An Attempt to Study Foundation Anchoring Conditions in Sedimentary Estuaries Using Integrated Methods

Abstract

The search for and knowledge of the best conditions for anchoring the foundations of certain structures such as bridges, tunnels and quays in sedimentary estuaries is a challenge, for both scientists in general and engineers in particular. Indeed, wharves are structures that receive a lot of stresses and therefore require anchoring to avoid tilting and to guarantee their stability during service. This work, based on the analysis of data from seismic refraction methods, mechanical soundings and laboratory tests, characterises the terrain of the Wouri estuary in Central Africa. The objective is to determine and present the subsurface layers encountered as well as their characteristics, in order to define the best conditions for anchoring the foundations to ensure the stability of the quays to be built there. The seismic refraction campaign shows that the study area is relatively heterogeneous over the first 25 m, with velocities measured in the range 1520–1750 m/s; modulated in two distinct ranges, between 1520–1580 m/s characteristic of mud and loose sediments (alternating layers of clay, sand, loose silt) and the range 1580–1750 m/s corresponding to the signature of sandy-silty or compact clays. The mechanical tests show sedimentary soils, with alternating layers of sandy clay and clayey sand over the 42 m drilled, loose over the first 30 m in the bank area and over the first 15 m in the canal or dredge area, with a limit pressure of less than 1 MPa. Similarly, the soil samples taken and tested in the laboratory show that the soils are clayey over the first 30 metres, plastic and liquid with respect to their water content, respectively, below and above the liquidity limits, confirming their loose character. The results of seismic refraction, mechanical soundings and laboratory tests show that, in estuarine areas characterised by alternating sandy clay and clayey sand, there are not always hard formations in the first 25 metres of depth but, from a depth of 30 m, the soils become moderately compact and begin to form an anchoring layer sufficient to guarantee the stability of the quays against earth pressure forces.

1. Introduction

The development of various types of trade in estuaries is accompanied by the need to provide these estuaries with large engineering structures such as bridges, quays and ports [1,2]. However, the sedimentary deposits generally encountered there, which are sometimes loose in consistency, pose the thorny problem of their recognition in order to guarantee the stability and durability of the foundations of the future structures that will be built there, which are essential conditions for their operation, durability and economic profitability [3,4,5]. The study of foundation anchorage conditions for structures in sedimentary estuaries is therefore a challenge for scientists in general and engineers in particular [1,2,3,4,6]. The Wouri estuary (Figure 1) is a large estuary in Central Africa, where several rivers join and flow into the Gulf of Biafra [5,7,8]. Given its connection to the Gulf of Guinea, it is the crossroads of many national, sub-regional and international economic exchanges [9]. The development of these numerous activities requires the construction of special civil engineering works, including wharves [10,11,12]. Indeed, wharves are land-supporting structures used for mooring ships and for the landing of cargo and people. The function of wharves therefore exposes them to numerous stresses which generally result in thrust forces that make their construction very complex, particularly their stability, which depends on the control of their embedding point or their anchoring in the ground [13,14]. The embedding or anchoring in the ground is dependent on the physical-mechanical characteristics of the supporting soil, which poses the problem of foundation depths influencing the stability of future quays [15,16,17]. This work, based on seismic refraction, mechanical soundings and laboratory tests, characterises the terrain in the Wouri estuary in Central Africa. The aim is to determine and to present the subsurface layers encountered and their mechanical characteristics to ensure the stability of the docks to be built there. The development of various types of trade in estuaries is accompanied by the need to provide these areas with major engineering structures such as bridges, quays and ports [1,2].

2. Materials

2.1. Study Area Description

The Wouri estuary, or Cameroon estuary (Figure 1), is located in Central Africa in Cameroon, where several rivers join and flow into the Gulf of Biafra [8,18,19]. This area consists of recent sandy-clay alluvium [20] and is located in the Douala sedimentary basin [21,22,23,24], whose Cretaceous transgression led to the deposition of the first ammonite sediment [25]. A second transgression in the Miocene led to the deposition of these detrital deposits, which consist mainly of sands and clays. The large depths are due to the reduction of the wetted section after the infilling of the western side in the absence of dredging [26,27]. In the hydrogeological point of view, the Wouri is a coastal river of Cameroon that rises in the western heights of the country, about 200 km north of Douala, at the foot of the volcanic massif that forms the western border of the territory. Thirty kilometres from its mouth, it widens into an estuary flowing from North-East to South-West and is about 1.2 km wide. The area explored in this estuary is about 0.3 km square, where the crossing structures are concentrated and where people and goods are unloaded daily and then transported to the factories located on the quays built on the banks of the estuary. Figure 1 shows the geographical location of the site.

