Using Dynamic Cone Penetrometer Tester to Determine CBR and Bearing Pressures of Subsurface Soils in Parts of Owerri , Southeastern Nigeria

Using dynamic cone penetrometer tester (DCPT) to determine the CBR and bearing pressure of subsurface soils in parts of Owerri, southeastern Nigeria were investigated in this study. Six (6) DCPT were measured to the depth of 6 m. The data obtained from field DCP test was plotted on the graph of penetration resistance (mm/blow) versus penetration depth (m); which revealed the soil profile of three layers with different penetration consistencies and resistances. The highest PR (mm/blow) recorded was 11.4 mm/blow and the lowest is 55.5 mm/blow.  The layers encountered during the measurement ranges from loose, medium and dense soils, where the average thicknesses of loose layers ranges from 0.1 to 0.5 m, medium layers 0.5 to 3.0 m and dense layers 3.0 to 6 m. The average CBR values recorded at interval of 1m to 6 m depths are as follows: 5%, 8%, 12%, 15%, 16% and 16%. Accordingly, the averages bearing pressures calculated were 104.8 KN/m2, 165.5 KN/m2, 231.9 KN/m2, 283.3 KN/m2, 291.5 KN/m2 and 301.1 KN/m2. It shows significant increase in strength with depths.


I. INTRODUCTION
Urbanization and infrastructural development is rapidly taking place in Owerri (study area) ranging from road construction and erection of high-rise buildings/edifices at strategic locations in the municipality, thus knowledge of soil strength characteristics in the study area has become important.
The dynamic penetrometer consists of a metal rod with a conical tip at one end, an anvil or strike plate around the rod and a sliding hammer with a fixed mass at the other end.The cone is pushed into the soil by successive blows of the sliding hammer against the anvil.The striking of the hammer provides a known amount of applied kinetic energy.
Herrick and Jones [18] described a dynamic cone penetrometer for use in soil investigation, enabling cheap, repeatable soil strength assessments in the field.
DCP test is simple, cost effective equipment and has been recommended to be an appropriate technology for use in developing countries [32].In spite of limited research on DCP test for estimating allowable bearing stress for shallow foundations design and construction, Sowers and Hedges [34], Sanglerat [32], Cearns and Mckenzie [15] and Ampadu Published on December 3, 2018.
A. C. Nwanya is with Arab Contractors Nigeria Ltd and consultant at Scope Geotechnical Laboratory in Owerri, Imo State, Nigeria.(e-mail: nwanyatony@gmail.com).
O.C. Okeke is with the Department of Geology, Federal University of Technology, Owerri, Nigeria.(e-mail: ositachris@yahoo.com)[5] have made significant strides in that aspect.In response to the need for simple and rapid device for characterization of subgrade soils, Kleyn [20] contributed to DCP development in the area of its application to determine insitu properties of road pavement layers in South Africa.He further considered the use of DCP in pavement evaluation and monitoring in terms of structural evaluation of pavement, comparing in-situ CBR and structural monitoring.Abu-Farsakh et al. [2] recognized DCP as excellent and reliable device to use in evaluating the strength (stiffness) of tested materials.It is inexpensive, easy to use, and records a continuous profile of the stiffness of the material throughout the depth tested.
Moreover, the DCP can test to a greater depth; therefore, the DCP is an excellent tool for assessing unbound base and subgrade stiffness.Brevik [12] evaluated a dynamic penetrometer with a pocket push penetrometer to see which one had the most reproducible readings.The results indicated that the dynamic penetrometer gave more consistent, reproducible results when compared to the pocket penetrometer.DCP is useful for site investigation during the initial exploration stage and it is widespread [17].He noted that DCP is employed to determine the stratigraphy, thickness, the soil type in terms of consistency, and the lateral extent of the lithologies.At detailed site investigation stage, DCP test is still employed to determine some geotechnical design parameters [23].
The Penetrometer constant energy (blow) would develop a higher penetration index or penetration resistance (PR) value in soil layer of low strength bearing pressure as compared to a soil layer of high strength bearing pressure [32], [33], [30], [11].Ampadu and Arthur [6] investigated compaction verification using the DCP in lateritic soil for construction of sub-base for roads and obtained a relationship.After using a DCP to evaluate the in-situ strength of many pavement projects, Kleyn et al. [20] found that DCP testing can be applied to construction projects to evaluate the potentially collapsible soils, construction control, efficiency of compaction, stabilized layers, subgrade moisture content.The Wisconsin Department of Transport (DOT) (Crovetti and Schabelski) [16] applied DCP and rolling wheel deflectometer testing for construction acceptance and found that both are viable tools for identifying poor areas of in-situ subgrade.The compact design of DCP makes it very practical to be used in the field, especially in rugged terrains [21].The dynamic cone penetrometer is still being used for road testing and specified in ASTM D6951 [9].This study had provided information of the stratigraphic properties, CBR and bearing pressure of undisturbed subsurface soils in the study area through field Dynamic Cone Penetrometer Test (DCPT).

