Fitil drenler

Abstract

Bu çalışmada bir liman yapısının inşası amacıyla yapılan dolgu altındaki zeminin konsolidasyonunun hızlı bir şekilde tamamlanabilmesi ve bu vesileyle zeminin kayma mukavemetinin arttırılması amacıyla daha önce kullanılan ve önyükleme süresinin oldukça uzun olduğu metodlara alternatif olarak radyal drenaj ve dolayısıyla radyal drenajın oluşmasına neden olması beklenen düşey kolonlar incelenmiştir. Bahsedilen düşey kolonlar genellikle, uzun yıllardan beri olduğu gibi zemine kum enjekte edilmesi yoluyla oluşturulan kum drenlerden ibarettirler. Zemine enjekte edilen bu kum kolonlar konsolidasyonun olması istenen tabakada radyal drenaja neden olarak, konsolidasyonun hızlı bir şekilde gerçekleşmesini sağlar. Bu da bize, ön yükleme süresinin kısalması gibi büyük bir avantaj sağlar. Son yıllarda yapılan araştırma ve çalışmaların doğal sonucu olarak, kum drenler yerlerini yavaş yavaş fitil dren olarak adlandırılan geosentetik malzemelere bırakmaya başlamışlardır. Fitil drenlerin kum drenlere göre pek çok avantajları bulunmaktadır ve bunlar göz önüne alındığında gelecekte fitil drenlerin, kum drenlere karşı tamamen üstünlük sağlayacakları anlaşılmaktadır.

A great problem occurs when we want to make a construction on the base consisting of very soft clay, due to low bearing capacity and the shear strength of the soil. Here, the main problem is the time that we can wait for the 90% of consolidation. Normally, there are many methods to make the soil consolidated. But, as it was said, the main problem is the time here. So, what can we do to accelerate the speed of the consolidation and make the consolidation faster? It can be seen that, there is a need for reducing the time of consolidation. Nowadays, many firms use radial drainage methods. This method consists of using some drains such as sand drains, wick drains by inserting them into the soil. And then, radial drainage exists through these drains. And, when the radial drainage starts, consolidation becomes faster. Prior to about 1975, it was generally implemented by installing vertical columns of free draining sand in the soil to be consolidated, and then placing the permanent fill and/or a temporary preload on the ground surface. The drainage path length is now the distance between sand columns and quite often a coefficient of consolidation higher horizontally than vertically can be taken advantage of. Analogously to Equation 1. For vertical flow, we have one for radial flow. t^TjjJüe2 (1) Where t : Time for consolidation. de: The center to center spacing between sand drains. Th: The time factor for horizontal drainage. Ch: The horizontal coefficient of consolidation for horizontal drainage as mobilized by vertical compression. Values of Th have been solved for a wide variety of sand drain spacings de and sand drain diameters dw, as shown in Figure 2. q, is sometimes considerably larger than Cv (particularly in sedimentary deposits of a continious nature), but not always. Specific tests should be conducted to determine its value. If no direct data are available, the value of Cv determined from standart consolidation tests should be the maximum value used. This technique, properly called vertical sand drains, has been used in a wide variety of situations. However, ail sand drain surcharge installations have not been succesful. At one such site 18 inches diameter sand drains at 7.5 ft. spacings were installed in a triangular pattern to consolidate 66 ft. of organic, clayey, silty soil. As surcharge fill was being placed, a shear failure in the foundation soil was mobilized. The failure plane eventually propagated up through the fill at one end, and well beyond the toe of the slope at the other end. Deformations were such that the sand drains were undoubtedly sheared off and the site had to be developed by alternative methods. Some comments are in order about how sand drains installed since their performance is influenced to some degree by the method of their installation. Three different techniques are common, although many others are available: 1. MANDREL DRIVEN. In this method, a close end hollow pipe (the mandrel) is driven to the bottom of the soil to be consolidated, it is filled with sand which is placed under approximately 100 lb/in2 of air pressure, and then it is gradually lifted out of the ground. The close end, being a hinged valve, opens, and the sand, under pressure rushes out. The pipe continues to be lifted until it reaches the ground's surface. The entire process is then repeated for the next sand drain. The cycle takes 2 to 10 min, depending upon local conditions. 