SOIL MECHANICS LAB

1. DETERMINATION OF IN-
SITU UNIT WEIGHT OF
SOIL
EXPERIMENT NO 2 & 3
Soil Mechanics Laboratory
CE PC 594

2.  It is defined as the weight
of any substance per unit
volume
Unit of unit weight is
kN/m3

3.  There are FIVE types of unit weight of soil.
i. Bulk unit weight (γbulk)
ii. Saturated unit weight (γsat)
iii. Dry unit weight (γdry)
iv. Submerged unit weight (γsub / γ’ )
v. Unit weight of solid (γS)

4. i. Bulk unit weight (γbulk)
It is the ratio of total weight of soil to total volume of soil
γbulk =
𝑊
𝑉
=
𝑊𝑊+𝑊𝑆
𝑉𝑆+𝑉𝑊+𝑉𝑎
ii. Saturated unit weight (γsat)
It is the ratio of weight of saturated soil to volume of soil.
S = 100%, No air will be in the void. na = 0% i.e. Va=0
γsat =
𝑊𝑠𝑎𝑡
𝑉
=
𝑊𝑊+𝑊𝑆
𝑉𝑆+𝑉𝑊

5. iii. Dry unit weight (γdry)
It is the ratio of weight of dry soil to volume of soil
S = 0%, No water will be in the void. na = 100% i.e. VW=0
γdry =
𝑊𝑑𝑟𝑦
𝑉
=
𝑊𝑆
𝑉𝑆+𝑉𝑎
iv. Unit weight of solid (γS)
It is the ratio of weight of soil solid to volume of soil solid.
γS =
𝑊𝑆
𝑉𝑆

6. v. Submerged unit weight (γsub or γ’ )
It is the ratio of effective weight of soil mass
to volume of soil
γsub = γsat- γw
 Order of unit weight:
γS > γsat > γbulk > γdry > γsub
W
FB
Effective weight
W’ = W- FB

7. Particle density- It measures the mass in a given volume of
particles.
Particle density focuses on just the soil particles themselves and
not the volume they occupy in the soil.
Bulk density- It includes both the volume of the solid (mineral
and organic) portion of the soil and the spaces where air and
water are found.

8.  Soil particle density depends on the chemical composition and
structure of the minerals in the soil.
 Most mineral particles in soils have a particle density ranging
from 2.60 to 2.75 g/cm3.
 However, the density can be as high as 3.0 g/cm3 for very dense
mineral particles, and as low as 0.9 g/cm3 for organic particles.

9. Unit weight help you to evaluate several
properties of soil, which are-
1. Effective stress
2. Settlement
3. Bearing capacity
4. Permeability
5. Shear strength of soil
6. Slope stability
7. Compaction
8. Earth pressure of retaining wall etc.

10. i. Bulk unit weight (γbulk)
γbulk =
𝑊
𝑉
ii. Dry unit weight (γdry)
γdry =
𝛾𝑏𝑢𝑙𝑘
1+𝑤
Here, W= total weight of soil
V = Volume of soil
w = Moisture content of soil

11.  There are six tests to determine unit weight of soil.
i. Core Cutter Method
ii. Sand Replacement Method
iii. Water displacement method
iv. Submerged mass density method
v. Rubber balloon test
vi. Gamma ray method
 We will discuss only first two of the tests to determine in-situ
unit weight of soil

12. APPARATU
S
Special
9 Kg

13. APPARATU
S
General
i. Balance of capacity 5 Kg and sensitivity 1 gm.
ii. Balance of capacity 200gms and sensitivity 0.01 gms.
iii.Scale
iv.Spade or pickaxe or crowbar
v. Trimming Knife
vi.Oven
vii.Water content containers
viii.Desiccator.

14. APPARATUS
Specification
100
mm
127.4 mm
• Volume of cylinder = Volume of Cutter = V
• V =
𝜋
4
× 𝑑2
× ℎ
• Here, d = diameter of cylinder = 100 mm =10 cm
h = Height of cylinder = 127.4 mm = 12.74 cm
• V =
𝜋
4
× 102 × 12.74 = 1000.597 cc ~ 1000 𝑐𝑐
• We assume,
• Volume of cutter = Volume of soil = 1000 cc

15. Procedure:
i. Measure the height and internal diameter of the core
cutter.
ii. Weight the clean core cutter (W1).
iii.Clean and level the ground where the density is to be
determined and use grease at inner surface of cutter.
iv. Attach Dolly over the cutter
v. Press the cylindrical cutter into the soil to its full
depth with the help of steel rammer and stop until
dolly penetrates 1.5 cm below ground level.
W1

16. Procedure:
vi. Remove the soil around the cutter by spade.
vii.Lift up the cutter.
viii.Trim the top and bottom surfaces of the sample
carefully.
ix. Clean the outside surface of the cutter.
x. Weight the core cutter with the soil (W2).
xi. Remove the soil core from the cutter and take
the representative sample in the water content
containers to determine the moisture content W2

17. CALCULATION:
γbulk =
𝑊
𝑉
=
𝑊2−𝑊1
𝑉𝑆
γdry =
𝛾𝑏𝑢𝑙𝑘
1+𝑤
NOTE:
1. This method is not suitable for cohesionless soil (Gravel & Sand)
2. It is also not suitable for dry clay and silt because such soil can’t
be held inside the core cutter while lifting it.
3. It is suitable for cohesive soil which is highly sticky.