A. Location of the study Area
The study area; Owerri comprises Owerri Municipal, Owerri-west and Owerri-north Local Government Areas with parts of Mbaitoli and Ikeduru, It lies between latitudes 5 0 20 ʹ to 5 0 32 ʹ N and longitudes 6 0 51 ʹ to 7 0 08 ʹ E, covering an area of about 740 km 2 (Fig. 1).The area is predominantly low-lying with a good road network (Fig. 1) and the terrain is characterized by two types of land forms namely, the undulating in the northeast and nearly flat topography in the southwest [19].The topographical map of the study area shows that the highest point in the area is 121.9 meters above mean sea level while the lowest point is 45.7 meters above mean sea level.

B. Geology of the Study Area
The study area is underlain by Benin Formation, which is a significant geologic unit of Niger Delta Basin (Fig. 2).The Benin Formation is Oligocene in age.It is composed of continental flood plain sands and alluvial deposits.It is estimated to be up to 2000 m thick [35].The uppermost part of Benin Formation has been altered by interglacial transgressions during the Quaternary period [27].The upper layers of the Late Quaternary Delta are comprised of interbedded layers of sand, silt and clay deposited during changes in sea level [3].The Benin Formation is better known for its hydraulic properties and groundwater yielding potential [4], [29], [25].According to Abam et al. [1] Benin Formation is relatively on the surface and encounters most physical infrastructural development that entail significant foundation engineering work, this makes the understanding of its geotechnical properties imperative.
The stratigraphy of the southeastern Nigeria (part of the study area) has been studied in detail by Uma and Egboka [37].The stratigraphic succession of rocks in the study area (Table I) consists of Nsukka Formation, being the oldest formation and followed by Imo Shale, Ameki Formation, Ogwashi-Asaba Formation and Benin Formation [37].a) The Nsukka Formation: This was deposited during the Maastrichian -Upper Cretaceous age [37].This formation consists essentially of dark shale, sandy shale, carbonaceous shales with thin coal seams.The grains sizes are medium to coarse.It has some sandstone members reported to be up to 15 meters.Nsukka Formation is the oldest formation with the thickness of more than 300 meters [26].b) The Imo Shale: Imo Shale of Paleocene age was deposited during transgression period that followed the Cretaceous [37].It developed as thick blue to greyish clay with a maximum thickness of about 1000 meters.Uma and Egboka [37] reported Imo Shale as laminated clayey-shale which is impervious and characterized by lateral and vertical variation in lithology.It dips at angles 17 o to 25 o to the southwest and the south [36].It includes three constituent sandstones: The Igbabu, Ebenebe and Umuna Sandstones outcropping at Imo River.The Umuna sandstone is composed of thick sandstone units and minor shales and is generally less than 70 meters thick.The Ebenebe Sandstone occurs as a lens in the northwestern extremity.It is similar in lithology to the Umuna sandstone but is relatively thicker with a maximum thickness of 130 meters [36].c) The Ameki Formation: Overlying the Imo Shale is the Ameki Formation, which extends far south to the Okigwe area where most of it is covered by the Benin Formation.It was deposited during Eocene age [37] and consists essentially of greenish-grey clayey sandstone, shale, and mudstones with interbedded limestone.Uma and Egboka [37] reported Ameki Formation as grey clayey-sandstone and sandy claystone.The lithologic units of the Ameki Formation fall into two general groups [31], [39], [7]; an upper grey to green sandstone and sandy clay and a lower unit with fine to coarse sandstone and intercalations of calcareous shales and thin shaly limestone.d) The Ogwashi-Asaba Formation: This was deposited during Tertiary-Oligocene-Miocene age [37].It is made up of variable succession of clays, sands and grits with seams of lignite.Uma and Egboka [37] reported this formation to be made up of unconsolidated sand with lignite at various layers.e) The Benin Formation: This was deposited during Miocene-Recent age [37].The Benin Formation consists of coarse-grained gravelly sandstones with minor intercalations of shales and clays.The sand units which are mostly coarse grained; pebbly and poorly sorted contain lenses of fine grained sands [28].The sands and sandstones are coarse to fine, partly unconsolidated with thickness ranging from 0 to 2100 m [10].Benin Formation is in part crossstratified with the forset beds alternating between coarse and fine-grained sands.The petrographic study on several thin sections shows that quartz makes up more than 95 % of all grains [28].

