Presentation on theme: "ندوة التشققات في الأبنية القائمة على التربة المنتفخة اللجنة العلمية – نقابة المهندسين 18-12-2005 د. أيّوب أبو ديّة: رئيس جمعية حفظ الطاقة واستدامة البيئة."— Presentation transcript:
ندوة التشققات في الأبنية القائمة على التربة المنتفخة اللجنة العلمية – نقابة المهندسين د. أيّوب أبو ديّة: رئيس جمعية حفظ الطاقة واستدامة البيئة الأردنية ومدير مكتب هندسي استشاري، ص0 ب عمّـان الأردن E mail:
Expansive Clays The two main factors defining expansive clays are: Clay fraction % and Plasticity Index % Four classifications of clays are recognized, extending from the very low expansive clays region which lies below the medium expansive clays region till the very high expansive ones, as shown in the previous figure. So, how do these types of soils respond to water content changes? Mineralogy of samples of expansive clays vary, tested samples in Jordan proved to constitute less than 10% of both sand and kaolinite, while silt percentage ranged from 40-47, and finally showed that the percentage of Mica-Smectite ranged about the same as the silt.The degree of heave or shrinkage the soil mass is subjected to is controlled by many factors, such as the degree and conditions of confinement of the soil mass, the soil’s initial water content and the increase in its water content, its degree of expansiveness and the amount of vertical confined pressure exerted on it.
التشققات السطحية في فصول الجفاف
انماط تقليدية من التشققات في الأبنية القائمة على التربة الطينية المنتفخة هبوط في الأرضيّات
أنماط التشققات في الأبنية القائمة على التربة المتحركة
Summer SeasonWinter Season Water Content% Depth Of Soil From Surface In Centimeters Water Content% Depth Of Soil From Surface In Centimeters Table 1. Typical Clay Water Content % Seasonal Variations
Evaporation in one sunny day/treePlantation Type 500Eucalyptus Macarthuri 350Poplar (Fully grown) 250Acacia Mollissima One litre/m2Highveld Grass (Themeda) Approximate height after 15 years (m) Approximate mature height (m)Common name of tree 1240Douglas fir 930Lombardy poplar 1525White willow 1025Scots pine 925Silver birch 825Oak 725Beech 725Lime 625Cedar 920Weeping willow
Damage to side walks
حماية تربة الأساس من أثر جذور الأشجار
A Case Study A case study of a typical one storey structure, where a 15 cm solid slab spanning over 40 x 40 cm2 squared columns spaced at 3 meters center to center is designed to support 4 floors. Foundation dimension is 180 x 180 cm2. Existing foundations were designed to support four floors, whereas: working design load / m2 = 1000 kg/m2. andSoil allowable stress = 1.48 kg/m2. Load per column = A1xL1xS1xM1 Where, Area of slab carried by column in meters L1Working design load in kilogrammes per squared meter S1Self weight allowance factor M1Moment allowance factor Load per column = 3x3x1000x1.1x1.2 kg/floor = kg/floor. When only one floor is constructed, the actual stress applied on the soil underneath the foundations is equals to: s = L2 / A2 Where, the Actual Soil Stress L2Working load on column at foundation level A2Area of foundation contacting the soil Therefore = / 180 x 180 kg/m2 = 0.37 kg/m2.
Comparing this stress value on the soil with the possible uplift pressure which can exceed 2 kg/cm2 will show that if the soil was of expansive clay quality classified as of very highly expansive characteristics, then it will be subjected to an uplift. The degree of heave is dictated by the original water content ratio and the subsequent incoming water into the soil matrix from so many different sources around a building and underneath it. Please note that the existence of trees around a building will further escalate the dilemma and will increase changes in water contents in the soil strata at even deeper levels. A uniform uplift along the whole plan area of the structure will probably induce no visible cracks; Neither in the structural members nor in the finishes. But uniform uplifts are quite rare, the most frequent in Nature are non- uniform movements, upwards, downwards and at an inclined angle, as a result of changes in the soil’s water content and as a consequence of uneven changes occurring in the soil's matrix. Cracks in structural members are controlled once the movement in the soil itself is controlled.
