Aspects of the engineering geology of Maputo City, Mozambique.
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Date
2011
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Abstract
The geological formations of Maputo City, which are mainly unconsolidated materials with soil
like properties, are described in terms of their engineering geological and geotechnical
characteristics with relevance to their distribution patterns and spatial trends. Problematic
conditions such as collapse potential characteristics, loose aeolian sand dune deposits and loose
sand plains characterize many of the materials. The geological characteristics combined with
anthropogenic interference such as intensive urbanization with inappropriate land use,
construction in sensitive areas like steep sandy slopes has led to many problems including slope
stability. Foundation problems with building settlement and gully erosion also occur. The aim
of this research was to study the engineering geological characteristics and the geotechnical
properties of the geological formations of Maputo City and various related problems. Special
relevance has been given to the understanding of three specific problems: building damage,
gully erosion and slope instability.
The geological formations are predominantly sandy (coarse to very fine sand) with very low
clay content, are non-plastic and are classified as from the group SP-SM which are poorlygraded
sand with silt. The majority of the materials are loose and normally consolidated with a
high level of residual strength. Assessment of collapse settlement through double consolidation
technique indicated soil compressibility and significant sensibility to collapse upon wetting.
Truly collapsible soils that show full collapse of the soil structure were identified in 33% of the
tested materials where the highest collapse behaviour reached values above 5%, predicted to
cause moderate trouble in foundation design. Some of the bonded materials are bonded (evident
in 67% of samples tested). Bonding was confirmed by comparing the compressibility of the
undisturbed and remoulded samples. The remoulded samples showed a significantly higher
compression than that of the bonded materials as part of the stress applied is carried by the
bonds themselves, as the bonded material is stiffer than the same without bonds. The curves of
the remoulded samples were used to establish the limit between the stable and meta-stable states
of the material.
A qualitative evaluation of the erosion susceptibility was investigated by physical tests such as
the crumb test, shear strength and chemical indicators while a quantitative evaluation of the
erodibility characteristics was obtained using a flume test. Some correlations were found
between the results of various methods. Almost all samples that were found to be dispersive
with ESP were also dispersive with TDS vs. %Na and SAR. Results of the flume erodibility
test have very little correlation with the chemical properties related to dispersion revealing that
the erosion susceptibility and gullying in Maputo City have more relation to the physical
processes than to the dispersion related chemical properties of the soils. The positive
identification of dispersive and erodible soils can only be carried out using a combination of
various techniques. Therefore, a new rating system for erosion susceptibility of sandy soils
combining the physical and chemical factors of dispersion is proposed including the flume test,
crumb test, TDS/%Na, SAR and ESP. The proposed rating system was applied to the tested
soils of Maputo City. Fifteen samples (83% of the rated samples) were classified with
intermediate susceptibility to erosion while 3 samples (17%) were classified as having a low
susceptibility to erosion. The highest rating scores were obtained by the same samples that
showed dispersive behaviour with SAR, ESP and TDS/%Na. This group of samples was of
intermediate erodibility in the flume test.
The slope instability mechanisms observed in Maputo City are predominantly rotational failures
with a mass of soil sliding along a curved surface of rupture followed by sand flow at the toe as
failure occurs in the presence of excess water. Four groups of factors account for the slope
instability problems in Maputo City: geomorphological causes, physical and meteorological
causes, geological and geotechnical properties of soils, and anthropogenic causes. The
mechanism of failure is mostly due to the loss of matric suction of soils in the presence of
rainwater and possibly from destruction of bonding agents. Factors of safety values indicate
that the slopes are generally unstable with the control being the slope angle.
The slopes in the Polana-CaniƧo and FerroviƔrio Quarters show high factor of safety values but
is the area most affected by slope instability. Slope failure in these areas is intrinsically caused
by anthropogenic factors related to inappropriate land use planning. The gully sidewalls are
unstable as the slope created is very steep. The slope at Friedrich Engels Avenue causes most
concern due not only to the slope height and angle but also to the size and number of buildings
constructed at the crest, mainly high rise buildings along the Julius Nyerere Avenue, the
integrity of which could be threatened by a landslide event (this slope has recently been affected
by active landslides).
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2011.
Keywords
Engineering geology--Maputo (Mozambique)., Theses--Geology.