1. A Review of Geohazards in Jazan,
Saudi Arabia
William Shehata and Mohammad Al-Rehaili

2. Jazan is the
second major
Saudi sea port
on the Red Sea

3. Jazan
In the
1950’s
Jazan
Jazan dome is 4
km2 and 50m In the
above sabkha 1990’s
level

4. A section across the Red Sea

5. GEOLOGY
Old Jazan is covered by rock salt piercing the cap
rock of gypsum, anhydrite and shale of Baid
formation and the more recent coralline limestone.
The rocks are locally covered (in depressions) by 6-
15m thick layers of fine sand and loess.
New Jazan is covered mainly by sabkha. Sabkha is
loose or soft soil varying from non-plastic fine sand
to highly plastic organic clay.

6. RED Sand
SEA
Salt
Dome
Sabkha
1 km
Simplified Geological Map

7. Eolian Deposits Sand
m Sabkha
0
Rock
Shale & Sandstone
Salt
2000
Gypsum &
Anhydrite
4000 Salt, Gypsum
F F F
& Anhydrite
20 km
Geologic section across Jazan
(modified after Londry, 1979)

8. Salt diapir

9. Inclined
gypsum beds
deformed by
salt diapir
intrusion

10. Loess beds

11. Salt concentrations in sabkha crust

12. GEOHAZARDS
Subsidence in old Jazan
Sabkha problems in new Jazan
Seismicity in the general region

13. SUBSIDENCE IN OLD JAZAN
Subsidence caused dramatic building
damages in an area of approximately 40,000
square meters.
More than 100 buildings were affected.
Damaging process were usually accentuated
after rain storms.
Complexity of the geologic setting adds to
the severity of the problem.

14. Reason of Subsidence
Dissolution of salt and the formation of
cavity into which the overlying soil are
washed by groundwater. Loss of circulation
frequently noticed during drilling at the top
the salt.
Wetting of the superficial loess deposits. A
collapse potential of 7% was calculated for
the loess*.
*This fact cannot account for all the collapses. Hodgson et al.(1985) reported
2m differential settlement during 15 years period in a 25m long building.

15. m
psu e
Gy on
d st
San
Mixed Eolian
Material Deposits
Cavity
Rock Salt
Cavity formation and collapse of eolian
deposits in old Jazan

16. LOAD, kN
20 40 60 80 100 120
4
Plate loading
test results
DISPLACEMENT, mm
8
12 (plate
Natural
16 Inundated diameter =
300mm)
20
24
(After Erol, 1989)
28

17. Behavior of loess upon wetting

18. Salt diapir
Differential settlement is inevitable for a building
constructed on this rock-soil complex

19. N
Beach sand
Eolian deposits
Cap rock
Sabkha
Damaged
buildings
(Modified after
500 m Erol, 1989)
Geology of old Jazan & the locations of damaged buildings

20. Examples of
damaged
buildings

21. Remedial Measures for Old Jazan
Due to the complexity of the geology on the dome,
site specific ground investigation and foundation
design are required.
Provision of effective drainage and sewage services
are essential.
Where the rock salt is exposed, cut-off walls,
impermeable membrane, etc. should be used to
prevent salt solution.
Where rock salt is at greater depth, soil replacement
and/or bored pile foundation is recommended.

22. SABKHA PROBLEMS IN NEW
JAZAN
The salt crystallization between the soil particles
may cause heave.
The transformation between gypsum and anhydrite
may cause heave or collapse.
The sabkha salt crust is stable but tends to be weak
when wet due to the dissolution of the binding salts.
The salts present in the soil and the shallow
groundwater cause corrosion to both the concrete
and the rebars.
The low water infiltration rate causes flooding.

