What benefit from the clam shells are you seeking, and what kind of garden (flowers, vegetables, native plants)? A soil amendment should not be needed unless the results of a laboratory soil test show either particular nutrient deficiencies, very low organic matter content, or a pH (acidity level) that a particular plant would need adjusted for it to thrive. You would not want to change soil pH if the plants growing in that area are doing well and not showing any indications of a nutrient deficiency (even if they were, a soil test would still be helpful before any additives are used).

The only reliable way to prevent wildlife damage to a vegetable garden is to enclose it with fencing (some gardeners construct an entire "room," complete with roof, since some pests can climb, like groundhogs). You can see a small-scale example of an enclosure built around a tomato bed in a Master Gardener demonstration garden in the linked video. Repellent sprays might not work all the time, and some formulations also might not be safe to use on edible plants; they also need regular re-application, whereas fencing is more-or-less set once put in place.

Miri