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	Sub-base layers

Introduction   Materials  - Granular    - Coverage  - Bound  - CBMs   Std. sub-bases   Price Guide   Construction

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Introduction and General Notes Principles The 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...
Unbound Granular Material 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-base The 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 "804", a name derived from the fact that the full specification is to be found in clause 804 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-base This 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 run This 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 Bound Materials This group refers to those materials that use cement as a binder, ie, a 'glue' to stick together the individual particles. Mass Concrete This 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 - 5 There 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 SHW This 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 SHW This 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 SHW These 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.

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