, the aggregate is a good deal smaller. Therefore it does not generate as much interlock as a Type 1 and is consequently not as strong. In some parts of the country, a sand/gravel mix qualifies as a Type 2, provided it meets the grading requirements shown below.

Grading Range for a Type 2 material

As can be seen from the comparison chart below, the "grading envelope" for a Type 1 material (red chart) allows much less fine material, that is, the material in the 10mm, 5mm and 600 µm ranges, to be included than is acceptable for a Type 2 (green chart).

Comparison of Grading Envelopes for DTp1 and DTp2

For those of a mathematical or scientific persuasion (or saddoes like me) the skewing to the right of the Type 2 grading envelope in the greenchart indicates that more of the finer material is passing through the sieves.

50mm Crusher runThis is very similar to the DTp1 discussed above and may be slightly cheaper. There is less rigid control of the fines:solids ratios, and it may be a slightly 'softer' or less competent rock. The name refers to the sieve size that the material has passed through following crushing. It is usually a suitable alternative to DTp1, and is more than adequate for light-duty applications such as patios or paths. Coverage rates are same as, or very similar to, those for DTp1.

 

Coverage Calculator

Other Calculators

Click here for a pop-up calculator that calculates the quantity of granular material required to cover a known area at any given thickness.

 

Cement (Hydraulically) Bound MaterialsThis group refers to those materials that use cement as a binder, ie, a 'glue' to stick together the individual particles.

Mass ConcreteThis is only normally used in applications where:-

exceptional loads are anticipated the paving units are to be directly bedded, such as setts, cobbles or duckstones bad ground may be a problem

It is more expensive than either of the above methods. A 'slab' of mass concrete, usually 100mm or 150mm thick is spread and compacted, then allowed to set. A few days later (2-7 days, depending on time of year and weather conditions), the paving is laid on a bedding material over the now solid concrete.

A medium strength concrete is used in most applications. Concrete is best supplied ready-mixed to ensure thorough mixing and a correct ratio of cement to aggregates. Strength C20 is the usual grade, and a 25mm slump is reasonably stiff and not too 'runny' to handle. If your application warrants a concrete sub-base, it would be wise to consult a paving /concrete contractor, or civil engineer for accurate specification. 1m³ concrete covers 10m² at 100mm thick and 6.5 m² at 150mm thick. Concrete is ordered and delivered by volume, not by weight.

CBM - Grades 1 - 5There is a range of cement bound materials that are not as inherently strong as the mass concrete described above, but are considerably stronger and stiffer that unbound crushed rock sub-bases. These have limited applications for small paving projects such as driveways and patios, but are used on larger commercial applications, such as pedestrian schemes and car parks, as well as roadways and filling station forecourts.

The Specification for Highway Works (SHW) arranges CBMs into 5 categories described below...

CBM 1 - clause 1036 of SHWThis is the 'weakest' of the CBMs and was previously known as 'soil cement'. The aggregate, which can be almost any granular material, including recycled materials, is mixed with cement and water and then compacted in place. The quantity of added cement has to be sufficient to achieve a 7-day crushing strength of 4.5N/mm². It can be batch mixed, which offers control over weight/volume of aggregates and cement, or mixed in-situ by, effectively, rotavating cement into an existing layer of suitable material. There is a slightly stronger version, CBM1a, which has a 7-day strength of 10N/mm² It is most commonly used as a capping layer or low-grade sub-base beneath flexible pavements.

CBM 2 - clause 1037 of SHWThis is a coarser and stronger material than CBM1, being based on a 40mm down granular aggregate or blast furnace slag, and sufficient cement to reach a 7-day strength of 7N/mm² CBM2 can be batch mixed or mixed in-situ. Again, there is an 'enhanced' version, with the stunningly original title of CBM2a, and this would have a 7-day strength of 10N/mm² This, too, is used as a capping or sub-base layer beneath flexible pavements.

CBM 3, 4, and 5 - clauses 1038 and 1039 of SHWThese materials are closer to what is commonly thought of as concrete, and are what used to be called 'lean mix'. They are mixed in batch plants, using approved natural aggregates (BS812 and BS882) although they can incorporate suitable recycled aggregates, such as crushed concrete and ggbfs (ground granular blast furnace slag). Typical size of aggregate is either 40mm or 20mm, and 7-day strength is a minimum 10N/mm² for CBM3 and 15N/mm² for CBM4. These materials can be used as road bases beneath heavy-duty paving, such as major highways, as well as being used as rigid sub-bases or high strength capping layers. Because they form an integral part of the final structure, quality control is paramount and so these materials must be batch mixed and they are typically laid by a slipform paving machine.

Typical Sub-base Designs

Here are four different types of sub-bases. Note that these are not 'official' classifications, in that they are not recognised by national building and civil engineering authorities. They are a simple generalisation, to aid comprehension of the various specifications for sub-bases discussed within this web-site.

