An Evaluation of the Geotechnical Characteristics of the Abutments of a Proposed Bridge Across a 400-Meter River Channel in the Lower Niger Delta, Nigeria

The Niger Delta is situated in the southern-most section of Nigeria and forms the major crude oil reservoir of Nigeria. However, it is in most parts devoid of transportation routes. A proposed Trans-Kalabari Highway is expected to connect some of the communities located within the Mangrove (“Transition”) hydro-meteorological zone of the delta that is characterized by a network of rivers, creeks and rivulets. The left abutment of one of the bridges across a 400-meter wide river indicates that the sub-surface down to a depth of 30.00 meters at a distance of about 40.00 meters from the edge of the river channel has six (6) different soil layers as against four (4) at a comparative distance on the right abutment. These soil layers are basically dark-gray organic silty clays (OH) underlain by grayish silty clayey sands (SC) which are further underlain by poorly graded silty sands (SP). A dark, highly plastic, grayish silty clay layer (SC-SM) underlies the above strata at a depth of between 12 to 18 meters. Well-graded sands and gravel (SW) layer further underlies the above and extends to depths in excess of 30.00 meters. Borings situated between 40.00 meters and the edge of the shoreline of the river channel all have five layers with poorly graded sands (SP) and well-graded sands and gravels (SW) underlying the organic silty clays and silty sands. Steel piles for the bridge support are expected to be borne at depths of between 20 and 25 meters corresponding to the well-graded sands and gravels (SW) layer. This paper describes the detailed geotechnical engineering properties of the sub-soils at both abutments, gives a bathymetric profile of the river bed at the bridge crossing and recommends the design parameters for the piles such as the end-bearing-capacities for the piles to be used for the bridge support

Subsurface Geotechnical Engineering Investigations for a 9.833-Km Long Road and 130-Meter Wide Bridge in a Karst Topography in South-South Nigeria

A proposed 9.833-kilometer road to evacuate quarried limestone materials to a state cement factory near the city of Calabar in the south-south sub-region of Nigeria passes through a 130-meter wide Kwa River, six meters deep with one and half kilometers of swampy terrain on either sides. The right-of-way of the road on both sides of the river is composed of 1.00 meter thick brownish lateritic Sands and Silty Clays (SC) underlain by 2.00 meters of gravelly lateritic Sands (SW). Below these are indurated shales and limestone (SH/LST) that make up the Mfamosing limestone sequence. Borings made at both abutments of the 130-meter wide Kwa River indicated that 2.00 meters of Organic Silty Clays (OL), 4.00 meters of Well-graded Sands and Gravels (SW) and 6.00 meters of Sandy Silty Clays (SC) form the lithology at the left abutment of the proposed bridge across the Kwa River, while 1.00 meter of greyish Sandy Silty Clays (SC) and 5.00 meters of organic Silty Clays (OL) form the materials at the right abutment of the same bridge. These materials are all underlain by indurated Shale and Limestone to depths beyond 30.00 meters at both abutments of the bridge. Geotechnical engineering properties of the Sandy Silty Clays (SC), the organic Silty Clays (OL) as well as the well-graded Sands and Gravels that lie on top of the underlying Shale and Limestone indicate that lengths of piles that will bear the weight of the proposed bridge should be about 15.00 meters on the left abutment and about 10.00 meters on the right abutment. The paper describes in details, the lithology and engineering properties of the sub-grade materials at intervals of 0+500 meters along the entire length of the road as well as proffers the construction methods to be adopted in emplacing piles in the karst topography

Geotechnical Investigations for Design of Foundations for Erosion and Flood Control Structures at Unwana Beach, Afikpo, Ebonyi State, South-Eastern Nigeria