2. HOLLOW STEM - CONTINIOUS FLIGHT AUGER. Here a continues flight auger of the same diameter as the intended sand drain is rotated into the soil to the specified depth. Since the auger is formed around a hollow pipe, sand, under pressure, can be introduced into it. As the auger is backrotated out of the hole, the sand is left behind and forms the sand drain. Cycle times for this installation method is 5 to 20 min. 3. JETTED HOLLOW OR CLOSED-END PIPES. The driving into soil either hollow or close end hinged pipes via water water jetting is also used for sand drain installations. After pipe placement, the sand is introduced under pressure as the pipe is withdrawn. Cycle times are 5 to 20 min. The problems of soil disturbance and smear around the periphery of the sand drain come up in regard to each of these construction methods. A list of those concerns is given in Table 2. Which method best avoids the problems is a subject still open for discussion-but the answer may already be merely academic, since a completely different style of drain using no sand at all, properly called a drain wick, has begun to dominate the market. tx 2. RADIAL DRAINAGE WITH STRIP (WICK) DRAINS. Generally, geosynthetic materials (polypropylene, polyester, polyethylene, etc.) do not wick. These polymers by themselves are quite hydrophobic, meaning that they actuallly repel water. If the fabric pore structure is full or partially saturated with water, it will exist in the voids and wait for some external source like gravity or pressure to initiate its movement. So why do most people refer to the subject of this section as 'wick drains '? The answer is probably that when these geocomposites are vertically inserted in the ground with their ends protruding at the ground surface and with water being forced out of them under pressure, they resemble a set of giant wicks! The mechanism of flow, however, is not by wicking as with a candle, so it must be clearly elaborated upon. This section adresses the situation, and by so doing attempts to establish a new topic heading called 'strip drains' instead of the commonly used 'wick drains'. The method of rapid consolidation of saturated fine-grained soils (silts, clays, and their mixtures) has been actively pursued using sand drains since the 1930s. The practice involves placement of vertical columns of sand (usually 20 to 45 cm. In diameter) at spacings of 1.5 to 6.0 m. centers throughout the subsurface to be dewatered. Their lengths are site-specific but usually extend to the bottom of the soft layer(s) involved. Once installed, a surcharge load is placed on the ground surface so as to mobilize excess pore water pressures in the water in the soil voids. This surcharge load is placed in incremental lifts, and with each increment (and simultaneous increase in excess pore water pressure in the underlying soil) drainage occurs via the installed sand drains. The water takes the shortest drainage path-which is horizontally radial-to the sand drain, at which point flow is vertical and very rapid since the sand is many of magnitude greater in permeability than the fine grained soil being consolidated. Critical in this method of consolidating soils rapidly (it does little insofar as the amount or magnitude of settlement is concerned) is the rate at which surcharge fill is added. Surcharge fill placement is controlled by the effective stress equation: a' = a - uw (2) where: a': Effective (or intergranular) stress, a : Total stress, and uw : Excess water pressure ('excess' since it is higher than normal hydrostatic conditions.) The increase in excess pore water pressure increment (via the surcharge) must never be higher than the increase in total stress increment. Thus the effective stress (directly related to the soil's shear strength) is never decreased, and as surcharge loading proceeds it actually increases. Surcharge loads are usually earth fills, but have been accomplished using a geomembrane-contained water loading. With this brief background of the concept, and recognizing that millions of sand drains have been installed, there are nonetheless the following shortcomings: ? There is a distinct possibility that the sand drain may not be continuous if the installation mandrel is withdrawn too fast or if sand runs out during the withdrawal. ? There is a definite vulnerability to a shear failure as surcharge load is being placed. Since the small-diameter sand drains offer essentially no shear resistance, the technique is limited to very slow placement of the surcharge. Often this is as little as 1.2 kPa per day (approximately 76 mm. per day), and even then instrumentation