Negatif çevre sürtünmesinin kazık taşıma gücüne etkileri

Abstract

Kazıklı temeller, konsolidasyonunu tamamlamamış ya da üzerine dolgu gelen sıkışabilen zeminler boyunca inşa edildiklerinde, negatif çevre sürtünmesi olarak tanımlanan, taşımak zorunda oldukları ilave bir yüke maruz kalırlar. Bu yük bazen kazığın taşıması öngörülen servis yükü kadar olabilir. Bu çalışmada, izmir Çevreyolu - Aydın Otoyolu projesinin bir parçası olan Çiğli şantiyesinde, viyadük ayaklarının oturacağı zeminin negatif çevre sürtünmesi değerini saptamak için gerçekleştirilen negatif kazık yükleme deneyi sonuçlan ile araştırmacıların önerdikleri metotlarla hesaplanan değerler karşılaştırılacak, ayrıca negatif sürtünmeyi oluşturan etkenler, negatif sürtünmenin kazık boyunca dağılımı ve bu etkiyi azaltmak için kullanılan yöntemlerden söz edilecektir.

This study deals with negative skin friction which is a serious problem for piles installed through a soft layer or a fill overlying a soft layer. Negative skin friction is due to the relative pile ground displacement. It is an additional load that the pile must carry whilst positive friction is a supporting force. Ground settlement and dewatering are two main factors causing negative skin friction. A value of 4-5 mm settlement for clays is enough to mobilise it. But for the sites completed their consolidation the problem is not seem serious. In addition of these, pile displacement time must to be considered in the design. Mostly the problem is the case of pile installed through the soft layer. But pile installed through a fill causes greater negative skin friction values. The reason that it makes is the high negative skin friction capacity of the non cohesive fill. Pile is even installed trough a fill overlying a soft layer or through a soft layer then the fill is set. In the case of soft soil overlying even a non compressible hard soil or compressible bearing layer negative skin friction occurs only through the soft layer. But for a pile driven through the soft soil underlying a fill it occurs through both two layers. The value of the negative skin friction changes with depth and so is not constant. In general it increases with depth but this is not a linear increment. Pile settlement neutralise pile ground displacement so after a certain depth the decrease begins. When the pile is loaded the increment seen at 1 or 2 meters below the ground surface is eliminated with elastic compression of the pile. xn Negative skin friction value neutralises at a point near the tip for a pile driven to non compressible tough rock layer since the ground settlement is not allowed. But for a compressible bearing stratum, it neutralises at an upper point. In the case of soft soil underlying a fill a more speed elimination can be expected since both two layers move down. The assumed point where negative skin friction is totally eliminated is called neutral point. Great values of negative skin friction occur when a pile is installed through a soft layer overlying a hard non compressible stratum. This event may also cause damage at pile tip. If the pile length exceeds 40-50 m floating (friction) piles must replace point bearing piles. Another solution to the problem is to drive separate protection piles around, an in some cases below the structure. In order to protect piles against negative skin friction several methods can be applied. But first thoughts are about pile structure. Enlarging pile diameter and tip dimensions, changing their lengths and constructing stepped piles are of this kind. Coatings are also used as precaution for driven piles. Asphalt applied with a thickness of 1mm for a steel and 2 mm for a concrete pile can reduce negative skin friction about 25% of the undrained shear strength. Damage occurring while driving is the main problem. This can be prevented by enlarged pile points, precoring or predrilling. Bitumen is another choice for coating. It is used with bentonit slurry. Researchers taking into account pile spacement had succeed to reduce up to 90% negative skin friction value. But in order to not reducing end bearing capacity, at lower parts coating will not be applied. For silty clay and clayey silt, electro - osmosis method proposed by Bjerrum can also be used. A reduction value of 50% for negative skin friction is obtained. It's cost is very high and not applicable to all type of soils. X11I Many researchers has studied the subject and proposed several methods to calculate negative friction value. Bjerrum acclaimed that negative skin friction is not relevant with undrained shear strength of the clay. His equation considers an axial stress distribution that's why opposed by many scientists. Meyerhof defended that negative skin friction is relative to a parameter named J3 varying with depth and effective stress in piles driven to hard clays. Terzaghi and Peck acclaimed that negative skin friction depends on drained pile- ground adhesion, lateral earth pressure coefficient and effective vertical stress. In general form negative skin friction value can be estimated from equation a-Ks-tant))'. Ks.tan<|)' term depends on the soil type and ground settlement rate. For clayey soils liquid limit is an additional factor, clays having high plasticity (liquid limit higher than fifty per cent), values correspond approximately to half undrained shear strength obtained by field vane tests which also gives the undrained cohesion. Another approach takes into account only undrained cohesion of soils. This value is taken same until 0.25 kg/cm2 but not greater than 0.5 kg/cm2 in cases that happen. Undrained cohesion can be obtained from field vane test as mentioned above and from empirical relations between SPT blow numbers and undrained shear strength so undrained cohesion. As an alternative and reliable method pile load tests customised to calculate negative skin friction can be applied. Pile is driven until neutral point calculated theoretically and cast through a casing filled with bentonit slurry. The latter eliminates possible positive skin friction. In order to obtain realistic load-settlement and time - settlement curves, load increments are kept small. While evaluating the test, found negative skin friction value is subtracted from ultimate bearing capacity. Negative skin friction value calculated to the depth where xiv pile - ground relative displacement becomes null is not subtracted from working load. The subject of this study is comparing calculated negative skin friction value from negative pile load test with proposed methods and discuss the solutions recommended to reduce negative skin friction for the site of Izmir Ringroad - Aydın Motorway project at Çiğli. After geological surveys and sedimantalogical studies, it is proved that the site is an old delta of Gediz. The stratum which give negative skin friction is organic silty clay deposits formed at upper zones of the site. The test is carried on a pile of 19.6 meters length. Tensile piles are driven to 23 - 26.3 meters and cast in dry conditions while there is a fill of 2 meters height around. In order to eliminate tip resistance, last 0.7 meters of the pile is filled with bentonit and concrete is applied after it gains tixotropic character. Load increment are limited up to twenty five tons. First the pile failed about 135 tons but it is decided to reload because of high relative displacement of the pile at first stage. Then, the pile failed about 95 tons. In order to keep safety negative skin friction value is admitted 100 tons. This corresponds to one third of the working load estimated from a pile load test realised nearby to the negative skin friction test. This is an important proof that negative skin friction value must to be considered in the design. The pile failed before settlement criterion so it is not considered in load estimation. High additional loads for small settlement values requires settlement limitation for superstructure. Comparison of the negative skin friction values calculated from pile load test with proposed Ks.tan<|)' values and undrained cohesion obtained by SPT - undrained shear strength (so undrained cohesion) relations shows that the result of pile load test is xv close to SPT relations. This support the thesis that negative skin friction is due to undrianed cohesion for loose, water saturated soft soils.