Geotechnical Engineering

DHARM

754 GEOTECHNICAL ENGINEERING

(c) Using the seismic velocity information, give the probable earth materials in the two
layers.

(a) The time-travel graph is shown in Fig. 18.19, Critical distance dc = 60 m.

Velocity in the upper layer, V 1 =

()

(.. )

60 15

0 10 0 025

−

− = 600 m/s

Velocity in the lower layer, V 2 =

120 60

012 010

−

(. −. ) = 3000 m/s

0.12

0.11

0.10

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

15 30 45 60 75 90 105 135 150

Distance, metres

Time, seconds

d = 60 mc

Fig. 18.19 Time-travel graph (Example 18.6)

(b) Thickness of upper layer,

H 1 =

dV V

VV

c

2

60 2 3000 600

3000 600

30 2

3

(^21) 10 6
21
−

• = −


• 
  

  (/)()==
  ()
  m = 24.5 m
  (c) From the seismic velocity values, the probable materials are hard clay overlying
  sound rock.

  
  

Summary of Main Points

1. Site investigation and soil exploration involve field and laboratory investigations required to
   obtain necessary data for the soil strata existing at a site where an engineering construction is
   proposed. Reconnaissance, study of maps and aerial photography are the important steps in site
   investigation.


2. Test pits, trial pits or trenches are direct methods, borings are semi-direct methods, and sound-
   ings or penetration tests and geophysical methods are indirect methods.


3. Planning an exploratory programme involves the fixation of spacing and depth of bore holes.
   Record of boring data is usually given in the form of a boring log.


4. Taking out soil samples from soil strata for laboratory testing is known as ‘soil sampling’. A
   sample may be disturbed or undisturbed (relatively speaking), the latter being necessary for the
   evaluation of certain engineering properties like strength and compressibility. Sample disturbance