2.2. In Situ Measurements

The present site has been subject to various data collection methods (Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5). Bathymetry and side scan sonar were used to map the reverbed relief and to identify possible obstacles. The surveys were deployed over an area of 850 m × 450 m. In order to meet the objectives of the geophysical campaign, i.e., to determine the thickness of the surface layers, the flute refraction method was used. The study area is divided into two parts: the area of the quay, on the eastern side, for which the depth of investigation is 30 m below the river bottom, and the area of the western channel, concerned by the dredging, for which the depth of investigation is 15 m below the river bottom. Twenty-five seismic refraction profiles, each 235 m long, with 48 hydrophones spaced 5 m apart, were carried out, 11 of which were perpendicular to the coast and 14 parallel to the coast and along 6 main lines. For each of them, 6 shooting positions were carried out. As regards the mechanical test pits, six pressure test pits (PMT) and eight standard penetration test pits (SPT) were drilled. Intact soil samples were also taken from the same boreholes for laboratory identification.

3. Methods

3.1. Bathymetry and Sides-Can Sonar

The GPR multi-beam bathymetry system is based on the latest technological advances in seabed mapping [28]. Positioning data is recorded by ultra-accurate RTK GPS and motion reference unit (MRU), bathymetry and sides are recorded simultaneously, allowing for accurate target location and the production of detailed maps and 3D models. The system provides excellent bottom coverage (up to 12 times the water height) with full nadir coverage (no slack). It is lightweight, easily transported and can be installed on boats of different sizes. Sides can sonar surveys provide acoustic imagery of the riverbed or the submerged part of structures such as dykes and dams, based on acoustic waves. To measure the topography of the submerged riverbed, bathymetry uses the emission of acoustic waves (from 30 to 200 kHz) to determine the height of the water column.

3.2. Seismic Refraction by Streamer

The seismic refraction method, due to its versatility, is one of the most commonly used geophysical methods in engineering, mining, groundwater exploration and environmental investigations. Based on favourable density contrasts that generally exist between geological materials, the refraction method is used to provide detailed information on the distribution and thicknesses of subsurface layers with characteristic seismic velocities. Overburden and basement rocks may be classified to some degree to discriminate for example, surficial sediments from consolidated soils or highly fractured rock from competent rock. This technique is widely used for the assessment of pipeline cable burial and for the assessment of river dredging. Operations involve laying out a seismic streamer with several pressure-sensitive hydrophone receivers (usually 24 or 48), at the takeout points on the streamer. Hydrophone spacing is strongly dependent on the depth of search and the desired resolution for a given survey (Figure 2). A pattern of shot points is then executed within and off the ends of the cable and the seismic wave arrivals for each hydrophone are recorded in the seismograph. The key piece of recorded information is the time of the first arrival. This arrival is the direct wave, or more commonly, the refracted wave which occurs when seismic energy propagates along a geological interface having a sufficiently great velocity contrast. This contrast must consist of a higher velocity zone underlying a lower velocity zone, fortunately the most common geological condition.

Interpretation of the seismic data involves resolving the number of velocity layers present, the velocity of each layer, and the travel time taken to travel from a given refractor up to the ground surface. This time is then multiplied by the velocity of each overburden layer to obtain the thickness of each layer at that point.

According to Equation (1) [29], the method consists of laying a receiver array (generally a streamer made of 2 × 24 hydrophones spaced 5 m) on the riverbed and performs shots with an air gun seismic source at specific locations: central shot, in the centre of the streamer, near shots, at both extremities of the streamer, in close position to hydrophones 1 and 48.

$$\frac{sin{\theta }_1}{V_1}=\frac{sin{\theta }_2}{V_2}=p$$

(1)

${\theta }_1$ is the incident angle, ${\theta }_2$ the angle of refraction, $V_1$ and $V_2$ are, respectively, the velocity of the first and second layer, $p$ is called the ray-path parameter.

The acquisition boat carries positioning and navigation systems, the recording system and a radio link for the source triggering. The acquisition boat is moored close to the end of the array to ease connections with the seismic recorder. The operation boat carries air-gun, compressed-air bottle and streamer. The streamer is deployed and recovered by hand and its position is recorded at three to five locations marked with surface buoys. The contact of the streamer with the riverbed is ensured through small dead weights. Air gun is deployed close to riverbed. Shots are performed successively at each of the specific locations. Both ends of streamer (i.e., position of hydrophones 1 and 48) and each shot are positioned using DGPS positioning at each mooring line (Figure 2).

Figure 2. Seismic refraction principle. (a) Propagation of refracted; (b) Deployment of refracted; (c) Array and shot distribution.

Figure 2. Seismic refraction principle. (a) Propagation of refracted; (b) Deployment of refracted; (c) Array and shot distribution.

