**Answer:**

To determine the average (D50) size of particles of the soil that will develop negative pore water pressure, we need to use the effective stress principle and the soil classification parameters: Cu (coefficient of uniformity) and Cc (coefficient of curvature).

**1. Effective Stress Principle:**
According to the effective stress principle, the negative pore water pressure in the soil is developed due to capillary action. The effective stress (σ') can be calculated using the equation:

σ' = σ - u_a

Where:
σ' = Effective stress (kN/m2)
σ = Total stress (kN/m2)
u_a = Pore water pressure (kN/m2)

In this case, the given pore water pressure is -9.81 kN/m3. To convert it into kN/m2, we need to multiply it by the unit weight of water (γ_w = 9.81 kN/m3).

u_a = -9.81 kN/m3 * 9.81 kN/m3 = -96.06 kN/m2

**2. Coefficient of Uniformity (Cu):**
The coefficient of uniformity (Cu) is a measure of the particle size distribution of the soil. It is calculated using the following equation:

Cu = D60 / D10

Where:
Cu = Coefficient of uniformity
D60 = Particle size diameter corresponding to 60% passing
D10 = Particle size diameter corresponding to 10% passing

**3. Coefficient of Curvature (Cc):**
The coefficient of curvature (Cc) is a measure of the shape of the particle size distribution curve. It is calculated using the following equation:

Cc = (D30)² / (D10 * D60)

Where:
Cc = Coefficient of curvature
D30 = Particle size diameter corresponding to 30% passing

**4. Determining the Average (D50) Particle Size:**
To determine the average (D50) particle size, we need to find the particle size corresponding to 50% passing. We can use the following equation to calculate it:

D50 = √(D10 * D60)

**5. Calculation:**
Given that Cu = 7 and Cc = 2, we can calculate the particle size distribution as follows:

Cu = D60 / D10
7 = D60 / D10

Cc = (D30)² / (D10 * D60)
2 = (D30)² / (D10 * D60)

Using these two equations, we can solve for D10, D30, and D60.

D10 = D60 / 7 --> Equation (1)
D30 = √(2 * D10 * D60) --> Equation (2)

Substituting Equation (1) into Equation (2), we get:

D30 = √(2 * D10 * 7 * D10) = √(14 * D10²) = √14 * D10 --> Equation (3)

Substituting Equation (1) into Equation (3), we get:

√14 * D10 = D60 / 7

Simplifying, we get:

D60 = 7 * √14 * D10 --> Equation (4)

Substituting Equation (4) into Equation (1), we get