Copy Export. Details You May Also Like. Details: Creators:. Rixner, J. Corporate Creators:. Corporate Contributors:. United States. Federal Highway Administration.
Resource Type:. Tech Report ;. Geographical Coverage:. United States ;. Final report; Sept. Contracting Officer:. DiMillio, A. Corporate Publisher:. Turner-Fairbank Highway Research Center ;. NTL Classification:. This volume presents procedures and guidelines applicable to the design and instal tion of prefabricated vertical drains to accelerate consolidation of soils.
The contents represent the Consultant's interpretation of the state-of-the-art as of August The volume is intended to provide assistance to engineers in determining the applicability of PV drains to a given project and in the design of PV drain systems.
The information contained herein is intended for use by civil engineers familiar with the fundamentals of soil mechanics and the principles of precompression. The volume includes descriptions of types and physical characteristics of PV drains, discussion of design considerations, recommended design procedures, guideline specifications and comments pertaining to installation guidelines, construction control, and performance evaluation.
This volume is the first in a series. PDF ;. DTFHC ;. Collection s :. US Transportation Collection. Main Document Checksum:. File Type:. But these were relatively expensive, and inconvenient to place at close spacing. They are relatively inexpensive, provide higher conductivity up to 30 times more effective than a mm diameter sand drain and can easy be installed at close spacing, thus shortening the path of the pore water in the impermeable soil and expediting the consolidation process.
The core is a highly flexible polypropylene extrusion, having maximum water flow capacity along the grooves formed longitudinally on both sides of the core. Strict quality control is employed to insure the extrusion is consistent. The geotextile fabric serves as a filter to allow passage of groundwater into the drain core while preventing piping of fines from the adjacent soils.
The filter also serves as and outer skin to maintain the cross-sectional shape and hydraulic capacity of the core channels. The equipment is comprised of a structural mast which in some cases also serves as equipment housing , a mandrel and mandrel propulsion equipment.
The mandrel and the wick are then driven into the ground to the desired depth. The anchor plate serves two purposes in the operation. First, it prevents soil from entering and clogging the mandrel as it is being driven into the ground, and secondly, it anchors the drain in place at the desired depth as the mandrel is being retracted. When the mandrel has been withdrawn, the wick is cut off above the ground surface, leaving a tail approximately mm long. Then a new anchor plate is installed, the mast is repositioned over the next location and the cycle is repeated.
There are various means of driving the mandrel, including a simple cable pull powered by a conventional crane, a vibratory head attached directly to the mandrel although this technique is not recommended because of its detrimental effects on the surrounding soil , and a hydraulic cylinder powered by the hydraulics of an excavator. The hydraulic system has a mechanical advantage, allowing deeper penetration and greater applied force. When stiffer soils are encountered extra weight can be added to the mast to assist penetration or holes can be predrilled before the mandrel and drain are inserted.
As the ground surface in areas requiring prefabricated vertical drains is often soft and unstable. It may be necessary to prepare a working mat to facilitate mobility of the installation equipment. This mat also serves a second purpose of providing a free draining layer for the water being discharged from the drains. This working mat is generally constructed of sand, as part of the preload or structure fill, and is typically mm or more in thickness.
In the event that good quality natural materials for the working mat are scarce, several options exist to both improve the stability of the surface soil and reduce the thickness of the working mat.
Geotextiles or geogrids are very effective in strengthening the sub-base, and can significantly reduce the amount of fill needed to provide a suitable and stable working mat. Efficiency may also be increased by replacing the free draining sand mat with prefabricated horizontal drains connected to the protruding wick tails. This is a very effective method to ensure fast and complete removal of all water discharged from the wick drain project.
When the applied preload becomes part of the structure, as in an embankment or bridge approach, the materials used for the preload are usually selected accordingly. However, when the ultimate required elevation is already close to the anticipated elevation after settlement, the added costs of applying the preload, and removing it again after consolidation has occurred, often renders the project economically unviable. The system consists of installing prefabricated vertical drains, individually connected below the surface to vacuum transmission pipes.
These pipes are then connected at surface level to a horizontal tubing system by means of specially developed airtight T-couplings.
The so-called drainage screens, a row of vertical drains that are connected at the top to a horizontal line, are brought outside the surcharge if any and connected to a combined vacuum air pump that has been developed in-house. The applied vacuum produces the same pressures as a traditional preload system of up to m in height. The compression test is the most common method, and the Ch value is a function the Cv value thus found. Drains are most efficiently placed in a triangular pattern, but they can be arranged in a square pattern although a triangular pattern is nearly 2.
The diameter of the sphere of influence D in formula 1 is based on the existence of a soil cylinder, and to account for overlap and gaps between such cylinders. The design drain spacing for triangular pattern should be 0.
The drain diameter d in formula 1 assumes a cylindrical drain. The flow towards a flat drain is less efficient than flow towards a cylindrical drain, and therefore, the drain diameter is conservatively taken to be 50mm. The average degree of consolidation U is usually expressed as a percentage, or a value between 0 and 1. Experience has shown that the drain spacing is usually grater than 1. Therefore, the following simplified formula is often preferred. A sand column with a diameter of mm has a qw of approx.
The formula only offers the possibility of determining the consolidation at a given depth z. The following graph shows how consolidation time varies at various depths for unilateral flow and a 20m thick layer of soil. The following example demonstrates use of the graphical solution. From this point, draw a vertical line to the oblique line corresponding to a depth of 20m, and from there a horizontal line to the scale along the edge indication the drain spacing.
The graphical solution shows that the required drain spacing is 1. In terms of drain quantities, these spacing represent ratios of 3.
Therefore, it can be seen that the drain capacity can be of considerable influence on the quantity of drain required. This decision will depend upon how settlement varies with depth and upon the different in Ch values at various depths. This is demonstrated in the example of a motorway which is planned through an area underlain by a 40m thick compressible layer. The following solutions are feasible : 1. Installation of drains down to 40m.
Degree of consolidation then becomes 0. For a square pattern, formula 1 then yields a drain spacing of 2. For the entire area this means the installation of 87,m of drain. It is assumed that the first 0. For a square pattern, this requires a drain spacing of 1. The drain length will be 20m. Installation of drains down to 25m. Besides the saving s in the quantity of wick, the shorter length of the wicks will also result in saving in the costs of installation. Furthermore, the variation of the Ch value can be of influence on the drain spacing.
There have been situations where the rate of consolidation is not influenced by the drain spacing. This is believed to be the result of layering of the soil. If there are closely spaced highly permeable layers of sand between the clay or pet layers.
The overstressed pore water will find these sand layer and follow this path to the nearest drains. The concept of vertical drainage utilizing prefabricated drains has been applied to many non-traditional applications and end uses.
CeTeau encourage innovative proposals from our clients for new and novel applications. Our involvement with a wide variety of geosynthetics such as geomembranes, geotextiles, geogrid, geocell, gabion and prefabricated drains uniquely qualifies us to combine prefabricated vertical drains with other products for economical and effective solutions to complex geotechnical problems. Construction: Where shown on the plans, or as directed by the Engineer, vertical drains shall be installed subsequent to the construction of the sand drainage blanked, and prior to placement of the surcharge material, or permanent embankment.
The Contractor shall take all reasonable precautions to preserve the survey stakes.
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