3.3. SPT Type Mechanical Borehole

The standard penetration test (SPT) is the oldest and most practiced mechanical test in the world. This test consists of beating a core barrel of defined characteristics and dimensions into the ground at the bottom of a borehole. After the borehole has been drilled and held in place by a mud or tube, the core barrel is lowered into the borehole and beaten in three stages. The number of strokes N_i required for each 15 cm drive is recorded, i.e.,: N₀ (0 to 15 cm test drive); N₁ (first test drive from 15 to 30 cm); and N₂ (second test drive from 30 to 45 cm). The number of significant impacts N, also known as the penetration resistance [30,31,32], is defined by Equation (2):

$$N=N_1+N_2$$

(2)

If, after more than 50 blows, the penetration does not exceed 15 cm, the test shall be stopped and the corresponding penetration noted. At the end of the test, the core is recovered in order to assess the lithological nature of the soil tested. From thousands of tests, carried out in particular in the United States, correlations have been established between N and the following characteristics: the compactness of the sands and their angle of internal friction; the simple compressive strength of the soils; the bearing capacity of the foundations; and the risk of liquefaction of the sands.

3.4. Pressuremeter Test

The pressuremeter test consists of the stepwise expansion of a cylindrical probe in the soil [30,31,32,33,34]. The tests are generally carried out every metre in a preliminary drilling. A stress-strain curve of the soil in place is thus obtained, which makes it possible to determine three parameters characterising the soil: the creep pressure $p_f$; the limit pressure $p_l$ and the pressure modulus $E_M$. Generally, only these three parameters are provided. If there is any doubt about the correct conduct of a test, the expansion curve should be requested. In practice, the net creep pressure ($p_f^{*}$) and the net limit pressure ($p_l^{*}$), are defined by Equations (3) and (4):

$$p_f^{*}=p_f-p_o$$

(3)

$$p_l^{*}=p_l-p_o$$

(4)

where $p_o$ represents the total horizontal stress in the soil at the time of the pressure test. When its value is not specified in the geotechnical report, it is calculated by the relationship (5):

$$p_o=u+{\sigma }_{vo}^{\prime }\ast K_o$$

(5)

where ${\sigma }_{vo}^{\prime }$ is the effective vertical stress in the soil at the level considered; $u$ is the pore pressure at the same level; $K_o$ is the coefficient of buoyancy of the soil at rest of the formation concerned, the value of which, in the absence of any other indication, may be taken as 0.5.

In addition to the classification of soils, this test is used to design the foundations of a structure.

Table 1. Conventional soil categories from the PMT [8,13,14,15,30].

SOIL CLASS		NATURE of SOIL	LIMIT PRESSURE P1 (MPa)
CLAYS SILTS	A	Loose clay and silt	<0.7
	B	Solid clay and silt	1.2–2.0
	C	Solid to stony clay and silt	>2.5
SANDS AND GRAVELS	A	Loose	<0.5
	B	Moderately compact	1.0–2.0
	C	Compact	>2.5
CHALKS	A	Loose	<0.7
	B	Altered	1.0–2.5
	C	Compact	>3.0
MARLS, MARLY-LIMESTONES	A	Soft	1.5–4.0
	B	Compact	>4.5
STONES	A	Altered	2.5–2.40
	B	Fragmented	>4.5

summarises the classification of soils by means of the PMT.

4. Results and Discussion

4.1. Riverbed Topography

Prior to seismic operations, side scan sonar and update bathymetry surveys were carried out (Figure 3). The results of bathymetry survey are presented in Figure 3a and side scan survey in Figure 3b.

Figure 3. Bathymetric map. (a) Side Scan Sonar Survey of the Douala’s Estuary area from [35], modified; (b) Bathymetric map of the Douala’s Estuary area from [35].

Figure 3. Bathymetric map. (a) Side Scan Sonar Survey of the Douala’s Estuary area from [35], modified; (b) Bathymetric map of the Douala’s Estuary area from [35].

The bathymetry survey shows a range of depth comprise between 0 and 19 m below the water level. We notice a maximum deepening in the North-West corner. The near shore refraction survey followed previous bathymetric and side scan sonar surveys, carried out in August 2014, evidencing two mains areas in the North East: the first one, with plant fragments at sea estuary, and the second one showing a punctual obstacle.

4.2. Seismic Sounding and Profiles

The Figure 4 shows the overlay of the bathymetric surveys and the layout of the 25 profiles listed in

Table 2. Coordinates of the transverse seismic profiles.

