Geotechnical Investigation of Chico, California Report (Assessment)

Problem statement

The city council of Chico, California is planning to construct a multistoried shopping complex on 341B, Main Street. Since the soil and the underlying rock are expected to carry the weight of the building, a geotechnical study is needed. To determine the nature of the foundation to be used onsite information about soil, rock, and ground water conditions are required. This investigation established the site state.

Literature review

Geological Setting

Based on the Geologic Map of the Chico Quadrangle, the geology underlying the site is comprised of Modesto Formation alluvium fluvial. This formation is characterized as alluvial terraces and some alluvial fans and abandoned channel ridges comprised of un-weathered gravel, sand, clay, and silt.

Regional Faulting

Fault Activity Map of California and Adjacent Areas (Jennings, 1994) shows that the closest known active fault which has surface displacement since Holocene time is the Cleveland Hills fault. It is located about 25 miles south of the site.

Method

Field Investigation

We observed and described the surface and subsurface soil, rock, and ground water conditions at the site using the procedures cited in the American Society for Testing and Materials. The soil color was described using the general guideline procedure in the Munsel soil Chart. Engineering judgments were used to extrapolate the observed surface and subsurface soil, rock, and groundwater conditions to areas located between and beyond the subsurface exploratory locations.

Subsurface investigation

The subsurface coil conditions were investigated by hand auger exploratory borings and performing a dynamic cone penetrometer test at the site.

The other important method to be used is Percussion boring. A machine will be used to loosen the soil by repeated blows of heavy chisel or spud, and the resulting slurry will be removed by shell or washing. The struck of each blow was controlled mechanically.

Laboratory Testing

Laboratory tests on selected soil samples were performed. The procedures used were the American Society for Testing and Materials (ASTM) and the Caltrans test Method (CTM) guidelines. The following are the test and procedures used.

1. ASTM –Particle Size Gradation (sieve and hydrometer), Soil Classification by the USCS, Soil Description (Visual manual Method), and Atterberg Plasticity Indices.
2. CTM- Resistance value (R-Rule).

Observations

Surface conditions

It was observed that the surface of the site is flat (<1degree slope angle) consisting of asphalt cover.

Subsurface Soil conditions

The following observations were made during percussion boring.

Stable rock was found at 10 feet below ground surface.

Type of soil

SM, Silt Sand; the soil consisted of 55% of sand, and 45% of plasticity silt and clay size particles. The soil was predominantly brown with a Munsel Color Chart designation of (7.5YR 4/5). The soil was loose to medium dump dense and dry to dump at the time of the subsurface investigation.

Groundwater conditions

No shallow groundwater was observed within the exploratory borings (maximum of 10 feet) during the site investigation. Local shallow groundwater conditions at the site were controlled by the seasonal elevation of Big Chico Creek adjacent to the proposed area.

Conclusions

The conclusions are based on information developed from the field study and laboratory investigations.

1. The site is suitable for the proposed shopping complex provided that the recommendations are incorporated into the earthwork and structural design.
2. Before construction, the investigation team should be allowed to review the proposed earthwork grading plan and structural plan to confirm that the geotechnical engineering recommendations have been incorporated.
3. The soil is predominantly loose to medium dense for noncohesive soil and soft to medium stiff for cohesive soil in the upper 10 feet.
4. The soil conditions below the existing ground service generally consisted of loose Silt Sand (SM).
5. There is no shallow groundwater within a depth of 10 feet. The depth of groundwater at the site is expected to fluctuate seasonally at depth of 12 to 14 feet below the ground surface on the seasonal flows of Bog Chico Creek.
6. The silt and sand soil has the following engineering material properties; moderate cohesion, no plasticity, nonexpansive, and medium bearing capacity.

Recommendations

1. Clay, sand, and silt must be excavated to start the foundation on stable ground. The excavation will be 10 feet deep from the ground surface.
2. The spread foundation should be used for the construction of the shopping complex. Based on the initial design, a spread foundation is the most suitable.
3. Reinforced squire pads should be used where columns will stand. Unstable soil must be removed from these spots to allow for the safe transfer of load.
4. The level where the foundation will stand should not be more than 11 feet below the ground surface. This will ensure that the foundation is free from groundwater.