of the site is mandatory. ? There is need for a relatively large crane to install the sand drains. This, in turn requires a substantial soil layer to be placed over the site to begin the job. ? The sand for this soil layer and for the sand drains themselves may be difficult, and costly, to obtain. ? Material for the surcharge (which should be somewhat greater in its final ground surface pressure than the contact pressure of the proposed structure) might be difficult to obtain. Surcharge fill heights of up to 9 m. are not uncommon. ? Once the site is consolidated, most of this surcharge fill must be removed, which is sometimes difficult to do and often expensive. By contrasting sand drains to the alternative of geocomposite strip drains, a number of interesting features are revealed. The strip drains, usually consisting of plastic fluted or nubbed cores that are surrounded by a geotextile filter, have considerable tensile strength. Typically, the braking strength of a 100 mm.-wide strip drain is 4.5 to 13.5 kN. When threaded throughout a site on centers of 1 to 2 m., they offer a sizable reinforcing effect. Furthermore, they do not require any sand to transmit flow, or large construction equipment for installation. A rig called a 'sticker' is used for installation. It is relatively lightweight compared to typical sand drain installation cranes. Thus the need for a thick sand layer is eliminated or at least minimized. Although surcharge must still be placed in the conventional manner, strip drains offer a number of advantages over sand drains, as indicated by the number of competing products. Regarding the installation of strip drains, they arrive at the site in rolls and are placed on the installation rig in dispensers like a huge roll of toilet paper. The end is threaded down inside a hollow steel lance, which must be as long as the depth to which the strip drain is folded around a steel bar or other type of base plate. The purpose of the base plate is to keep the strip drain down at the bottom of the lance and at the same time to keep the soft soil through which it will be placed out of the bottom of the lance. The entire assembly (lance, base plate, and strip drain) is now pressed into the ground to the desired depth. If a hard crust of soil or a high- strength geotextile or geogrid is at the ground surface, it must be pre-augered or suitably pierced beforehand. When at the desired depth, the lance is withdrawn, leaving the base plate and the strip drain behind. The process is repeated at the next location. It is a very rapid construction cycle (approximately 1 rrun.), requiring no other materials than the strip drains and the base plates. Concerning the design method for determining strip drain spacings, the initial focal point is on the time for consolidation of the subsoil to occur. Generally, the time for 90% consolidation is desired, but other values might also be of interest. Two approaches toward such a design are possible. The first, an equivalent sand drain approach uses the strip drain to estimate an equivalent sand drain diameter and then proceeds with design in a standard manner. This is done by taking the actual cross-sectional area of the candidate strip drain and making it into a void circle. This void circle is then increased using the estimated porosity of sand to obtain the equivalent sand drain diameter. In summary, it can be seen that strip drains offer so many advantages over sand drains that strip drains will be used exclusively in the future. Strong in their favor are the following items: XI ? Tensile strength is definitely afforded to the soft soil by installation of the strip drains. It is, however, a difficult, three dimensional problem to quantitatively assess. ? There is no resistance to the flow of water once it enters the strip drain. This is not the case with conventional sand drains. ? Construction equipment is generally small, imparting low ground contact pressures on the soft soils. ? Installation is simple, straightforward, fast, and clean. Regarding additional research into strip drains, the main items are the effects of soil smear and kinking. Soil smear includes the distortion of the soil due to installation, withdrawal, and collapse of the in-situ soil on the strip drain. Its effect is mainly on the horizontal coefficient of consolidation (ch) and it is yet to be understood, although work is ongoing in this regard. Kinking refers to the shortening of the strip drain during the consolidation process. In some strip drains the tendency might be to fold into a tight S-shape, that is to 'kink' thereby cutting off flow.