GEOPHYSICAL SURVEYS
Coordinates of the Realised Profiles
Transversal Lines
Lines/Profiles	Start of Profile		End of Profile
T1	910,940.95	1,449,419.4	910,710.53	1,449,421.7
T2	910,656.41	1,449,279.5	910,929.63	1,449,275.59
T3	910,944.06	1,449,471.7	910,717.15	1,449,474.93
T4	910,949.94	1,449,383.2	910,702.68	1,449,379.69
T5	910,901.23	1,449,217.3	910,665.33	1,449,222.55
T6	910,935.68	1,449,319.9	910,704.72	1,449,317.14
T7	910,942.96	1,449,650.5	910,709.98	1,449,651
T8	910,938.23	1,449,600.8	910,716.13	1,449,599.7
TA1	910,944.6	1,449,554.8	910,713.04	1,449,552.84
TB2	910,910.11	1,449,079.7	910,677.55	1,449,089.42
TB3	910,887.71	1,448,979.9	910,662.64	1,448,984.86
Number of profiles realized	11

and

Table 3. Coordinates of the longitudinal seismic profiles.

Longitudinal Lines
Lines	Profile	Start of Profile		End of Profile
L1	1st profile	910,930.55	1,449,795.66	910,926.8	1,449,555.46
	2nd profile	910,934.16	1,449,544.78	910,932.89	1,449,311.97
	3rd profile	910,931.3	1,449,298.61	910,904.2	1,449,056.47
L2	1st profile	910,953.03	144,449,639	910,949.97	1,449,406.84
	2nd profile	910,949.84	1,449,381.27	910,927.41	1,449,152.69
L4	1st profile	910,920.59	1,449,815.5	910,924.06	1,449,558.62
	2nd profile	910,911.86	1,449,544.46	910,921.15	1,449,312.39
	3rd profile	910,916.56	1,449,252.8	910,901.6	1,449,022.78
L5	1st profile	910,846.66	1,449,833.82	910,849.63	1,448,602.29
	2nd profile	910,847.75	1,449,576.22	910,844.48	1,449,341.64
L7	1st profile	910,745.75	1,449,844.45	910,747.71	1,449,612.96
	2nd profile	910,747.57	1,449,596.55	910,746.33	1,449,363.16
	3rd profile	910,748.12	1,449,253.09	910,749.59	1,449,066.67
L9	1st profile	910,663.18	1,449,519.53	910,664.54	1,449,746.14
Number of profiles realized	14

showing that, the study was carried out according to 25 seismic refraction profiles, 11 of which were transverse and 14 longitudinal.

Figure 4. Layout of the seismic profiles.

Figure 4. Layout of the seismic profiles.

The geodetic parameters and transformations used for the implementation of the profiles are summarised in

Table 4. Geodetic parameters and transformations [36,37,38].

Local Datum Geodetic Parameters
Ellipsoid	International 1924
Semi-major axis:	a = 6,378,388.000 m
Inverse Flattening:	1/f = 297.00
Datum Transformation Parameters from international 1924 to Local Datum
Shift dX: −28.209999 m	Rotation rX: 0 arcsec
Shift dX: 132.589996 m	Rotation rY: 0 arcsec
Dy:
Shift dZ: 77.730003 m	Rotation Rz: 0 arcsec
Projet Projection Parameters
Map Projection:	Universal Transverse Mercator
Grid System:	UTM Zone 31
Org Scale:	0.999
1 parallel:	33°00′00″
2 parallel:	45°00′00″
Longitude:	10°30′00″
Latitude:	00°00′00″
False Easting:	1,000,000 m
False Northing:	1,000,000 m
Units:	Meter

.

4.3. Velocity Calculations

The seismic compressional velocities, according to

Table 5. Range of Vp.

Name arrays	L1.1	L1.2	L1.3	L2.1	L2.2	L4.1	L4.2
Velocity (m/s)	1520	1530	1530 to 1580	1530 to 1580	1530	1530 to 1540	1550
Name arrays	L4.3	L5.1	L5.2	L7.1	L7.2	L7.3	L9
Velocity (m/s)	1530	1530	1530	1530	1530 to 1570	1530 to 1750	1630
Name arrays	T1	T2	T3	T4	T5	T6	T7
Velocity (m/s)	1520 to 1620	1520 to 1580	1520	1520	1520	1530	1530
Name arrays	T8	TA1	TB2	TB3
Velocity (m/s)	1630	1530	1520	1530 to 1750

, are between 1520 and 1750 m/s for the entire network.

4.4. Time-Distance Curves and Seismic Sections

The time-distance curves and the seismic sections obtained are presented in Figure 5. This figure shows that, the seismic compressional wave velocities can be divided into two ranges: the 1520 to 1580 m/s range is predominantly for the first arrivals (profiles L1, L2, L4, L5 and L7, L9, T2, T3, T4, T5, T6, T7, T8, TA1 and TB2) and the 1580 to 1750 m/s range is predominantly for the second arrivals (profiles L7, T1 and TB3).

Figure 5. Time-distance curves of the seismic profiles and seismic section. (a1–a9) seismic profiles, (b1–b9) Seismic sections.

Figure 5. Time-distance curves of the seismic profiles and seismic section. (a1–a9) seismic profiles, (b1–b9) Seismic sections.

