Small and medium diameter vertical wells - general aspects

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Description

Wells are used in deep landslides. They are necessary when the water table or landslide surface are deep and when the soil is not homogeneous but is characterized by horizontal layers of different permeability, among which more permeable ones must be captured. Wells are usually divided in (Fig. 1).

• wells of small diameter (< 800 mm);

• wells of medium diameter (1200 - 1500 mm);

• wells of large diameters (> 2m) or structural wells.

Small diameter wells can work without pumps or by means of pumps or siphons. The medium and large diameter wells usually allow the drawdown of the water by means of gravity drainage through the bottom and of the sub horizontal drains (well diameter > 3 m) (Fig.1). The cost is higher than the other drainage systems, especially when pumping is necessary. The construction of small and medium diameter wells is the same and is described below. Large diameter wells are described in fact sheets 4.5.6 and 4.5.7.

Technology for small and medium diameter wells

Small and medium diameter wells consist of a drilled hole; a screen or slotted pipe section to allow entrance of ground water; a bottom plate; a filter to prevent entrance and ultimate loss of aquifer material; a riser to conduct the water to the ground surface; a check valve to allow escape of water and prevent back flooding and entrance of foreign material; backfill to prevent recharge of the formation by surface water; and a cover and some type of barricade protection to prevent vandalism and damage to the top of the well by maintenance crews, livestock, etc (Fig. 2).

The hole should be vertical so that the screen and riser can be installed straight and plumb. The hole is drilled large enough to provide a minimum thickness of 10 – 15 cm, depending on the gradation, of the filter material. The methods of providing an open boring in the ground are:

• Standard Rotary Method (Fig. 3b): Standard rotary drilling consists of rotating a cutter bit against the bottom of a boring, while a fluid is pumped down through the drill pipe to cool and lubricate the bit and return the cuttings up the open hole to the ground surface. The fluid must be biodegradable, organic; no bentonitic clays are used in the drilling fluid.

• Reverse-Rotary Method: This method is generally considered to provide the most acceptable drill hole and should be used whenever possible for the installation of permanent wells. In the reverse-rotary method, the hole for the well is made by rotary drilling, using a similar cutting process as employed in standard rotary drilling except the drilling fluid is pulled up through the drill pipe by vacuum and the drilling fluid reenters the top of the open boring by gravity. Soil from the drilling is removed from the hole by the flow of drilling fluid circulating from the ground surface down the hole and back up the hollow drill stem from the bit.

• Bailing and Casing (Fig. 3a): Where standard or reverse-rotary drilling is not successful, especially in caving alluvial sands and unconsolidated palaeochannel deposits, an equally acceptable method of drilling consists of bailing while driving a steel casing into the hole to stabilize the boring walls. This method is economical in some materials, and it does not inject deleterious materials into the formation. Loose to medium dense, clean, granular materials can be bailed economically. Thin layers of cohesive materials, or cemented materials within the formation, can preclude the advance by bailing and may also produce smear along the sides of the drill hole which could impair free flow into the well.

• Bucket Augers: Under certain conditions drill holes for relief wells can be made with a bucket auger. The method has been successfully employed where cobbles up to 254 mm have been encountered. A bucket with side cutters is employed, and only water is used as the drilling fluid.

Once the boring is completed and the tools withdrawn, the well screen and riser pipe can be constructed at the site in varying lengths. The lengths of screen are connected together as they are lowered into the hole. The riser and screen sections should be centred in the drill hole by means of appropriate centring devices to facilitate a continuous filter around the well screen. Then the filter may be placed. A tremie should be used to maintain a continuous flow of material and thus minimise segregation during placement. After the tremie pipe or pipes have been lowered to the bottom of the hole, they should be filled with filter material and then slowly raised to keep them full of filter material at all times.

Extending the filter material at least 60 cm above the top of the screen will depend on the depth of the well to compensate for settlement during well development. The level of drilling fluid or water in a reverse-rotary drilled hole must be maintained at least 2 m above the natural ground-water level until all the filter material is placed. If a casing is used, it should be pulled as the filter material is placed, and the bottom of the casing kept 60 - 300 mm below the top of the filter material.  Development procedures include both chemical and mechanical processes. Development of a well should be accomplished as soon after the hole has been drilled as practicable. Delay in doing this procedure may prevent a well being developed to the efficiency assumed in design.