18. Precautions:
i. Steel dolly should be placed on the top of the cutter before
ramming it down into the ground.
ii. Core cutter should not be used for gravels, boulders or any hard
ground.
iii.Before removing the cutter, soil should be removed around the
cutter to minimize the disturbances.
iv. While lifting the cutter, no soil should drop down

19. APPARATU
S
Special
SHUTTE
R
CONE

20. APPARATU
S
General
i. Balance of capacity 15 Kg and sensitivity 1 gm.
ii. Standard sand-graded between the No.25 (600 micron) and No.52
(300 micron) B.S. Sieves
iii.Spade or pickaxe or crowbar
iv.Trimming Knife
v. Oven
vi.Water content containers
vii.Desiccator.

21. APPARATUS
Specification
100 mm
127.4 mm
• Volume of cylindrical calibrated container = V
• V =
𝜋
4
× 𝑑2 × ℎ
• Here, d = diameter of cylinder = 100 mm =10 cm
h = Height of cylinder = 150 mm = 15 cm
• V =
𝜋
4
× 102
× 15 = 1178.09 cc
• We assume,
• Volume of cylindrical calibrated container =
Volume of sand = 1178 cc

22. Procedure:
I. Weight of Sand Occupying the Cone of the Sand
Pouring Apparatus
1. Pour sand into the apparatus with valve closed and
determine weight of apparatus and sand (W1).
2. Place the apparatus on a smooth glass plate and open
the valve to fill the conical portion.
3. After the sand stops running, close the valve sharply, and
weight the sand that fill the cone of the apparatus (W2).
4. Replace the sand removed in the apparatus and again fill
the sand pouring cylinder with valve closed.
W1
W2

23. Procedure:
II. Density of Sand
1. Fully filled Sand Pouring Apparatus with known weight
of sand and place it concentrically on top of the
calibrating cylinder.
2. Open the shutter and allow the sand to drain out.
3. When no further movement of sand takes place in the
apparatus, close the shutter and weight the sand
remaining in the apparatus (W3).
4. Again fill the sand pouring cylinder with valve closed.
W1
W3

24. Procedure:
II. Density of Sand
• Weight of sand in calibrated container = 𝑊1 − 𝑊2 − 𝑊3
• Unit weight of sand =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑛𝑑
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑎𝑛𝑑
• We know,
Volume of sand = Volume of calibrated container =V
• 𝛾𝑠𝑎𝑛𝑑 =
𝑊1−𝑊2−𝑊3
𝑉
W3
Calibrated
container

25. Procedure:
III.Density of soil
1. Prepare the surface of the location to be tested so
that it is a level plane.
2. Keep the soil tray firmly on the place surface.
3. Excavate with hand tools a hole with diameter equal
to that of the hole of the plate and about 10cm in
depth with smooth walls and rounded bottom edges.
4. Place all loosened soil in a container being careful to
avoid losing any material.
10 cm
W

26. Procedure:
III.Density of soil
5. Seat the already weighed apparatus with sand on the
hole of the tray.
6. Open the valve and after the sand has stopped flowing
close the valve.
7. Weigh the apparatus with remaining sand (W4)
8. Weigh the material that was removed from the test hole
(W).
9. Mix the material thoroughly and weigh a representative
sample for moisture determination.
W4

27. Procedure:
II. Density of Soil
• Volume of excavated soil = Volume of sand poured in
excavated place = Vsoil =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑛𝑑 𝑖𝑛 𝑒𝑥𝑐𝑎𝑣𝑎𝑡𝑖𝑜𝑛
𝑈𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑛𝑑 (𝛾𝑠𝑎𝑛𝑑)
• Vsoil =
𝑊1−𝑊2−𝑊4
𝛾𝑠𝑎𝑛𝑑
• Vsoil =
𝑊1−𝑊2−𝑊4
𝑊1−𝑊2−𝑊3
𝑉
W4

28. Procedure:
II. Density of Soil
• Unit weight of soil (γ) =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑜𝑖𝑙
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑖𝑙
=
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑜𝑖𝑙
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑒𝑥𝑐𝑎𝑣𝑎𝑡𝑒𝑑 𝑠𝑜𝑖𝑙
• γ =
𝑊
𝑉𝑠𝑜𝑖𝑙
=
𝑊
𝑉𝑠𝑜𝑖𝑙
• γ =
𝑊
𝑊1−𝑊2−𝑊4
𝑊1−𝑊2−𝑊3
𝑉
W4

29. Precautions:
1. This method is suitable for cohesionless soil (Gravel & Sand)
2. It is generally adopted in construction of roads and highways.