Soil, Earthworks and Foundations

Classification and Placement of Soils and Fill Materials

In earthmoving operations, it is essential that the materials in use comply to the classification and placement criteria designated in the contract and are tested in compliance with the relevant clause of the British Standard BS 1377. An earthworks design is extremely important to a construction as all of the subsequent operations are based on the integrity of the placed materials and rely on the anticipated strength of the operation. The classification of fill material is primarily driven by the grading analysis (BS1377-2) and is normally dictated by the Specification for Highway Works Volume 1 Series 600. In this document all of the permissible fill classes are tabulated and required characteristics are ascribed to each. For all classes, a grading envelope is given along with a plasticity (BS1377-2) requirement in most cases and other attributes are detailed for materials of more specific use. These include electrical conductivity, shear strength, and Moisture condition Value (MCV) (BS1377-4) amongst many others.

The requirements for each classification are decided based on the use of the fill material and it is imperative that the parameters are met. For this reason, regular sampling and testing of placed earth is highly important and is generally, closely monitored on site for compliance.

Simtec is able to provide experienced staff to sample earthworks materials and carry out on-site tests such as MCV and in-situ density using the Nuclear Density Gauge or electromagnetic Soil Density Gauge the latter UKAS accredited to D7830/D7830M-14.

Plasticity Index – Atterberg Limits

The soil characteristics that most affect the behavior of placed earthworks material are the Atterberg limits which consist of a plastic limit, a liquid limit and the calculated plasticity index. Some materials are not going to perform adequately in certain situations and therefore it is imperative to classify the soil before use. Since • content the density can be calculated and again, as in the SRD method compared to the maximum dry density for the material under test.

This method is restricted to cohesive soils as it relies on the sample being able to be removed intact from the layer. Any aggregate content will interfere with the clean driving of the core and so the sand replacement method would need to be employed.

Strength and Loading Characteristics

Compacted soils and unbound aggregates placed for specific usage are tested in-situ by various physical methods. Crane platforms and piling rig locations are often tested using the plate loading test or the lightweight deflectometer (LWD).

For cohesive materials, either as present in undisturbed ground or placed and compacted as part of an engineering project the hand shear vane can be used to gain stiffness information. Although only used at the surface, for a consistent layer of soil it gives a reliable assessment. Simtec tests to the New Zealand Geotechnical Society Inc August 2001 specification and holds a UKAS accreditation for the operation. The DCP, as described below, can also be used to evaluate the stiffness of lower layers using a dynamic method.

Plate Loading Testing

The plate loading test developed by the US Corp of Engineers can be used to assess the in-situ strength of naturally occurring ground for structural purposes or to test the integrity of placed layers of material for compliance purposes.

The first of these tests is generally referred to as the “Plate Loading Test” where a fixed load is applied to a thick steel plate by means of a hydraulic jack and the reaction is monitored over time using digital or rotary gauges. The load applied is normally determined by the anticipated loading the ground is to be subjected to, as in a crane placement, a bridge or building construction etc. Deflection and permanent deformation can be measured very accurately using this method.

In the second application, usually identified as the “Plate Bearing Test” a steadily increasing load is applied to the plate in stages and the reaction of the material beneath the plate to this increasing load is measured. This modulus of reaction can be used to calculate the “California Bearing Ratio” (CBR) value, often a requirement of construction contracts and is normally carried out to BS1377-9.

The depth to which the load is applied in either case is a function of the plate area. The ground is stressed to approximately one and a half times the plate diameter and the plates used in testing generally range in diameter from 100mm to 600mm making the test able to assess ground depths from 150mm to 900mm.

This assessment method has been in use successfully since the 1930s and remains a “stock” test in highways and construction monitoring.

Typical Plate Bearing Test

The graphical output from this test shows the ground settlement under stress. The second load shows whether further compaction has resulted from the first loading, a ratio between the two reactions can be used to assess whether more rolling is required.

The plate bearing test can also be used to assess existing ground conditions during geotechnical investigations. In this way structural design for a project can be fine tuned to take into account the kind of formation that will be encountered during construction.

Stiffness Measurement with the Lightweight Deflectometer (LWD)

The “Portable Falling Weight Deflectometer” or “Lightweight Deflectometer” as it is also known (LWD for short) is a rapid assessment tool for carriageway layers, both existing and newly placed. For pavement design it is necessary to know the stiffness of the foundation platform and this can be quickly achieved by use of the deflectometer.  Giving a direct readout in Megapascals (MPa) this equipment is an invaluable tool for pavement engineering.

The portable apparatus consists of a weight which is dropped a fixed distance down a rod onto an anvil attached to a thick plate. The underlying material’s dynamic reaction to this drop is measured by means of a geophone, Bluetooth connected to a computer.

A test is carried out at the desired location by placing the machine over the location and releasing the weight by means of a trigger. This process is repeated several times and the computer, having collected the geophone data calculates an average value for the stiffness of the material beneath the plate. The process takes only a few minutes at each location and so a large area can be assessed in a short space of time.

This equipment is now accepted by Highways England and appears in the Specification for Highway Works as a tool for the evaluation of highway foundation platforms. The same process can be carried out on existing pavements to gain stiffness information when designing maintenance strategies.

Since the development of a generally accepted conversion equation some Authorities have now adopted this test to replace the Plate Bearing Test to assess sub base compaction in highway construction and repair works. The stiffness values can readily be converted into estimated CBR values. Although measuring in two different ways, a more precise correlation is possible between the Plate Bearing Test and the Lightweight Deflectometer for most commonly used road materials. This enables continuous monitoring of such works as pavement stabilisation, cement bound road bases and recycled road materials such as “Foambase” and cement bound recycled material. The more sophisticated type of LWD has a range up to 7000mPa and can therefore be used to evaluate whole pavements in site investigation routines.

Dynamic Cone Penetrometer Investigation

It is possible in both site investigation and multi layered placement operations to evaluate strata using a mechanical ground penetrating system known as the Dynamic Cone Penetrometer (DCP). This apparatus, developed for site investigation in undeveloped areas by the Transport Research Laboratory, is able to return data from subsequent layers by driving a rod and cone device vertically into the soil in a location. Simtec works to an in-house method, TPSM022, based on SHW IAN 73/06. The equipment consists of a steel cone mounted on a set of detachable rods which are driven into the ground by an in-line mounted drop hammer. This method enables stiffness values to be gained from a considerable depth. Software has been developed to convert the cone resistance to a CBR value and present it in graphical format. This can be invaluable for pavement design, foundation design and for compaction assessment.

Frequently Asked Questions

The size of sample depends on the material and the reason for the testing. Simtec Materials Testing can offer advice once we know the nature of the tests required. The more information you can supply, for example, client, site name, date sampled, material type, specification etc the more accurately we can advise you. A purchase order is normally required at this stage.

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