In most cases, one of the following types should be suitable for residential projects. The notes on each paving type will refer to one of the following sub-bases. In extreme conditions, e.g. very heavy loads expected on paving, or bad ground conditions, a special sub-base may need to specified for your particular situation. More details available by email

Sub-base Type A (light, foot traffic only - paths and patios)

75mm compacted thickness of DTp1 or of 50mm crusher run, compacted with a vibrating plate compactor (Wacker plate) or vibrating roller, if space permits. Both these items are available from local hire centres. A reasonable alternative is to compact the material by dropping a sledge hammer or asphalt punnel directly down onto the material, using the weight of the tool to aid the compaction. A compacted sub-base should not move when you walk upon it.

Sub-base Type B (light foot and vehicle traffic - domestic drives, paths and patios)

100mm compacted thickness of DTp1 or of 50mm crusher run, compacted with a vibrating plate compactor (Wacker plate) or vibrating roller. As this sub-base is intended to carry the weight of a vehicle, it is essential that compaction is thorough. It is money well-spent to hire a plate compactor or roller to eliminate the chance of future settlement. The finished surface of the sub-base should be 'tight' i.e. not open textured or with surface voids. Fill any such openness with fines or grit sand.

Compacting a 100mm sub-base

Sub-base Type C (foot traffic, cars, vans and small pick-up trucks not exceeding 7.5 Tonnes)

This is exactly the same as the Type B sub-base above, except it is 50mm thicker. 150mm of DTp1 is used, and this may be compacted as two 75mm layers or one 150mm layer. Compact thoroughly and make sure the final surface is 'tight' before continuing with the bedding material.

Compacting a 150mm sub-base

Sub-base Type D (foot traffic, cars, vans and small pick-up trucks etc. not exceeding 20 Tonnes)

Simply a slab of mass concrete i.e. there is no reinforcement in the concrete. If this is your required sub-base, seek professional advice before continuing. Usually 100mm thick or 150mm for heavier users such as tractors or wagons or when covering bad ground. In these cases, reinforcement via polyester fibres or steel mesh is usually added to the slab.

Price Guides

The following prices are intended for guidance only. Prices vary throughout the UK, check with your local suppliers. Price includes material as stated, all necessary labour and hire of suitable compaction equipment.

Type of Sub-base 30m² Rate per m² 100m² Rate per m²

50mm DTp1 £90 £3.00 £200 £2.00

100mm DTp1 £180 £6.00 £400 £4.00

150mm DTp1 £270 £9.00 £600 £6.00

100mm C20 Concrete £380 £12.70 £1,060 £10.60

150mm C20 Concrete £570 £19.00 £1,590 £15.90

Construction

The sub-base material should be spread out in layers no thicker than 150mm and roughly levelled with spades, rakes or similar. Once level, it should be compacted by multiple passes with the compacting equipment, both longitudinally and transversely, until full compaction is achieved. See tables above for minimum number 'passes' for each layer of sub-base. A road roller should be used for heavy-

duty applications

Once compacted, the level of the sub-base layer should be checked. Any part of the sub-base deviating from the required level by ± 10mm should be raked off or topped up with additional material and re-compacted to the correct level.

Sub-bases of thickness greater than 150mm should be constructed in layers not exceeding 150mm. For example, a 250mm thick sub-base would be constructed as a 150mm layer, laid and compacted and a 100mm layer laid and compacted over it.

Roadways and parking lots consist of several different layers. Subbase – Typically local soils which vary greatly in ability to support a load determine the required thickness of the overlaying layers. Gravel Base – Types and thickness vary. Original Pavement – Types and thickness vary.

Traffic loads should be factored in when your pavement is designed. If a pavement is under-designed or overloaded, it may tend to deteriorate or even fail sooner than intended.

Bituminous SurfaceThe uppermost part of a blacktop road is called the bituminous (or wearing) surface. This layer is about 6.5 cm (2.5 inches) thick and made of a mixture of asphalt (6%) and aggregate (94%). The aggregate particles are about 2 cm (0.75 inches) in diameter. The bituminous surface can be removed and replaced as it "wears" out.

 Bituminous BinderThe intermediate layer of a road is called the bituminous binder. It is about 7.5 cm (3 inches) thick. Like the bituminous surface, it is a mixture of asphalt (6%) and aggregate (94%), and is designed to be the main load-carrying layer. The aggregate particles are about 4 cm (1.5 inches) in diameter.

 

AggregateA road’s base layer consists entirely of aggregate. It is about 30 cm (12 inches) thick and made of various sizes of aggregate. The largest particles are about 5 cm (2 inches) in diameter. The base layer dissipates the weight of cars and trucks into the underlying layer, the subgrade.

 SubgradeThe very bottom of a road is its subgrade. This is the local soil that has been compacted with a roller. The subgrade provides the stable base upon which pavement is constructed.