4.5. Determination of Investigation Depths and Seismic Velocities

For the profiles where only the first wave of velocities was collected, the calculations of the investigation depths were made on the basis of the assumption of a two-layer soil model with an average velocity of 1530 m/s for the first terrain and a maximum velocity of 1750 m/s for the second terrain, showing terrains from 0 to 25 m and more than 25 m, respectively [39,40,41] (Figure 6).

4.6. Material Identification

The combination of the seismic refraction results (

Table 6. Typical P wave velocities for common materials.

Material	Vp (m/s)
Air	330
Water	1450–1530
loess	300–600
Soil	100–500
Sand(Loose)	200–2000
Sand (dry, loose)	200–1000
Sand (water saturated, loose)	1500–2000
Glacial Moraine	1500–2700
Sand & Gravel	400–2300
Clay	1000–2500
Estuarine mods/clay	300–1800
Floodplain alluvium	1800–2200
Sandstone	1400–4500
Mudstone	1600–5000
Limestone	1700–7000
Dolomite	2500–6500
Anhydrite	3500–5500
Shales	2000–4100
Granite	4600–6200
Basalt	5500–6500
Gneiss	3500–7600

) shows that the are similar to mud and loose sediments while the are similar to very loose sands and silts or compact clays [42].

4.7. Mechanical Borings

The layout of the boreholes is shown in Figure 7.

4.8. PMT Results

The PMT results obtained from the boreholes (Figure 8 and

Table 7. Results of pressuremeter tests.

N°	Borehole N°	Depth (m)	Soil Description	Borehole Pressuremeter Testing
				${\mathit{p}}_{\mathit{l}}^{*}$ (MPa)	EM (MPa)	$\mathbf{EM}/{\mathit{p}}_{\mathit{l}}$
1	SP1	2.30 to 9.00	Fine to fair sand with plant debris	0.29–0.33	2.77–3.57	8.39–12.13
2		9.00 to 18.00	Fine sand	0.22–0.76	2.76–16.93	8.12–41.36
3		18.00 to 26.00	Aletenating fine mica sand with clay	0.32–0.49	4.69–10.75	9.57–33.59
4		26.00 to 30.00	Coarse to fine sand with litle clay	0.89–0.91	6.26–9.01	6.88–10.12
5		30.00 to 36.00	Coarse sand with litle gravels	1.04–1.08	7.00–18.69	6.73–17.31
6		36.00 to 42.00	Fine mica sand with thin clay layers	0.83–1.17	5.51–10.64	6.43–9.09
1	SP2	1.65 to 4.00	Sandy clay with mica fine sand	0.39	1.19	4.90
2		4.00 to 7.00	Mica fine sand with thin layers of mica silt and plant debrits	0.33–0.40	1.41–1.90	4.27–4.75
3		7.00 to 12.00	Clay alternating with fine sand	0.39	1.90	4.87
4		12.00 to 16.50	Greyish fine sand	0.40–0.90	6.08–1.36	3.40–6.14
5		16.50 to 19.00	Fair to fine sand	0.70	6.02	6.48
6		19.00 to 38.00	Alternating fine mica sand with clay	0.70–1.28	6.02–17.88	5.70–22.07
1	SP3	3.7 to 10.00	Fine sand with plant debrits	0.4–0.56	3.22 and 6.22	8.05–11.14
2		10.00 to 13.00	Clay	0.3–0.36	1.35 and 1.69	4.50–4.60
3		13.00 to 28.00	Fine mùica sand alternating with clay	0.3–0.36	1.35–1.69	4.50–4.60
4		12.00 to 16.50	Greyish fine sand	0.27–1.13	1.25–14.99	5.49–20.30
5		28.00 to 32.00	Coarse, gravelly sand	2.33	12.12	5.20
6		32.00 to 34.00	Fine sand	0.8	4.09	5.11
7		34.00 to 41.00	Stiff clay to claystone	1.82–2.93	29.57–39.58	10.09–21.75
1	SP4	2.08 to 7.00	Fine mica sand alternating with clay	0.29–0.33	3.49–3.99	10.58–13.76
2		7.00 to 14.00	Fine sand withb plant debrits	0.16–1.07	1.26–9	7.88–8.50
3		14.00 to 23.00	Fine sand alternating with thin clay layers	0.28–0.89	1.24–46.21	4.43–61.61
4		23.00 to 29.00	Fine and	0.77–0.81	23.21–32.44	28.65–42.13
1	SP5	1.91 to 6.00	Marine mud with litle sand	0.12	0.60	5.00
2		6.00 to 11.00	Mica fine sand with plant debrits	0.15–0.33	2.80–9.10	8.84–60.67
3		11.00 to 32.00	Fine sand alternating wiyh thin clay layers	0.55–1.06	2.12–32.67	5.33–59.40
4		32.00 to 35.00	Mixture of fine fair, gravelly sand with litle clay	0.80	5.23	6.54
5		35.00 to 42.00	Coarse sand with gravel and cobbles	1.16–1.33	5.66–11.70	4.88–8.80
1	SP6	3.00 to 18.00	Mica fine sand alternating with thin clay layers	0.15–0.91	1.64–9.10	6.07–24.59
2		18.00 to 23.00	fine sand	0.99	27.56	27.84
3		23.00 to 28.00	clay	0.54–0.90	7.54–14.05	12.38–20.07
4		28.00 to 34.00	Alternating fine sand with clayand trace gravels	0.65	4.93	7.58
5		34.00 to 40.00	Mixture of fair, coarse, gravelly sand with cobbles	0.61–1.41	6.83–14.92	5.25–17.36

