Dry-density/water-content relationship

The laboratory environment can get complicated. Running the correct tests in the right way, keeping track of samples and which laboratory technicians worked on each sample can take up a lot of time. With Lab-i, a laboratory information management system (LIMS) from business solution provider QBCon, you can focus on the most effective methods of analysis without worrying about time-consuming administration.

This week's blog entry focuses on the dry-density/water-content relationship. Why don't you focus on the test and let QBCon take care of your laboratory?

Dry-density/water-content relationship

The aim of the test is to establish the maximum dry density that may be attained for a given soil with a standard amount of compactive effort. When a series of samples of a soil are compacted at different water content the plot usually shows a distinct peak.

The maximum dry density occurs at an optimum water content.

The curve is drawn with axes of dry density and water content and the controlling values are values read off:
rd(max) = maximum dry density
wopt = optimum water content

Different curves are obtained for different compactive efforts

Explanation of the shape of the curve

For clays

Recently excavated and generally saturated lumps of clayey soil have a relatively high undrained shear strength at low water contents and are difficult to compact. As water content increases, the lumps weaken and soften and maybe compacted more easily.

For coarse soils

The material is unsaturated and derives strength from suction in pore water which collects at grain contacts. As the water content increases, suctions, and hence effective stresses decrease. The soil weakens, and is therefore more easily compacted.

For both

At relatively high water contents, the compacted soil is nearly saturated (nearly all of the air has been removed) and so the compactive effort is in effect applying undrained loading and so the void volume does not decrease; as the water content increases the compacted density achieved will decrease, with the air content remaining almost constant.

1. SCOPE

The maximum dry density and optimum moisture content, as defined below, is determined
by establishing the moisture-density relationship of the material when prepared and
compacted with a vibratory hammer at different moisture contents.

1.1 Definitions

Maximum density: The maximum density of a material for a specific compactive effort is the highest density obtainable when the compaction is carried out on the material at varied moisture contents.

Optimum moisture content: The optimum moisture content for a specific compactive effort is the moisture content at which the maximum density is obtained.

MDD & OMC

Soil compaction can be a very economical method of soil improvement, and it is often used to make ground suitable for the foundations of roads and buildings. It is also used in the placing of soil fills and in the construction of earth dams to ensure suitable soil properties. The compaction is normally achieved through the input of energy into the soil by impact, kneading, vibration or static means.

The extent of compaction depends on the moisture content of the soil and the compactive effort used. In a compaction test the object is to determine the optimum moisture content and maximum dry density achievable with a given compactive effort. A plot of dry density versus moisture content indicates that compaction becomes more efficient up to a certain moisture content, after which the efficiency decreases. The maximum dry density is obtained at this optimum moisture content.

If the compaction process were completely efficient, it would be possible (but not necessarily desirable) to expel all the air from the voids, in which case the dry density would correspond to a zero-air voids state (ie. the sample would be saturated with water). Since perfect compaction is not possible (except at high moisture contents and low dry density) the compaction curve will always fall below the ideal or zero-air voids curve.

It should be noted that there are a number of standards for compaction tests, each differing in the amount of energy input into compaction. For a given soil the different tests will produce different maximum dry densities and optimum moisture contents (ie. these parameters are NOT soil properties). The maximum dry density and optimum moisture content are only relevant for a specified compaction procedure which should be stated when presenting the results.

In earthworks it is common to specify a dry density within a certain percentage of the maximum determined from a specified compaction test. For this to be a sensible procedure it is important that the compactive effort used in the laboratory is comparable to that supplied by the field equipment.

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The laboratory toolset

Every industry has basic tools enabling businesses to function effectively. These tools can include computers with software for writing documents. E-mail application’s to communicate with employees, partners, clients or financial management software. Most of these tools are no longer a luxury. We rely on these tools in the day to day running of our businesses.

Tools like equipment and software are more specialised for each industry. Commercial laboratories are diverse, but the basis of successful operations must be consistent. Laboratories rely on trained staff, clients (samples), formulas, chemicals, testing equipment, and processes. Combining these with good management will give you the basis for your success.

What if you could increase your probability of success?

Laboratory information management systems (LIMS) are often overlooked when laboratories put together their toolboxes. LIMS provide opportunities for improvement of your lab. If used correctly, LIMS allow a number of tangible benefits, including increased productivity and quality, faster and better service, and can increase your bottom line.
QBCon’s Lab-i will do just that. What sets us apart from our competitors? We have been in the industry for 18 years and we have a home grown advantage. We can customize our systems to satisfy our customer’s needs whilst ensuring a short turnaround time.

Visit the QBCon home page for more on Lab-i http://www.qbcon.com/

Geostrada invests in Lab-I

Geostrada Engineering Materials Laboratory (Geostrada) provides laboratory testing for civil engineering projects. A business unit of specialist technical services consultancy, Aurecon, the Hatfield laboratory processes a large number of samples for analysis every month and required a laboratory information management system (LIMS) that would minimise the amount of time spent managing tests and samples, as well as reduce paperwork.

Subsequently, Geostrada took a decision to invest in Lab-I, a LIMS development by business solution provider QBCon. The LIMS will replace the current manual system at the Hatfield laboratory and will be implemented in all Geostrada laboratories in the near future.
Geostrada opted for Lab-I because the customizable software could be adapted to meet their unique requirements. Additional equipment integration functionalities will be added to the standard laboratory software, as well as technical and graphically reporting requirements.

Biometric identification functionality

Lab-I’s biometric identification functionality, which is already part of the system, enhances the security aspects of the application. This fingerprint identification functionality allows management to know precisely who was responsible for each phase during the laboratory process. This aspect is of critical importance for ISO 17025 accreditation purposes.

Equipment integration functionality

The equipment integration functionality will add to test result validity and accuracy, leaving no space for human error. An example of this is the weighing of samples on a balance. Previously, a user would enter results, whereas equipment integration functionality means the reading will be automatically recorded into the application, minimising room for mistakes.

A customisable computerised LIMS will allow Geostrada to streamline laboratory processes, improve service delivery and increase their profit margin. Laboratory staff will be held accountable for tests performed and automation will ensure greater accuracy – all benefits that equip the laboratory to provide an enhanced service to its clients.