A. Field Measurements and Sample Collection
Dynamic cone penetrometer Test (DCPT) method as designated in ASTM D 6951/D 6951 -09 [9], covers the measurement of the penetration rate of the dynamic cone penetrometer with 8 kg hammer through undisturbed soil to the depth of 6 m in the study area.DCP measurements were done in parts of Owerri, and average of four readings taken from each test location was recorded as one measurement for the area.Six (6) DCP testing were measured, samples collected and Global Positioning System (GPS) coordinates taken at the same locations with sample numbers: Sp1, Sp2, Sp3, Sp4, Sp5 and Sp6 (Table II and Fig. 3).The DCP testing readings were recorded at intervals of 0.5 m to 6 m depth.
In the field, the number of blows required to advance the cone by 0.5 m depth into the soil is what was measured and recorded.DCP testing are expressed in terms of the penetration index or resistance (PR), which is defined as downward vertical movement of the DCP cone produced by one drop of the sliding hammer (inch/blow or mm/blow).DCP testing consists of using the DCP's free-falling hammer to strike the cone, causing the cone to penetrate the base or subgrade soil, and then measuring the penetration per blow, also called the penetration rate in mm/blow.This measurement denotes the stiffness of the tested soil layers, with a smaller penetration index or resistance (PR) number indicating a stiffer material.In other words, the PR is a measurement of the penetrability of the subgrade soil.The CBR of the undisturbed soil in the study area was calculated from the empirical method in equation ( 2) as stated by the U.S Army Corps of Engineers (USACE) and Webster et al. [38].log() = 2.465 − 1.12 log () (2)

Sp4
where, CBR = California Bearing Ratio (%) PR = Penetration Index or Resistance (mm/blow) The soil bearing pressure (KN/m 2 ) was calculated using Dutch formula as stated in equation (3).Dutch formula is a dynamic formula which is based on the assumption that kinetic energy delivered by the hammer during penetration by dynamic penetrometer is equal to work done by the entire penetrometer system.It is founded on the newton's principle of energy and impact motion.At point of striking the hammer, most of the kinetic energy is transferred from hammer to the anvil which in turn is transmitted to the lower rods and finally to the cone which dissipated in the soil.Sanglerat [32] work involved the use of Dutch formula for the dynamic resistance.