) show that, up to a depth of 42 m, the soils in the Wouri estuary are essentially sedimentary and consist of alternating sandy-clay and clayey-sand layers [42]. This heterogeneity is characterised by a strong dispersion of the pressure parameters. In fact, the limit pressure varies from 0.12 MPa to 2.93 MPa and the pressuremeter modulus from 0.6 MPa to 46.21 MPa.

4.9. SPT Results

The results obtained from SPT are summarised in Figure 9 and in

Table 8. Results of SPT.

N°	Borehole N°	Test Depth (m)	SPT Resistance N°	Nature of Soil	Classification
					Granular Soil (1–5)	Cohesive Soil (A–F)
1	SP1	3.85–1.20	1 $\le$ N $\le$ 2	Fair sand with mica	1
2		12.0–17.0	3 $\le$ N $\le$ 7	Fine sand with little mica	1–2
3		17.0–26.0	4 $\le$ N $\le$ 8	Clayey fine sand to fine sand with thin clay layers	2
1	SP2	2.985–6.90	1 $\le$ N $\le$ 3	Sandy clay to fine mica sand with peat		A–B
2		6.90–16.35	4 $\le$ N $\le$ 9	Stiff clay to fine sand		B–C
3		16.35–19.45	10 $\le$ N $\le$ 15	Fair to fine sand with thin clay layers	3
4		19.45–24.15	N = 3	Compact clay with thin layers of fine sand		B
5		24.15–26.15	N = 9	Fine sand with thin layers of clay	2
1	SP3	5.25–7.15	N = 4	fine sand thin layers of plant debris	2
2		7.15–13.25	2 $\le$ N $\le$ 6	Clay to mica fine sand		B–C
3		13.25–25.29	4 $\le$ N $\le$ 12	Fine sand altermating with thin clay layers	2–3
4		25.29–33.05	5 $\le$ N $\le$ 15	Coarse gravelly sand to fine sand	2.3
5		33.05–4.06	12 $\le$ N $\le$ 89	Greenish stiff clay		D–F
1	SP4	3.63–4.08	N = 1	Clay with thin layers of fine sand	2	A
2		4.08–7.92	N = 2	Fine sand alternating with thin layers of clay and plant debris	4
3		7.92–14.62	7 ≤ N ≤ 9	Fine sand	2
4		14.62–20.71	7 $\le$ N $\le$ 9	Fine sand with thin clay layers	2
5		20.71–26.01	11 $\le$ N $\le$ 13	Fine sand	3
1	SP5		N = 0	Marine mud	1
2			0 $\le$ N $\le$ 6	Fine sand with mica and plant debris	1–2
3			N $\le$ 1	clay		A
4			N $\le$ 3	Fine sand	1
5			N $\le$ 5	Fine sand alterneting with thin layers of clay	2
6			1 $\le$ N $\le$ 3	Clay alternating with thin layers of fine sand		A–B
7			N $\le$ 2	Fins sand	1
8			N $\le$ 6	Clay alternating with thin layers of fine sand		B
9			1 $\le$ N $\le$ 13	Mixture of fairn gravelly sand with little cobbles	1–3
1	SP6	3.1–5.9	1 $\le$ N $\le$ 2	Fine sand alternating with thin layers of clay	1
2		5.9–12.28	1 $\le$ N $\le$ 4	Clay alternating with thin layers of fine sand		A–B
3		12.28–15.98	1 $\le$ N $\le$ 2	Clayey fine sand to fair sand with trace coarse sand		A
4		15.98–20.03	6 $\le$ N $\le$ 7	Fine sand alternating with thin compact clay layers	2
5		20.03–26.06	2 $\le$ N $\le$ 5	Clay with thin layer of fine sand		B
6		26.06–29.56	3 $\le$ N $\le$ 5	Fine sand alternating with thin clay layers with gravels at botton	2
7		29.56–38.24	8 $\le$ N $\le$ 16	Mixture of fair, coarse and gravelly sand with little cobble	2–3
1	SP7	3.8–4.25	N = 2	Fine, fair reddish black sand	1
2		4.25–14.05	3 $\le$ N $\le$ 4	Fine sand with thin layers of plant debris	2
3		14.05–18.45	N = 2	Gray fine sand	3
1		4.05–6.50	N = 2	Fine sand with thin layers of clay with trace gravels	1
2		6.50–8.75	N = 1	Clay with alternating thin layers of fine sand		A
3		8.75–10.03	N = 3	Peat with fine sand	2
4		10.03–19.03	4 $\le$ N $\le$ 8	Fine sand with thin layers of clay with plant debris	2
5		19.03–22.24	N = 4	Clay with thin layers of fine sand		B
6		22.24–23.94	N = 6	Fine sand alternating with thin clay layers	2
7		23.94–25.64	N = 23	Clay with thin layers of fine sand		D
8		25.64–39.60	12 $\le$ N $\le$ 24	Mixture of fine, fairn coarse and gravelly sand with little cobbles	3
9		39.60–41.80	N = 28	Fine to fair sand with plant debris	3

