Problem 2 Shallow Foundations on a Sand Site A tower will be

Problem #2- Shallow Foundations on a Sand Site A tower will be built at a site where the soil conditions consist of loose, uniform sand (SP) with little fines. The average corrected SPT blowcount is Noo 4. The groundwater level is at a depth of 2 ft. a. Soil Properties (30 pts) Based on the provided information, select or estimate the following properties for the sand. Feel free to assume saturated conditions to determine total unit weight. Provide justification for your values. Referencing the class notes is fine. Effective stress friction angle, , (deg)\" Effective cohesion, c, (psf) = Total unit weight, (pcf)- b. Bearing Capacity (120 pts) The tower legs will be inclined at an angle of 14 deg from vertical. Two options are being considered for the shallow foundations to support the tower, as shown in the sketch below. Both options will have Dr 2 ft and a total load, including Ws, of 240 kips. Load 240 kips 14 deg x 12 ft 8 ft x 12 ft 14 deg Option 1 - Inclined Footing For each option, use the Vesic bearing capacity equation to determine the ultimate bearing capacity and factor of safety for the conditions shown. Let G--1 Option1 Option 2 - Inclined Load Option 2 F (as designed)- F (as designed)-

Solution

Given that corrected N₆₀ =4

and given soil is SP (Poorly graded sand)

RELATION BETWEEN SPT (N) VALUE WITH DIFFERENT SOIL PROPERTIES IN CASE OF COHESION LESS SOIL

SPT VALUE (N)

COMPACTNESS

ANGLE OF FRICTION ()

UNIT WEIGHT () in T/m3

0 – 4

Very loose

< 28

1.1 – 1.8

4 – 10

Loose

28 – 30

1.4 – 2.0

10 – 30

Medium

30 – 36

1.7 – 2.2

30 – 50

Dense

36 – 41

1.7 – 2.3

> 50

Very dense

> 41

2.0 – 2.3

From table   = 28⁰

and = 1.7 t/m³

since very less fine is there you can assume that C= 5 kpa

SPT VALUE (N)	COMPACTNESS	ANGLE OF FRICTION ()	UNIT WEIGHT () in T/m3
0 – 4	Very loose	< 28	1.1 – 1.8
4 – 10	Loose	28 – 30	1.4 – 2.0
10 – 30	Medium	30 – 36	1.7 – 2.2
30 – 50	Dense	36 – 41	1.7 – 2.3
> 50	Very dense	> 41	2.0 – 2.3