Water proofing At concrete density 2300 kg /m3 the thermal conductivity (K-value) is as follows: K= 1.02 W/m2.ْ 0% water content by volume At 5 % water content by volume K= 1.75 (Joint 1) At 25 %water content by volume K= 2.75 (Joint 2)
Capillary attraction and diffusion
Picture 1, crystallization of salts deteriorating concrete in walls
Joint 1 details
In Joint 1, a normal exterior concrete wall of 25 cm in thickness. In normal conditions it posses about 3 % water content by volume (K-valve = 1.8 w/m2). The heat losses at the corner is 138 W/m in the horizontal direction and 46 W/m in the vertical direction, where as the temperature at the corner where the panel tile is situated is around 14 c. ْ Joint 2 details 12
In joint 2, the same corner is at a temperature of 12 when the wall posses 15 % water content by volume due to water in filtration due to capillary attraction. The K-valve is approximately 2.8 w/m2.ْ c. The heat flow increases to 180 w/m in the horizontal direction and to 50 W/m in the vertical direction. ْ When walls are saturated, the temperature drops much further. This action induces condensation at the these surfaces at a relative Humidity of the internal ambient air of around 70 %. Looking at any psychometric chart we can see that condensation can only start at around 80 % in the first condition (Joint 2), whereas condensation starts at only 70 % Relative Humidity in the second wall (Joint 2). This makes us reach a conclusion that waterproofing is essential to stop water infiltrating exterior walls in order to reduce heat losses, water vapor condensation, fungus growth and of course damage to the walls. Damage due to crystallization of salts diluted in ground water when dry (see picture), or rust to steel reinforcement, cracks, etc.
Recommendations and Repair strategy.Simple architectural design with minimal complicated discontinuous units Basement areas are recommended to cover as much as possible of the plan area of the ground floor. Two complete floors with a deeper foundation level are expected to function much better in comparison to a single ground floor with a small basement covering only a small part of the plan area. Symmetry in the design plan will enhance uniform settlement or uniform heaving, thus cracks will be expected to be minimal. Landscaping should be studied in accordance with the recommendations discussed earlier in order to reduce moisture changes inside the soil matrix, thus reducing movement in the structure due to either contraction or expansion in the soil supporting the foundations. Loans can be watered lightly on daily basis rather then watering them heavily at long intervals Plantations around and within buildings require thorough analysis to take into consideration the effect of shading, fallen leafs, wind circulation and tree roots as some of the many factors affecting soil movement. Chopping trees can lead to heaving in the next few years due to the tendency of the soil to balance itself out with the surrounding in terms of its water content. Roots can be directed towards a water source in exactly the same way its fine roots seek moisture around manholes and floor traps. Once a source has been established away from the building, the roots will find no reason to invade the soil matrix near the foundations. We can call them Tree Root Traps. These traps can help stabilize the soil all year round by controlling its moisture content.
Deep reinforced concrete walls and chemicals can be introduced to stop or kill the existing roots and keep them away in the future. We can call these Root Barriers. Manholes can be introduced to add fresh quantities of pesticides as frequently needed. Directing roof rain waters, controlling watering systems and designing topography of garden’s surface soil away from foundations. Avoiding long exposures to soil at foundation level before pouring blinding concrete, as well as avoiding expanding the excavation area and minimizing concrete curing waters. Allowing sufficient cover to the foundations ranging from meters deep depending on the degree of expansivity of the clay. A layer working as a vapor barrier can be used to cover walkways in existing buildings. Reinforced concrete walls connected to foundations are expected to function in a superior mode compared to isolated columns connected to hollow concrete blocks. Openings in structural walls induce high stresses at the corners, and thus reduce the wall’s resistance. Reinforcing steel should be introduced at the openings sills as well as at the lintels. Special details are needed in lightly-loaded structures susceptible to movement. For example, allowing for soft comprisable materials between the soil and lightly loaded concrete beams carrying block work, in order to allow for any heave in the soil. Concrete beams adjacent to walkways around the periphery of buildings should be detailed to identify a soft frictionless membrane fixed to its sides. This helps to reduce friction initiated from the contact between the concrete’s rough surface and the moving adjacent soil around the building due to changes in its moisture content.
Reinforced Concrete slabs carrying walkways are usually laid on superficial soil or on a layer of base-coarsed materials. The slabs remain susceptible to settlement and uplift forces due to volumetric changes all year round as a result of severe climatic changes. The best way to protect a walkway is to suspend it, or to support it on little concrete pins or columns which are in turn supported by small foundations at a reasonable depth in order to increase the actual pressure on the soil to the extent where uplift pressures cannot lift them upwards. Providing rigid enough, yet flexible enough pipes and manholes inside the structure and around its peripheries. Securing water tightness from all sources of waters, such as water storage tanks, Cess-pools, manholes, pipelines, etc. Reinforced concrete members designed to resist light vertical and horizontal loads are not usually strong enough to stop cracks in members subjected to continuous movement due to unstable soil underneath. Expansion Joints are recommended. All members in contact with moisture and vapor should be water proofed to avoid water or moisture movement in members by absorption, adsorption, thus reducing structural damage, deterioration and heat losses in winter.