. Similar to the pressuremeter tests results, the SPT confirm the sedimentary nature of the soil, as well as the alternation of sandy-clayey and sandy-clayey soils; in addition, a strong mechanical heterogeneity is observed, characterised by the strong variation of the number of cuts N, which ranges from 0 to 89.

4.10. Laboratory Tests Results

Soil samples were taken during the pressuremeter tests and analysed for identification in the laboratory.

Table 9. List of samples taken per borehole.

Borehole N°	N° of Samples	Depth (m)
Borehole SP1	4	UD1 (6.25 to 8.00)
		UD2 (10.10 to 11.10)
		UD3 (19.40 to 20.40)
		UD4 (6.25 to 8.00)
Borehole SP2	5	UD1 (7.65 to 9.20)
		UD2 (13.45 to 14.95)
		UD3 (20.20 to 23.70)
		UD4 (27.0 to 28.50)
		UD5 (35.50 to 37.00)
Borehole SP3	7	UD1 (7.65 to 9.20)
		UD2 (13.45 to 14.95)
		UD3 (20.20 to 23.70)
		UD4 (27.0 to 28.50)
		UD5 (34.75 to 36.25)
		UD6 (40.06 to 41.56)
		UD7 (40.86 to 42.36)
Borehole SP4	5	UD1 (6.23 to 7.73)
		UD2 (12.42 to 13.92)
		UD3 (17.17 to 18.67)
		UD4 (22.06 to 23.56)
		UD5 (27.41 to 28.91)
Borehole SP5	8	UD1 (6.9 to 8.40)
		UD2 (11.0 to 12.50)
		UD3 (15.05 to 16.55)
		UD4 (21.24 to 22.74)
		UD5 (24.84 to 26.34)
		UD6 (30.52 to 32.02)
		UD7 (39.71 to 41.21)
		UD8 (39.71 to 41.21)
Borehole SP6	6	UD1 (7.10 to 8.60)
		UD2 (12.83 to 14.33)
		UD3 (16.18 to 17.68)
		UD4 (21.48 to 22.98)
		UD5 (26.41 to 27.91)
		UD6 (32.13 to 33.63)
Borehole SP7	0
Borehole SP8	4	UD1 (2.55 to 8.60)
		UD2 (15.08 to 16.58)
		UD3 (25.04 to 27.14)
		UD4 (32.09 to 34.39)

summarises the number of samples taken for each borehole and the depths explored.

The results of the laboratory tests are presented in

Table 10. Results of laboratory tests.

