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.
RED Sand SEA Salt Dome Sabkha 1 km Simplified Geological Map
Eolian Deposits Sand m Sabkha 0 Rock Shale & Sandstone Salt2000 Gypsum & Anhydrite4000 Salt, Gypsum F F F & Anhydrite 20 km Geologic section across Jazan (modified after Londry, 1979)
GEOHAZARDS Subsidence in old Jazan Sabkha problems in new Jazan Seismicity in the general region
SUBSIDENCE IN OLD JAZANSubsidence caused dramatic buildingdamages in an area of approximately 40,000square meters.More than 100 buildings were affected.Damaging process were usually accentuatedafter rain storms.Complexity of the geologic setting adds tothe severity of the problem.
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.
m psu eGy on d st San Mixed Eolian Material Deposits Cavity Rock Salt Cavity formation and collapse of eolian deposits in old Jazan
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.
SABKHA PROBLEMS IN NEW JAZANThe salt crystallization between the soil particlesmay cause heave.The transformation between gypsum and anhydritemay cause heave or collapse.The sabkha salt crust is stable but tends to be weakwhen wet due to the dissolution of the binding salts.The salts present in the soil and the shallowgroundwater cause corrosion to both the concreteand the rebars.The low water infiltration rate causes flooding.
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 1-6zone 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.
Corrosivity* Sulfate ChlorideSoil (%) 0.12 – 14.90 0.12 – 10.72Groundwater 1,098 – 3,876 14,200 – 97,625(ppm) *Sabkha soil and groundwater can generally be classified as very corrosive (Hodgson et al., 1985)
Sabkha corrosion action on structures(Examples from Jeddah)
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.
Wadi Jazan & Wadi Dhamad flow towards the city
A picture taken in the 1980’s for flood in Jazan sabkha
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.
SEISMICITY OF THE REGIONThree earthquakes occurred with epicenters close toJazan between 1941 and 1955 with magnitudesranging between 5.5 and 6.2.72 shocks were recorded in the area withmagnitudes ranging between 0.3 & 3.0 during onemonth period using 5 portable seismographs(Merghalini, 1979).No correlation could be made between the locationsof the epicenters and a fault that extends parallel tothe Red Sea shoreline east of Jazan.
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.
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
Jabal Al Abadel Example of slope failure at Jabal Al Abadel, east of Jazan
Remedial Measures for SeismicHazards 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.
GENERAL ASSESSMENTThe engineering geological mapping of Jazanshould be done.The different hazards and their involved risksshould be assessed.The seismic events should be recorded, thefocal mechanism determined and the surfacefaulting checked.The possible locations of flood control damsshould be reinvestigated.