Erosion occasioned by annual floods along the Cross River at Unwana Beach, Afikpo, situated on latitude 8° 00’ 00’’ North of the Equator and longitude 5° 40’ 15’’ East of the Greenwich Meridian, has threatened the location of a Water Pumping and Treatment Station nearby. Geotechnical investigations carried out using the Shell-and-Auger rig along the shoreline of the Unwana Beach indicated that the subsurface consists of between 3.00 to 8.00 meters of Yellowish brown silty clay layer (CL) underlain by about 3.00 meters of Dark, clayey sands (SC). These are further underlain by between 0.00 – 4.00 meters of Reddish gray, mottled silty clay (CL) and between 1.00 – 1.50 meters of Black stiff silty clays (MH). Underlying all these is a Black fissile Shale layer that extends beyond the limiting 20.00-meter depth of boring prescribed by the clients. Standard Penetration Tests (SPT) indicate that the upper-most Yellowish brown silty Clay layer has N-values of between 15 and 45, while the underlying Dark clayey sand layer has average N-values of 20. The Reddish brown, mottled silty clay layer (CL) beneath has average N-values of 22, while the Black fissile shale layer has N-values more than 50 ( that is, refusal ). The computed allowable bearing capacities (based on N-values and Terzaghi’s classical soil mechanics approach) for the subsurface materials at the project site indicate that the upper Yellowish brown silty clays (CL) have q(allowable) of 236.06 and 139.51 kPa respectively; Dark clayey sands (SC) have 188.84 and 195.67 kPa respectively and the Reddish gray mottled silty clays (CL) have 173.11 and 201.60 kPa respectively. Bathymetric surveys carried out perpendicular to the shoreline at five sections indicated that the maximum depth to the river bed at the proposed site for a Landing Jetty, at the date of investigations ( 11-22-2002), was 6.20 meters. Steel sheet piles were recommended and used as foundation systems for the shore protection works with the length of sheet piles equal to H + Df + h, where H = depth to the bottom of the river bed at low-low water, Df = depth of embedment of pile into the bearing medium and h = height of sheet pile above the river bank cliff (free-board) at the time of investigations ( 18th – 28th November, 2002). Wales of steel-type were used as reinforcement for the emplaced sheet piles, with their vertical separations approximately equal to 1.50 meters. Steel tie-backs were used to restrain the emplaced sheet piles from undergoing flexural and / or buckling failures, with tie-back vertical separations equal to 1.50 meters and tie-back horizontal separations approximately equal to 2.00 meters. Additionally, anti-corrosion protection for the tie-backs was asphaltic materials and concrete encasements

Alignment and Design of a 73-Km Long Coastal Road in the South-Central Segment of the Niger Delta, Nigeria

A 73-km East-West Coastal Highway that traverses six major rivers within the Mangrove and Coastal Hydro-meteorological Zones of the Niger Delta is to be built. The total number of river crossings along the five intervening sections of this road is 36.The Niger Delta Sub-region lies at the southern-most portion of Nigeria. Geotechnical investigations along the road profiles showed between 10-18 meters of thick Organic Clays (OH) underlain by 2.50-4.50m thick Silty-clays (OL) along the first three Sections (A,B,&C) of the road. These have saturated densities (γsat) of 10-15.40 kN/m2; PI ~15.00-35.00%; cohesion (c) ≤24.50-68.50kPa, low strength (qult ≤ 12.00 kPa) and relatively high settlement values of δult ~ 0.056m-0.072m. Poorly-graded sands (SP) and well-graded sands with high bearing capacity values (482 – 4,250kPa) lie beneath these at depths of 20m and 30m, respectively. Most of the road alignments were submerged, with few points lying 0.30m above water level during the time of the investigations (December – March). Sections D &E of the road have relatively thinner soft layers (2.00 – 2.50m thick) underlain by sands (SP and SW) with relatively high bearing values of 582-4,250kPa. The large thicknesses of compressible layers underlying most portions of the road alignment require special pavement construction techniques such as: (i) Excavation of 2.50m of the soft layer materials; (ii) Emplacement of vertical pre-fabricated Geo-drains; (iii) Emplacement of woven geotextiles atop the pre-fabricated Geo-drains, (iv) Emplacement of about 4.50m high sand-dump on top of the woven geotextiles, (v) Allow for settlement of the underlying soft layer corresponding to t50, in this case ~1.14 years. Settlement computations obtained prior to- and after pre-loading phases were 0.0608m and 0.670m, respectively. Geosynthetic reinforcements were to be used in the pavement construction of the highway in order to attain a four-fold pavement structure consisting of: (a) Bound layers made up of (i) Overlay, (ii) Surface layer and (iii) Binder layer course; (b) Either bound or Unbound made up of (i) Base; (c) Unbound layers made up of (i) Sub-base, (ii) capping and (iii) Protection layer; (d) Sub-grades made up of (i) Stabilized sub-grades and (ii) Sub-grade proper. For most portions of the remaining Sections D and E, where the thin upper soft layers are less than 1.25m these are to be scraped off before emplacement of the Bound layer directly on top of Sub-grades. This paper describes the geotechnical characteristics of the sub-soils along the entire 73-km of the road alignment and the pavement design considerations adopted