N°	Borehole N°	Depth (m)	MECHANICAL TEST					Type of Soil	N°	Borehole N°	Depth (m)	MECHANICAL TEST					Type of Soil	N°	Borehole N°	Depth (m)	MECHANICAL TEST					Type of Soil
			Atterberg Limits (%)		Density (T/m3)							Atterberg Limits (%)		Density (T/m3)							Atterberg Limits (%)		Density (T/m3)
			WL	IP	Yd	Yh	Ys					WL	IP	Yd	Yh	Ys					WL	IP	Yd	Yh	Ys
1	SP1	4.00–5.00	Nm	Nm	1.11	1.61	2.62	Black Silty sand	1	SP2	7.65–9.20	Nm	Nm	1.21	1.66	2.62	Black silty sand	1	SP3	12.76–14.26	69.42	37.23	1.15	2.03	2.61	Grayish clay
2		10.10–11.10	57.6	33.7	0.69	1.28	2.55	Black muddy clay	2		13.45–14.95	Nm	Nm	1.47	1.92	2.65	Black coarse sand	2		20.50–22.0	60.6	28.1	1.15	1.64	2.64	Grayish muddy clayey sand
3		19.40–20.40	Nm	Nm	1.11	1.56	2.64	Black coarse sand	3		22.2–23.70	70.54	28.24	1.34	1.61	2.62	Grayish silty sand	3		23.34–24.84	Nm	Nm	1.17	1.58	2.63	Grayish coarse sand
4		25.00–26.00	Nm	Nm	1.29	1.69	2.67	Black coarse sand	4		27–28.50	52.92	21.93	1.08	1.47	2.45	Grayish silt	4		29.13–30.63	Nm	Nm	1.51	1.92	2.6	Grayish coarse sand
5		-	-	-	-	-	-	-	5		35.5–37	67.84	22.89	1.03	1.53	2.69	Drayish silty sand with gravels	5		34.74–35.19	53.6	24.4	1.34	1.85	2.62	Yellowish, gravelly clay
6		-	-	-	-	-	-	-	6	-	-	-	-	-	-	-	-	6		40.06–41.56	58.02	24.69	1.08	1.78	2.62	Grayish, gravelly clay
7	-	-	-	-	-	-	-	-	7	-	-	-	-	-	-	-	-	7		40.86–42.36	50.29	25.29	1.27	1.73	2.62	Grayish stiff clay
1	SP4	6.23–7.7	Nm	Nm	1.47	1.86	2.64	Black coarse sand with plant debris	1	SP5	6.9–8.40	-	-	-	-	-	Black coarse sand with plant debris	1	SP6	7.1–8.6	Nm	Nm	1.22	1.68	2.66	Grayish coarse sand
2		12.42–13.92	Nm	Nm	1.12	1.47	2.64	Grayish fine sand	2		11.0–12.50	-	-	-	-	-	Grayish silty sand	2		12.83–14.33	38.88	20.7	0.84	1.48	2.61	Grayish, muddy clayey, sand
3		17.17–18.67	Nm	Nm	1.35	1.79	2.66	Grayish coarse sand	3		15.05–16.55	41.1	16.5	1.57	1.95	2.64	Grayish silty sand	3		16.18–17.68	Nm	Nm	1.53	2.02	2.65	Grayish coarse sand
4		22.16–29.56	Nm	Nm	1.43	1.92	2.62	Grayish coarse sand	4		21.24–22.24	Nm	Nm	1.06	1.67	2.52	Grayish silty sand	4		21.48–22.98	Nm	Nm	1.46	1.98	2.62	Grayish gravelly sand
5		27.41–28.91	Nm	Nm	1.35	1.98	2.64	Grayish coarse sand	5		24.84–26.64	66	24.3	0.98	1.45	2.49	Grayish compact silt	5		26.41–27.59	58.24	32.16	0.96	1.58	2.6	Grayish muddy clay
6	-	-	-	-	-	-	-	-	6		30.52–32	-	-	-	-	-	Grayish silty sand	6		36.09–37.59	Nm	Nm	1.36	1.94	2.68	Grayish coarse sand
7	-	-	-	-	-	-	-	-	7		35.15–36.65	Nm	Nm	Nm	Nm	2.76	Rolled gravel	7	-	-	-	-	-	-	-	-
1	SP8	2.55–4.06	Nm	Nm	1.59	2.02	2.73	Black silty sand
2		15.64–58	39.5	17	1.2	1.58	2.63	Grayish silty sand
3		25.64–27.14	38.5	23.2	1.9	2.5	2.67	Grayish silty sand
4		32.09–34.19	Nm	Nm	Nm	Nm	2.71	Rolled gravel

IP: plasticity index; Y_s: weight of solid grains; Y_d: dry weight; Y_h: density; W_L: liquid limit; Nm: Not measurable.

, confirming the clayey and sandy nature of the soil revealed by the pressure and penetrometric soundings; moreover, they show that the earth pressure on the future building platform will be quite high, taking into account the wet density which varies from 1.28 t/m³ to 2.5 t/m³. The Atterberg limits of the clayey soils encountered show that they are plastic, which translates into a very high deformability and constitutes a poor anchoring ground for the foundations of the structure.

5. Conclusions

The objective of this study is to determine and present the subsoil layers encountered as well as their characteristics, in order to define the best conditions for anchoring the foundations of the quays to be built in a sedimentary environment such as that of the Wouri, using seismic refraction, mechanical soundings and laboratory tests. The results of the seismic refraction studies are presented in the form of a lens and make it possible to define two groups of speeds: 1520 to 1580 m/s; 1580 to 1750 m/s. The low velocities characterise loose soils for the first arrival group without distinction between sand and clay layers. The second group characterises the compact formations, probably consisting of sand over the 30 m surveyed. The mechanical boreholes show an alternation of sandy clays and loose clays over the 42 m depths investigated. Seismic is not able to identify this alternation of layers, but we measured an average value characterising the loose formations. According to the objectives of the geophysical study and the calculated theoretical depth of investigation, there is no compact formation in the first 25 m in the bank area and in the first 15 m in the channel area. Finally, the study shows that the anchoring of the quay can only be considered from 30 m.