Principles of foundation engineering The Islamic university - Gaza Faculty of Engineering Civil Engineering Department Principles of foundation engineering Eng. Safaa Abu El-Aish

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Geotechnical Properties of Soil Foundation Engineering ECIV 4352 Lecture No. (1) Geotechnical Properties of Soil Phases of soil: There are various relations for soil properties can be derived as follow: Important relations: Phase Diagram of soil element:

QUASTION

Atterberg limits: نسبة تواجد الماء في التربة لها أثر كبير في تغيير خصائص التربة، فوجود الماء بنسبة كبيرة يجعل التربة بحالة شبيهة بالحالة السائلة، و بتقليل هذه النسبة تمر التربة بعدة حالات من اللدونة إلى الصلابة وهكذا، ومن هذا المنطلق وضع العالم أتربيرج معايير معينة تحكم تحول التربة من حالة إلى حالة فيما يعرف بـ :

Hydraulic conductivity of soil: The hydraulic conductivity of soil is related to permeability of soil which has a coefficient called permeability coefficient measured by (cm/sec) and denoted by (k). For sandy soil the value of (k) is relatively high to the clayey soils. In the laboratory we have two types of tests to determine (k): Constant head test for sandy soil. Falling head test for clay.

Concept of effective stress: عند تعرض التربة للأحمال فإن هناك جزأين يقوموا بتحمل هذه الأحمال، الماء بين فراغات التربة بالإضافة إلى حبيبات التربة ذاتها. لكن بعد مرور وقت معين على تأثير الحمل على التربة يحصل تسريب للماء فتبقى حبيبات التربة وحدها تقاوم هذه الأحمال، ولذا يتوجب علينا معرفة الإجهادات التي يتحملها الجزء الصلب فقط من التربة أي حبيبات التربة وهو ما يعرف بـ: Soil Effective Stress

Settlement: There are 3 types of settlements on soil, the two main types are: Immediate settlement. Primary consolidation settlement. immediate settlement: The settlement that occurs immediately after applying the loads on the soil.

m1 = Length of Foundation (L) / Width of Foundation (B)

consolidation settlement: عند تعرض التربة المشبعة بالماء للأحمال، فإن الماء يتحمل الحمل بالكامل، ولكن بعد مرور زمن يبدأ الماء بالتسرب من بين حبيبات التربة فيتوزع الحمل بين حبيبات التربة و الماء في الفراغات، فتبدأ حبيبات التربة بالتضاغط و بالتدريج حتى يحدث التسريب الكامل للماء من بين الحبيبات فيحدث الهبوط الانضغاطي بشكل كامل للتربة. According to consolidation, soil can be classified to two types: Normal consolidation: Present pressure=Past pressure.

<= c Over consolidation: Past pressure > Present pressure. OCR: Over Consolidation Ratio.

Time rate of consolidation: t: The required time to reach a specific degree of consolidation. Hdr: Height of drainage. Cv: Coefficient of consolidation. To find Cv: K: Permeability coefficient. mv: Coefficient of volume compressibility. To find Tv: Look Table 10.5 P293 Or from equations:

Example : A soil profile as shown, the pre-consolidation of the clay is 3400 psf. Estimate the primary consolidation settlement that will take place as the result of surcharge equal 2200 psf. Assume Cs = Solution Find the unit weights of the soil layers: For dry sand: For saturated sand: For saturated clay: e = 2.7 x 0.3 = 0.81

Find the present effective pressure: Find Cc and Cs: Cc = 0.009 ( 35 – 10 ) = 0.225 Cs = Cc / 5 = 0.225 / 5 = 0.045

shear strength Soil strength is the resistance to mass deformation developed from a combination of particle rolling, sliding, and crushing and is reduced by any pore pressure that exists or develops during particle movement. The shear strength is measured in terms of two soil parameters: cohesion "c" and angle of internal friction "φ“ The general equation to calculate the shear strength at failure is coulomb equation:

By determining the values of at plane (ab), if it lies at point A, it means that no shear failure will occur at plane (ab), but if it lies at point B it means that it will fail at plane (ab), Point C can’t be achieved because the soil will be failed before it.

There are many laboratory tests to obtain these parameters, the famous tests are: Direct shear test:

Triaxial shear test:

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