IV. RESULTS AND INTERPRETATION
The DCP measurements reported a highest penetration index or resistance (mm/blow) of 11.4 mm/blow and lowest of 55.5 mm/blow (Tables IV,V,VI,VII,VIII,IX).The calculated average CBR (%) and bearing pressures (KN/m 2 ) of the soils' layers in the study area are presented in Table III.The results showed significant increase of strength with depths.
The graphs of penetration index (mm/blow) versus penetration depth (m) revealed averages of three layers of loose, medium and dense soils, though 99 % is mediumdense soil layers (Fig. 4 to 9).It also revealed the statistically uniform layers that can be identified at the points where the directions sharply reverse, indicating border between uniformed layers.The thickness of the loose soil layers in the study area ranges between 0.1 m and 0.5 m depths, medium soil layers ranges from 0.5 m to 3.0 m depths and that of dense soil layers ranges from 3.0 m to 6 m depths.Penetration index values increased with decreased in CBR (%) and bearing pressure (KN/m 2 ) as recorded in Table III.The AASTHO pavement design guide [8] described a method of determining uniform unit and boundaries of uniform section of the sub layer and it was employed in this study.The graphs of penetration index (mm/blow) versus penetration depth (m) were plotted to show general soil profile and strata characteristics (Figs. 4 to 9).The penetration index values were used to estimate the CBR (%), bearing pressure (KN/m 2 ) and strata thicknesses (m) (Figs. 4 to 9).The Minnesota Department of Transport (Kremer) [22] developed a specification stating that CBR values of > 8 % calculated from penetration index (PR) does not need remedial procedures.The average CBR values calculated in the study area, between 1 m to 2 m depths revealed weak layers that need remedial procedures (Table III).It is clearly that CBR values recorded from 2 m to 6 m depths in the study area meet the requirement as stated in Kremer [22].
The B.S 8004 [13] outlined presumed bearing pressure (KN/m 2 ) values for categories of soils and types (Table X).
Base on the presumed bearing pressure outlined in B.S 8004 [13], the soil in the study area fall within the categories of non-cohesive soils of loose to medium-dense sandy gravel.
The graphs of penetration index (mm/blow) versus penetration depth (m) shows soil of different layers in the study area ranging from loose, medium and dense (Fig. 4 to  9).
According to Building Code [14] as shown in Table XI, granular soil layers with number of blows > 30 mm/blow is classified as dense soil layers (Class 3a ), >10 to 30 mm/blow is medium soil layers (class 3b) and < 10 mm/blow as loose (class 6) soil layers, respectively (Table XI).The soil samples collected from the study area are granular (fine to medium sandy gravel); hence 99 % of the layers' densities fall within class 3b and class 3a (medium to dense).The shape and the sharp reverses recorded on the graphs of penetration index (PR) versus penetration depth, revealed three layers of soil in the study area (Figs. 4 to 9).The AASHTO pavement design guide [8] described a method of determining uniform unit and boundaries of uniform section of the sub layer.
The knowledge of soil bearing pressure and geotechnical characterization of an area is important in order to map areas of weak subsoil layers and potentially collapsible soil.V. CONCLUSIONS Field measurements identified three layers of soil to the depth of 6 m and 99 % of the layers are mediumdense soils.The soils in the study area are granular and noncohesive mediumdense sandy gravel.From the CBR results, weak layers were identified within 0.1 m to 1 m (loosemedium soils) while strong layers from 1 to 6 m (mediumdense).The bearing pressures in the study area increased with decrease in penetration index or resistance.

Using
Dynamic Cone Penetrometer Tester to Determine CBR and Bearing Pressures of Subsurface Soils in Parts of Owerri, Southeastern Nigeria Anthony C. Nwanya, and O. C. Okeke

Fig. 2 .
Fig. 2. Geological Map of Imo State, Nigeria Showing the Study Area

TABLE I :
STRATIGRAPHIC SUCCESSION OF ROCKS IN THE STUDY AREA

TABLE II :
SAMPLE LOCATION POINTS WITH GPS COORDINATES IN THE STUDY AREA

TABLE IV :
DCP READING FOR SAMPLE 1 IN THE STUDY AREA

TABLE V :
DCP READING FOR SAMPLE 2 IN THE STUDY AREA

TABLE VI :
DCP READING FOR SAMPLE 3 IN THE STUDY AREA

TABLE VII :
DCP READING FOR SAMPLE 4 IN THE STUDY AREA

TABLE VIII :
DCP READING FOR SAMPLE 5 IN THE STUDY AREA

TABLE IX :
DCP READING FOR SAMPLE 6 IN THE STUDY AREA

TABLE X :
BUILDING CODE CLASSIFICATIONS OF SOILS LAYERS IN TERMS OF DCP NUMBER OF BLOW (AFTER BC, 2008)

TABLE XI :
BRITISH STANDARD PRESUMED BEARING PRESSURE FOR CATEGORIES OF SOILS (AFTER BS, 8004)