Need for Prior Geotechnical Engineering Studies for Foundation Design: Cases of Collapsed Buildings in Port Harcourt and Environs, Nigeria

Cases of collapsed buildings have been on the increase in the city of Port Harcourt and environs in Rivers State, in particular and other major cities in Nigeria in recent times. A critical evaluation of the modes of failures indicates that absence of and / or inadequate subsurface geotechnical investigations have been responsible for these building foundation failures. Case histories of four major building foundation failures within the municipality of Port Harcourt and environs in the southern Niger Delta sub-region of Nigeria in recent times are presented and discussed in this paper. The first case history involves a five-storey building that collapsed because it was constructed across a river channel that had sand and gravels as major subsurface materials beneath the building site. As a result of excessive increase in groundwater table during the rainy season and the attendant excessive pore water pressures build-up that led to a rapid loss of the bearing strength of the subsurface materials, it collapsed in the form of a “punching failure”. The second case was a bearing capacity failure due to rapid construction that did not leave enough time for the dissipation of pore water pressures to allow the foundation soils gain shear strength. It collapsed soon after construction was completed. The third case failed as a result of lack of sufficient time to allow for curing of the block materials used for the building. This was a case of structural failure. The fourth case failed as a result of a complete lack of soil investigations that prevented a detailed foundation design for the residential buildings near the banks of a creek at Opobo town, a suburban settlement along a tidal creek. The paper presents and discusses in details the geology, hydrogeology and modes of failures of these four structures and draws attention to the need to carry out detailed subsurface investigations and abide within the building codes (if any)

PHYSICAL MODELLING IN ROCK SLOPE STABILITY EVALUATIONS

In addition to the well-known planar and wedge type failure modes in rock slopes, several secondary modes of failure may also occur in open pit mines. These include slide-toe-toppling, slide-head-toppling and cantilever-type failure. In this research these failure modes were studied using both physical modelling and analytical techniques, with the physical modelling technique incorporating an inclinable base-friction table. Results indicate that the major controlling factors for both slide-toe- and slide-head-toppling failure modes are (i) dip of the bedding plane; (ii) interblock and blockbase friction angles and (iii) block weight ratios (BWR). The factors that control the stability of the cantilever-type failure mechanism are (i) length of protrusion of cantilevered block; (ii) thickness and weight of cantilevered block; (iii) weakness planes, their distribution and frequency in the cantilevered strata and (iv) the weight of rock overlying the cantilevered section. Some field behavior patterns of rock materials on a slope prior to failure were also studied using physical models. In this work, emphasis was placed on the shape and configuration of the slope blocks. Results from these tests indicate that (i) the failure of a rock slope is time-dependent. A general relationship obtained which relates elapsed time and cumulative horizontal displacement of any rock slope block, has the form LogU = mLog(t/t(,1)) where U = cumulative horizontal displacement of slope blocks; t = elapsed time; t(,1) = time intercept and m = time exponent (slope of obtained straight line); (ii) there is commonly a gap opening between slope blocks in the top and sometimes intermediate layers of a given slope geometry; (iii) a possible backward rotation of top and intermediate layer slope blocks, commonly results in the closure of earlier formed tensile openings; (iv) an initial vertical displacement (positive or negative) of fore-slope blocks due to slumping (or rotation) and (v) eventual progressive failure of the rock slope beginning with the fore-slope working inward with time