Chemical development is applied usually in the case where special drilling fluids are utilised and chemicals are injected into the well to aid in the dissolution of the residual drilling fluid in the filter. After the chemicals have been dispersed, the well should be pumped and the effluent checked to ensure that the drilling fluid has completely broken down. The purpose of mechanical development is to remove any film of silt from the walls of the drilled hole and to develop the filter immediately adjacent to the screen to permit an easy flow of water into the well. The result of proper development is the grading of the filter from coarsest to finest extending from the well. The effect of proper development is an increase in the effective size of the well, a reduction of entrance losses into the well, and an increase in the efficiency of the well. Basically there are three methods used in development: a) Water Jetting, b) Surging, c) Pumping.

During the development process, sand and silt will be brought into the well. When the depth of sand collected in the bottom of the screen reaches 30 cm, it should be removed by bailing. The remainder of the hole should be filled with either a cement-bentonite mixture tremied into place or concrete. In both cases, a 30 cm layer of concrete sand or excess filter material should be placed on top of the filter before placement of grout or concrete. A tremie equipped with a side deflector will prevent jetting of a hole through the sand and into the filter.

Materials for wells

Well screen (fig.2): Commercially available well screens and riser pipes are fabricated from a variety of materials such as black iron, galvanised iron, stainless steel, brass, bronze, fibreglass, polyvinyl chloride (PVC), and other materials. How well a material performs with time depends upon its strength, resistance to damage by servicing operations, and resistance to attack by the chemical constituents of the ground water. PVC appears to be completely stable, and it is easy to handle and install; however it is a relatively weak material and easily damaged. A variety of slot types are available in most types of well screens. PVC screens with open slots of varying dimensions consisting of a series of saw cuts are typically available. The size of the individual openings in a well screen is dictated by the grain size of the filter. The openings should be as wide as possible, yet sufficiently small to minimise entrance of filter materials. The open area of a well screen should be sufficiently large to maintain a low entrance velocity of less than 3 cm per second at the design flow. In general, the slot width (or hole diameter) of the screen should be equal to or less than the 50% size of the finest gradation of filter.

Filter: The filter gradation must meet the stability requirement that the 15% size of the filter should be not greater than five times the 85% size of the aquifer materials. The design should be based on the finest gradation of the foundation materials, excluding zones of unusually fine materials where blank screen sections should be provided. If the aquifer consists of strata with different grain size bands, different filter gradations should be designed for each band. Each filter gradation must also meet the permeability criterion that the 15% size of the filter should be more than three to five times the 15% size of aquifer sands. Either well graded or uniform filter materials may be used. The filter should consist of natural material made up of hard durable particles.

Well-characteristic curve

Pumping tests are necessary to obtain: (a) well-characteristic curve and (b) hydrogeologic characteristics of aquifer (permeability, K , trasmissivity, T,  etc...). The well-characteristic curve is  the relation between the decreasing water level in the well respect the initial piezometric level at equilibrium and the flow pumping, and in particular to know the optimal flow to pump. In order to stabilize a slope, if the decreasing of the piezometric level is realized by means of wells, the characteristic curve provides the flow to remove from aquifer to reach that ground-water level. The well-characteristic curves are shown in figures 4a and 4b, respectively for freatic and artesian aquifer.

However if the characteristics of the aquifer are known: the permeability influences radius of the cone of depression and the thickness of aquifer, Thiem equations can be used to link the pumping rate to depth of water in the well while pumping (Fig.5a, b).  Derivations of the foregoing equations are based on the following simplifying assumptions: 1)  uniform hydraulic conductivity within the radius of influence of the well; 2) the aquifer is not stratified, 3) for an unconfined aquifer, the saturated thickness is constant before pumping starts and for a confined aquifer, the aquifer thickness is constant; 4) the pumping well is 100% efficient, that is, the drawdown levels inside and just outside the well bore are at the same elevation and Head losses in the vicinity of the well are minimal; 5) the intake portion of the well penetrates the entire aquifer; 6) the water table or piezometric surface has no slope; 7) laminar flow exists throughout the aquifer and within the radius of influence of the well; 8) the cone of depression has reached equilibrium so that both drawdown and radius of influence of the well do not change with continued pumping at a given rate.  For details about Thiem equations see Thiem, 1906 Hydrologische methoden, Leipzig.

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Design methods

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Functional suitability criteria

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Reliability and feasibility criteria

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Urgency and consequence suitability

References

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