23. Sabkha Properties*
Layer Average Description SPT
Thickness (Soil class)
(m)
Crust 1.0-1.5 Fine sand-silt 9-16
cemented with salts (ML-SM)
Compressible 8.0-10.0** Non plastic fine sand 1-6
zone to highly plastic (SM, CL, SC,
organic clay OH)
Base ? Dense to very dense Variable up to
fine sand refusal
* Modified after Hodgson et al. (1985) & Dhowian et al. (1987)
** The thickness increases toward the south.

24. Corrosivity*
Sulfate Chloride
Soil (%) 0.12 – 14.90 0.12 – 10.72
Groundwater 1,098 – 3,876 14,200 – 97,625
(ppm)
*Sabkha soil and groundwater can generally be classified
as very corrosive (Hodgson et al., 1985)

25. Sabkha corrosion
action on
structures
(Examples from Jeddah)

26. Flooding
The city of Jazan is located in the downstream area
of Wadi Jazan and Wadi Dhamad.
Jazan (Malaki) dam regulates 80 Mm3 out of the 90
Mm3 of mean annual runoff.
No flood control structures exist on Wadi Dhamad;
the mean annual runoff of 37 Mm3 is uncontrolled.
The low permeability nature of the sabkha soil and
the shallow groundwater condition retards
infiltration and allows flooding to stand a longer
time.

27. Wadi Jazan & Wadi Dhamad flow towards the city

28. A picture taken in the 1980’s for flood in Jazan sabkha

29. Remedial Measures for Sabkha
A 4m of granular fill will improve the ground
condition of the sabkha soil.
The 4m thick fill will break the capillarity of the
saline groundwater and protect the foundations
from corrosion.
Raft foundation is suggested to reduce the potential
damage due to differential settlement.
A 4m fill will also elevate the structure above the
flood plain.
A flood control dam is suggested on Wadi Dhamad.

30. SEISMICITY OF THE REGION
Three earthquakes occurred with epicenters close to
Jazan between 1941 and 1955 with magnitudes
ranging between 5.5 and 6.2.
72 shocks were recorded in the area with
magnitudes ranging between 0.3 & 3.0 during one
month period using 5 portable seismographs
(Merghalini, 1979).
No correlation could be made between the locations
of the epicenters and a fault that extends parallel to
the Red Sea shoreline east of Jazan.

31. Seismicity of southwestern Saudi Arabia

32. Seismic Activity Possible Impacts
The compressible non-plastic fine sand zone
with low SPT values in the sabkha area are
the most susceptible soil to liquefaction.
Rock falls and landslides in the mountains
east of Jazan are triggered by seismic
activities as well as rain storms.
Subsidence in old Jazan can be accentuated
by the seismic activity.

33. Liquefaction potential evaluation charts
(modified after Seed, 1971)
SPT (Blows/ft) SPT (Blows/ft)
0 20 40 60 0 20 40 60
0 0
Groundwater level Groundwater level
de
nitude
on soil
il
20 Liquefaction 20 Liquefaction
n so
pen agnitu
very unlikely very unlikely
Depth (ft)
Depth (ft)
Liquefaction very likely
ds o
ke mag
m
epends
Liquefaction very likely
acti quake
40 40
a
e
earthqu
on d
ction d
Liqu & earth
Liquefa
ef
type &
type
60 60
Maximum ground surface acceleration = 0.15 g Maximum ground surface acceleration = 0.25 g
80 80

34. Jabal Al Abadel
Example of slope failure at Jabal Al Abadel,
east of Jazan

35. Remedial Measures for Seismic
Hazards
A 4m of granular fill may significantly reduce the
liquefaction potential.
Raft foundation is suggested to reduce the potential
damage due to liquefaction induced settlement.
A building code should be established especially
for high rise buildings.
Slope stability analysis of the mountain road cuts is
essential.

36. GENERAL ASSESSMENT
The engineering geological mapping of Jazan
should be done.
The different hazards and their involved risks
should be assessed.
The seismic events should be recorded, the
focal mechanism determined and the surface
faulting checked.
The possible locations of flood control dams
should be reinvestigated.