F. ALFOUZAN ET AL.
72
surface soil layer. Loose spots of clayey sand were
formed due to the loss of fine material being washed
away. The subsurface soil was subjected to great hydrau-
lic head difference created by topography and rainfall.
When soils got wet and liquefied, interstitial pores in-
crease, hollow tiny voids and vugs develop into fissures,
cracks, big voids and then collapse by time (Figure 2).
Chemical disintegration of subsoil material can have a
significant role in cavity formation especially when cal-
cium and salts including carbonates and sulfates are pre-
sent.
Calcium carbonate (CaCO3) reacts with CO2 to form
soluble calcium bicarbonate; Ca(HCO3)2. This can cause
solid material to reduce and wash away with water.
This soluble compound is then washed away with the
rainwater. This form of weathering is called 'Carbona-
tion”.
Calcium oxide can react with basic oxides to give cal-
cium sulfites.
CaO + SO2 → CaSO3
with oxidation of the CaSO3 gives CaSO4 or Ca
SO4·N(H2 O).
2. Geology Setting
Arabian Peninsula is formed of two main structures, the
first is the Arabian Shield, which covers nearly 40% of
the Arabian Peninsula in the West, and the second struc-
ture is the sedimentary formation which covers the re-
maining parts of the Kingdom, located dominantly to-
wards the East. The formation of the Arabian Shield con-
sists of solid basement rocks of [2] Proterozoic Eon
(Precambrian), which is overlain by rocks that return in
age to Paleozoic, Mesozoic and Cenozoic eras, forming
rocks of the sedimentary basin in the sedimentary cover.
The study area lies on recent deposits of Quaternary
Period in age, directly above Sara Formation of Early
Silurian. Sara Formation is characterized and strongly
influenced by lifting tectonic movements. Sara Forma-
tion is formed of different constituents, which include the
two main components:
Figure 2. Typical sink hole close to ground surf ac e .
1) Shale and silt, exposed at Jabal Khanasir Sara in Al
Qassim Region.
2) Tillite sandstone, exposed at Jabal as Zarqa, east of
Hail. These rocks consist of heavy distribution of cross
bedding structures.
The local site geology of the study area indicated the
presence of Quaternary deposits of sand, clay and silt
followed by weakly cemented sandstone. The sandstone
cementation is improving with depth and getting sound
and intact beyond 10m below ground level.
The area under investigation lies on recent deposits of
an active khabra, of Quaternary Period in age, Figure 3.
It is located in a low area, compared to the surrounded
topography. During the rainy season, the khabra is
crossed by surface runoffs that drain rainfall and water
flowing from the upstream drainage basin, in which the
silt and clay carried by wadis settle, and natural vegeta-
tion flourishes.
The khabra deposits are typically silty and clayey with
small amount of eolian sand and no pebbles or gravel. It
looks like sand sheets of different sizes.
3. Methodology
3.1. Geotechnical Tools
The field work included advancing four boreholes that
range in depth between 23 to 30 m, below earth's surface,
and also few open test pits excavated to a depth of 2 to 3
m below grade level. The boreholes and test pits were
distributed in a way to cover the area next to school
which showed many cavities and sink holes. The term
sink holes used to denote cavities near surface in which
the arching collapsed leaving an open hole. Figure 4
presents the drilling instrument used (Acker AD II) type,
fixed on board International, 4-wheel drive truck.
Figure 3. A geologic map of the study area within recent
khabra deposits.
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