Consider the following statements: 1. According to Terzaghi, the average angle of wall friction may be taken equal to two-thirds of the respective angle of internal friction. 2. Wall friction increases the active earth Pressure and also decreases the passive earth resistance of the soil 3. Wall friction decreases the active earth pressure and increases the passive earth resistance of the soil. Which of the above statements is/are Correct?

Option 4 : 1 and 3 only

If the friction between the soil and the back of the wall is considered in the analysis of earth pressures, the earth pressure σ and its resultant P are inclined from the back of the wall by the angle δ called wall friction. This wall friction depends on many parameters such as type of wall material, degree of roughness of the back of wall, type of soil, density of soil, vibrations and shocks etc. Refer following figure:

With increasing value of Wall friction (δ) the value of active earth pressure decreases, i.e. the resultant force of active earth pressure deviates from the normal direction and vice-versa in case of passive earth pressure system.

Its value may range from 0⁰ to ϕ (internal friction angle). Its value varies from δ = ϕ/3 for smooth walls to δ = 2/3 ϕ for rough walls. δ = 0, when backfill soil is subjected to vibrations.

However, δ = 2/3ϕ is a good approximation as per Terzaghi’s.