Proctor density

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The Proctor density ( ) is a term used in geotechnical engineering , construction and geology and is the highest achievable under a defined compression work density of a soil, which in the standardized test (Proctor test DIN 18127) at the optimum water content is adjusted. However, it is not the fundamentally highest density that a soil can occupy; however, this is often wrongly assumed or formulated in this way.

The Proctor test was introduced in August 1933 by the American civil engineer Ralph R. Proctor .

The remaining reduction in the proportion of pores or the permanent increase in the dry density of a soil is called compaction . Full saturation is hardly ever achieved and the dry density remains below the saturation line . The dry density that is achieved with a given soil depends not only on the water content but also on the compaction work performed and a certain type of compaction.

Proctor trial

A soil sample, the dry density of which has been determined beforehand, is supplied with predefined energy in a defined container (Proctor pot or Proctor compacting device) according to a specified working process via a Proctor compactor (drop weight with guide / handle bar) and the density achieved is then determined. The experiment is carried out at least five times with different water contents. If the densities achieved are plotted against the associated water content , a curve results which first increases, reaches a maximum and then decreases again. The maximum of this curve is the Proctor density of the soil with the associated optimal water content . A clear connection between compressibility and water content becomes visible.

The degree of compression can be expressed as follows:

The degree of compaction is given as a percentage of the maximum possible density in the Proctor test. It serves as a reference value for the storage density that can or has been achieved on a construction site. Due to the increased compression performance compared to the standardized Proctor test, Proctor densities of over 100% can also be achieved. Often - depending on the type of soil - a degree of compaction of at least 95% or more (up to 103%) is required. The degrees of compaction required in earthworks are also specified in the ZTV E-StB (Additional Technical Contractual Conditions and Guidelines for Earthworks in Road Construction).

The aim of this test procedure is to reduce the proportion of the third phase (air) of the three-phase system of a soil (air, water, solid matter) to a minimum. This occurs during compression by reducing the pore space by "pushing together (compressing)" the solid particles and filling the remaining pores with water (constant increase in the water content). The increase in the dry density by increasing the water content also results from a significant reduction in the frictional forces between the solid particles - the water acts as a kind of "lubricant", so that the fixed, constant compression energy has a better effect at higher water contents. This effect ceases as soon as the optimal water content is exceeded and an unfavorably high water content is reached. The applied compression energy then no longer has the effect of a denser deposit of the grains, but goes into the excess amount of water, so that water escapes or splashes out during compression.

Experimental setup and example

A gravel sand (up to 10 mm grain size) is examined successively with 6, 7, 8, 9, 10 and 11% water content. In the prescribed cylinder with a diameter of 10 cm, about 2 kg of soil are filled in three layers. Each layer is compacted with 25 blows of a 2.5 kg tamper with a diameter of 5 cm from a height of 30 cm. The dry density (quotient of dry matter and volume) is then determined for each sample and recorded in a curve diagram (x-axis = water content in%; y-axis = calculated dryness in t / m³ or g / cm³). This results in the optimal water content for compaction; the highest dry density can be with a gravel sand z. B. 2.12 t / m³ (g / cm³) and the water content are 7%.

Individual evidence

  1. ^ Wiltshire, Richard L., 100 Years of Embankment Dam Design and Construction in the US Bureau of Reclamation, Denver 2002
  2. Compaction requirements according to ZTV-W 205 for earthworks
  3. Quality control of the earth and foundation work according to ZTVW and ZTVE-StB
  4. Compaction requirements according to ZTVE-StB 94, see slide 9